Construction - Insulating with Rice Hulls - The Planning Stage

SPOILER ALERT -- Summer, 2024

Do not use rice hulls for insulation.  We did and lived to regret it.  Here's our story.

As a methodical and cautious early adopter, I thoroughly researched rice hulls and thought I understood the risk of rice hull weevil infestation.  My contact person at the mill assured me that weevils cannot survive the parboiling of the whole grain that is done before separating it from the hulls.  In the absence of any information to the contrary, despite considerable research, I decided to take a chance on them.  

My research told me that weevil reproduction required a grain of rice into which an egg is laid and the larva develops until emerging as an adult weevil.  In the process. the rice grain is consumed.  If there were rice grains in the sample bag of hulls sent by the mill, they were so inconspicuous we did not see them.  Consequently, we assumed that, even if a few existed in the insulation, they would soon be consumed by baby weevils until no more grains existed.  Besides we did not see any weevils in the sample, although, in retrospect, they were much tinier than we expected and were simply overlooked. 

My research also revealed that diatomaceous earth is an insecticide for bugs with exoskeletons by finding its way under their skeletons and dehydrating them.  So a cup of diatomaceous earth was mixed with each bag of rice hulls as they were blown into he wall and ceiling cavities, all the while thinking that this extra effort was probably overkill but advisable since the stakes were high and our knowledge so limited. 

Three years later, we are beginning to think that we know more about rice weevils than those speaking and writing about them at the time we did our research.  Clearly they reproduce in the absence of rice grains.  The "hulls" would have had to include a huge amount of grain in order to support such exponential infestation.  Moreover, we are not only seeing a large number of adult weevils outside the confines of the wall, principally on the window sills, we are seeing quite a few viable larva a well.  They are larger than most adult weevils, but since they have no means of locomotion, they must be transported out into the open by adults.

A major oversight on my part is that diatomaceous earth as a backup would prevent an infestation.  However, the adults obviously live long enough to reproduce before dehydrating and the eggs each female is capable of laying numbers in the hundreds.  Therefore, new bugs are developing faster than the old bugs are dehydrating.  I must say that the vast majority of adults that we see are dead, many with their feet in the air.  So it is safe to assume that, no sooner do they emerge into the open, they die from dehydration.  Supposedly most are able to fly but we see very few actually doing so although some of those on the sills flap their wings like they are trying.  But, even when prodded, they are so debilitated as to be incapable of flying.

There will be no solution to the weevil infestation short of opening up the affected walls and ceilings, removing the rice hulls and re-drywalling over conventional insulation.  Fortunately, because we designed the exterior walls for minimal air leakage, the only easy egress for the weevils is through the openings in the sub- window sills through which the hulls were blown into the spaces below the windows.  Currently I am sealing these and watching for any other escape routes.  These efforts will suffice for the limited time we have left (ages 88 and 91) but our heirs will need to take more definitive action.

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So far, there are just under 140 individual posts to this blog.   Two posts have attracted far more visitors over the past several years than the others.  Most visited has been the post about the plumbing rough-in under the concrete floor, including the homeruns for the PEX water supply system.  The second-most popular, with about

half as many hits, is the one about rice hull insulation posted back in the spring of 2016 that introduced the unfamiliar concept of insulating with rice hulls.  This post and, perhaps as many as three additional posts, describe how we planned for and bought a semi-trailer-load of rice hulls, the equipment needed to blow them into the wall and ceiling cavities, the atypical sequencing of the drywalling necessary for their use and several other hints and observations that we learned about the hulls.  To be sure, the whole process has been an adventure. 

Insulation Blower

Early on, I learned that the insulation blowers available at the big box stores were incapable of handling rice hulls due to their finer texture and their slightly heavier per-unit weight.  My good friend, Keith, being a master innovator, accepted the blower dilemma as a challenge and began experimenting with non-insulation blowers, such as hand-held and tractor-mounted leaf blowers, before giving up and searching the web for alternatives.  Eventually, he learned about a commercial machine that at least one person had reported using successfully with rice hulls.  The ideal model for our situation, FORCE ONE , was one of several models made by the Intec Corporation located in Frederick, CO.  When we called the company we found that that model had been discontinued but newer models would be equally effective but at a much higher price.  Ultimately, through Keith's effort, we were able to find a used Force One in good condition on Ebay.  

Special Effort to Prevent Leakage of the Hulls

Unlike conventional insulation, rice hulls are small enough

ZIP tape sealing junction
between the electrical box
and the drywall

to escape the wall and ceiling cavities through small openings such as those in and around electrical boxes and around plumbing stub-outs.  So we spent extra time eliminating such openings with spray foam and Zip tape.  Doing so also helped to air-seal the drywall but, as discussed in previous posts a seal at the drywall side of the wall is not especially critical since, on the exterior side of the truss walls, we sealed the joints between plywood sheathing panels with ZIP tape and, on the interior, sprayed foam insulation along the mudsill, in transitions between concrete and stick-built walls and around penetrations for electrical wires and water lines.  The extra sealing on the drywall side however will help to minimize moisture penetration into the wall and ceiling cavities that, in excess, could compromise the R-value of the insulation, encourage mold and, as explained below, compromise the insecticidal effectiveness of the diatomaceous earth that we will be blending into the insulation.

Reduced size cover plate
used as a guide for cutting
the tape back enough to be 
hidden under a normal size
cover plate, shown here before
cutting.

