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Building Science

Air Leakage at Electrical Switches and Outlets

How I blew a gasket and exposed the hole in one air-sealing method

Image 1 of 2
Testing a light switch for air leakage with a blower door. The smoke is blowing straight out from the switch as a lot of air leaks through, despite the presence of an air-sealing gasket behind the cover. (See the video in this article for an even more convincing demonstration.)
Image Credit: Energy Vanguard
Testing a light switch for air leakage with a blower door. The smoke is blowing straight out from the switch as a lot of air leaks through, despite the presence of an air-sealing gasket behind the cover. (See the video in this article for an even more convincing demonstration.)
Image Credit: Energy Vanguard
Holes in an electrical junction box are part of a three dimensional air leakage network that connects the inside to the outside.
Image Credit: Energy Vanguard

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 a recent 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 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.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. Kristopher Steege-Reimann | | #1

    Why should I fix my leaky outlet?
    Hi Allison,
    You mentioned that several years ago you put gaskets in all the switches and receptacles in a house and that the before-and-after blower door tests were pretty much identical (within the uncertainty of the equipment). Have you done a before-and-after blower door test after sealing holes in the junction box with fire caulk and sealing the gap between the junction box and the drywall? My intuition (that I have no good basis for) is that, in a typical house, caulking all of your outlets is hardly going to lower energy costs.

    However, I realize air sealing also is important to keep humid air from getting into places it shouldn't be. Is it common for leaky outlets to cause moisture problems in a house? I know air leakage in basements and attics can cause big problems, but I haven't heard of problems with leaky outlets.

    I guess my bigger question is: What's a good reason to tell home owners that it is important for them to go fix their leaky outlets?

  2. JoeW N GA Zone 3A | | #2

    q & a
    Allison, re: your last para -- you don't mean put foam (even fireblock) inside did you? Doesn't code require the interior of a box be left open? Of course, this could be yet another sign of my ignorance.

    A short cut for those who use the foam gaskets -- plug the child protector covers into the outlets for a "sorta" fix.

  3. User avater GBA Editor
    Allison A. Bailes III, PhD | | #3

    Response to Kristopher Steege-Reimann
    No, I haven't done that type of before-and-after testing. It would be interesting to see how much you could reduce the air leakage by making all junction boxes airtight. I think it would be measurable, but I'll have to do it and see.

    If you're in a cold climate, you want to air-seal the interior side of all walls to keep water vapor out. Unlike worrying about which side to put a vapor retarder on, which can reduce vapor transport by diffusion, doing the air-sealing can make a big difference. I'm not in a cold climate myself, but I've heard from many people that moisture problems in walls are always caused by air leakage, never by diffusion. Chris Dorsi told me this week that he's remodeled thousands of homes in Montana, where they have 8000-9000 heating degree days, and he's never seen a moisture problem caused by diffusion.

    With existing homes, air-sealing the outlets and switches would be one of the last air-selaing measures I'd suggest. Go after all the big and medium-sized holes first before you ever start worrying about pulling switch covers off. Even then, go after the small holes that are high or low in the house because they're more affected by the stack effect.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Kristopher Steege-Reimann
    Your instincts are correct. The first places you want to concentrate on when it comes to sealing air leaks in your house are the attic and the basement. Leaks around electrical boxes don't matter as much as leaks in the attic and basement.

    That said, leaks through electrical boxes can cause comfort complaints, especially when the temperature drops below zero Fahrenheit. Moreover, these leaks can raise the risk of moisture problems in walls.

  5. Luke Morton | | #5

    I'll third those e-notions
    Just a further confirmation of what Martin and Doctor Bailes said.

    Back when I was an Earthcraft House inspector, I had the occasional house where the blower door wasn't passing, and we were grasping at anything we could to get it to pass due to inaccessibility of source of leaks (e.g. the aforementioned lack of sealing at top/bottom plates and drywall). So, we'd give gasketing and other forms of sealing at all the electrical outlet covers. Lo! and Behold! the improvement in blower door performance was barely readable within the noise of that test. In some cases, I know it was just being offset somewhat by more leakage underneath unsealed baseboards and along the linear 'filtered' leak that their carpet will become.

    Perhaps your neighbors at Southface have some hard data that they can share somewhere around their Eco-office there.

    As far as comfort issues-- definitely agree there. Cold weather with large stack effect can be sensible at those locations, and windy days are all the more so. Of course, when I was living in Montana last winter, it was hard to feel the little stream of air coming from the electric outlets when there were plenty of single-paned windows above to provide a nice advective cascade of cold air-- kind of a microcosm of sitting under the Arctic shelf and experiencing the full brunt of thermohaline circulation.

