Construction - AGS System for Passive Solar Heating and Air Conditioning

This post is the first of three on the design and installation of the AGS system.  It focuses on the conduits that carry the heat from the summer sun to the thermal mass under and around the house.  The other posts will be forthcoming as soon as the associated installations have been done.  One will cover the design for the solar collector and the excavations necessary to get it installed.  The last will discuss the construction of the shell for the collector, joining the conduits to the collector, insulating and backfilling the excavations.

AGS Design Review
The design of the Annualized GeoSolar system has been detailed in earlier posts (first post, second post, third post).  In a nutshell, the heat distribution system itself has two components -- a solar collector for harvesting the heat from the summer sun and a series of conduits for conveying the heat from the collector to the soil under and adjacent to the house for storage.  Another important element is the insulation/watershed umbrella extending +/-20' outward from the house about two feet below grade so as to insulate and keep dry a thermal mass larger than the footprint of the house.  The necessity for dry soil is covered in the three posts cited above and more recently in one of the posts on French drains.  The construction of the umbrella will be detailed later after it is installed, which will take place after all of the concrete work is done and before wall construction begins.

Conduit Configuration
The conduits comprise two types of 4" pipe -- rigid smooth-walled Schedule 40 PVC and flexible un-perforated corrugated drain pipe.  The smooth pipes extend from the collector to just under the front foundation where they are joined to the corrugated pipes then again in back of the house between the corrugated pipes and daylight above the north grade which will be nearly two-stories high. The corrugated pipes are limited to the area under the slab floor.  

The heavy-duty smooth pipes serve two functions -- to withstand the weight of 8 - 14' of backfill and their smooth walls should expedite the flow of heated air from the collector. The corrugated pipes maximize heat transfer from the conduits to the soil by creating air turbulence. They can be utilized for this purpose because the weight of backfill is rendered moot by filling the trenches with gravel and pouring a concrete floor over them.

Designing the diameter, number, depth and inclination of the pipes was an educated guess on my part after reading the article describing AGS (Don Stephens paper) and visiting one of the houses he designed near Spokane (the Mica Peak residence mentioned in the paper) and research on earth homes.  In our iteration, nine conduits were installed which means they are about 10' apart after flaring out under the floor. As for depth, they leave the collector 6' below floor level, pass under the front foundation at 5' below floor level, pass under the back wall of the house 3' below floor level and run to daylight behind the house at a 45 degree angle.  Except for the latter, the inclination is about 3 degrees above horizontal which should be enough slope for passive convection of the heated air but not so much that the flow rate is too fast for efficient heat transfer to the soil.

The decision on the diameter of the pipes was based primarily on a good book on earth sheltering that I have since lost track of and on Hiat's book, "Passive Annual Heat Storage".  Before reading them, it seemed reasonable to me that the larger the pipe the more heat transferred to the soil, so my early thinking was to use at least 6" pipes. However, it turns out that a large pipe with a given internal volume has less external surface and therefore, as a conduit, less contact with the earth than several small pipes whose combined internal volume equals that of the large pipe. The situation is analogous to human body types.  Compared to heavier people (endomorphs), skinny people (ectomorphs) typically tolerate summer heat better and tend to chill more in cold weather because they have a high ratio of skin to body mass and therefore more surface area for heat transfer.  I also reasoned that going smaller than 4" might not suck the heated air out of the collector fast enough to keep the collector from overheating unless the number of pipes was increased beyond reason.

Installing the Conduits
Installing the conduits was a cakewalk compared to installing the French drains.  Our

Installation of the corrugated pipes

contractor, Brian Hayes, brought in an industrial-strength trencher.  As soon as he dug a trench we dropped in the corrugated pipe, connected a smooth pipe to it at the north end, backfilled to within a foot or so of floor level with man-made pea gravel then backfilled to grade with soil.  Most of the soil will be removed during the final grading for 4" of rock sub-base and 4" of concrete.

