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home page » passive solar design chapter » HTM exterior detail page ... you are here
HTMs are very adaptable to many different styles of exterior fascia from structural stucco over EPS blueboard foam to aluminum siding. Naturally, the south face is dominated by the full floor-to-ceiling glass in a cold climate, but the other three sides are open to personal taste. These pictures outline the standard, low-maintenance covering of surface bonding cement. This method is very low-tech in that all you need is a small barrel mixer, some water and a strong back. Application of the surface bonding cement is done with a trowel, just like stucco.
Mind you, there are several ways to design the exterior, but the first step involves waterproofing all footers and walls with two mop on coatings, or better yet, a sheet bithumine such as Grace Ice and Water Shield®.
Surface bonding cement is waterproof without additional treatment, but we recommend taking extra precaution, especially with bermed walls.
Water transference is not something you want to promote and hope to preserve air quality.
The inside of all interior concrete (block or poured-in-place) walls must also be made non-porous.
This is a step often overlooked by novice designers and is the root cause of negative comments about underground homes being damp and moldy.
Surface bonding cement and/or a very high quality non-porous latex paint is normally spec'd to ensure water transference does not occur.
Molds and mildew must have fuel and water to grow - a sealed concrete wall is nearly sterile, as compared to drywall.
Add good ventilation and warm walls to the equation for healthy air.
All exterior wall surface areas (block or concrete especially) must be insulated to ensure thermal isolation.
The typical minimum specification is four inches of extruded EPS blueboard R20 on walls and six R30 on the roof.
Blueboard does not absorb water and is easily stuccoed or sided - plus, it is a True R rating.
Cellulose absorbs water, packs down, invites vermin and generally looses its ability to insulate over time.
Fiberglass itches, compacts and allows air to infiltrate and generally does not insulate as effectively as sheet foam.
Classic white Styrofoam® bead board and polyisocyanurate foam both absorb a little moisture, although newer generations of these foam materials are very promising.
And there is always straw - bales do make good insulation when used outside the walls of an HTM.
The normal construction method is to attach 2 by 4's (wood or metal) to the walls vertically then nail or screw the EPS blueboard foam to these "sleepers".
Be certain to make the top and bottom of your sleepers airtight with a blocker board to avoid convective energy loss.
It is possible to fill the gaps with cut pieces of 1-1/2 inch wide blueboard or use it as dead air space with a reflective coating inside sleepers.
After the insulation is glued and/or nailed to the sleepers, it is covered with chicken wire or stucco lathe and coated with a layer of latex modified surface bonding cement (SBC with a liquid latex modifier additive to help it stick better).
Alternative exteriors are not limited: siding is common, along with stone, plaster, mock logs and natural plasters over straw bales.
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Please note that an HTM is not always bermed (buried into a hillside). Underground house designs are always an option with an HTM, but most people opt for the conventional, totally above ground appearance. The same choice applies to sloped glass, just bear in mind that vertical glass does not supply nearly as much solar gain as sloped glass. You can expect up to 30% less direct solar gain in the deep winter months. |
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HTMs are easily adapted to full underground burial or completely above ground designs. The photo at right shows the north side of an HTM being made ready for application of a sod roof. The waterproofing "wing insulation" layers are not visible beneath the sand, but you can see the fabric wrapped "French drain" pipe as it is being laid against the roof. This pipe carries most of the water away from the ground around the home much like a gutter, keeping things dry and therefore warm. Later, the gravel you see will be extended full length and the roof and surrounding ground covered with a double layer of sod. Voila, underground earthhome! Sod on the roof really doesn't add anything to the home's energy efficiency, but the aesthetic qualities are wonderful. With a natural grass roof this home will be nearly invisible from the north. |
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Here's a look at a south face design sketch which incorporates a shed roof and a full overhang past the sloped glass.
Overhang designs still manage to generate full solar gain during the peak heating months while providing generous amounts of shade in the summer.
One advantage of this particular overhang design is that the shade cloth can now be hung vertically across the front of the posts instead of horizontally, awning style, out in front of the home.
This is but one example of the type of design choices you will have with your HTM. Don't be lead to believe that the home you see on this site is the only design we can provide, there is very little limit to your choices.
For example, you'll notice that the roof shown here is sketched as being built with standard BCI joists instead of T&G planks and logs.
Please note that this sketch outlines a very deep frostwall.
Some parts of the country call for frostwalls to extend up to four feet below the finished grade.
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Whether you install vertical or sloped glass, it is key to note that wood framed windows are common to high end housing, but you may wish to consider the benefits of vinyl windows.
If you are keeping the house for future generations, the resale value of wood is not an issue and vinyl is usually a lot less expensive.
Plus, vinyl windows can be made to any size you need.
All fixed vertical and sloped windows along the south facing planterbed and clerestory thermopane units should be at least 1 inch overall (two 3/16 inch glass with a 5/8 inch gap) or 7/8 inch overall (two 1/8 inch glass with a 5/8 inch gap). More than a 3/4 inch gap can lead to energy robbing convection currents within the space. Less than a 5/8 inch gap can more easily allow conductive heat/cooling loses. Residential windows are generally thinner, less effective against heat/cold transmission, more delicately vacuum sealed, not as well tempered and more prone to condensation problems. Patio door glass, for instance, is marginal for vertical windows, but fully unsuited for sloped applications. Buying thicker, commercial 3/16 inch thick tempered glass thermopane units (store window glass) is recommended. Using commercial components wherever possible, is one key to saving money while building a home. Your local Quik Stop/garage/warehouse was much cheaper to build than any local home, yet was built with heavier duty building materials, block walls, nice tile, big laminated windows, heavy duty garage door, large handicapped bathrooms and steel entry doors.
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