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Air Sealing FAQ

October 5, 2013 | Posted by Mitt Jones

Whether your house was built in 1890 or 1980, chances are you lose at least 10 percent of the energy it takes to heat and cool your home to air leakage. Air sealing not only reduces the waste but also reduces the drafts that many homeowners experience in winter. Read on for answers to frequently asked questions about air sealing.

Attic_chase_IR.jpgWhat is air sealing?

Air sealing is the process of reducing air leakage into and out of a house, essentially by plugging gaps and holes. The gaps and holes that come to mind for most homeowners are the ones that are easy to see inside the house, such as a bit of daylight around a door or maybe a cracked window pane.  The air sealing locations that make a much bigger difference are usually in places you don’t see, such as the attic. A well-trained home performance crew knows where to find these leaks and how to seal them.

Why should I care about air sealing?

Air sealing homes has several benefits, including reducing noticeable drafts, helping homes heat and cool more evenly, reducing energy bills, reducing the chance of condensation problems in attics and walls, and possibly increasing indoor air quality.

How does air sealing improve comfort?

Air sealing improves comfort in a few ways. The most obvious is by reducing drafts. Air sealing can significantly reduce the flow of air through your house, which increases comfort in winter.

Air sealing also improves comfort by helping to keep unconditioned air where it belongs—in unconditioned spaces. Let’s say you have an open area around a chimney that rises up through your house. In winter cold air from the attic sinks down to keep this space nearly as cold as the attic, which in turn is nearly as cold as the air outside. In this scenario you lose a great deal of heat through the interior walls around the chimney, which would also become another cold surface in your house that creates comfort problems. (See Understanding Home Energy Comfort Problems to learn more about the effects of cold surfaces.)

Eliminating these cold surfaces also helps the house retain heat more evenly, especially when large chases are present.

How does air sealing guard against condensation problems?

Warm air can hold much more moisture than cold air. Even if the air in your house feels dry in winter, moisture will condense from that air if it is allowed to come into contact with a cold enough surface.

That can easily happen when air leakage allows indoor air to escape through penetrations in the attic floor, for instance, or into en exterior wall cavity. Condensation can form on a roof deck in cold weather near large air leakage locations. That’s especially true when an attic has been well enough insulated that the roof deck is not drawing much heat from the house.

Doesn’t a house need to breathe?

Air leakage is more likely to lead to moisture damage in a house than to help maintain durability in any way, so the idea of a house needing to breathe is imperfect at best. The occupants of the house most certainly need to breathe, however, and for that reason there are well-defined ventilation requirements established by the Building Performance Institute (BPI).

Can a house be too tight?

In areas with relatively mild climates, the great majority of homes get their ventilation from unintentional air leakage. It’s possible but fairly rare for this unintentional air leakage to be below recommended levels.

In cold climates, the common mantra is “seal tight, ventilate right.” The idea is to tighten the house as much as practical and also to install mechanical ventilation to provide consistent, predictable ventilation performance. With that approach, no house is too tight.

Can I insulate now and air seal later?

It’s important to air seal the attic floor and the subfloor above the crawlspace before insulation is added to those areas. Insulation itself does not stop the flow of air and in most cases doesn’t even slow it down. If you insulate first, air sealing later will be more difficult and expensive. Performing air sealing after the attic is insulated will also damage your new attic insulation.

What materials are used during air sealing?

Air sealing crews make heavy use of gun foam, which is canned foam that installs in a simple spray gun. Crews also often use caulk and an assortment of rigid products including drywall, foam board, OSB, and plywood.

What do you seal?

Typical attic air-sealing targets include chimney chases, dropped soffits, electrical penetrations, plumbing penetrations, gaps next to top plates, and recessed light fixtures. If there is a vented crawlspace, sealing around the electrical and plumbing penetrations through the subfloor above it is key, as is sealing around heating registers. In an unfinished basement, sealing gaps and penetrations at the rim joist and sill plates can provide big gains. An experienced insulation crew will also detect air sealing opportunities inside the house, such sealing built-ins, weather stripping exterior doors, and caulking around windows.

How much energy will air sealing save?

The Environmental Protection Agency (EPA) estimates that air sealing and insulating homes typically reduces heating and cooling costs by up to 20 percent, and that figure can certainly be higher in older or especially leaky homes. For air sealing alone, 10 percent of the heating and cooling bill is a probably a good guesstimate. 

How do I know my house really needs air sealing?

Most houses should be air sealed to improve energy efficiency, occupant comfort, and building durability. Comparatively few houses have been built tight enough not to need air sealing, and most of those have been built in the last few years by builders that have chosen to focus on developing energy efficient homes. ‘

That said, you’ll know soon enough how tight your house is if you hire a home performance contractor. In most cases, a home performance contractor will run a blower door test during the home energy audit or before beginning work. The blower door test will measure air leakage in your home and may help identify specific air-sealing opportunities.

What is blower door assisted air sealing?

Depending partly on the house, an air sealing crew may run the blower door test occasionally throughout the day to gauge progress and help locate additional air-sealing opportunities. In some cases, this approach may help the crew avoid over-tightening the house, though it is more commonly used to maximize the air-sealing reduction.

What if my house is sealed too tight?

An experienced home performance contractor will know before work begins if there is considerable chance of over-tightening the house and will plan accordingly. For a simple, cost-effective approach, a timer can be added to one or more bathroom exhaust fans to allow them to provide the necessary ventilation. A more sophisticated solution might involve installing a heat recovery ventilator (HRV) or energy recovery ventilator (ERV).

How will I know the work was done right?

In most cases the home performance company performing the work will run a blower door test before and after air sealing is performed. If your house started with high air leakage, you should see a drop of at least several hundred CFM at 50 Pascals between the pre- and post-work blower door numbers.

Many home performance programs provide some level of quality control inspections to help keep contractors standards high, often verifying results for 5% to 15% of the jobs performed by any given contractor.

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