As will be described in a subsequent post on the drywalling phase, using rice hulls for insulation required an atypical sequencing of the drywall on the exterior walls.  We will start with the first course at the floor for the walls and next to the wall on the low side of the cathedral ceilings.  After the hulls are blown behind one tier of drywall, the next tier will be added and filled as well, working up the wall and across the ceiling.

Calibrating the Blower

A sample bag of rice hulls came in handy for validating the efficacy of the blower and preliminary calibration of it  by blowing the hulls back and forth between two appliance boxes that were separated by ~30 feet. It took only a few back-and-forths to determine the best size of the opening in the bottom of the blower hopper for a steady stream of hulls. The exercise also familiarized us with the remote controls on the blower.  We even took one of the boxes to the far corner of the second story to see if elevation slowed the flow of hulls through the 50' hose.  It did not. 

Friends Myron (at the hopper)
and Keith (with remote control
and hose in hand) testing
the blower.  

The sample hulls raised enough dust to warn us that mask-wearing while insulating would probably be necessary irrespective of COVID, particularly, as discussed below, when we include diatomaceous earth with the hulls.

Estimating the Quantity of Hulls

After vigorous stirring of the 50 lb sample bag in an effort to fluff up the hulls, we became skeptical that each bag would yield 7 cu ft when blown into the wall or ceiling as contended by the supplier.  Blowing the hulls back and forth between the boxes did not seem to increase the volume very much, if any, over just stirring.  So  I used 6 cu ft to calculate our needs.

The volume of rice hulls that we will need for the exterior walls (15" thick) and the ceilings (18" thick) turns out to be just under 5,000 cu ft.  A 53' tractor-trailer load comprises 768 bags (50 lb each).  The hulls are compressed for bagging such that a bag contains 5 cu ft.  If 768 bags expand to 6 cu ft when blown into the wall and ceiling cavities, the total volume for a truckload would be 4,600 cu ft, slightly less than our needs.  If they expand to 7 cu ft, a truckload might be even slightly more than enough.  The plan is to proceed with a truckload and see how far it goes then, if necessary, decide what to use to finish insulating.  If very little additional insulation is needed, perhaps locally-available cellulose would be the best choice for finishing.  If the amount needed is excessive and the price differential between hulls and cellulose is substantial, it might make sense to pay freight on a few more pallets of hulls.

Receiving and Handling the Shipment of Rice Hulls

In the March 2016 post on rice hulls for insulation, I was unaware that they could be purchased bagged and on pallets (if in fact they were actually available then).  I assumed that they would be delivered in bulk on a walking floor trailer.  Having them bagged, though more expensive, will greatly simplify their handling at every stage -- from truck to blower.

In order to avoid commercial warehousing fees and the inconvenience of off-site storage, it took a bit of head-scratching to figure out the best way to receive a truckload of 48 pallets on a narrow almost-one-way dead-end street in the heart of the hilly Mississippi River bluffs.  Finally, we settled on the following plan:  using a retailer friend's parking lot for a drop-off trailer, pickup trucks to move the pallets from the trailer to the storage sites in the garage and living space of the house under construction and a rental pallet jack.  The pallets will be double-stacked in the trailer.  Two double-stacked pallets weighs 1,600 lbs but we envisioned little difficulty moving them to the back of the trailer with a pallet jack then breaking them down so that bags could be handled individually. 

Rice Weevil Problem?

Also in the 2016 post, I was not yet aware of the potential problem of weevil infestation.  The ensuing years have provided time to research rice weevils.  The available information online is spotty and inconsistent but seems to indicate that, as a minimum, we should add diatomaceous earth (DE) to the hulls as they go into the wall and ceiling cavities.  (DE, also known as silicone dioxide, is the best green insecticide for weevils and most other insects with exoskeletons and works indefinitely as long as it stays dry.)  DE is the fossilized remains of microscopic diatoms that, to paraphrase Wikipedia, were protists, a cellular organism with a nucleus that is not an animal, plant or fungus.  The sharp edges on the fossils kill insects by scratching or piercing their exoskeletons, causing them to dehydrate.  (Check out the Wikipedia link for an electron microscopic image of DE particles.)

The hulls we will use come from parboiled rice.  The Riceland Foods, Inc. representative with whom I had been working, contended that parboiling kills all three forms of the weevil -- eggs, larva and adults. Again paraphrasing Wikipedia, parboiling rice makes it easier to process by hand, boosts its nutritional profile, changes its texture and makes it more resistant to weevils.  However, "resistant" is not total prevention and so far I have not found any studies that say unequivocally that parboiling eliminates weevils.

Diatomaceous Earth

In the absence of definitive information on parboiling and weevils, I decided to add DE to the hulls as we insulated but maybe not as much as would be the case if they were not parboiled.  As of this writing, our best source for the kind of DE that we need is a local farm and home store which stocks it as a livestock supplement.

A quick search online reveals that there are two kinds of DE.  One kind goes by various names -- industrial grade, filter grade, pool grade -- while the other is food grade.  The former is inappropriate for our use because it is heat treated or chemically treated that leaves it ineffective as an insecticide and tends to make it a health hazard, particularly with regard to silicosis, although several sources recommend dust masks when using food grade DE as well, not for fear of silicosis but to prevent airway irritation from its microscopic particle size. The amount of food grade DE recommended as an insecticide in grains for human consumption seems to be one cup per 50 lbs of grain, which is probably overkill for our purposes considering that parboiling probably leaves minimal or no weevils to worry about and the hulls will contain hardly any rice grains that weevils would need for long-term survival.  Nevertheless, we decided to go with 1 cup of DE added to each 50 lb bag of hulls.  It will then be in the wall and ceiling cavities indefinitely to control all types of insects with exoskeletons, not just weevils.  

