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.

*          *          *           *

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.
______________________
*  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).

Construction - Insulation/Watershed Umbrella for the Annualized GeoSolar System

The insulation/watershed umbrella is a concept advanced by John Hiat in his self-published book, Passive Annual Heat Storage:  Improving the Design of Earth Shelters".

The book is mandatory reading for anyone contemplating a project like ours but it is out of print and apparently available only secondhand online. Don Stephens later improved (in my view) Hiat's PAHS --  including Hiat's umbrella concept, -- and called his iteration "Annualized GeoSolar" (AGS) (also check out  three prior posts on AGS:  first post, second post, third post (the third post zeros in on the insulation/watershed umbrella)).

I began excavating for the umbrella at Thanksgiving-time so it is uncertain how much of it I can do before winter weather interferes.  The umbrella adjacent to the concrete walls will have to wait several months until the tops of the walls between the umbrella and the mudsills are insulated on the outside then backfilled to the level of the umbrella. 

Priority Dilemma
My concern is that all of the footings are protected from freezing this winter by completing the umbrella (highly unlikely) or burying the footings under at least 3' of dirt (behind the north wall) or with some other stopgap measure.  As it turns out, I resorted to covering the exposed footings with EPS foam insulation board with the intention of using the boards later to insulate the concrete north wall in the area of the garage.  Unfortunately, the umbrella will have to wait until after the Spring rainy season.  

Actually, a higher priority for what good weather we have left before the ground freezes is to remove the superfluous dirt from in front of the house in order provisionally to restore the pre-construction contour of the slope.  The grading will be finished in conjunction with installing the AGS umbrella in late Spring.

Most of the superfluous dirt came from the final grading for the floor of the
house but considerably more came from excavating for the AGS umbrella.
(Click on the image to enlarge it.)

Superfluous dirt has been moved behind the wall as backfill.

The backfill against the wall is +/- 6' deep in the middle and sloped
towards both ends for surface water drainage; notice  the AGS
conduits protruding upwards; after backfilling against the wall,
the remainder of the dirt from in the front of the house was stacked
to the right of the conduits to a height approximating the final
 backfill level when it is extended all of the way to the wall;
 eventually, the conduits will have to be extended even more in
order to reach daylight through the taller backfill.

Composition of the Insulation/Watershed Umbrella
The umbrella will be merely a "sandwich" made up of plastic sheeting, rigid foam insulation board and sand.  Starting at the bottom in the order in which the layers will be built up, the sequence is as follows:

• Sand over the soil to create a smooth bed on which to lay the umbrella
• 6 mil plastic
• Thin layer of sand
• Mostly expanded polystyrene insulation board; some extruded polystyrene board
• Thin layer of sand 
• 6 mil plastic
• Thick layer of sand
• 6 mil plastic
• Thin layer of sand
• Two layers of recycled synthetic carpet upside down
• Topsoil

I will be layering the plastic and insulation precisely as Hiat recommends except, of course, we have no earth sheltering on the roof.  Stephens' influence will be represented by the carpet overlay to protect the sandwich from

Illustration from Hiat's book (click on picture to magnify the details)

mechanical damage from such things as burrowing critters or thoughtless use of shovels and other tools.  It will also protect somewhat against the penetration of plant roots although the latter will mostly be inhibited by the fact that the only area that is not too dry to support them will be above the first layer of plastic.

As recommended by Hiat, the insulation will be thickest near the house and thinnest at the periphery in five steps corresponding to the width of 4' x 8' foam board --   4" thick for 8 ft,, 3" for 4 ft,  2" for 4 ft and 1" for 4 ft -- making the umbrella 20' wide. Where a single sheet of plastic is not at least 20' wide abutting sheets will be shingled with a large overlap. The sand is important for a couple of reasons.  First, to provide smooth interfaces between the layers and eliminate air spaces that might allow the umbrella to be damaged by heavy equipment or vehicular traffic, particularly punctures of multiple layers of plastic at the same place allowing through-and-through leakage. Second, to hold the plastic sheeting apart enough that any water finding its way through one layer is sure to flow downhill through the porous sand to the periphery.  Without the sand, the backfill over the sandwich would compress the plastic layers together and trap water, particularly where the plastic extends beyond the insulation.  The umbrella will slope away from the house in all directions and, in some places, end in a mini-French drain for faster drainage ("drainage gravel" in the illustration).

