Timeline - Deciding on the Amount of Earth Sheltering

Past Six Years

Earth Contact Walls and Roof
Concrete walls are included in the design of our passive solar home even though their extent is limited to the north wall and about half of the west wall, technically making the design "earth-bermed" instead of "earth sheltered".  The classical earth sheltered dwelling   

had only the south wall exposed.  The north, east and west walls would have earth contact and so would the roof to varying degrees. It took me a while to realize that so much earth contact is not necessary in order to eliminate conventional heating and air conditioning.  As you can see in the nearby photo of the house model that was built nearly five years ago and in the prior post on the house model, the original design called for half of the roof to be earth covered.

Going overboard with earth sheltering at that stage was the result of my confusion about the role of that part of the house envelope not facing south.  I bought in on the idea that maximizing earth contact seals off the hostile external environment and makes the surrounding ground temperature the default temperature for attenuating living space temperatures -- all of

which is true.  It took awhile and considerable research, however,  to realize that the story doesn't end there.  Conventional walls and ceilings can be air-sealed and super-insulated (R-50 range) to ward off the outside environment at least a well as contacting earth does. Then, Hiat's "Passive Annual Heat Storage" and Stephens' "Annualize GeoSolar System, made me realize that earth contact can be severely reduced then intentionally manipulated so as to raise the default ground temperature to a comfortable level for living spaces without any help from conventional heating or air conditioning.  

In addition, there were three practical reasons for downsizing the amount of earth contact.  First was the realization that building a two-story concrete wall using something like dry-stacked concrete blocks (like Rob Roy showed in his book, Earth-Sheltered Houses) was beyond my skill and endurance levels and beyond what any structural engineer would be willing to stamp, especially when half of the wall would be two stories

tall with no bracing from internal T-walls.  And not to mention that our budget made it necessary to limit the amount of concrete work we delegated to professionals. Secondly, there are two options for a roof that is strong enough to support earth, neither of which we could afford. One is concrete supported by concrete or steel (wood is not allowable).  Another is like Roy advocates -- 2-by tongue and groove decking supported by timber framing.  But for our rather large project, the timber frame would have to be professionally done -- again a budget buster. The third reason for avoiding an earthen roof is that, since the roof would not support earth moving equipment, the amount of the labor involved in wheel-barrowing enough soil to cover a 2,800 sq ft roof to an appropriate depth would have been formidable.  

According to printed and on-line resources, water leaks have always been a problem with earth contact roofs, at least in non-desert climates.  But this negative did not influence my decision against earth covering for the simple reason that I think Hiat is dead-on in saying that the insulation/watershed umbrella covering the roof and extending laterally from the house, as opposed to conventional waterproofing applied directly to the deck, is very unlikely to allow roof leaks.  And, since the insulation/watershed umbrella is critical for our AGS system, it would have been carried onto the roof if we had opted for earth contact.

Super-Insulation
The advent of "super-insulated" walls and roofs tipped the scales.  I am betting that our iteration of the AGS system will generate and store as many BTUs as the house will ever need.  So the only issue is to make sure as many BTUs stay in the house during cold weather (and out of the house in hot weather) as possible.  And I am betting that our meticulously air-sealed and super-insulated walls and ceilings will do a better insulating job than the earth contact walls and ceilings they replaced.  

The concrete wall is limited to the entire north side and
half of the west side of the house.  The north wall will
be backfilled to nearly a two-story height over more than
 half its length.

In our case, earth contact is limited to the north wall and half of the west wall. The walls have two functions. One function is to become part of the thermal mass but, in this regard, they are only a small part of a larger thermal mass comprising the soil under a 2,800 sq ft floor plus the soil extending 20' outward from the house in all directions under the insulation/watershed umbrella. The large volume of earth thus encompassed by the system is the primary thermal mass; the concrete walls play a minor role.  Actually, the concrete floor plays a far bigger roll than do the walls by a ratio of 2:1 in terms of earth contact area.  The two thirds of the concrete area that the floor represents lies just a few feet above the AGS conduits and thereby is in position to heat-sink more solar energy than the walls.

The second and most important function for the concrete walls is to serve as a heat transfer medium between the soil behind and west of the house and the interior environment.  For this reason, the inside of the walls will always remain unobstructed and accessible to air movement.  Situated just inside the north wall and part of the west wall of the house will be the utility area (aka, "vertical basement" by Stephens) which means that storage shelves will be attached to the stick built walls opposite the concrete walls in order to keep the latter completely exposed for transferring heat.

Design - Rice Hull Insulation

SPOILER ALERT -- Winter 2025

Do not use rice hulls for insulation without reservation.   We did and are not sure as of this writing it was a wise choice.  

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.

In our situation, it would seem that the weevil infestation should abate due to the limited number of rice grains sealed up in the envelope of the house that are available for food and for egg-laying (which is a different situation than weevils in a granary where there is plenty of food and egg-laying possibilities are infinite).  At the time of this writing, this seems to be the case.  The holes in the window sill sheathing that were necessary to deliver the hulls to the spaces under the windows should have been sealed with foam insulation before the sills were installed.  They were the only major patent openings in the envelope through which the weevils could escape.  So, window-by-window, we have removed the trim and sealed the openings. It is now rare to find weevils.  However, electrical boxes in the walls and ceilings are only remaining unsealed penetrations in the envelope such that it may yet be necessary to access the boxes and obliterate the spaces around the wires entering and leaving them. 