The next post will chronicle our experience with the hulls from receipt to incrementally insulating with them. 

Design - Solar Collector - Problem-Solving

This is fifth post on the solar collector but, undoubtedly, not the last since I will reporting on its performance over time. The purpose of this post is to report that it has not lived up to expectations and to suggest changes that will make it work.  

Lack of Performance

The glass for the collector was installed by the middle of July just in time for lots of clear skies and 90+ degree temperatures that should have maximized airflow from the collector into the conduits.  I gave the system a few days to rev up, thinking that it may take a while for the hot air pushing up the conduits to overcome the cold air dropping down.  During the early afternoon on a bright and hot day, I checked the air flow from the north ends of the conduits and found that, if there was any movement at all, it was so minimal that I could not detect it.  I waited for another sunny day but one without any wind in order to dangle thin strips of paper from the conduit outlets that would detect any air movement.  Still no detectable convection.

At our latitude, the temperature of the soil where the house sits would be in the mid-60s by mid-summer at the depth of the conduits.  However, since the house temperatures during the past couple of winters have stayed above 40 degrees (despite no insulation), it is reasonable to think that the temperature of the earth under the house surrounding the conduits is higher than it would be if not shielded by the house.  If so, the temperature of the air falling out of the conduits might be as high as the 70s but apparently still too cold for the heated air from the collector to overcome. 

The performance was disappointing but not totally unexpected given the dearth of practical information available in print and on the web, which means that our design had to be largely original.  I look at the situation as just another problem that needs to be solved and reported on just like many other surprises and challenges that we have encountered with such a unique build.

Are the Conduits the Problem?

In a previous post, I listed some of the unknowns that come into play.  "Assuming the design of the collector is adequate, its function is still at the mercy of many unknowns about passive air flow through the conduits.  Will 4" diameter conduits be the optimal size for sufficient airflow?  Are conduits that are nearly 90' long from collector to daylight behind the house too long to expect passive flow?  Do they angle upward enough from 10' below floor level when they leave the collector to a depth of 3 or 4' below floor level at the back wall of the house and then make 45 degree turns to daylight?  Will using the corrugated (rather than smooth) piping under the house (the intent for which is to cause turbulence in the air flow and thereby improve heat transfer to the soil) slow the flow too much?  Will the cooler soil during the first winter and, to a lessor extent, after each succeeding winter, cause cool air to flow backwards towards the collector to the extent that the warm air from the collector cannot passively reverse the flow?"   Of all of the items on the list, only the one in italics can be addressed at this point -- the rest are what they are.  At this juncture, I would add one other possibility.  The conduits terminate with two 90 degree fittings in order to keep rain out.  Perhaps if the conduits pointed straight up, there would be less resistance to passive flow.

Is the Solar Collector the Problem?

The problem could also be in the design of the collector rather than in the conduits.  Maybe one layer of galvanized roofing is not enough; maybe multi-layers are necessary.  Maybe there is so much space between the metal and the glass that the volume of heated air is insufficient for spontaneous escape up the conduits.  Perhaps the collector is not large enough to supply nine conduits passively.  Maybe it will be necessary to add to the system one or two what might loosely be called "solar chimneys" whereby the conduits would be brought together and exit to daylight through a common chimney, with or without the assistance of a fan.

At first I assumed the problem lies with the

Termini of the nine conduits.  The one in the middle was
modified to accept a vacuum hose for testing.

conduit portion of the system instead of the collector which means there is only one factor I can test -- the one in italics above. I manipulated the airflow in one conduit to see if it could be jump-started to overcome the effect of the cool soil by cutting away the double 90s in order to fit the end of the conduit with an end-cap having a hole the same size as a vacuum hose.  I pulled air through the conduit with a vacuum for a couple hours hoping that, when the cap was removed, I would feel warm air, or any air for that matter, coming out of the conduit.  Such was not the case. 

The next probable cause for under-performance that could easily be investigated was to measure the amount of heat the collector was producing.  Having assumed that the temperatures would be too high for plant growth, 

Thermometer resting on the metal is maxed out.

I began to suspect that heat generation was insufficient when a couple of plants sprouted along one edge of the collector.  I pulled the plants and placed a thermometer of the common type with a scale to 120 degrees inside the collector.  It recorded temperatures approaching 100 in the early morning when only the west half of the collector was sunlit and the thermometer was shaded by the east wall of the collector.  As the sun reached the glass fully, the temperature readings quickly rose and stopped at the maximum capability of the thermometer somewhat above 120.  And further plant growth has been non-existent.  So, pending the purchase of a thermometer with a higher range, the initial readings are encouraging enough to look elsewhere for ways to make hot air flow through the conduits. 

Reconfiguring the Terminal Ends of the Conduits

It is becoming obvious that the conduits will require redesigning at their terminal ends.  Instead of nine conduits exiting to daylight independently, I am now convinced that they need to converge into one or two solar chimneys fitted with a solar fan(s).  Since each conduit is nearly 90 ft long, I think two chimneys with four or five conduits each would be more efficient than one chimney located 40-50 feet from the termini of the most outlying conduits.  At the time of this writing, it is mid-September and completion of exterior trim for the house is the highest priority.  Reconfiguring the conduits will have to wait until spring.

