Image Credit: All photos: Pure Energy Coach This air handler has an unsealed return-air plenum. When the blower is operating, the leaky return plenum pulls attic air into the duct system. Weatherization contractors and home performance contractors call this kind of big hole a "bypass." Test smoke isn't going up the chimney, indicating weak draft pressure. The pressure boundary is re-established by capping this dropped soffit hole with rigid foam board and canned spray foam. Once the air sealing work has been completed, insulation will be blown over the whole area.
It’s important to keep attic air out of the house and house air out of the attic. That’s why the home performance industry and every above-code building program make it a top priority to fully separate attics from the rest of the building.
When the attic isn’t fully air sealed from the living space and the combustion appliance zone, three undesirable scenarios can occur:
- Attic air may contain contaminants, including mouse droppings, fiberglass, asbestos, and mold. If this contaminated air gets sucked into the living space, it can cause health problems for the occupants.
- Air from the living space below can carry water vapor into the attic, hitting a cold place in the attic and causing rot and mold growth.
- Leaks of conditioned air from the home into an attic waste energy.
The stack effect
How does air from the living space get into the attic?
In the winter, when the heat is on, heated air rises and expands and pushes against the top ceiling of the building. If there is a way for the air to get into the attic, it will.
Air leaks out at places where the builder didn’t seal, such as around the tops of walls near the ceiling (through and around leaky top plates), around attic hatches, through dropped soffits above cabinets, around recessed lights, and around chimneys.
As air leaves the building through these cracks, air also gets sucked out of the basement, typically around the chimney chase.
This air movement due to the stack effect is the biggest reason for heat loss in the winter.
An open wall top
Image #1 (at the top of the page) shows an open wall top — that is, a partition wall without a top plate. Heated air from the living space gets into this wall and rises into the attic.
This wall also delivers moisture to the attic, which can lead to roof rot and mold. And the leaking air can suck on the combustion appliance zone (the area of the house where the furnace and water heater are located), causing these appliances to backdraft.
In some cases, cold attic air can drop into the living space through these openings, leading to frozen pipes.
Return air ducts with leaky seams
How does air from the attic get into the living space? As mentioned above, it cold attic air can fall down open chases, especially when the house is depressurized by exhaust fans.
If there are ducts in the attic and the return ducts have holes in them, attic air can also be sucked into the house through those duct leaks. See Image #2 (below), which shows an unsealed return plenum in an attic.
Anything that causes unbalanced room air pressures can cause bad air from the attic to mix with good air in the house.
The stack effect can depressurize your basement
Can an unsealed attic cause the combustion appliance to backdraft? Yes, when a bypass (such as a chimney chase) isn’t sealed in the attic, the stack effect can suck air out of the basement.
Since most of our heating appliances and water heaters are in basements, their flue pipes can be the source of the make-up air that is being sucked out the basement.
This is very dangerous! For more information on this issue, watch our YouTube video on Zonal Pressure Diagnostics.
Recommendations from the Building Performance Institute
What does the Building Performance Institute (BPI) say about attic air sealing? BPI standards are very clear about this issue, providing the following recommendations:
- Don’t add attic ventilation without first verifying that the attic is fully air sealed from the rest of the building. Adding attic ventilation without ensuring that there is a solid air barrier between the attic and the living space will make air and heat from the house leak out faster, because the attic will be colder in the winter and the rate at which air leaves the building will increase.
- If the house air entering the attic is carrying moisture with it, the moisture may condense on the cold roof deck and cause wood rot and maybe mold.
- Unbalanced house pressures can cause natural draft heaters and water heaters to backdraft fumes. The fumes may contain carbon monoxide (CO). CO is very dangerous and can cause death.
BPI expects technicians to verify an effective air barrier between the living spaces and the attic spaces by doing visual inspections and by using a blower door to perform zone pressure testing with a manometer and smoke.
An infrared camera can also be a very effective way to find remaining air leaks.
The benefits of having a complete air barrier between attic spaces and the rest of the building are:
- Reduced air flow, so the house is less drafty and people are more comfortable.
- Lower energy use.
- Reduced chance of mold and roof rot.
- Pollutants in the attic stay in the attic and don’t enter the living space.
- Reduced air suction on the combustion appliance zone, so natural draft combustion appliances have a better chance of drafting properly and carrying fumes outside instead of backdrafting. Pollutants in flue gasses (including CO) stay in the flue and chimney and don’t enter the house.
- An air barrier allows the attic to be insulated and ventilated.
- An air barrier keeps moisture in the basement or crawl space where it can be controlled by dehumidification (or, in some climates, by ventilation).
A. Tamasin Sterner is president of Pure Energy Coach in Lancaster, Pennsylvania and The Pure Energy Center in Montana. She has worked in the energy efficiency and energy conservation field for over 30 years. Tamasin is a nationally recognized expert in energy efficiency. She was selected to advise President Obama on the proposed federal Home Star program.
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