I Blew a Gasket — Air Leakage at Electrical Switches and Outlets
One thing that invariably surprises people when I walk them through a house during their first blower door test is how much air leaks in through the electrical switches and receptacles. On Friday, we went out to do the last home energy rating in our latest HERS 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 on 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 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.
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, shown at right. Rather than being made for ease of wiring with no consideration given to airtightness, these junction boxes have knockouts that are sealed until knocked out. Once you do insert wires, they 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 photo at left shows the LESSCO cover with a double gang junction box. The advantage here is that it's made to be airtight whereas a junction box clearly is not. The wiring penetrations in this polyethylene box 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.
Mind the Gap - Air Leakage at the Top Plates
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The 3 Rules of Air Leakage (Plus a Bonus!)