Construction - Solar Collector - Safety Fence

The previous post covered the functioning part of the collector.  This, the fourth post on the collector, describes the safety fence surrounding it, the design for which needed to meet

The framework for the safety fence around the collector shell.

several criteria.  It needed to be tall enough to meet code for handrails, it needed to be as attractive as possible considering its prominence in front of the house and it should not block the view from the house any more than necessary.

(Reminder: click on any picture to enlarge it for better viewing.)

Upgrading the Top of the Wall
When the collector shell was built five years ago, the top of the walls were covered with 2 x 12 pressure treated planks that were anchored with bolts embedded in concrete.  While they protected the top of the walls, they left much to be desired aesthetically and begged to be replaced.  It took only a few minutes at the landscape supply store to decide in favor of concrete pavers instead of pricey capstones.  I dry-fitted them to determine where the posts for the fence should be located in order to minimize laborious notching of the pavers.  I then installed the posts and notched the pavers using a diamond blade in the radial arm saw.  I also custom crosscut a few pavers as necessary to filled the gaps that would not accommodate full sized units. 

Post Placement
Like the solar collector framework, the pressure treated lumber for the fencing was stickered and air-dried for several months so that it remained straight and would accept and hold paint.  Then, before assembly, it was undercoated on all six surfaces and final coated on at least three surfaces.  The 4 x 4 posts extend below the ground to the depth of 12 - 16" so they needed to be ground-contact rated; the pressure treated lumber for the rest of the railing system needed only to be rated for above-ground use.  The corner posts were "V"-shaped at the bottom so as to rest on the top of the wall and straddle both surfaces of the wall below.  Two long and robust Tapcon concrete screws through each surface was more than adequate to anchor them firmly and, with the sometimes help of composite shims, hold them plumb.  Instead of "V-ing" the bottoms of the intervening posts (south and north sides), they were reduced in thickness by a half so as to rest on top of the wall and extend downward where they were anchored by four screws each.  Backfilling and tamping the soil around all of the posts will help to support them as well.

With the posts in place I could lay the pavers in mortar.  The weather was hot and I got in a hurry to get done so I laid them without the benefit of a mason's line.  The result, I am sorry to say, looked pretty amateurish.    

East and West Sides 
The wood framework for the enclosure supports a wire grid fashioned from cattle fencing that I cut from panels that come 4' high and 12' long.

The fencing for the east and west sides of the enclosure presented a challenge in that it had to be stepped to follow the contour of the stepped walls and its top had to slant to follow

Temporary layout board.  A few of the cap stones
 are yet to be mortared in.

the slope of the stepping.  Since the cattle panels were 4' high, I installed a temporary board 4' above the lowest step and leveled it.  Off of that I could measure the amount that the fence panel would have to be reduced over each step uphill from the lowest step. These measurements, along with the length of each step and the slope at the top, were laid out on a cardboard pattern.  After dry-fitting and tweaking the pattern, I laid it on a section of fencing that had been cut to length and marked the wires to follow its edges.  An angle grinder with a metal-cutting diamond blade easily handled the 4 gauge wire.  The cardboard was accurate enough to be used as a guide for cutting and fitting the support board at the top of the fence.  I used 2-by blocks under the wire panel to hold it off the wall slightly as I attached the board at the appropriate height to catch enough of the top of the fence board for secure fastening.

Dry-fitting the cardboard pattern to the east end of the 

The cardboard pattern in place over the cattle fencing
for the east end of the collector.

collector showed that, with one minor adjustment, it could be used for cutting the east fencing panel. I added the top board on the east at a height that matched the board on the west.  Confident that metal panels were installation-ready, I set them aside so that they would not interfere with laying the pavers with mortar.  

North and South Sides
Compared to the east and west sides, the north and south sides were easy.  For the north side, I installed the top boards at a height that matched the northern ends of the side boards and the bottom boards at 1 1/2" above the pavers.  I added 2x2 nailers to the sides of the corner posts to receive the vertical edges of the

Fencing completed except for caprails.  The ladder
used during construction is still in the collector.

fencing panels.  As an cosmetic touch after the panels were fastened to the top and bottom boards and to the posts, I added  1-bys on the collector side of the entire framework.

I decided to delay the fence for the south side until the glass was installed on the collector in order to leave plenty of access with the heavy glass panes.  I entertained the idea of building an access gate into the south fence but decided against it based on looks, which means that we will have to drop an extension ladder into the shell when access is necessary for maintenance.  The design of the south fence was then identical to the north fence.

Post Caps and Cap Rail
Finally, I added store-bought pressure treated post caps to the tops of the posts -- for aesthetics and to protect the end-grain of the 

In lieu of a gate in the fence, a ladder will have to
be used for servicing the collector

posts from deterioration.  The caps for the top rails will be 2 x 6s that I customize with bi-sloped tops to shed moisture eventually.  Their addition is being postponed while more urgent projects on the house are handled.

Construction - Solar Collector - Functioning Part

The preceding two posts described the science behind the solar collector.  This is the first of two detailing its construction.  

