Air Leakage at Electrical Switches and Outlets
How I blew a gasket and exposed the hole in one air-sealing method
One thing that invariably surprises people when I walk them through a house during their first blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. is how much air leaks in through the electrical switches and receptacles. On a recent Friday, we went out to do the last home energy rating in our latest HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. rater class, and we got to see something even better. But first, let's talk about that air leakage. We have a number of surprises waiting.
First, people are surprised that so much air leaks in through those holes. Second, they're surprised when I show them that just as much air leaks in through switches and outlets on interior walls as on exterior walls.
Why? Because the wires that came to that switch or outlet probably came through holes drilled through the top or bottom of that wall. In older homes, those holes weren't sealed. Even in newer homes, where building inspectors generally enforce fire codes that require those holes to be air-sealed, attic air still gets into the wall from the gap between the top plate and the ceiling drywall.
I blew a gasket!
In the house we visited that Friday, the owner told us that he'd put gaskets behind the switch and receptacle covers. So we looked at them during the blower-door test to see how well they worked. Check out the video below.
That's quite a breeze blowing the smoke during the test. Yes, the gaskets may have reduced the amount of air leakage, but if you asked me if it were worth the cost of installing them (which is mostly labor because the foam gaskets are cheap), I'd say no.
Several years ago, I put gaskets in all the switches and receptacles in a house. The before-and-after blower door tests were pretty much identical (within the uncertainty of the equipment). So why don't gaskets help much?
The main problem is that gaskets seal the parts that are already sealed. Air doesn't leak in through the plastic cover itself, yet that's where most of the foam is. They do help reduce leakage a bit for covers that don't fit snugly against the wall, but they do next to nothing about the holes in the switches and receptacles.
Why do switches and receptacles leak?
Switches and receptacles themselves aren't airtight. The holes you see in front (around the edges of the switch and right through the receptacle) are connected to holes in the back (where the wires plug are connected). These electrical devices sit inside an electrical junction box, so that connects air in the house to air in the junction box.
To get electricity to the switch or receptacle, wires have to come into the box. The photo below shows the back of a typical junction box, with all its holes. Take a look at all those holes. There are four knockouts for each switch/receptacle, and even the ones that haven't been knocked out still leak like a sieve.
Those holes in back connect the holes in front with the air in the wall cavity. As I said at the beginning, the wall cavity in many homes is connected to air in the attic, basement, crawl space, garage ... all those unconditioned places you don't want to be connected to. In other words, all those holes are part of a three-dimensional network of air leakage that connects the inside to the outside.
How can you stop this air leakage?
For new homes, the best way to do this is to find an airtight junction box or use a cover that you mount before installing the junction boxes. An example of the former is the one made by Airfoil. Rather than being made for ease of wiring with no consideration given to airtightness, these junction boxes make the air sealing very easy by including a gap above the box where you spray a little bit of foam. (See their installation photos to get a better idea of how this works.)
EFI sells junction box covers made by LESSCO that can hold up to a triple-gang junction box. You install regular junction boxes inside their cover. The advantage with these covers is that they’re made to be airtight whereas a standard junction box clearly is not. The wiring penetrations in the polyethylene cover are easily sealed.
In existing homes (or new homes if it's too late to do the above), you'll need to seal all those holes in the junction box with fire caulk. If you get those holes sealed up and also seal the gap between the junction box and the drywall, you'll do a much better job of stopping air leakage at your switches and receptacles than you would by installing gaskets.
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