Over time, watch this space for updates on the weevil problem. 

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                                                ORIGINAL POST

Why Rice Hull Insulation?

The short answer is economy.  The rice mills have a hard time disposing of the hulls so they are only too happy to sell them by the truckload cheaply.  In fact, the cost of the hulls is

insignificant compared to trucking costs from the Mississippi delta (Missouri boot-heel, Arkansas, Mississippi or Louisiana) to our St Louis area.  But there are many other advantages of rice hulls over conventional insulation which are covered in this longer-than-usual post.

(From a sustainability perspective, cellulose is the best choice among low-cost conventional insulating materials.  Therefore, it is the one most used in the following paragraphs for comparison to rice hulls.)

Rice Hull Properties

My serendipitous discovery of rice hulls is explained in a previous post --  design evolution - insulation.  My new awareness then lead to the definitive paper on the rice hulls for insulation by Paul Olivier, PhD, "The Rice Hull House".  The information below comes entirely from his paper including the following quote from the opening abstract. 

"The rice hulls are unique within nature.  They contain approximately 20% opaline silica in combination with a large amount of the phenyl propanoid structural polymer called

lignin......Recent ASTM testing.....reveals that rice hulls do not flame or smolder very easily, they are highly resistant to moisture penetration and fungal decomposition, they do not transfer heat very well, they do not smell or emit gases, and they are not corrosive with respect to aluminum, copper or steel.  In their raw and unprocessed state, rice hulls constitute a Class A or Class I insulation material, and therefore, they can be used very economically to insulate the wall, floor and roof cavities....."

Olivier's paper goes on to explain in detail why rice hulls are ideal for insulation. Their R-value compares favorably with cellulose and loose fiberglass at a value greater than R-3 per inch. Their natural fire resistance precludes the addition of large quantities of flame and smolder retardants as with cellulose.  Nor is the addition of anti-fungal agents necessary since the amount of moisture absorbed from the air is very low compared to most organic materials that moisturize in equilibrium with the surrounding humidity.  The high concentration of opaline silica on the outer surface of the hulls makes them very hard but, lignin within the hulls adds flexibility and elasticity, making them far more resistant to settling and compression than cellulose. Also their "tiny tips, edges and hairs interlock........( to produce a) peculiar bonding of rice hulls under mild pressure......(such that) no further settling is possible". According to Olivier, cellulose, can settle "as much as 25%" despite stabilizing additives such as (un-green) polyvinyl acetate or acrylic adhesive.  Finally, "since rice hulls require no shredding, hammer-milling, fluffing, fiberizing, binding or stabilizing, they possess far less embodied energy than even cellulose".  And they are durable enough to be recycled indefinitely.

Freight Costs 
Olivier goes on to analyze the total cost -- hulls plus transportation.  "At an installed density of 9 lbs per cubic foot, one ton of rice hulls will insulate 222 square feet of a 12-inch wall cavity........A standard 53 -foot trailer attains optimal transport efficiency at its maximum legal weight of 24 tons.  If.......we pay an average trucking fee of $1.45 per mile, it would cost approximately $15, $30, $45, $60, $75 and $90 to transport one ton of rice hulls 250, 500, 750 1000, 1250 and 1500 miles respectively.......the (freight) cost per square foot would be $0.07, $0.14, $0.20, $0.27, $0.34 and $0.41 respectively.......Those living less than 200 miles from rice hulls should have a hard time justifying the use of any other type of insulation material".  

He goes on to say that, even if the hulls cost $25 per ton (five times the cost at his location in Louisiana at the time the paper was written),........the purchase price of the rice hulls per square foot of wall insulated is on $0.11.......(when this price for the hulls is added to) the cost of transport over these same distances, we arrive at a total cost (per cubic foot of wall space of) $0.18, $0.25, $0.32, $0.38, $0.45 and $0.52 respectively".  (In this analysis, cubit foot is interchangeable with square foot because the wall thickness is 12".)

Cost Comparison with Cellulose
Olivier says that "cellulose insulation in a dense-pack application (reaches) a density of approximately 3.5 lbs per cubic foot (and) will insulate 571 sq ft of our proposed 12-in thick wall.  At an average delivered price of $540 per ton, cellulose insulation costs roughly $0.95 per sq ft of wall insulated........roughly five times the price of rice hulls transported 250 miles and twice the price of rice hulls transported 1,500 miles".

Real Costs

Rice Hulls:    When I called the rice mill in SE Missouri that was the closest to Collinsville three years ago, the price for the hulls was $94/ton.  When I emailed this figure to Olivier, he assured me that they could be bought for $15/ton.  Indeed, the company that handles the rice hulls for a large rice co-op recently quoted me $15 out of Greenville, MS.  