As I was building out the collector, I was designing and redesigning on the fly.  So I think it is worth repeating what I wrote a couple of posts back:  "....... I did a Google search on using solar collectors for passive solar heating and air conditioning and found nothing that resembled our situation.  Even the sources with which I was already familiar were short on details.  So I am erring on the side of too much detail in case someone out there is contemplating Annualized GeoSolar and could benefit from our experience."   I hope the focus on minutiae is not too off-putting for the majority of readers.

Sand Bed
Couple of years ago, I used the trackloader to dump 4 or 5 bucket-fulls of sand into the solar collector shell while it was still possible to reach it without scuffing up the landscaping with the loader tracks.  As luck would have it, the amount was just about right to serve as a foundation for the functioning part of the collector.  Before building the framework for the collector, though, the sand had to be reconfigured so that it tilted upward from south to north at about the same angle as the glass will take when the collector is finished.  Doing so got it out of the way of building the framework on the low side.  

I think that I almost wore the sand out moving it.  After the framework was done, a lot of sand had to be moved temporarily from the inside of the frame to the walkable space south of the frame in order to position foam insulation board.  Then it had to be shoveled back again and smoothed out before the corrugated roofing could be laid on top of it.  But these details are getting ahead of the story.

Building the Framework That Supports the Glass
As explained in the previous post, the angle for

Birdseye view of framework.  The arrow
points to the 18 degree slant of the frame;
the "X" is over the walkable space

the glass was 18 degrees off horizontal.  So I used a chalkline to lay out that angle on the east and west walls of the shell so that the high north end of the layout would be about the width of a 2 x 4 higher than the tops of the nine 4" PVC pipes (conduits) that exit the north wall of the shell, fan out under the house and run to daylight behind the house through which heated air will leave the collector and warm the thermal mass under and behind the house.  Pressure treated 2 x 4s that had been dried by  stickering and storing out of the weather for at least a couple of months were then cut to fit and painted on all six surfaces before installing.  Their length was determined by the length of the salvaged glass panels that were a Craigslist find many years ago -- almost 5" long.  Once the east and west boards were fastened with concrete anchors, pre-painted 2 x 4s were attached to the north wall so that their tops lined up with the east and west boards and their bottoms were at or just above the tops of the conduits.  That then left the more complicated south part of the frame to design and install.

The South Framework
The south framework had to be free-standing

Arrow points to the air intake space 
between the 2 x 6 wrapped in plastic 
and the 2 x 4 top piece that supports 
the glass; encircled is the one of the
footings for the frame

in order to preserve the walkable space between the collector and the south wall of the shell.  In addition, it had to be perforated to allow the intake of air that exceeds the air flowing through the conduits.  And, being free-standing, it needed to be supported in several places.  

The south frame comprised  2 x 4s on the top and  2 x 6s below with an air space in between.  They were dry-fitted in place then suspended at grade level for final assembly.  I used gussets and metal angle iron to fasten them together.  Knowing that the 2 x 6 would be buried partially in sand and knowing that its pressure treatment was not rated for ground contact, I wrapped it in 6 mil black plastic (maximum resistance to UV degradation vs. clear plastic) and taped the seams.  I am hoping that the degree to which it gets wet will be so minimal that rot will not be an issue especially since the buried side will be inside the collector where the environment should be too hot and dry to support mold. 

The finished south frame was lowered to place and supported temporarily at both ends while four concrete piers were fitted into holes in the dirt such that they were resting on undisturbed soil and not quite in contact with the frame.  I then filled the gap between the piers and the 2 x 6 with mortar for a passive fit.  And I stapled galvanized hardware cloth over the air intake to thwart curious creatures. 

Support For the Glass
Rather than try to balance the glass panels directly on the 2-by

Laying in the foam insulation after 
screeding the sand base.

framework, I attached 2 x 2s flush with the lower edge of the framework's 2 x 4s on which to lay the glass.  This arrangement brought the glass more in alignment with the tops of the conduits for what I imagine will improve the passive flow of air between the air intake and the conduits.  I then stapled the thickest and spongiest (non-rubber) weatherstripping I could find to the 2 x 2s to cushion the glass and make up for any discrepancies in the levelness of the 2 x 2s.  The 2 x 4s on top of the south framework were beveled at 18 degrees and held flush with the tops of the 2 x 2s so that the glass resting on them overhangs enough to dump rain water into the walkable space instead of inside of the collector. 

Cautionary Hiatus 
Once the framework was done, I moved on to building the enclosure around the top of the shell which had to be done before the glass was installed in case something fell into the shell that was heavy enough to break the glass .  The next post details with the assembly of the enclosure.

I postponed the construction of the south side until the glass was in place for a couple of reasons:  having the south side open made installing the glass easier and the walkable space served as a buffer between the south wall of the shell and the glass below, making it unlikely that the glass would be damaged by falling objects.

Sand Bed, Insulation and Corrugated Roofing 
There was too much sand inside the framework to lay

Sand bed ready to receive corrugated roofing

the insulation at the proper level so I moved the excess to the walkable space then used a 1-by board to "screed" the remaining  sand smooth to support the recycled 2" foam board.  Then I returned most of the sand in the walkable space to the collector to cover the insulation which I then "screeded" smooth to receive the roofing panels.  I nestled the steel panels into the sand in such a way as to maximize contact with the sand.  As it turned out, another couple of inches of sand under the roofing would have been ideal from the standpoint of narrowing the space between the glass and the roofing.  A narrower space, according to Bernoulli's Principle, would probably speed airflow but there was no easy way to add any without using bagged sand and the number of bags that it would have taken to make a difference was easily disincentivizing.