The white expanded polystyrene (EPS) foam board will suffice for all areas except under the driveway and the garage floor where pink 250 psi extruded polystyrene (XPS) will be a better choice for supporting vehicles.  The sand in the sandwich will go a long way towards stabilizing and supporting the weaker EPS. The two reasons for selecting EPS for most of the umbrella insulation are price and EPS's long history of use in wet environments (think insulated concrete forms and flotation for boat docks). I trust that the plastic sheets will keep the foam pretty dry but there will be a few small leaks inadvertently created during installation that will admit trickles of water through a given layer that will have to travel in association with the foam and sand layers to an exit at the periphery of the next layer of plastic, so it is prudent to anticipate some exposure to moisture.

The reason for the plastic is to keep the soil under the umbrella dry.  Doing so lowers its thermal conductivity and inhibits transfer of heat from the thermal mass to the outside environment.  Through dry soil, it takes 6 mos for a unit of heat to move 20' which is the basis for making the umbrella 20' wide.  As Hiat explains, even if some moisture would somehow find its way through three layers of plastic, the negative impact on the ground below would be inconsequential, that the saturation would be so spotty and limited that it would not be enough to compromise the efficacy of the AGS system.

Garage Serves As Part of the Umbrella

Insulation of the wide footing is completed by the addition
of  XPS foam board horizontally; it was covered with sand
before the backfilling; EPS would have worked as well

The entire floor of the garage will be insulated for a couple of reasons. One is to complement the other measures that will make the garage warmer, viz., the insulated foundation, the thick walls and ceiling filled with insulation and the insulated overhead doors. The other is to serve as the umbrella for the north half of the east end of the house.  

In order to fulfill the requirements for a frost-protected shallow foundation, the garage foundation footing below the insulated concrete forms will have to be insulated where it is not protected by the house and garage floors or the umbrella next to the south half of the east end of the house. Likewise, the wide footing for the east end of the concrete wall was poured against vertical insulation but it needed to be insulated on top, which was simply a matter of fitting foam board over it, sandwiching it between two thin layers of sand (to carry any water from behind the damp proofing membrane to the footing drain) and backfilling over it. Eventually, a retaining wall will be resting on it.  

Horizontal insulation in place and the backfilling started

I insulated the narrow footing under the garage foundation in conjunction with extending the footing drain to daylight downhill. I enlarged the overdig next to the footing so that it would accomodate foam board 3' wide and embedded the drain in sand, much like along the backside of the wall, screeded the sand level with the top of the footing and laid in the insulation. I did the same procedure for that part of the front footing for the garage that will not be protected by the umbrella, except there was no drain to worry about and the EPS was only 2' wide since it faced south and direct sunshine should warm the backfill enough to make for a warmer footing anyhow.

The footing drain at the other end of the house merely needed to be extended downhill to daylight, bedded in sand and backfilled.  

The extension of the footing drain for the west concrete wall.

With respect to the exposed footing inside the garage that will be covered by the floor eventually, I used soil to fill the overdig flush with the floor grade and laid foam boards against the north wall and the insulated concrete forms for the other three walls and weighted them down with stones. That should protect the footing this winter; the floor will go in next summer.

For some reason, the follow-up post on construction of the insulation/watershead umbrella was posted out of sequence and is dated 11/28/15.  Please drop down four or five posts to find it.

Design - Exterior Walls

Priorities

Being on a strict budget will precipitate a lot of compromises.  However, one issue is not negotiable and that is a super-tight and super-insulated envelope for the house.   Probably this means we will have laminate or wood counter tops instead of natural or man-made stone, or site-made interior doors instead of prehung, but that's okay.  Energy conservation will remain our highest priority.