Deck of a walking floor trailer

Freight:  The actual costs for us will be more than Olivier's paper would suggest due to higher freight costs-- the trailer capacity is less and the per-mile rate is higher than his analysis.  The fluffy hulls are hauled in enclosed 53 ft trailers with "walking floors". Instead of a dumping action, the trailer deck has three sets of slats that move in a coordinated manner such that load is conveyed towards the rear of the trailer until it falls out. Unfortunately, the trailers actually hold only +/-18 tons instead of the 24 that Olivier described and, since truckload freight rates are based on mileage, less tonnage means higher trucking cost per ton. Accordingly, It will cost about $3,600 to ship 18 tons the 450 miles to Collinsville or $8/mi instead of the $1.45 that Olivier used for his examples.  I checked with a local trucking company only to find that, since its grain trucks have half the capacity of the walking floor trailers, trucking cost from Greenville would be higher due to having to make two hauls instead of one.  I hope that, when it comes time to buy the hulls, more research and comparison shopping will turn up a source closer to home that will save on transportation.

Total cost:   Our cost for a trailer-load will be 18 tons x $15/ton = $270 plus $3,600 for freight, making a total cost of $3,870.  If, as Olivier says, a ton of hulls will insulate 222 cu ft of wall space, 18 tons will do just under 4,000 cu feet.  If we were to insulate the garage walls with hulls as well as the house, we would need about 2,400 cu ft for the walls and 2,800 for the ceilings or 5,400 altogether.  This means one trailer-load will not meet our needs and supplementing with conventional insulation for the garage will be necessary.

Cost comparison with cellulose:  In 2013, a local insulation company quoted our project when the design was still in flux to the extent that we were at a ceiling thickness of 12" instead of 15" and a wall thickness of 7 1/2" instead of 15".  The quote for dense pack cellulose was $4,700.  Extrapolating, our current design would have been quoted at $6,130 (plus 3 years of inflation).  This amount would be more than a third higher than insulating with hulls.  This amount also figures out to be $1.23 per cu ft which is a little more than 20% higher than Olivier's figure of $0.95.

Weight Factor
Rice hulls at 9 lbs per cu ft are almost three times as heavy as cellulose at 3.5 lbs per cu ft. Although Olivier does not discuss the weight factor, it seems reasonable that half-inch drywall screwed to rafters or joists would not adequately support hulls piled thick enough for a high R-value.  Either the drywall would have to be thicker or applied in layers.  In our case, tongue and groove pine ceilings were already planned before considering rice hull insulation so the weight factor will be moot. However when estimating rice hull insulation, I think the cost of a more robust ceiling should be factored in.

Non-monetary Advantages 
The numerous advantages of rice hulls compared to cellulose are covered above in the paragraph about their properties.  Another advantage they have for our DIY project -- and I consider it to be very important -- is that they can be installed incrementally in conjunction with building the walls and ceilings as opposed to an all-at-once job by an insulation company.  For example, the walls can be filled as the drywall goes up and the cathedral ceilings can be filled as the tongue and groove pine is installed.  With visual access to the cavities, the chances of voids will be minimized.

Our Plan

Our plan is to use a blower to fill the walls and ceilings. However, the blower used for cellulose is not strong enough for hulls so a custom blower will have to be assembled similar to the one shown in Paul Olivier's slide show on the Rice Hull House concept. As to where to store the hulls between delivery and installation, my current thinking is to have them dumped onto the slab floor of the future garage then protect them with a tarp while we rush to get the garage undercover.
_________________

Update November 2020

We are finally in the process of insulating with rice hulls in ways that resemble the plan just described very little.  Here is the link that will bring the reader up to date --  The Planning Stage.  It is the first of several posts explaining the surprises we encountered, like sticker-shock on the price of the hulls, dealing with the rice weevil infestation issue and how incredibly dusty the hulls were coming out of the blower hose, as well as atypical structural modifications and drywalling sequencing that were mandated by the hulls.  And one of the posts parses the advantages and disadvantages of using rice hulls for insulation versus cellulose. 

Construction - The First Retaining Wall

Our plans call for four retaining walls with the first one next to the west concrete wall being the most challenging.  It needed to be 4 - 6 feet high and insulated and waterproofed as part of the insulation/watershed umbrella.  The insulating and waterproofing were a challenge while building the wall was easy, thanks to 9 energetic volunteers.  But that's getting ahead of the story.  (Click on the photos to enlarge them for more detail.)

Insulating the Concrete House Wall
The first book I bought when contemplating an energy neutral home was "Earth Sheltered

Wall insulation in place

Houses" by Rob Roy.  Through it I learned early on that insulation should be inserted between a retaining wall and a concrete house wall in order to keep the retaining wall from sucking heat from the house.  In our case, I was already planning to insulate the concrete walls of the house that would not protected by the insulation/watershed umbrella. Since the umbrella will dip down behind and go under the retaining wall, the house wall adjacent to the retaining wall will fall outside the umbrella and would need to be insulated.