Solar Chimney Discarded
The original design called for bringing all nine conduits to daylight at the north end via a solar chimney.  However, during installation, I decided to dispense with the solar chimney and run the conduits to daylight separately.  The change was driven by several things but mostly by my fear that the system might prove so efficient that the house would overheat. Stephens mentioned this concern in his paper and suggested incorporating thermometers

AGS conduits headed towards daylight; trenches filled
and the site graded for the sub-base and slab

in the soil below and behind the house in order to monitor the heat uptake and make corrections (presumably by closing some of the conduits at the collector). He suggested it might take several years in any case for the thermal mass to reach a stable year-round floating temperature so overheating would not be a problem for at least a couple of years for our project.  I decided that it made more sense to leave the conduits separated so that some could be capped for all or part of a summer following a winter with an overheating problem rather than trying to manage the problem at the collector end.  And it would be possible to reverse engineer a chimney later if proven necessary.

The other reason for keeping the conduits isolated was to eliminate the cost  and complexity of tying the conduits together and leading them into the solar chimney.

Timeline - Design Evolution - Wall Cladding

Past Three Years

Preconceived Ideas

Dorothy and I are not fond of vinyl siding.  It is unappealing because it is ubiquitous, looks cheap, is petroleum-based, has a lot of embodied energy, is subject to wind and hail damage, has a short life span and mostly ends up in land-fills. The commonplace 4 x 8 sheets of cedar-veneered plywood would not be a bad choice except for requiring perpetual maintenance.  Real cedar clapboards are beautiful but have the disadvantages of requiring ongoing care, coming from old growth trees and being expensive.  Our search for alternative cladding that was inexpensive, low-maintenance and green took some unexpected turns.

Fibercement -- the New Green
My green building research kept pointing me towards fibercement siding as the greenest

Fibercement lap siding

choice for cladding.  I even went so far as to buy a used electric nibbler to use for cutting it someday.  It is made from cement and renewable FSC-certified (Forest Stewardship Council) wood, holds paint two or three times longer than wood, is virtually wind and hail proof, has a long lifespan and comes with a reasonably low level of embodied energy.

SmartSide
At one point, a building supply salesperson tried to sway me away from fibercement with a product called SmartSide that appears to be an OSB-like board that can stand the weather. He gave me a sample and told me to take it home and soak it in water for a while to demonstrate its durability. I did and it swelled.  I emailed the company and was told that it was not designed for underwater, just wetting as would be expected with any cladding.  Fair enough.  It is indeed a green product since it comes from renewable wood chips from plantation trees and the finish comes with a long-term warranty.  For green-ness, its other attributes pretty much match those of fibercement.

Metal Siding?
Until two years ago, metal siding had not entered my mind. When my stepson , Keith, said that he was going to use it on their energy efficient house, my first reaction was, "Are you kidding?"  Bu

Siding underway for Dawn and Keith's house;
 note that, for energy conservation, the amount
 of glazing on north and west sides is minimal

t after I helped install it, I came to realize that he had made a wise choice -- it's definitely an unique finish for homes, it's DIY-friendly, it's virtually maintenance-free, it lasts for plus or minus a century and it has a recyclable end-life. If there is a knock against metal siding, it is that it has fairly high embodied energy which, to some degree, is off-set by its recycled content.

Comparative Pricing

When the various options for cladding were compared, the SmartSide was the most expensive, the steel siding was the least expensive and fibercement was intermediate. After being so impressed with the steel siding that we installed on Keith and Dawn's house, the price sealed the deal for steel.

Installation
The panels that we installed on Keith's two-and-a-half story house were long -- some were over twenty feet -- which were a struggle using ladders instead of scaffolding. All of our walls are single story height which will make installation easier. The color we have in mind is white because of its timeliness and its high reflectance against solar gain in summer.

Well glazed southern exposure

The one thing that I would change from the typical installation is the trim at the corners and around the openings.  If the appearance of the metal trim sold with the panels could be modified to look more like the trim used with old-fashioned clapboard or fibercement siding, the house would for us appear more "residential" and less "commercial". Accordingly, I plan to rabbet well-dried pressure treated 2x's to receive and conceal the edges of the metal panels just like the metal trim does. Then, in order to minimize maintenance, I will paint the wood on all four sides with the best paint I can find and hide the metal J-mold (that would ordinarily be exposed around the openings in typical installations) under the rabbets.

