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BACKDRAFTING BLUES
Courtesy: The Energy Conservatory



While a well-sealed building enclosure will enable optimal thermal insulation and energy-efficient operation, there is a concern that making the basement too tight could cause the furnace to backdraft.


“’Too tight’ is a phrase that we hear a lot,” relates Kevin Colwell, president, BE RETROFIT, Newton, Mass. “In a home with a traditional-style atmospheric hot water heater and heating system, ‘too tight’ is absolutely a concern. However, this same house with all sealed combustion mechanical equipment does not have the same risk.”


The key here, says Colwell, is properly ventilating the home, which is where blower door testing comes in. By depressurizing or pressurizing the home, the blower door mimics the effects which changing outdoor temperatures and wind pressures will ultimately have on the home. This way, both air leaks, on the one hand, and the potential of backdrafting, if the home is too tight, can be measured.


“Based on the test results, mechanical ventilation may then need to be added to a home to provide enough combustion air to a furnace,” explains John B. Smith, P.E., global platform leader, environmental construction, Johns Mansville Technical Center, Denver.


In addition to a blower door and combustion safety test, which is done once the home has been sealed, ASHRAE Standard 62.1 2010, Ventilation for Acceptable Indoor Air Quality is an excellent guide for ensuring proper ventilation levels.


Of course, on the other end of the spectrum is the blower door’s ability to identify pesky air leaks in the envelope. As the operating blower door sucks area out of the space, a smoke pencil or infrared camera is used to pinpoint any leakage paths.


“In retrofit work, the blower door is a fun tool because we ‘test in’ and ‘test out,’ which allows us to immediately quantify the air leakage reduction as a result of our work,” says Colwell.


At the same time, Smith points out that the blower door will only locate air leakage, but not measure how well it is insulated.


 


Blower door testing is an important aspect of any attempt to identify air leaks, and to determine whether tightening the basement could lead to backdrafting.


 


Whereas builders have traditionally it used batt insulation and covered it with a wallboard, this is no longer acceptable. Rather, as Socolean explains, “scientific evidence has shown the best approach to be water-resistant insulation—usually foam— installed up against the concrete foundation wall while wallboard is used as an interior installation.”


Similarly, Smith recommends foam insulation or a hybrid system with foam against the foundation. “With hybrid systems, unfaced fibrous products should be used to permit the insulation system to dry to the inside,” he adds.


Meanwhile, Colwell points out that the most important aspect of this installation is to ensure continuity with the above-grade wall system air and insulation barrier.


Consequently, when using SPF, he explains that the band joist and box ends must be insulated and connected to the subfloor of the first story.


“Using high R-value rigid insulation board—poly-isocyanurate—is another good solution and works well on poured concrete with a flat surface,” he says. “Using rigid board adds a second air sealing step. A kit system polyurethane foam air sealant should also be used to air seal the rigid insulation board to the first story sub-floor in order to create the necessary air barrier and thermal barrier continuity.”


 


 


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