Stick-Built Walls Using Salvaged Lumber

In a perfect world, we would be using structural insulated panels or insulated concrete forms for the exterior walls but they far exceeded our budget and are limited on  the amount of insulation they can provide.  And we have a lot of free
salvaged dimension lumber that would go to waste if it were not used for the exterior walls.  The trouble with salvaged 2 x 4s however is that the nails often split the ends during the salvage process, especially when toe-nailed.  Typically, a couple of inches must be sacrificed from one or both ends of the boards during the denailing process, resulting in many studs that are too short for typical 8' wall construction.

Wall Trusses

Early on, I decided to use wall trusses instead of individual two-bys as "studs" for several important reasons.  One, with trusses, the salvaged 2 x 4s could be utilized for an 8' wall even if shortened as long as they remained at least 93" long.  Two, some of the 2 x 4s were not entirely straight (but neither is a lot of new lumber these days) but could still be utilized quite well for trusses.  Third, trusses would essentially eliminate thermal bridging. The last and most important reason for using trusses is that R-value for the walls is directly proportional to the thickness of the insulation and trusses can be designed to house whatever thickness of insulation needed to hit a given R-value target.

Truss Design

The 2 x 4s in the trusses will be turned 90 degrees from the way a stud normally sits in a wall and arranged in pairs, one facing outward and the other inward.  They will be tied together with short  2 x 4s at the top and the bottom and braced with three pairs of gussets cut from 3/8" or 1/2" plywood or OSB.  When arranged in the wall on 24" centers, the result will be a wall filled with 15" of insulation and virtually no thermal bridging.  What bridging does occur will be limited to the short 2 x 4s at the tops and bottoms of the trusses and through the skinny gussets.

As far as straightening the boards, my original idea was to assemble them at the tops and bottoms with the bows pointed away from each other.  Then I planned to use clamps to pull them together until they fit the precut gussets.  Once the gussets were attached, the truss would be straight.  

Using Jigs
However, while researching rice hulls as insulation, I came upon a most interesting

Wall truss jig

slide presentation  (Rice hull house) showing the use of wall trusses similar to what I envisioned but being built in jigs for ease of construction and for standardization.  At the time of this writing, a jig was already in service (the subject of another post) that facilitated any straightening that needed to be done.

Top and Bottom Plates
Characteristically the top and bottom plates are as wide as the wall is thick, e.g., 2 x 4s for 3 1/2" walls and 2 x 6s for 5 1/2" walls.  Unfortunately, the plates then become conductive thermal bridges because they are exposed simultaneously to both the exterior and interior environments.  In order to solve this problem, our 15" walls will have double 2 x 6 mudsills side-by-side and  2 x 6 top plates that are double in a side-by-side sense as well as two courses on top of each other in the typical fashion.  The side-by-side configuration of the mudsills and the top plates will allow 4" of insulation between them, thereby arresting thermal bridging. 

Design - Windows and Doors

Even with our tight budget, we intend not to compromise on window and door quality because they are the weak link in heat transfer in and out of the building envelope.  For example, the R-value of double pane glass is a paltry 1.5 - 2 while the R-value for our exterior walls will be over 45.  The best way we can maximize the thermal performance of the windows and doors is by controlling air infiltration between the wall framing and the window.and between the window frame and the window sash.  Then we need to control thermal bridging through the window frames and through the glass itself.

Air Infiltration
The two ways air infiltrates windows is between the window and the framing and between the window sash and the frame.  In order to hold the window back from the plane of the exterior wall (see below), we will have to use replacement windows that do not have nailing flanges like that of new construction windows.  Nailing flanges are the first line of defense against air leakage between the window frame and the framing.  We will have to compensate for the lack of flanges with caulking and minimal expanding foam insulation and precise fitting of the inside and outside trim.


Air infiltration between the frame and the sash is easier to fix.  Instead of sliding windows

Casement window

(single- or double-hung or horizontal sliders) we will use hinged windows that close against and compress a semi-rigid air seal that doesn't leak. Sliding windows are built with more tolerance between the window and the frame in order to facilitate sliding.  The air sealing is done with flexible air seals that tend not to be as airtight. For us, all but a couple of windows in the garage will be casement windows.