Cementitious board before adding the lower section

The exact way I insulated the house wall will be covered in detail in another post but suffice to say at this juncture, I used 3 5/8" steel stud track to support 3 1/2" of expanded polystyrene for an R-15 on the outside of the wall.  (The inside of the wall will be insulated in a similar fashion eventually for a total of R-30.)  I then fastened 1/2" old-fashion, heavy, hard-to-cut cementitious board and parged it with top-coat stucco, the latter primarily to cover the junctions between boards and to make the exposed areas of the wall more aesthetic.  Six mil plastic separated the

Wall parged to height of retaining wall;
horizontal insulation in place (part of
which already covered with sand);
vertical insulation supported from
 behind with steel fence posts

galvanized steel from the concrete on one side of the insulation and from the cementitious board on the other side.  Six linear feet of the wall were insulated and covered from the top of the wall down to meet the horizontal insulation already in place over the footing.  

Integrating the Retaining Wall with the Insulation/Watershed Umbrella 
As part of the Annualized GeoSolar system, the insulation/watershed umbrella should extend 20' outward from the house in all directions.  On the west side of the house, it has be convoluted in order to accomodate the retaining wall.  Above the wall it will slope gradually southward

Insulation wired to
steel fence posts

towards the wall then, dip sharply downward behind the wall, go under the wall and finally blend with the umbrella in front of the house. 

I smoothed out the soil under the wall then covered and leveled it with sand to provide a base for a 4' wide wall. Next, I laid down plastic sheeting (6 mil) such that it extended several feet beyond the prospective wall in the up-slope and down-slope directions.  Then came a thin layer of sand over the plastic where the wall would rest followed by two panels of 4' x 8' x 2" insulation board next to the house and another panel of 2" lateral to it such that the insulation under the wall would be 4" thick for the first 8' then 2" thick for the last 8'.  Ideally, the insulation should have extended 4' further to satisfy the 20' width for the umbrella but the original excavation did not accomodate it.  

Three layers of plastic -- one between
insulation and soil contact then two
between insulation and the outside
environment

The vertical insulation was also 4" thick for the first 8' from the house then 2" thick for the last 8'.  To support it, I drove four steel fence posts into the ground behind them and wired the insulation to them.  In retrospect,  all of the insulation should have been the stronger extruded polystyrene (pink) instead of expanded poly (white), especially the vertical pieces, one of which cracked while building the wall and had to be held together by hand until it could be supported by sand in front and dirt behind.

I next liberally covered the horizontal insulation with sand and added two more sheets of 6 mil plastic with a layer of sand between them. Finally, I covered the plastic with a heavy layer of sand into which the stones of the wall could be nestled without damaging the plastic.

Building the Retaining Wall
Three or four years ago, we salvaged foundation stones from a 19th century barn in such

Some of the volunteers at work

quantity that we do not have to use them in a miserly way. Consequently, I decided to make the wall massive enough that the rocks could be laid randomly and still resist the pressure of the backfill behind it.  The final dimensions were roughly 4' wide at the base, 2' wide at the top, 5' high and 16' long.

A view from the loader; step-son Keith
(left) and my good friends Dave and Pat
lifting rocks out of the bucket

The wall came together in less than three hours.  As fast as I could track loader the rocks to the wall site from the rock-pile, the volunteers could set them, including packing sand into the crevices between rocks, laying a sandy base for the next level and compacting the sand with hose water.  Some of the stones weighed quite a bit north of 100 lbs so I was more than grateful for the help. (Parenthetically, the crevices between the rocks will be filled someday with plants native to our area.)

The result

Backfilling Behind the Wall
Backfilling immediately before it rained was critical otherwise water pooling behind the plastic might create sufficient hydraulic pressure to move the wall. I did the backfilling the same day the wall went in. As it was filled, the grades next to the house wall and the rock wall were intentionally tilted to create a swale for carrying runoff safely around the retaining wall.  As the back-fill settles over time, it may have to be tweaked to protect the wall.

Initial backfilling

  
Eventually, the umbrella and a couple of feet of topsoil will be added over the initial backfill after which the rock wall will be protected from runoff permanently. However,  chances are the wall will need a couple of courses of stones added on top to accomodate the horizontal insulation in the umbrella and the topsoil over it.  It also looks as if I should have made the black damp-proofing membrane higher on the concrete wall.  Not to worry, it can be extended later.

Design - Whole Wall Insulation

Whole Wall R-Factor

The R-factor for the whole wall is less than the R-factor attributed to the insulation itself due to conductive heat loss or gain through the structural members (thermal bridging). The conventional R-factor also disregards convective heat loss/gain through the wall (air infiltration), which has even more potential than thermal bridging for diminished whole wall performance. Furthermore, it disregards heat loss/gain through windows and doors. Whole wall R-factor reconciles all three.

The subject of air infiltration pops up in many prior posts and will keep popping up in future posts.  It is hard to discuss green design and construction of exterior walls, cathedral ceilings, windows and doors without proper attention to air-sealing.

Our present discussion, though, is mostly about conductive heat loss through a wall or cathedral ceiling whether through the insulation or through the structural members. For a more complete discussion of convective vs. conductive heat loss, link to another post, Odds 'N Ends - Whole Wall R-value.

Modern Walls

Two modern methods for super-insulated wall construction are structural insulated panels and insulated concrete forms.

