QUESTION: We are building a house and spending a bit more effort on insulation, hoping to save on summer/winter energy costs. We are planning on building a double exterior wall. The exterior will be 2x6, then a one-inch gap, and then a 2x4 interior wall. We plan to put 11/2 inches of rigid board on the outside, against the sheathing, over which we will put acrylic stucco. Inside the exterior wall we will spray with closed cell foam, a minimum of three inches, and then after all the electrical is done we will fill the balance by blowing in cellulose.
So, instead of the traditional vapour barrier being the plastic sheet hung on the interior wall, the vapour barrier now becomes the spray foam in the exterior wall.
I am interested in your thoughts and concerns about this type of design? Do you know of someone that you feel would have enough experience or know more about this kind of thing?
-- Warren Loewen
ANSWER: I appreciate your interest in my comments on your suggested "super-insulated walls," but you may not welcome my opinion on the subject. While it's very admirable to do all you can for energy conservation in a new home, the wrong approach can be both costly and a waste of materials and labour.
Exterior walls in a typical wood-framed home are now built with 2x6 lumber for one main reason -- increased insulation capacity. Since most building codes adopted by various municipalities have required a minimum of R20 insulation level for the last few decades, these walls have had to use larger studs to create a cavity large enough to hold enough traditional fibreglass insulation to meet this criteria. This will increase the strength of the walls relative to older 2x4 construction, but that's an unnecessary bi-product of the desire for higher insulation levels. Most older homes built with 2x4 exterior walls had more than enough lumber to properly support the roof structure and have held up well.
The drawback of requiring an increase in thermal resistance of exterior walls, without adopting new insulation technologies, is the increased demand for framing lumber. Because builders still relied primarily on somewhat outdated fibreglass batt insulation to meet the newer requirements, many more trees had to be cut down to build the thicker walls. This not only increased the cost of house construction, it also required more resources to cut, mill and ship the larger boards.
So, instead of improving the insulation technology to accommodate the newer requirements, we just added more of the same old stuff to thicker walls. This also required all doors and windows be built with thicker jambs and frames, further increasing material use. A better approach would have been to improve the insulation itself -- to use products that could do just as good a job or better with less material.
In the last decade or two, insulation with a higher density and better properties of thermal resistance and air sealing have come into wider use. All three types of insulation you propose using fit into this category, but your planned combination of these materials is where your plan errs. Both the high density blown-in foam and the rigid insulation you site should have better thermal resistance and higher R-values than the loose cellulose. They also should be much better air/vapour barriers. Because of these two properties, it does not make sense constructing an entire extra wall, just to house the cellulose, rather than simply increasing the thickness of the other insulation. That way, you would also not have to deal with the issue of the proper location for the air/vapour barrier. The obvious reason for your suggested method is likely the higher cost of the foam insulation, but that should easily be negated by the cost of the second, inner frame wall. To make matters worse, you will have to build ridiculously wide jamb extensions for all the doors and windows, use longer floor joists and roof trusses, more sheathing, shingles, etc. just because the walls are thicker. In that case, your efforts to conserve energy to heat the building will require a substantial waste in additional building materials required for the double wall.
I'd recommend contacting a builder and/or a knowledgeable insulation contractor who supplies blown-in foam insulation to see what practical choices you have. You may be able to achieve your desired level of thermal resistance in the exterior walls by using a combination of blown-in and rigid foam insulation with a single 2x6 framed exterior wall. That way, you will have the best of both worlds. You will be able to minimize energy consumption and costs without wasting precious resources building a redundant inner wall.
Even if the cost does turn out to be slightly higher to insulate with the more expensive, modern insulation, the long-term benefits to the environment, your home and your bank account should be worth it.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors - Manitoba (www.cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at www.trainedeye.ca.