Learning from your energy audit

Posted Wednesday, June 20, 2012 in Sustainable Maine

Learning from your energy audit

A Passivhaus thermogram

by Paul Kando

Where does your heating energy escape? A  professional energy audit may show, for example, that your walls account for 24.2 percent of your heat losses; attic/roof 30.1 percent, basement 12 percent, pipes and ducts 0.7 percent, windows/doors 4.8 percent, and air leaks 28.2 percent. The report will provide a number of recommendations  along with estimates of first-year savings from each recommendation. The more energy-wasting your house, the greater your opportunities for savings.

Whatever you do to improve the house, you must address not only heat loss, but also the movement of air and moisture.

In essence, you must emulate the Passivhaus standard, which combines features that reduce energy consumption over 90 percent. You may not come close to that performance, but key elements of Passivhaus apply to any energy upgrade. If you burn 1,000 gallons of $3.70-a-gallon fuel oil per winter, saving 25 percent will still leave you with an annual fuel bill of $2,775. Saving 50 percent would bring your cost down to $1,850; and saving 78 percent would leave you with a bill of just $814 per year (if oil prices don’t rise).

So, is it possible for your house to be upgraded to Passivhaus performance, perhaps even zero energy? The short answer is probably yes, the long one is more complicated. 

A newly built Passivhaus structure has sufficient insulation (R-60 all around in our climate), no thermal bridges, sufficient air tightness, ample ventilation to control indoor moisture levels and provide fresh air, and super-efficient, multi-glazed, thermal-bridge-free windows and doors. Several thousand existing structures around the world have  been upgraded. Once Passivhaus-level improvements have been realized, the remaining heat load is no higher than 15 kWh/square meter (4,746 Btu/sq. ft.) of floor space per year, which is easily taken care of by renewable means.

In contrast, your conventionally built house, on average, has zero insulation underneath, R-13 in its 2x6 framed walls (R-9 if 2x4 framed). It has thermal bridges through wooden framing members. It is not airtight and loses over a third of its heating energy through air leaks. It has R-1 to R-2 windows. As a result, the average house has a heat load 12 times that of its Passivhaus equivalent. The difference is systemic: A new Passivhaus is constructed by a fundamentally different process than a conventional house, in tightly controlled steps. There are few, if any, subcontractors at the building site. Everything is sealed and performance tested along the way.

Your choice is to either “plug holes,” so to speak, in the existing house, in quest of marginal improvements, or you can, from the outside, wrap the existing house in airtight super-insulation, replace all windows and doors and add heat-recovery ventilation. Such upgrades have been found feasible — there are thousands of Passivhaus-rehabbed structures in all climate zones — when done as a part of major remodeling or when roofing or siding needs replacing.

Is Passivhaus performance attainable short of such “radical surgery”? Not very likely, because of problems inherent in the conventional building system. For example, conventional structures often have air leaks and thermal connections between the outdoors and partition walls and between floors. These spaces are often difficult to seal. Insulation cannot be blown into spaces where active knob-and-tube wiring or recessed light fixtures are present. Super-insulating from the inside can be expensive, messy, disruptive, and unlikely to fix the problems just described.  

It is possible to phase a close-to-Passivhaus upgrade. If headroom permits and the basement is dry, several inches of foam board can be laid atop the basement floor and a thin concrete slab or other floor atop that. Heat-recovery ventilation can be installed. Foam insulation (or structural insulated panels) can be added to the roof as part of re-shingling. Outside walls can be covered with sealed-together foam boards (or structural insulated panels) under new siding. One by one all windows can be replaced, bringing window-insulation values up to R-9 or R-10. 

It does not all have to be done at the same time, but it is important that each step, phased or not, be accompanied by rigorous air and vapor sealing verified by blower-door testing.

Bringing an old house up to 21st century standards is a challenge. Start with an energy audit and rely on professionals with Passivhaus knowledge and experience. Years of traditional construction experience without a willingness to re-learn is likely to be a disadvantage here.   

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