Structural Insulated Panels (SIPs)

SIPs are basically a sandwich with OSB board for the bread and solid foam plastic for the meat.  They typically are fabricated off-site and "flown" to place at the job-site with a crane.  When the joints between panels are caulked and foam sprayed, air infiltration is virtually eliminated.  Their R-value per inch is more like the solid foam board found on the rack at the home center -- much higher than fiberglass or cellulose.  And the foam core is available up to nearly a foot thick for a variety of R-values.

With regard to sustainability, SIPS rank high.  OSB is an engineered wood that comes from sustainable tree plantations and contains no VOCs while the core, expanded polystyrene, no longer requires ozone-depleting manufacturing processes. They are so strong that they do not need traditional framing for support which saves finite resources (and costs).  Offsite fabrication is more sustainable than on-site stick-building.  And on-site labor costs are less because they go together so fast.

The major downside to SIPs is initial expense (the cost of the crane alone for the time it takes to assemble a house is substantial).  

Our first choice was SIPs but they were ruled out early on the basis of cost.  My labor is free so there was no sense paying someone else to build walls.  Also, I have a substantial stash of (free) recycled lumber for wall construction that shouldn't go to waste.

Insulated Concrete Forms (ICFs)

An ICF is another sandwich.  The bread is 2.5" of solid foam insulation and the meat is reinforced concrete of varying thicknesses.  The whole wall R-value is +/- R-22 for the brand with which I am most familiar.  The forms are stacked and braced then the concrete is poured inside much like pouring basement walls with metal forms.

 Our ICF frost protected shallow foundation

The downside to ICFs is that the foam comes in only one thickness so their R-value is what it is.  Another is that concrete walls complicate wiring and plumbing and are hard to remodel later.  Also, unlike SIPs, they are not suitable for roofs.  Their upside is that their R-value exceeds the recommended of R-18 for our climate zone, are relatively easy and inexpensive to construct and are gang-busters in hurricane- and tornado-prone areas.  R-22, good as it is, does not qualify as "super-insulated" so I felt that there would be considerable risk in using ICF construction in conjunction with our passive solar Annualized GeoSolar system (in lieu of conventional HVAC) whereby conservation of every BTU counts.

However, we did use ICFs for our frost proof shallow foundation under our truss walls.The cost was little more than for a conventional concrete wall with foam board DIY-bonded to both sides.  And, at least for a short foundation like we needed under the stick-built walls, their R-factor is acceptable and they were fast and very DIF-friendly.

A more recent approach to whole wall insulation than SIPs and ICFs is the "super-insulated envelope" that I discuss in the next post.

Design - Whole Wall Insulation (Cont'd)

Super-Insulated Envelope

Our Annualized Passive Solar system, which figures in dozens of previous posts, qualifies

our project as passive solar even if winter solar gain is only adjunctive. Johnston and Gibson in "Toward a Zero Energy Home" have this to say about good passive solar design:  ".....thermal load of a building can be reduced by 90% primarily through super-insulation, an air-tight envelope, good windows, and heat recovery ventilation."  Further along, they state that "the National Renewable Energy Laboratory (NREL) advocates a simple formula when it comes to insulation:  30-40-50.  In colder climates, zero energy homes start with R-30 for floors, R-40 for walls and R-50 for ceilings/roofs.  Further north where it's really cold, green builders are using even higher figures." 

I am assuming that, despite global warming, our St Louis climate still fits what they call "colder climates".  Accordingly, our design should be right at R-48 for our truss walls with almost no thermal bridging.  We should achieve about the same R-rating for our cathedral ceilings with a thermal bridging factor that is unfortunately slightly higher than for the walls due to the need for structure to carry the weight of the roof. Essentially, the walls will be overkill and the ceiling right at the "super-insulated" threshold.  Our floors are already taken care of by the AGS system. Our windows will be high-end fiberglass and we will have energy recovery (instead of heat recovery) ventilation. 

Super-Insulate with What?

Consistent with my philosophy of sustainability, the last insulation I would want to use is, unfortunately, the most effective -- sprayed-in-place foam such as closed cell polyurethane at R-6 per inch.  Some brands are touted as being soy-based but competitors say that the claim is greenwashing in that the amount of soy in it is a pittance. In any case, spray foam is a turn-off for me because it contains fossil fuel and it is the most expensive product.  Hopefully, the manufacturers claims are legit when assuring that the toxic VOCs (that necessitate space suits for the installers) dissipate rather quickly. 

Cellulose has a lot going for it.  For walls, it is most often mixed with a little water and polymer then sprayed into wall cavities for a very dense configuration held together by the polymer until supported by drywall.  For attics, it is merely blown in like loose fiberglass.  For cathedral ceilings, it is installed with a process called dense-pack so that it is much more compacted than if it were sprayed.  And, since cellulose is ground-up newsprint and other post-consumer paper, it qualifies unequivocally as a green product.  It was our first choice for walls and cathedral ceilings after giving up on structural insulated panels and spray foam.  However, it still exceeded our budget, did not lend itself so well to DIYing and has some downsides that I will compare with rice hull insulation in a near future post.

