What is Infiltration?
Infiltration is outside air entering and leaving the house, taking conditioned air with it.
A lot of people are surprised by the above photo, assuming air leaks happen predominately around windows and doors. In actuality air is driven through our homes by the stack effect – warm air rising. This means the attic and basement are most critical for preventing air leaks and infiltration. Infiltration is a bad thing: not only is it a huge energy waste, it brings in air from the dirtiest places like attics and crawlspaces, and spreads that contaminated air through the living space.
The key to stopping infiltration is creating a good air barrier.
Think of a home’s insulation like a wool sweater. On a calm fall day the sweater is enough to keep you warm. If a breeze picks up, though, the cold wind will blow right through the wool and you will probably reach for your windbreaker. In a home we call the windbreaker layer the air barrier, and it is just as important as the insulation. Insulation limits heat transfer through the walls and roof, but only when paired with an effective air barrier.
Stop Infiltration – Air Barrier Rules
- The air barrier needs to be totally continuous. If you take a cross-section plan of the house you should be able to draw the air barrier all the way around the house without lifting your pen.
- The air barriers, such as drywall, should be in direct contact with the insulation. This often breaks down in locations like walls under staircases, behind fireplaces, and under tubs where there is (hopefully) insulation but no drywall air barrier.
Infiltration costs hundreds of dollars per year in heating and cooling bills. Fixing the biggest air leaks requires expertise and attention to detail, but it’s worth the effort.
Where Does Most Infiltration Occur?
The image above shows data from the U.S. Department of Energy highlighting where air leaks occur in average homes.
There are three critical types of air leaks to watch out for:
- Big holes. Some common design elements can result in big holes in the air barrier. For instance, a dropped soffit under an attic is a great pathway for air leakage. Tubs and fireplaces on exterior walls can create similar holes if a solid piece of rigid insulation isn’t installed behind them. Floor joists bays that extend from conditioned space to a garage, or to a porch overhang are another way to blow open the air barrier. While these locations can be air-sealed and insulated, good design would eliminate the potential for big holes altogether.
- Cracks. Every building has a number of cracks that seem minor when taken on their own, but add up to a big air leak. These cracks occur between the sill plate and foundation, at exterior wall bottom plates, between adjacent studs, and around window and door frames.
- Penetrations. Every hole cut in the exterior envelope (ceiling drywall, exterior sheathing, top plates below attic) creates a potential air leak. Penetrations include plumbing pipes, duct registers, can lights, exhaust fans and exhaust ducts, and electrical wiring. In the graphic above, 13% of air leaks are from plumbing penetrations.
Air is relentless: it will find any and every pathway into a home. Sealing 50% of the apparent leaks will not cut 50% of the infiltration because air will find another way in. Good air sealing aims to seal 90% of the leaks. It requires patience, attention to detail and the expertise to recognize tricky air bypasses. It also requires a clear understanding of the thermal envelope, especially at complicated architectural details.
Tips for Successful Air Sealing:
- Good air sealing requires a plan, and should be a priority during the design phase. Ask yourself where is the air barrier? Can you draw it without lifting your pen?
- During construction, air sealing should be the responsibility of all the trades. Air is persistent, and the whole project team needs to be just as thorough in fighting it.
- A good rule for a job site is if you cut a hole, you seal it. It is easier for each trade to seal their own holes, rather than relying on one person to find everyone else’s holes.
- Fire-stopping is not necessarily air sealing. Fire-stopping material like rock wool does virtually nothing to stop air infiltration. Use caulk or foam to air seal.
In our follow-up post we cover how air leakage is measured with a blower door test and what a good target is.