*    *     *    *     *     *
Update - November 2019
We did indeed follow through with white steel siding with pressure-treated trim boards painted white.  For details, go to "Construction - Steel Siding".

Timeline - Design Evolution -- Earth Contact North Wall

Last Four Years

As a DIYer, it was hard for me to imagine ahead of time the structural complexity of a high concrete wall that is essentially a retaining wall, -- backfilled on one side and not supported on the other. It took Steve Rehagen, who drew our house plans and Mark Bachetti, our structural engineer, to educate me.  Prior to Steve and Mark, I went through a couple of budget-driven iterations that would never be stamped by a structural engineer as required by the Building Director.  

The wall is 92' long, 54' of which is 12' tall with the remainder 8' tall.  In order to maximize earth contact for the AGS system, the inside of the wall must remain open to air circulation. Consequently, a long, narrow storage area will abut the wall with all of the storage taking place on its south wall so as to leave the north wall unencumbered.   The disadvantage of this arrangement from a structural standpoint is that there will be no right-angle interior stem walls to brace the north wall.  Hence, its similarity to a retaining wall.

Dry-stacked Concrete Blocks

Originally, we envisioned more earth sheltering than we ended up with and the

First three courses of dry-stacked blocks
for our solar collector for the AGS system 

quintessential text on the subject is Rob Roy's "Earth-Sheltered Houses" which was an early acquisition and influencer.  He advocates using dry-stacked cinder blocks for earth contact walls -- primarily 12" thick rather than the usual 8" thick. Apparently, dry stacked (mortar-less) concrete blocks originated with the Corps of Engineers.and produces a wall that is not only stronger than a mortared wall but a wall that rivals a poured concrete. So our first vision was a 12" dry-stacked concrete wall.  Dry-stacking would be by far the cheapest approach and the most DIY-friendly but the amount of labor involved with stacking 60 lb blocks 12' high would be formidable. Also such a wall would be hard to insulate. Insulation appended to the outside tends to be disturbed by backfilling and insulation on the inside is not as effective because it is on the wrong side of the thermal mass.

At the time of this writing, we were constructing the walls for the solar collector for the AGS

Parging dry-stacked blocks with
 fiber-bonded cement

system using dry-stacked blocks. The photo above was taken while the first horizontal bond beam course was being filled with concrete and horizontal rebar.  A second bond beam coarse was similarly used higher up in the wall and many of the cores in the blocks were filled with concrete and vertical rebar.  As is typical, both sides of the walls will be coated with fiber bonded cement which makes the joints between blocks stronger than 3/8" mortar joints. Dry-stacking is perfect for this small project but I am glad that we cannot use it for the north wall because it is not as straightforward and easy as it might seem -- minor variations in the size of the blocks complicate stacking them level and plumb, particularly when half-blocks are mixed in with full-sized blocks.  

Complete Blocks 

Last year, I came upon a St Louis start-up making insulated blocks for house walls (Complete Block Company).  After several visits with Herb Walters, the inventor, I become convinced that Complete Blocks were exactly what we needed for our project. With proper equipment, the 200 pound blocks could be dry-stacked to form our long wall in

Dry-stacking the blocks with lifting equipment; notice the
 stamped concrete exterior; the mating surfaces are sealed
with an elastomeric material as the blocks are seated

one or two days.  They would be poured off-site with or without insulation in them. As can be seen in the lower photo, they fit together in tongue and groove fashion then vertical rebar is added after stacking.  It is threaded and epoxied into holes in the slab or footing then tensioned from above.to pull the blocks into tight contact with with each other and with the slab or footing.  Our project would require insulated blocks only at the periphery then solid blocks for most of the wall in order to give thermal mass for the AGS system.  As mentioned above, the intrinsic insulation would be more effective if it were on the exterior side like with insulated concrete forms (post on insulated concrete forms).