Loose fiberglass should be blown to an R-50 in an attic.  And, according to the engineer who quoted our job, it can also be dense-packed into cathedral ceilings for a higher R-rating than dense-packed cellulose.  We did not discuss walls.  However, based on the quote for the ceilings, it still exceeded our budget.  (In my view, fiberglass batts are a joke and should not be mentioned in the same breath as super insulation.)

Rice Hulls

This subject is covered in detail in prior posts --  early thinking (best post for details) and exterior walls.  A couple of near-future posts will explore rice hulls in even more detail.  Suffice it to say that rice hulls are cheap and very effective, being highly resistant to fire, pests and mold and similar to cellulose with regard to conductive heat loss. Their R-value is slightly over 3 per inch allowing us easily and inexpensively to meet the NREL recommendations for walls and ceilings.  The only kicker is one of logistics -- how do we get them from the Mississippi delta to inside our walls and ceilings?  For this, watch future posts.  It is a challenge we look forward to meeting.

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Spoiler alert!  As explained in the updates in the posts on rice hill insulation referenced above, hulls should not be considered for insulation due to the uncontrollable rice weevil infestation.

Construction - Pre-Made Trusses for Exterior Walls

"Super-insulated" seems to be the green building catch-phrase for exterior walls that greatly surpass the recommended R-factor.  For our passive solar design that eliminates convention HVAC, super-insulation is not an option.  We need to turn away as much heat as possible in summer and retain as much heat as possible in winter. Consequently, we are using wall trusses in lieu of "two-by" studs in order to minimize thermal bridging and maximize the space for wall insulation. (Reminder: click on any photo to enlarge it for detail.)

The trusses are thick enough to hold 15" of rice hull insulation at slightly over R-3 per inch for a total of R-45+.   By contrast, the R-value for fiberglass batts in 2 x 4 walls is 15 and, for 2 x 6 walls, 19.

Truss jig; the aluminum angle "iron" provides rigidity
when crooked salvaged 2 x 4s are forced to fit the
jig in order to give absolutely straight trusses; notice
the pre-cut truss parts in the background

Another important design feature from an energy conservation standpoint is that the trusses eliminate through-the-wall penetration of two-by structural members, thus minimizing thermal bridging (see prior post, whole wall R-value).   For the sake of consistency and reproducibility, the trusses are built in a jig that is not our original idea (as explained in a previous post, stick-built exterior walls and in the original reference for the trusses).

Truss Configuration
A finished truss has 2 x 4 vertical members arranged flat-ways, i.e., rotated 90 degrees from the usual.  The tops and bottoms are joined by short 2 x 4s. Then there are six gussets, three to a side, made from 3/8 or 1/2" thick OSB or plywood to impart structural integrity while limiting through-the-wall thermal bridging.  All components come from recycled lumber so a certain amount of twisting and bowing is to be expected (but not unlike new lumber today, right?).  And recycled lumber comes with many nail holes on the 1 1/2" side, so exposing the 3 1/2" side for new nailing is another advantage for using trusses.  The second photo below is a prime example of the nail hole problem.

Fabrication
On rainy or cold days and other such house construction downtime, I have been making trusses in front of the

Framing nailer used to fasten 2 x 4
components together; the long 2 x 4s are
the bowed ones shown in the photo below;
notice how straight they are after the short
2 x 4s are wedged to placew

double garage workshop next door to the building site. At the time of this writing, most of those that will be needed have been assembled and stored.

The components are pre-cut to master patterns.  The long and short 2 x 4s are placed edgewise in the jig and fastened together with a framing nailer -- one nail at each corner. Then, with a trim nailer, three gussets are attached with 2" nails -- 11 nails in the end gussets and 8 in the middle gusset.  Since trim nails are virtually headless, they are driven at various off-angles to provide more holding power. The truss is pried out of the jig, turned over and forced back into the jig for identical nailing and gusseting of the second side.

When the long 2 x 4s are bowed, they are positioned in the jig with the convex side against the jig.  Then the short 2 x 4s are driven to place at each end to straighten them. Because the trusses are held jig-straight by the gussets,  the future sheathing and the drywall will lay perfectly smooth.  When bowed in the other direction, straightening is unnecessary.  A typical 2-by stud with only a 1 1/2" nailing surface has to be straight in order to catch enough of the sheathing and drywall panels for secure nailing.  By

Driving the short 2 x 4 to place at
the far end of a bowed board pulls
it away from the jig on nearby end.
Forcing a short 2 x 4 to place on the
 nearby end creates a straight truss
 that the gussets fixate 

contrast, each truss offers 3 1/2" of nailing surface so almost any amount of bowing left or right is acceptable.

Other Green Features
As with 2 x 6 construction, a wall will be plenty rigid with trusses on 24" centers tied together with side-by-side 2 x 6 mud sills and side-by-side 2 x 6 top plates.  The span between nailing surfaces, by virtue of the 90 degree rotation of the vertical members. will actually be 3" narrower than with 2 x 6s.  The double sills and plates are necessary because there are no 2x boards wide enough for a 15" walls -- which is a blessing. Having to tandemize the mud sills and top plates leaves a sizable gap that can be filled with insulation and thereby provide two more breaks (mud sill and top plate) against thermal bridging.  

