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home page » passive solar design chapter » wing insulation detail page ... you are here
The home pictured throughout our site was bermed to help it stay sustainable through our long and harsh Rocky Mountain winters. In most climates, an underground home is more of an aesthetic choice than a sustainable design necessity. Above ground HTMs with a shallow footer wing insulation design function worlds better than any conventional home. Please note that bermed homes are ones which have been backfilled to the top of the first floor; wrap soil around to the front of an HTM, add a Swedish sod roof, and it becomes an "underground" home.
The Underground Space Center at the University of Minnesota did some excellent research in the late 70's and early 80's pointing to the fact that horizontal "wing" insulation was preferable to vertical foundation wall insulation. Wing insulation was shown to keep the earth near foundation walls dry, greatly increasing the insulation's efficiency. Expanding greatly upon this concept, John Hait published Passive Annual Heat Storage (PAHS) in 1983. This work was dedicated to a very basic concept that is ignored by nearly all architects: keep the ground under and around the home dry and you can use it for an amazingly effective heat sink and seasonal temperature moderator. After all these years, I am still shocked and amazed at the ignorance shown by designers who insist on leaving the foundation walls in contact with wet earth. Frost-protected shallow foundations (wing insulation) protect the area under and around the footers, allowing for a lower foundation depth - making it perfectly suited for slab on grade (monolithic slab) construction. The cost savings from less excavation and concrete in the footers can be dramatic. Frost-protected shallow foundations have been the standard since the 1950s in cold climate Scandinavia countries.
One of the most important design factors for sustainable, high thermal mass (HTM) construction is desiging a frost-protected shallow foundation (wing insulation) around the perimeter of the home. As the shallow footer (monolithic slab) sketch below outlines, we stress the need for waterproofing and insulating up to a 20 foot* perimeter around the home (*less than 4 feet is a waste, 8 feet is better, 12 feet is an economic compromise, 16 feet is typical up to 20 feet in extreme climates). Wet earth near your foundation acts as a constant drain pulling energy away from your home. This is a mistake you simply cannot afford, literally, when buidling a sustainable home.
HTMs do not employ any exotic building materials or methods. Construction details are more common commercial, if anything. Electrical wiring in conduits and pipe in chaseways; there's just not alot of wood framing to drill through, is all. A single story, bermed, concrete block home is the easiest to build and the most efficient passive solar design for many reasons. The key to a single story HTM's total sustainability is having the floor grounded directly to the incredibly large heat / cooling storage mass that the earth provides. This is the main reason why a two story home is inherently not as sustainable. Two story homes will always need some type of mechanical heating/cooling system to service the floor above. And, they're more technically difficult to build - special skills are needed that the average homeowner-builder does not possess.
If a key interest is being energy independent, never build an attached garage with any type of home (home and garage share a door). When the two structures share concrete footers/foundation/walls, energy flows easily and the garage constantly drains heating/cooling from the home. Thermally separating walls and foundation by heavily insulating underneath garage floor and between the two structures helps, but the physics works against you. There is a finite amount of passive solar gain potential - why use stored energy resources to indirectly heat/cool the garage and ground around it? We always recommend detaching the garage by at least 12 feet, preferably 20 feet, or more from the home. The integrity of the home's wing insulation is key to thermal mass performance. Allowing energy to be drained off and escape through garage foundation is not sustainable. A covered breezeway between the two structures is very common design element to span the gap. These breezeways are often enclosed against the weather (covered patio), adding to the integrity of the wing insulation (ground above warm and dry). A greater separation of public and private areas goes along with stretching the layout with a breezeway. And, the cost of building the garage portion is greatly reduced.
The drawing below, outlines a conventional poured-in-place frostwall foundation footer design. You can build an HTM with poured-in-place walls - it requires special equipment and is more technically difficult the first time than stacking blocks, but concrete is more common from local contractors. If you don't plan on building your own home, we can work with you on a poured-in-place design instead of surface bonded, stack block walls. The main advantage of dry stack block construction is its ease by the average homeowner / builder and their friends. Putting up your own walls will save you a lot of money and make it a more personal project. When labor amounts to around two-thirds of your home's total cost, it is an important consideration.
Here's some basic building terminology to know:
The Uniform Building Code (UBC) is regionally modified to prevent frost heaval of your foundation walls.
In the Colorado high country, the top of backfill (finish grade) extends a minimum of forty inches from the exterior finish grade (your yard surface) to the bottom of the footing (footer).
When floor joists run across the top of a short frostwall, it's called a crawlspace.
When you pour a frostwall tall enough to allow headroom (7'6" minimum), it's a basement.
If the basement floor has a door that leads directly outside (no steps up) it's a slab-on-grade walkout basement.
Slab-on-grade construction refers to pouring the concrete floor (slab) directly on the ground (grade).
In the picture above, the floor slab will be poured directly on the top of footer elevation.
You'll find it very handy to know the vernacular when talking with builders and architects.
With any foundation design, it is key to note:
As noted above, roof runoff should be gathered and directed away from the foundation. Moisture under and around your foundation creates an amazing heat sink that results in a lot of energy loss during both the heating and cooling seasons. We always suggest connecting the gutters to underground pipes and taking the roof runoff at least 20 feet away from the foundation and disposing of it in underground drywells. Please see drywellinstallation.htm for details on these drywells.
The most important factor to successful earthtubing is DRY EARTH. As the sketch below outlines, we stress the need for waterproofing and insulating up to a 20 foot perimeter around the home. Wet earth acts as a constant heat sink pulling energy away from your foundation walls. Dry earth under and around an HTM stores an amazing amount of energy. Earthtubes utilize this energy by allowing fresh, incoming ventilation air to passively gain or lose heat energy before it enters your home. This sustainable ventilation system exchanges indoor air more often, keeping your home's environment fresh without the drawback of "losing all that energy".
All exterior foundation walls should be insulated with 4 inches (minimum) of EPS ("blueboard") foam to prevent energy loss. You need to make the foundation energy efficient - we would most highly recommend wing insulation. Your house plans should call for a minimum 2 percent grade away from the foundation for at least an 8 foot perimeter (minimum) up to 20 feet. We normally suggest 2 inches of EPS foam and three waterproofing layers as shown in the sketches above. Wing insulation keeps the perimeter of the home dry and insulated. This is critical to creating a viable heat sink under and around the home. Dry earth stores heat/cool; wet earth steals energy form the foundation and footers. A French drain should be placed along the far edge of the wing insulation to direct ground runoff to the drywells 20 plus feet away from the foundation. This wing insulation concept is critical to high thermal mass housing and will pay off in the long run. It is not common to conventional construction yet, so don't let yourself get talked out of it. Topsoil above wing insulation is one foot minimum or go with a xeriscaping design with sand, rocks and stone. You can adjust this as needed to fit the site's needs and the availability of backfill. We suggest one foot as a minimum only.
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