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home page » passive solar design chapter » HTM exterior detail page ... you are here
HTM high thermal mass exterior walls 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.
click here for the main chapter of our free Passive Solar Design eBook
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. |
HTMs exteriors are not limited to any particular exterior appearance. Whether stucco, brick, stone, wood or log siding, aluminim or steel paneling, the same basic design principle apply to the construction of the building underneath. Please do not interpret the emphasis on block and stucco work here as an endorsement of that particular approach. The exterior of an HTM is not that relevant to its function. Just be sure to ashere to ALL the basic design principles outlined in this free eBook. Block and poured-in-place concrete homes and businesses can be found in every town in the world. That's what makes an HTM so economical and functionally sensible - the common commercial style of block or poured-in-place concrete construction. Your local Quick-Stop grocery/filling station was block built for the sake of economy and strength. Add exterior insulation (for heating and cooling retention) and a nice fascia and throw in a little passive solar design (maybe some shade cloth) - and you have an HTM. Any local contractor should be more than capable of working on the project ... nothing really "alternative" about it other than the passive sustainable function with design items like wing insulation. You take what free solar gain there is available in your area and supplement the rest, just like any other home.
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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.
click here for the main chapter of our free Passive Solar Design eBook
initial design consultation with packet $195*
Questions? Concerns? order an initial consultation to discuss your project
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$195* initial design consultation
Whether regarding HTMs specifically or passive solar in general, our initial consultation will address your concerns.
After reviewing the material, we encourage you to drop us an email with your questions.
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First, we email an extended outline of tips, tricks, and technical details, many not found on the website.
Then, we ship you a two inch binder jam-packed with the following printed information:
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plan review and HTM consultation for $1495*
after initial consultation, we can perform plan review with personal consultation
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$1495* plan review & HTM consultation
This isn't ongoing personal project consultation, and is limited to five or six emails, but we trust the plan review and follow-up consultation puts your project on the right track. |
We request that you start with the initial consultation packet for $195* to ensure a solid feel for HTM principles and get a chance to know each other first. Later, after your construction blueprints are finished, sign up for plan review consultation and send us two complete sets for study. For $1495* we will carefully pick over your plans for possible errors, better flow, wiser use of space, and alternative design possibilities. When pointing out options, we view your whole house as an independent "holistic system" chock full of inter-related, functionally inter-dependent parts from the ground up. With a project as large and complex as a new home, it's extremely easy to save more money than you spend on a good consultant. Building a home is the greatest investment of time, energy, and money most of us will ever make. Having an experienced ally giving you relatively unbiased answers to your questions is priceless. Please note that our plan review and consultation service has never included custom blueprinting, engineering or load calculations. Custom drafting and structural engineering services are best supplied by locally licensed professionals and we do not offer referrals, sorry. Always hire professionals familiar with your area's building regulations and who can physically visit your property to inspect soil, construction and materials. Here is a short biography page: whoweare.htm as way of personal introduction and our standard contract is online at: Terms of Service Agreement. We look forward to being of assistance. |
in-house consultation and personal HTM home tours are no longer available
hourly and flat-fee on-going HTM consultation services are also discontinued
don't want any consultation? DVD only for $9*
we include a free copy of the website on CD, but no phone or email time
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$9* DVD only
One hour long video on universal DVD format
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First half hour walks you through the sustainable design, HTM passive solar earthhome featured on our website. While not a How-To video, the second half does include detailed Infiltrator chamber leach field septic system installation and Sun-Mar composting toilet footage. |
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