Doors, Windows, Corners and T-Walls
Trusses that frame openings for doors and windows will have to be modified to carry headers and, for windows, sills.   The plan for headers and sills is to make regular trusses, minus the gussets on the side facing the opening, then let the headers and sills into the truss 2 x 4s to a depth of 1 1/2" to give the same amount of support as a jack stud aside a king stud.  I plan to tie a window-supporting truss to the closest regular truss with horizontal 2 x 4s in line with the header and sills to support the header/sill truss in one direction.  The opening itself  will be lined with OSB or plywood, not only to provide anchorage for the window or door, but to support the header/sill truss in the opposite direction.

When a "T- wall" intersects a truss wall between trusses, horizontal 2 x 4 blocking will join the two trusses and provide fastening for the T-wall, much like what is done with advanced framing techniques.

Gussets are fastened with 2" nails using a
nailer

Truss walls intersecting at corners will utilize three trusses arranged so that the entire corner is accessible to 15" of insulation and thermal bridging is held to a minimum just like the rest of the walls.

Fire-Blocking, Electric Cables and Plumbing Pipes
Fire-blocking is not possible with trusses but rice hulls are virtually impossible to ignite (paper on rice hulls as insulation, page 3) thereby rendering fire blocking in exterior walls moot.

Running electric cables and plumbing pipes inside of trusses will be a joy since holes in studs are not necessary as with stud walls.  The worst case scenario is that an occasional gusset would require drilling. What's more, plumbing supply pipes can be held to the interior side of the wall to isolate them from the exterior with more than enough insulation to prevent freezing.

Design - Roof and Cathedral Ceilings

The design of cathedral ceilings is an interesting, complicated and confusing issue and one that has caused much waffling on my part.  Apparently, the majority of cathedral ceilings extant today have been constructed with the top-to-bottom combination of roofing, sheathing, rafter/insulation and drywall.  And the outcome has been that most have failed, are failing or are destined to fail because of moisture condensation at the top of the insulation. So what should do we do different?

Red Flags

The entire roof for the living quarters of our house will be shed type with cathedral ceilings (no attic).  My research on cathedral ceilings has led me to conclude that the sheathing should not be applied directly to the tops of the rafters.  The emerging consensus is that 6 mil moisture barrier on the bottom side of the rafters is no answer (actually should be avoided in our climate) but that there should be ventilated space between the rafters and the sheathing -- a "mini-attic" if you will. 

And there are alternatives to a mini-attic .  A good one, if you can afford it, is to fill the entire space between sheathing and drywall with spray foam insulation, as opposed to using compacted fiberglass or compacted cellulose.  Spray foam completely blocks air infiltration and the moisture that it carries.  Another approach that apparently works as well or nearly as well is to hold conventional insulation back from the top few inches under the sheathing and fill the space with spray foam insulation.  This would seem to be an economical alternative to all spray.  Most of the other solutions that crop up online are mostly iterations on the mini-attic approach.

We plan to use rice hulls for insulation for both the walls and the ceilings,  And, since I have had no information on rice hulls as an air barrier, I plan to equate them to dense-packed cellulose and dense-packed fiberglass.  This means that, at the 15" thickness of our walls and ceilings, the hulls will probably stop most of the moisture-bearing air infiltration that penetrates beyond the air-tight drywall detailing. 

Spoiler alert:  Do not use rice hulls due to the likelihood of rice weevil infestation.  For details, read the spoiler alert at the top of the original post on rice hulls. 

For more on the thinking that went into the final design for the roof, check out an earlier post:  Timeline - Design Evolution - Energy Efficient Roof.

Solving the Moisture Problem

In order to create a ventilated "mini-attic" above the insulation and avoid the moisture problem, I plan to fasten salvaged 2 x 4s on top of  the 2 x 12s rafters then nail the sheathing to the the 2 x 4s.  When insulation is added later, it will be held level with the tops of the rafters to create a 3 1/2"  space between them and the sheathing. Ventilation will occur when the "mini-attic" allows convective air movement between vents in the soffet at the lower end of the shed roof and vents at the upper end located next to the wall between rafters.

The ceiling that will support the rice hulls will be wood (see below).  It will have to be installed a little at a time and insulation blown in.  The question becomes, how do I keep from filling the 3 1/2" mini-attic with insulation instead of holding it back level with the tops of the 2 x 12s?  The best answer so far seems to be stapling some sort of strong fabric, such as fiberglass screening or weed barrier, to the tops of the 2 x 12s before adding the 2 x 4s.  It would not impede air and moisture transfer through the ceiling but would be strong enough to control the rice hulls.  

The disadvantage of using fabric is that the 2 x4s would have to be installed from above while balancing on the rafters rather than from below from a scaffold or ladder.  Or the fabric would have to be installed one rafter at a time as 2 x 4s were fastened.  This choice could be avoided altogether by using sheet goods, such as plywood or OSB board, over the the rafters instead of fabric.  The moisture accumulating at the top of the insulation would be pulled through the sheet goods by the air movement in the mini-attic but not quite as fast as through fabric.  The sheet good approach would have one other perk that interests me.  Air-sealing tape could be used over the junctions between sheets for an easy way to eliminate air-infiltration through the ceiling or, failing that, caulk could be used from below. If I did this in conjunction with our plan for something similar with the exterior walls, the entire envelope would be sealed.

