Why Does Air Sealing Have to Happen Before Insulation?
As the home performance contracting industry moves to improve the energy efficiency, comfort, safety, and health of existing homes by applying building science principles, one of the most ubiquitous problems building analysts are finding is that, even recently, most homes are not built using airtight construction methods.
Air leakage in homes is measured in Air Changes per Hour (ACH), which compares the amount of air that a home exchanges every hour with the amount of air that can fit in the home. Ideally, a home should change over about 35% of the air inside of it every hour (0.35 ACH). This is enough air exchange to remove moisture, stale air, and contaminants, and provide sufficient fresh air for occupants to breathe. In addition to being delivered in the proper quantity, fresh air should also be of high quality. Unfortunately, almost all existing homes suffer from either excessive quantity or poor quality of ventilation due to unintentional air leakage, and many suffer from both. The goal of air draft sealing is to reduce the quantity of air leakage that takes place in the home while improving the quality of air that enters the home. Often, a ventilation system is required to deliver high quality fresh air in the appropriate quantities, but that is another topic entirely.
In older homes, a blower door test typically reveals air leakage rates of 0.6 to 1.0 ACH, or about twice to three times the amount of desired air exchanges. This large amount of air exchange comes from a variety of sources. Although we frequently associate drafts with leaky windows or doors, the easiest places to make the largest gains in reducing air leakage while spending the least money are the attic, crawl space, basement, and garage:
Air Sealing Before Insulation in an Attic
- Penetrations (holes) in the ceiling and wall frames where light fixtures, wires, plumbing, heating ducts, and other mechanical components of the home protrude into the attic
- Wall top plates – the joint where the drywall connects to the wall frames at the ceilings are usually 1/4 inch wide or larger on both sides, and there can be more than 200 lineal feet of walls. This adds up to about a 10 inch square hole!
- Dropped ceiling cavities and chases, often over cabinets or behind showers, can expose large areas of interior walls to the attic. A 1 foot wide, 8 foot long chase could expose over 100 square feet of interior surfaces to attic temperatures. This condition is called a thermal bypass because a relatively small hole allows air from one place to move into cavities where they can affect the surface temperatures of areas much larger than the hole itself.
- Open wall cavities, or open floor cavities in multi-level homes with knee wall attics. Open cavities are considered to be thermal bypasses, basically exposing the entire ceiling of the rooms below as well as the floors of the rooms above to attic conditions.
Air Sealing Before Insulation in a Crawl Space, Basement, or Garage
Penetrations as described above
- Large thermal bypasses around cavities, especially under bathtubs, showers, and staircases
Warm air tends to rise out of holes in the attic during winter, and be replaced by cold air that is pulled in from the basement. This driving force is called the stack effect, and it is the principle on which chimneys, furnace flue pipes, and smokestacks work. In order to reduce the stack effect in a home, workers start air sealing in the attic.
Not only do these holes allow air that we paid to heat leak directly outdoors, penetrations and thermal bypasses greatly reduce the effectiveness of insulation. Fiberglass and loose blown cellulose, the two most common insulation products, don’t work as they are designed to when air is moving through them, just as a wool sweater doesn’t keep a person very warm when the wind picks up. Finally, thermal bypasses allow attic or crawl space air to leak right past insulation, eliminating its effectiveness across large portions of the home.
Air Sealing Helps with Indoor Air Quality
In addition to all this excessive air leakage wasting energy and harming comfort, the attic and crawl space aren’t exactly places I would think of if I wanted to step out for some fresh air. Here are just a few of the particles residing in most attics and crawl spaces that might be in our “fresh air” stream:
- Rat feces
- Rat poison
- Mold and mildew
Air Sealing Is Difficult to Do After Insulation
Sealing air leaks from the attic and crawl space reduces the quantity of air leakage while improving the quality of air inside in most cases. And by far, the most logical and convenient time to perform air draft sealing in the attic is at the same time that new attic insulation is installed. The new insulation won’t work like it’s supposed to without doing the air draft sealing The air draft sealing costs less than the insulation. Also, it’s nearly impossible (and much more time consuming) to install effective air sealing measures without wrecking an insulation job. Installing insulation without taking steps to reduce unwanted air infiltration is similar to painting a car without addressing rust spots – it saves a little money in the short term, but the result is disappointing from day one.
Blower door test link: energy.gov http://energy.gov/energysaver/articles/blower-door-tests
Copyright September 2014 by Advanced Home Energy
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