The excessive amount of heat rising off of the

Corrugated roofing in place. 

roofing on sunny days, that could be felt even while standing on the ground above the collector shell, seemed to indicate that the collector would function as intended after the glass was in place. 

Finally the Glass
I built the framework to fit four pieces of glass that I found for free on Craigslist and stored in a safe place many years ago.  The collector was long enough that I was one piece short which was a good thing.  It gave me reason to get a glass company involved who could help in several ways.  First, a professional could tell me whether the Craigslist glass was appropriate for the collector, could supply the missing piece and, most importantly, would have the tools and experience for getting the glass safely into the shell and laid on the framework.

As it turned out, the CL pieces were plate glass when tempered glass would be four times stronger against large hail.  So I bought all new glass and was fortunate to have caught the dealer out of Plexiglas and therefore temporarily unable to work on its backlog of orders for Plexiglas barriers during the COVID-19 epidemic.  Otherwise, the collector glass might have been delayed until so late in the summer that the amount of heat collected before the AGS system had to be mothballed for winter would have been negligible. 

 

Parenthetically, by the time the pictures above

were taken, the bonding cement used in conjunction with the dry-stacked concrete block walls of the shell had been painted with white Drylock Waterproofing paint.  It served three purposes:  as a preservative, to improve appearance and to reflect sunlight into the collector.

Installing the Glass

The gods must like our project.  Otherwise, go figure how clouds moved in just before the glass installers arrived and cleared as they were leaving.  Otherwise the heat from the roofing on a 95 degree afternoon the second week of July would have been almost unbearable for the workers, shown in the photo installing the last panel of glass.

Getting the glass in place meant that, five years after the shell was built, the collector was finally functional -- a major watershed in our passive solar build!

Design - Solar Collector - Maximizing Solar Gain

The previous post on the solar collector for the AGS system dealt with the mechanisms for trapping solar energy and converting it to usable heat.  This post discusses the factors that go into maximizing the amount of solar energy collected.

Original Assumptions
My early thinking, that appears in at least one prior post, was that the best tilt for the glass and steel would be perpendicular to the sun angle for St Louis shortly after the summer solstice (June 21), say July 21.  Since the choice was empirical, I wanted to flesh it out with data if possible; hence, the following analysis based upon three factors -- optimal sun angle, warm weather collection period and available daylight.

Optimal Sun Angle
NOAA Solar Position Calculator is seemingly a useful tool for knowing the elevation of the sun angle from horizontal. However, Gary, my mathematician brother-in-law, calculated the angles and found the NOAA data to be incorrect. Following are his sun angle calculations (rounded up or down) for St Louis.  The optimal tilt for the glass and galvanized steel roofing in the collector would be 90 degrees from the sun angle during the warm season but the question is what date would be best to use as the default.  For the sake of discussion, the  figures for five scenarios are listed below :  

     June 21:    Sun elevation from horizontal = 75 degrees; collector glass angle = 15                       degrees from horizontal

     July 21:  Sun elevation = 72 degrees; glass angle = 18 degrees

     August 21:  Sun elevation = 64 degrees; glass angle = 26 degrees

     September 21:   Sun elevation = 52 degrees; glass angle =  38 degrees

     January 21:  Sun elevation = 32 degrees; glass angle = 58 degrees

A default date of July 21 with a sun angle of 72 degrees is only 3 degrees less than June 21 and, by observing the play of the sun in the collector shell for a few summers now, I think that the difference between the two is moot, i.e., the additional amount of sun entering the collector on June 21 vs. July 21 is negligible.  

The sun angle for January 21 was included above to contrast the difference between the typical passive solar design that uses the energy from the low-angle winter sun versus the high-angle summer sun that energizes our AGS system.  For solar gain in winter, the optimal tilt for our 38 degree St Louis latitude of, say, greenhouse windows, would be tilted 58 degrees off horizontal to be at 90 degrees to the sun angle compared to the 18 degrees off horizontal that optimizes the output of our solar collector on July 21 -- a difference of 40 degrees.

Parenthetically, the south facing windows of our house are 90 degrees from horizontal rather than the 58 degrees that is optimal for January 21. The 32 degree difference might be important for a classic winter-centric passive solar build but, as detailed in a prior post, maximizing solar gain in winter is not very important for our Annualized GeoSolar System, especially after the first couple of years.

Warm Weather Collection Period
In order to keep cold air out of the AGS system, the north ends of the conduits will be capped during the cold months -- roughly from the end of September until the beginning of April.  Consequently, the question becomes, "Shouldn't the default date for the sun angle fall in the middle of the six-month April through September collection period?"  If so, it would be sometime in June.  However, it is more likely that lingering cold weather in the spring will delay opening the conduits than early cold weather will cause them to be capped prematurely in the fall, thereby skewing the midpoint of the collection period backward to, say, sometime in July.  So, again, any advantage of a June default date over a July date is questionable, particularly in view of the amount of cloudy weather in the spring as discussed below and recurring warmer autumns due to global warming.

Available Daylight
In addition to optimal sun angle and which months are included in the collection period, it is useful to consider the available daylight during the collection period. NOAA's Sunrise-Sunset Calculator is helpful in this regard.  Here are some representative values spanning the seven months that would be in play as the optimal collection period.