If it weren't for having to maintain the space for the mini-attic, 16" tall I-joists could be installed to give an R-48+.  But, again, how do we maintain the attic space?  If I decide in favor of I-joists, it would have to be with a 12" height with the addition of screening or sheeting on top then 2 x 4s on edge.

Structural screws are available in a
variety of lengths

Fastening the 2 x 4s on edge to provide space for the mini-attic is structurally feasible and DIY-friendly due to the advent of structural screws and impact drivers.  Where long lag screws would have been used in pre-drilled holes in similar situations in the past, self-threading, "star"-driven construction screws are used.  Not only are they faster to use but are also considerably stronger at much smaller diameters than lag screws.  (The International code now even allows construction screw connections between rafters and the top plates in lieu of rafter ties.)

Increasing the Thermal Efficiency of the Roof
We can increase the thermal efficiency of the roof in two ways:  (1) top-to-bottom design that minimizes heat loss in winter then (2), adding foil-backed sheathing that also reduces heat gain in summer.

With rice hull insulation, at R-3+ per inch, flush with the tops of 2 x 12 rafters, the R-value for the roof would be 36+ which is 6 over the recommended minimum for our climate zone. However, buying good quality 2 x 12s in the lengths necessary to span the open areas of the house may be impossible or impossibly expensive.  Our original budget assumed that we would be using salvaged lumber* and I had not comparison-shopped dimension lumber vs. I-joists at the time of this writing.

In the unlikely event that another salvage opportunity comes up or 2 x 12s of sufficient length can be bought at prices that are considerably cheaper than I-joists, I would go ahead and use 2 x 12s. In which case, I am toying with the idea of adding edgewise 2 x 4s to the bottoms of them before attaching the tongue and groove pine ceiling.  The additional 3.5" of depth for the cathedral ceiling would increase the R-factor at least by 10, giving us a total R-factor approaching 50. (Parenthetically, the pine ceiling is both an aesthetic choice and a structural one -- to support to the rice hull insulation which is a mite heavier than fiberglass or cellulose.)

While salvaged 2 x 12s would have been about as green as it gets, engineered I-joists are greener than new dimension lumber because they minimize thermal bridging, are available in heights taller than dimension lumber to accomodate more insulation, are available in very long lengths and, best of all, are made from sustainable plantation trees. Moreover, they are manufactured to exacting standards. But their use requires special knowledge that experience with dimension lumber does not automatically impart -- a challenge I will have to meet if we use them.

While the "mini-attic" approach should solve the moisture problem, it does nothing to prevent the conduction of heat in and out through the rafters, i.e., thermal bridging.
I-joists 12" tall would be the ideal way to hold thermal bridging to a minimum because

Man-made I-joists

their vertical components are so skinny compared to dimension lumber. But, in case the I-joists are too expensive, I have been thinking about ways to avoid thermal bridging through conventional 2 x 12s. One solution would be to sandwich insulating shims between the bottoms of the rafters and the 2 x 4s installed below them. The shims could be cut from extruded polystyrene insulation boards and glued to place temporarily until the 2 x 4s could be fastened with long construction screws. When it comes time to do the final comparison shopping, I would not be surprised if the combination of dimension lumber, insulation and the pricey construction screws might make I-joists a reasonable choice after all.  More on this at the time of construction.

Actually, for our passive solar sun-drenched home, our worry is as much about heat gain through the roof in summer as heat loss in winter.  So I plan to pay a little more for OSB sheathing having a foil backing that will help to deflect the sun's radiant heat before it can raise the temperature in the mini-attic and challenge the R-factor of the rafter/insulation complex further down.

Steel Roof

Our roof will be a highly reflective light colored steel roof which, compared to asphalt shingles that are made from petroleum, lasts longer, has a recyclable end-life and is cheaper upfront.  I will underlay it with 30#felt then, instead of self-adhering bitumen-type material for the eave edges to prevent damage from ice dams, I will substitute unused roll roofing that was a Craigslist find at nominal cost. Unfortunately, our budget dictates exposed fasteners for the metal panels, as shown in the photo, instead of the more desirable concealed fasteners. For a more complete discussion of our choice of steel roofing, check out a prior post on roof cladding.
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*  Originally, the rafters were to have been 2 x 12s 22 feet long salvaged from an old implement shed at about half the cost of new ones.  Until the "rafter fiasco" (discussed in another post on Craigslist shopping), all of my experience with Craigslist has been nothing but positive. However, the outcome that I have to live with is that all of the rafters will probably have to be store-bought.

______________________

Update:  Winter 2021

This only one of several posts have dealt with the evolution and final design for the roof.  To fast-forward, the one we built was supported by roof trusses and double-sheathed so as to create a dedicated 3 1/2" mini-attic space between layers and provide a space above the ceiling that was tightly-packed with insulation that was either 16" or 18" thick (+/- R-60).