Thermal Bridging

Thermal bridging, a form of conductive heat transfer, occurs through both the glass and the frames.  Double glass marginally improves the R-factor of single pane glass by providing a dead air space between the panes.  An R-2 is not much to write home about. Since windows are essentially poorly plugged holes in the wall, a better strategy is to limit the number and size of windows, particularly on the north and west sides of the house (US Midwest).  Except for two or three small windows on the east, two of which are in the garage, all of our windows will face south.

The addition of argon gas between panes of glass also helps to reduce thermal bridging by impeding convection currents in the space between panes. The literature in my library and online information lead me to believe that argon is more hype than help because it escapes within a few years and therefore is not a good investment.  Recently, the sales rep for the window company that will supply our windows convinced me otherwise. The argon in his windows is guaranteed to be 80% effective for the first 20 years and costs only about $10 per window.  That sounds like a good investment after all.

Thermal bridging through the frame is easier to control than bridging through the glass by choosing materials that have low thermal conductivity.  Fiberglass and vinyl are better insulators than wood and wood is better than metal so, if our budget allows, we will opt for fiberglass for both windows and doors.  If not, vinyl will be a reluctant second choice at least for windows.  Nevertheless, choosing vinyl would be a double-edged sword from a green building standpoint; while it  minimizes thermal bridging it comes largely from petroleum. Fiberglass is a green choice for three additional reasons:  (a) it has the same coefficient of thermal expansion

Proprietary graphic comparing coefficient of thermal expansion 

as glass so the seal between the panes and between the panes and the frame lasts longer, (b) it is many times stronger than

Another proprietary graphic showing the strength of fiberglass relative to vinyl 

vinyl and (c) it is greener because it is made from the most abundant resource on earth -- sand. Also, we will specify warm edge spacers between the panes, which, at nominal cost, reduces thermal bridging through the edges of the glass.  However, we will not opt for fully insulated frames because the payback is not as quick here as in northern climates.

Winter Winds
Glass that directly faces winter winds is considerably more conductive than glass on the leeward side of the house because the wind removes the thin layer of insulating air on the exterior surface of the glass (wind-washing), thereby accelerating the loss of "fresh" heat through the glass.  We plan no windows directly facing the prevailing winter winds. Our exterior walls will be 15+ inches thick which will allow us to recess the south-facing windows into the wall as much as 6" thus sheltering them from wind-washing more than if they were mounted flush with the wall.

The fact that the house will be earth sheltered on the north side and half of the west side will pretty much neutralize cold winter winds.  Nevertheless, soon after purchasing our property years ago, we began establishing a shelter belt north and west of the future house as another buffer.  And the eastern red cedars that we planted are native to our area and very beneficial for wildlife. 

Tinting and Low-E

Exterior overhangs will shade the windows from the summer sun but will allow winter solar gain.  The gain will be maximized by using clear glass instead of tinted glass. However, we will specify a low-E coating in order to slow heat loss out through the glass in winter. Admittedly, low-E will diminish slightly the amount of solar gain in winter but our AGS system is the principal heat source and gain through the windows will be welcome but not essential.

That said, we may leave off the second level overhangs for the first one or two years.  It will probably take that long for the AGS system to charge the thermal mass with enough heat for a year-round constant floating temperature at a comfortable level.  Accepting some summer solar gain to augment the output from the solar collector might be a good strategy.

Also, after the AGS system reaches equilibrium, we will assess the need for thermal window shades for nighttime and gray day use.  If the system provides plenty of heat, they would be superfluous, if not, they can be added.  Their function would be to keep moving air (convection) away from the glass and thereby slow conductive heat loss through the glass.  Secondarily, less convection would make for a more comfortable living environment.  In order to be successful, the shades would need to be sealed as tightly as possible on all four edges.  Heavy drapes, regular window shades or louvered blinds help some but are not the same thing and, in some cases, actually enhance convection.