     April 1:  12 hours, 40 minutes

     May 15:  14 hours, 18 minutes

     June 15:  14 hours, 51 minutes

     July 15:  14 hours, 35 minutes

     August 15:  13 hours, 39 minutes

     September 30:  11 hours, 48 minutes

     October 30:  10 hours, 36 minutes

These data seem to indicate that a June 21 date in the middle of a six-month collection period -- April through September -- would provide more available daylight than would a mid-July date with a six-month collection period -- May through October.  
However, according to Climate for St Louis, there is much more rain in April through June than in August through October.  More rain means less available sunlight.  Less available sunlight in spring and early summer therefore makes May through October a more attractive collection period than an April through September period despite the latter's shorter days and makes July 21 a better midpoint than June 21.

Summary
Three factors influence the choice for the best date on which to base the angle for the glass and steel panels:  (a) sun angle from horizontal, (b) the timing of the collection period and (c) the available daylight during the collection period.  A close look at sun angles and collection periods results in essentially a wash between June 21 and July 21. But the third factor, the available daylight/sunlight during the collection period, seems to tip the scales in favor of July 21 which validates my original hunch.

Design -- Solar Collector - What Is It and How Will It Work?

This is the first of five posts on the solar collector.  The first two parse its design; the second two detail its construction and the fifth reports on its actual performance.  In preparation for writing these posts, I did a Google search on using solar collectors for passive solar heating and air conditioning and found nothing that resembled our situation.  Even the sources with which I was already familiar were short on details.  So I am erring on the side of too much detail in case someone out there is contemplating Annualized GeoSolar and could benefit from our experience.  At this stage, I cannot not be sure that it will work as planned so I am reconciled to the possibility that, even after completely installed, it will have to be re-configured.  If there are problems, I will report them in the fifth post and describe how we handled with them.

What Exactly is a Solar Collector?
Solar collectors capture the sun's heat, as opposed to photovoltaic panels that use the sun's light.  Solar collectors can take many forms, some more complicated than others.  By using a flat-plate solar collector, we benefiting from the simplest of forms -- entirely passive, having no moving parts.  As is typical of flat-plate designs, ours is a rectangular box with a glass cover and a heat-absorbent bottom.  Sunlight passes through the glass, warming the air inside the box and pushing the warm air through a series of pipes to heat the thermal mass under and behind the house.

Finally It's Time to Build Out the Collector

Two 2015 posts detailed the construction of the dry-stacked concrete block shell for the solar collector located in front of the house (as opposed to the photovoltaic array behind the house). The two posts were Construction of the Solar Collector and More on the Construction of the Collector.

Shell for the solar collector showing the AGS conduits

exiting the back wall of the collector 6' below the floor

 level of the house (line of holes near the bottom); the

glass top over the working part of the collector will be

 situated just above the conduits; notice the white conduits

 running to daylight at the back of the excavation for

 the house (beyond the blue tarps) at a distance of about

65 ft from the collector.  (Click any picture to enlarge it for

 better viewing.)

It is time now to convert the shell into a functioning part of our Annualized GeoSolar system -- something to which, until recently, I had not given much thought beyond realizing that it would not be as simple as covering it with glass as if it were a greenhouse roof.  Fortunately, our friend, Ben, a retired metallurgical engineer, has been sharing his knowledge.

The red arrows bracket the solar collector, also identifiable
by the ladder protruding from it.  Although it looks as if it
is attached to the house, it is actually 18-20' away from it.
Parenthetically, notice the berm (yellow arrows) that directs
 runoff to a rain garden (magenta arrow).  The garden is the
 third in an interconnected series of four with the first one
 situated beside the garage. 

(If you are new to the blog or unfamiliar with the concept of AGS and the role of its solar collector in eliminating the need for conventional heating and air conditioning, click on "Featured Post" in the left column then follow the links to other posts on the subject. Or, for a quick overview of AGS, go to Wikipedia.)

(Click on any picture to enlarge it for better viewing.)

How the Collector Will Work -- in Layman's Language
Our solar collector will heat a stream of air entering the collector on its south side and exiting through nine 4" conduits on the north side.  

North-south cross-section of the collector. (Click on the
drawing to enlarge it.)

The air will flow between transparent glass that traps the sun's heat and corrugated galvanized roofing panels a few inches below the glass that absorbs the heat then releases it to the passing air (see nearby sketch).  

The hot air passing through the conduits will heat the earth under the house and the earth under the insulation/watershed umbrella behind the house before exiting to daylight.  And, since the conduits are slanted upwards as they fan out and run north, the heated air rises through them passively with no mechanical assistance.

How the Collector Will Work -- in Engineer-speak
According to Ben, the sun must raise the temperature of some material in the collector above that of the air in the collector.  The heated air is then forced by convection out of the collector and into the AGS conduits (which, as mentioned above, are tilted slightly up -- leaving the collector at 6' below the floor level of the house and passing under the north wall of the house at 3' before bending abruptly upward to daylight behind the house.)  For maximum efficiency, the heated material in the collector must have high solar absorptivity.

However, it is not enough to be highly absorptive.  It is also important that the material readily transfers its heat to the air, i.e., be highly emissive.  The higher the ratio of absorptivity to emissivity, the more efficient a material is for solar collection. Ben has vetted an interesting Table of Absorptivity and Emissivity of Common Materials and Coatings that lists nearly a hundred materials with regard to the ratio (third column in the table) of absorptivity (first column) to emissivity (second column). There are only five materials in the table, such as metals plated with nickel oxide or plated with black chrome, having a higher ratio than the one Ben recommends and all of them hard to find and beyond our budget.

