11 - Wing insulation & shallow footer, frost-protected perimeter foundation design provides key benefits even when HTMs are not bermed into a hillside.
Bermed foundation wall and frost-protected shallow foundations should both be given at least basic frostwall concrete waterproofing and wing insulation. The home pictured throughout our site was bermed to help it stay sustainable through our long and harsh Rocky Mountain winters, but more importantly... the property was already sloping to the south. In most climates, an underground home simply an aesthetic choice, not a sustainable design necessity by any means, and certainly not cost-effective to do on a flat property. 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. Add a Swedish sod roof and wrap soil around to the front of an HTM, 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 basic concept, ignored by too many architects: keep the ground under and around foundation dry and it will retain energy better. Water transference problems aside (the classic wet, moldy basement), leaving uninsulated foundation walls in direct contact with wet earth is a huge, senseless energy loss. A giant thermal heat sink. Frost-protected shallow foundations with horizontal wing insulation protect the area under and around footers, allowing for a lower foundation 'frostwall' depth, making it perfectly suited for slab on grade, monolithic slab construction. Installing at least two foot width of 2 inch thick foam board insulation in a horizontal wing around the perimeter of ANY home for frost protection is wise. That technique has been a standard since the 1950s in cold-climate Scandinavian countries.
One of the most important design factors for sustainable, high thermal mass (HTM) construction is this frost-protected shallow foundation wing insulation around the perimeter. As the shallow footer monolithic slab sketch above outlines, we stress the need for waterproofing and insulating up to a 20 foot perimeter around the home. Should your site and/or budget dictate less than twenty feet of wing insulation, 4 feet is bare minimal in any climate, but even 2 feet will make a difference. Sheets of foam board insulation typically come 2 and 4 feet wide by 8 feet long (48 sheets of 2 inch thick, 4 foot by 8 foot per unit/pallet). To save money when purchasing, always get a wholesale bid on 'units' or 'bunks' 4 foot by 8 foot by 8 foot tall on a pallet and ignore the per-piece cost at building center. 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 building a sustainable zero-energy house plan. And once you have it dry, insulating your foundation is always a good energy investment. Always insulate the exterior of any home design and break ground contact at every opportunity.
HTMs do not employ any exotic building materials or methods. Construction details are common commercial, if anything. Like any block or concrete wall, electrical wiring is best in conduit within the wall. Knockouts can be made in block for outlets and switches or 'formed out' with poured-in-place walls. Plumbing usually takes advantage of chaseways above and 'wet walls' within the floorplan that are framed out. 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 often total sustainability is having the floor grounded directly to the incredibly large heat and cooling energy storage mass that the Earth underneath provides. This is the main reason why a two story home is inherently not as sustainable. Two story homes are HTM hybrids and will always need some type of mechanical heating and cooling system to service the floor above. And they're more technically difficult to build... special skills are needed to build a two story home that the average homeowner-builder does not possess. Skills like perching on top of a 30 foot roof.
If a key interest is being energy independent, never build an attached garage with any type of home where the home and garage share a door. When the two structures share concrete footers/foundation/walls, energy flows way too 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 your HTM home's wing insulation is key to thermal mass performance. Allowing energy to be drained off and escape through garage foundation is simply not sustainable. A covered breezeway between the two structures is very common design element to span the gap. These breezeways can be enclosed against the weather as a covered patio, adding to the integrity of the wing insulation and keeping the ground underneath 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 just requires special equipment and is more technically difficult the first time than stacking blocks. Concrete pours are more common from local contractors. 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 good 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. Connect your roof gutters to underground pipes and take all of the roof runoff water at least 20 feet away from the foundation. If you have a flat lot, dispose of the water in underground drywell leach pits. 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. 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 insulation to prevent energy loss. Your house plans should call for a minimum 2 percent grade away from the foundation for at least an 8 foot minimum perimeter 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 moderating storage area under and around the home. Dry earth stores heating and cooling energy, while wet earth steals energy from 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 please 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 and the availability of backfill. We suggest one foot as a minimum only.
Surface bonding cement SBC dry stack concrete masonry unit CMU block wall and monolithic foundation details are typical and common to any size building, but will naturally vary depending upon size of structure, soil and site conditions, and local building codes requirements. Conservative engineering practice is to design SBC wall reinforcement (rebar) in the same grid pattern as a mortared block wall, given site, soil, and foundation variables. In high thermal mass construction, it is standard practice to fill all cores. No hollow cores in an HTM. Non-structural cores can be filled with sand or similar. But in practice, if you are 'shooting cores' with a pumper truck, it is sensible to just go ahead and pour all cores with concrete 'grout' while you have the machinery on site. We no longer provide construction, consultation, or engineering services and any information presented on this website is for 'entertainment purposes only'. Site, soil, and local code requirements are only the first of many unknown variables. We do not warrant information for any errors or omissions. Plans are not presented as construction ready. Local engineering approval must always be obtained first, before building. Click on drawing for printable Adobe .pdf file.
The roof structure, cmu rebar pattern, and backfill depth dictate ultimate wall engineering design. While possible to avoid interior perpendicular walls and counterforts entirely, it is common practice to have one every ~18 foot when the roof structure does not provide adequate stability. Placed perpendicular, roof beams act as retaining. Placed in other direction (parallel with wall) roof beams provide no wall support. Then the interior perpendicular walls both retain and provide bearing for parallel roof beams. Thus the ~18 foot mark for allowing 20 foot beams to span. Using roof trusses instead of beams has much the same, but lesser retaining effect, but only when opposite wall provides anchor for far end of truss. Sometimes you see short 'stub' block walls (counterforts) used to retain, with wood frame walls extended past counterforts on interior. Boils down to whether exterior wall is retaining backfill against structure.
HTM Passive Solar eBook - Table of Contents
- Introduction to HTM design basics with the following pages going into detail
- photo gallery is packed with photos and commentary from our HTM Home Tour DVD video
- dry stack surface bonding cement construction photos and sample block layout sketches
- floor plans is an annotated progression of layout design choices for more functional HTMs
- roof detail chapter outlines T&G plank-style vented roof decking atop log purlin joist beams
- sun screens shade panel micro-climate passive cooling design tips and installation tricks
- heat storage with fiberglass water tanks providing a means to moderate temperature swings
- earth tubing is a simple, passive method for tempering a household's fresh air return intake
- solar orientation is important, but HTM designs excel in hot or cold climates alike... anywhere
- exterior fascia SBC stucco coatings over EPS foam board insulation and sloped glass details
- wing insulation details and shallow footer, frost-protected perimeter foundation treatments
- planter beds are central, functional features in greenhouse style HTM home construction
- links page is packed full of handy research references and relevant manufacturer websites