    PS-- I'll now apologize for calling you "Doctor," Doctor Bailes. Maybe a simple "Professor" would be a more acceptable honorific :)

  6. Jan Juran | | #6

    Air Sealing Options
    Hi Allison: you are quite correct; I too was surprised at the amount of air infiltration coming through electrical outlets (even those in interior partition walls) when my house was blower door tested. I sealed these using foam gaskets, however since most of the drywall cutouts were not plumb I caulked the perimeter of the gasket to the drywall in most cases. Since much of the infiltration came through the 3 prong holes (esp. the large ground prong), I installed child proof plastic safety caps in each plug, and used inexpensive 3 prong to 2 prong "grounding adaptors" to plug in lamps etc. with 2 prong plugs. Perhaps Cardinal Outlet Covers, which use a spring loaded sliding plate for 3 prong outlets might be superior? Or possibly the Air-Tite Outlet Covers could be a better alternative, see Perhaps you could put all three of these to the smoke pencil test during your next blower door test? Some empirical data re the best performing, practical retrofit method would be quite useful for existing homes.

  7. Sean Wiens | | #7

    Exterior Air Seal
    Great article Allison,

    The difficulties identified in this article are exactly why I am a proponent of an exterior air barrier approach combined with the installation of dense insulation in the cavities to avoid convection currents. It is much easier to install, usually is cheaper, and if covered over with exterior CI is pretty much hurricane proof.

  8. Dick Russell | | #8

    Or interior airseal
    I suppose it's a tossup as to whether to put the air barrier at the exterior or interior. In my case, I put it on the inside, using MemBrain "smart vapor retarder." During framing, I had the crew seal strips of poly under and over the inner of the double exterior wall frames and over the tops of all interior partitions on the upper level. When the MemBrain was installed, room by room, it was taped to the edges of these strips, giving a continuous air barrier. I also selected Airfoil electrical boxes for all exterior walls and upper level ceiling. Besides giving an excellent blower door test result, I also feel it goes a long way toward isolating interior humidity from the wall cavity. Some reports of modeling or testing of wall cavities have implied that thick walls filled with cellulose are "risky," but it was I think in the BSC report that they mentioned that the risk is mitigated or eliminated if the inside humidity is prevented from entering the cavity, which I take to mean "inside air barrier."

  9. User avater
    Roger Normand | | #9

    Carlon vapor tight single gang work box
    I see that Carlon introduced in Dec a new vapor tight single gang work box. I see that Home Depot carries the box online only (at least the local HD did not stock it) at $2.24 each. That seams a cheaper approach than others I have seen. Anyone used one of the new Carlon boxes?

  10. Colin Groundwater | | #10

    I simply don't understand air
    I simply don't understand air sealing electrical boxes in most cases. If you're using airtight drywall it makes sense but the vast majority of houses have the air barrier at the sheathing and use ADA for the attic. You can easily air seal electrical boxes in the top floor ceiling from the attic. I think it's more important to figure out how the air is getting into the walls and stop it there than it is to put band aid fixes on holes in the interior side of the walls

  11. Amanda Dawson | | #11

    Safety should be your main concern while repairing
    Turn off the circuit breaker or remove the screw-in Edison Base fuse protecting the receptacles that you will be working on. Safety should be your first thought when working around electricity. You won't be working inside the box, and there shouldn't be any reason to come into contact with the live terminals on the receptacles or switches, but it's best to deactivate the circuit just to err on the side of safety.

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Colin Groundwater
    I must admit that your comments made me chuckle.

    You wrote, "The vast majority of houses have the air barrier at the sheathing and use ADA for the attic."

    If only it were true! I'm not sure where you live... probably in a very unusual, progressive neighborhood... but you should get out more and visit a few job sites some day.

  13. Andrew Alden | | #13

    Response to Martin Holladay
    Could you explain your response a bit more?
    Do you take issue with "the air barrier at the sheathing" statement or the "ADA for the attic" one, or both?
    If the former, I'm confused. I'm trying to understand what's wrong with Colin's comment in general. with respect to addressing the leakage issues at the intended air barrier rather than the inner walls.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Andrew Alden
    There is nothing wrong with establishing an air barrier at the exterior sheathing. It's an excellent idea.

    There's also nothing wrong with using ADA for the ceiling under an attic. It's also an excellent idea.

    The funny part was when Colin Groundwater wrote that these features exist in "the majority of houses." Unfortunately, they don't. The majority of houses have leaky sheathing and leaky ceiling drywall.

  15. User avater GBA Editor
    Allison A. Bailes III, PhD | | #15

    Response to Colin Groundwater & Andrew Alden
    Air-sealing at the drywall layer isn't so important if two conditions are met: (1) The exterior sheathing is air-sealed, and (2) the house is NOT in a cold climate (IECC climate zones 5 or higher). If the house is in a cold climate and air leaks into the building cavity, it could find the cold backside of the exterior sheathing and start accumulating there, leading to rot and mold.

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