Despite the perfect match between Complete Blocks and our needs and a generous offer

Notice the tongue and groove mating and the horizontal
rebar; note also the intrinsic insulation and furring strips
 (for attaching interior finish materials such as drywall);
the vertical rebar on 16" centers is not apparent here

from Herb to help with installation for essentially the cost of the blocks alone, we decided to go a different route.  The system was so new that there were no structural data available that a structural engineer could use to stamp our plans. We are still a couple of months away from constructing the north wall so I recently contacted the company to see if data now existed and any engineers stood ready to stamp our plans.  The answer is still essentially "no".

Poured Concrete Wall
Meanwhile, the default position was to pour concrete in order to keep the project moving. The wall that Mark designed is 1' thick and rests on a monolithic footing (poured at the same time as the slab) that is 1' thick and 8' wide.  The amount of rebar extending from the footing into the wall and interlaced within the wall is mind-boggling -- literally tons of number 4, 5 and 6 rebar.   Presently, I am vetting potential vendors and do not know yet the cost of pouring such a tall wall.

As for insulating, a poured wall would be as problematic as a dry-stacked cinder block wall.

*     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *     *
Update
A month or so after this posting was published, I made the decision definitely to go with a poured wall and worry later about some creative way(s) to insulate it to at least R-20.

Timeline - Design Evolution - Roof Cladding

Since Two Years Ago

Steel Panels

While we waffled considerably on wall cladding, roof cladding turned out to be a no-brainer.  In retrospect, it is hard to understand why there are so few steel roofs except in the southern

Asphalt shingles

states.  I am not sure how the cost stacks up against asphalt or wood shake shingles when professionally installed but, for the DIYer, a standing seam metal roof is cheaper than buying and installing his or her own shingles. We did consider "barn tin" type corrugated galvanized metal roofing that that we have admired in books and magazines but local code prohibits it. 

Cost

The cost of metal roofing fluctuates with the thickness of the metal and the way it is fastened.  Obviously, the thicker the metal, the higher the cost.  As far as fastening is concerned, there are two ways to fasten standing seam roofing -- exposed fasteners and concealed fasteners. The first utilizes hex screws with heads matching the color of the steel.  The screws have neoprene washers under the heads to seal out water. The other system typically utilizes clips that are installed first then the panels are snapped to place over them. 

Pros and Cons of Fastener Styles

(The rest of my comments are based partly on my research and partly on my having helped my step-son, Keith. install a metal roof recently.)  

The advantage of exposed

Exposed fasteners (click to enlarge)

fasteners is that the system is a third cheaper than

Concealed fasteners (click to enlarge)

systems with hidden fasteners, which is what makes the cost of metal competitive with asphalt. And they are faster and easier to install, particularly for a DIYer. 

The disadvantage of exposed fasteners is that, unless care is used in setting the hex screws so that they are not over-tighten or fail to seat fully, water will eventually follow the screws through the panels. This is one of the reasons that 30# felt is used under steel roofing.  Another disadvantage of screwed down panels is that they cannot react to the thermal expansion of the metal as it heats and cools.  This is more important as the panels get longer.

The advantage of hidden fasteners is that there are no breaks in the surface of the panels that can leak. If the panels are installed correctly, they shorten and lengthen in response to temperature changes by sliding on the clips.  The disadvantages of hidden fasteners for the DIYer is that there is a steeper learning curve and the added cost.

Slippery Slope

Installing metal roofing on a steep roof is dangerous because, unlike rough asphalt, it makes a perfect sliding board.  In my view, a DIYer might want to think twice about installing it on a steep roof.  And, if s/he does decide to do it, a safety harness is an absolute must.