Ben's Recommendation

Galvanized roofing

New galvanized steel, with an aborptivity of 0.65 and an emissivity of 0.13, has a ratio of 5. "Exposure to weather" (whatever exactly that means) eventually causes the ratio to drop to 2.90 which is still high compared to most of the materials listed.  Perhaps, under the glass of the collector, the steel will "weather" slower than if it were in direct contact with the elements. And steel panels are cheap enough that replacing them from time to time will not be an issue if the need arises.

The galvanized steel panels will have to be supported by something.  Ben recommends using dirt or sand which will double as a heat sink. When the sun heats the panels, most of the heat will be carried away to the conduits by air movement but some will be conducted to the heat sink below.  When the sun is not shining, some of it will reverse-conduct into the cooler space of the collector and find its way into the conduits. 

Insulation
In order to be sure that most of the heat in the sand under the galvanized roofing is not lost to the ground below, I am considering laying down at least 2" of foam board insulation before adding the final layer of sand that supports the galvanized panels.  The insulation is depicted with dashes and its label with a question mark in the drawing above because, at the time of this writing, the decision to include it was still in limbo.  .

My inclination, though, is to use it since doing so is consistent with the way we insulated under the nine conduits running between the collector and the house.  In the top photo, notice the pink vertical insulation on the east and west sides of the excavation behind the collector shell. The same insulation had already been laid down under the conduits.  The insulation in the overlying  insulation/watershed umbrella insulates the top.  A single layer of foam board surrounding the conduits was deemed sufficiently insulating because the soil on the outside of the foam is already being warmed by the overlying umbrella but perhaps the case could be made for using more than one layer in the collector.

Air Flow
The collector will have to be designed so that air flows passively between the glass cover and the steel panels.  In order to make sure the volume of air entering the collector is more than enough to replace the warm air exiting into the conduits, the square area of the air intake on the south side of the collector will need to exceed slightly the total area of the openings to the nine 4" diameter conduits.  There is walkable space between the collector and the south wall of the shell that will not only provide a patent air intake but will also give access for clearing leaves and plant growth and for cleaning the glass periodically.

Assuming the design of the collector is adequate, its function is still at the mercy of many unknowns about passive air flow through the conduits.  Will 4" diameter conduits be the optimal size for sufficient airflow?  Are conduits that are nearly 90' long from collector to daylight behind the house too long to expect passive flow?  Do they angle upward enough from 10' below floor level when they leave the collector to a depth of 3 or 4' below floor level at the back wall of the house and then turn abruptly to daylight?  Will using the corrugated (rather than smooth) piping under the house -- that is intended to cause turbulence in the air flow and thereby improve heat transfer to the soil -- slow the flow too much?  Will the cooler soil during the first winter and, to a lessor extent, after each succeeding winter, cause cool air to flow backwards towards the collector to the extent that the warm air from the collector cannot reverse the flow?  We are only weeks away from having answers to these questions which I will report in the last post about the collector.

Sunken Configuration
The glass of the heat exchanger will be situated about 6' below the top of the back wall of the collector and nearly that deep in front due to its cant southward.  At first blush, it might seem that the sunken configuration will reduce the amount of useful sunshine reaching the glass.  While the east and west walls do indeed block some of the suns rays until mid-morning and after mid-afternoon during the majority of the summer collection period, their angle of incidence to the glass would be so low that most would be reflected from the surface of the glass instead of penetrating it.  Even then, the amount of glass that is shaded at 10:00 am and 4:00 pm comprises less than a third of the total.

Another reasonable objection to using a sunken configuration is that it would hold water, which could be a greater problem in the future

The top of the serendipitous French drain in the walkable space.

with intense storms associated with climate change.  It just so happens, however, that the excavation for the collector was deep enough to uncover one of the seven French drains that were installed early on to keep the soil under the house as dry as it has to be for the Annualized GeoSolar system.  The three rock formations seen in the second photo protect some of the French drains as they emerge to daylight.  The middle one passes through the collector shell.  Without it, we would have had to install a separate French drain for the collector.

While excavating for the house and trenching for the French drains and the AGS conduits, we encountered a layer of glacial till, also known as hardpan.  As will be discussed in a post on the actual build-out of the collector, we found that the dirt floor of the collector also comprised hardpan.  All along, rain falling into the collector must have been shunted by the hardpan to the French drain with the drain carrying it away fast enough to eliminate any pooling.  However, as part of the build-out, we removed the hardpan from the walkable space so that the concentrated flow of water from the glass panels of the collector would be carried away quickly -- by finding the French drain or by simply soaking into the newly exposed permeable soil.

Hail Damage?
One of the advantages of our southern-ish latitude is that the sun is more directly overhead, which is good for harvesting solar heat.  But it also bad because it means that the solar collector glass is more horizontal and therefore more susceptible to hail damage. The glass panels will be salvaged 1/4" thick tempered plate glass that will probably hold their own against normal size hail but with larger size maybe not so much.  If damage does occur, one option would be to switch to transparent fiberplass or polymer panels having UV coatings.  However, not only are they not as efficient for solar gain, UV degradation would limit their useful life-expectancy.

The next post will deal with the optimum angles for the glass and the steel panels.