Sustainability

Steel roofing is the most sustainable among roof claddings.  While it has fairly high embodied energy, it is less so than asphalt or aluminum.   It contains a high recycled content,, has a recyclable end-life and lasts longer.  It also is available in highly reflective colors which help to limit the amount of radiant heat penetrating the roof in summer. Asphalt shingles are petroleum-based, rarely have recycled content, almost always have a landfill end-life, have a shorter lifespan and lower reflectance. Shake shingles too often come from old growth trees, have a limited lifespan and end up in the landfill or, worse yet, are burned.

Our Choice

We will be using a light colored reflective steel roof.  The good-news-bad- news story is that the roof pitch is so low as to make slipping off the roof unlikely. but our budget dictates the use of exposed fasteners.

__________

Addendum, June, 2017
By the time I placed the order for the metal roofing, I had given up on the use of exposed fasteners due to a lower pitches to all of the roofs than originally envisioned.  

Odds 'N Ends - Managing Salvaged Lumber (Cont'd)

This is the second of two posts on managing salvaged lumber.  The first post discusses de-nailing; this one is about preserving the lumber until ready for use.

Storing De-nailed Lumber

At first, I stored salvaged lumber stacked tightly together, like on the racks at home centers, and under a heavy-duty Craigslist freebie pool cover.  Moreover, the hard-won boards were stacked on top of salvaged 4 x 4s to keep them off of the ground. Needless to say, termites found this arrangement convenient.  By the time I discovered my stupidity, their munching was still confined to some of the shortest and most expendable 2 x 4s -- but they sure got my attention.  And let me add that my experience with protecting things for extended periods of time with anything resembling a "tarp"  or plastic sheeting has made me aware that ultraviolet radiation always wins and coverings leak.  When they do, mold can be a problem, both from the standpoint of deterioration of the lumber as well as health concerns.

After my experience with the pool cover,  I followed a different  protocol.  I covered the ground under a prospective stack with something to control weeds and grass -- old tarps, old carpet, old carpet pads or black ground cloth. (as can be seen in both photos).  I then arranged salvaged concrete blocks in grid fashion.  Supported by the blocks were 2 x 4s or 4 x 4s cross-ways of the stack that were shimmed with the help of a long straight edge as necessary to compensate for the unevenness of the ground to ensure that the boards would lie perfectly flat lengthwise.  I am hoping that the 8" tall blocks, will dissuade at least the lazy termites. 

Stickering

When green sawmill lumber is stacked for air-drying, each layer is separated by narrow

Air-drying green lumber -- stickers between every layer

boards called "stickers" laid cross-ways.  It is important for drying green lumber to have all four sides exposed to air. However, for seasoned lumber, I reasoned that exposure to air on three sides should be enough to keep lumber that is stored outside dry, so I used stickers between every other layer for all the salvaged lumber.

At the time of this writing, one sizable stack of 2 x 4s and a stack of 1 x 8s have been dismantled for making wall trusses and concrete forms, respectively, and no deterioration is evident -- no termites, no rot, no mold, everything is cool except for lots of stink bugs (we are having a horrendous infestation stink bugs in the Midwest because there are few natural enemies to keep them in check (see food chain). As an example of stacking recycled lumber, the bottom photo shows 2 x 4s that were pre-cut for wall tursses then restacked with three sides of each board exposed to the air.

Covering

Using the pool cover is a bad idea because stacked lumber needs to breath.  I covered the new stacks with loose sheets of barn "tin" weighted down with heavy stones. And, since, our area is in the tornado alley of

Storing salvaged lumber -- stickers every other layer

the Midwest, I lashed each stack together by looping wire over the tin and under the stack then twisting it tight.  The loops are spaced about 5' apart along the length of the stack.  So far, we have had numerous tornado warnings and one close call but no direct hits to test the efficacy of the arrangement.

Of course, the tin does not fully cover the ends and sides of the piles but this is not important because the surfaces that are most prone to wetting are also the ones most exposed to air and sun.  The exposed areas turn gray but remain in good condition.

Spacing the Stacks
If I had it to do over again, I would have spaced the stacks so as to be able to mow completely around each stack with the riding mower.  As it it now, the mowing has to be augmented with a string trimmer, not one of my favorite things to do.