How to Air Seal Your House (Part 1)

You should be proud of yourself—the fact that you’ve come to this page looking for information about how to air seal your house means that you’re way ahead of the pack in terms of making smart energy-saving decisions. Let everyone else spend a fortune on new windows. Air sealing is probably the single most cost-effective intervention the average owner of an old home can make to start conserving energy and reducing their utility bills.

If you want to be really smart about it, take a few minutes before reading this article to learn about three crucial concepts: your thermal envelope, stack effect, and the prime importance of air sealing. You will get a lot more out of this “how-to” if you do. That’s because every old home has a different design and a unique renovation history—not to mention being located in a particular climate. So while this article covers the most important places to focus a typical air-sealing project, you will have the best results if you really take the time to understand those concepts—and how they apply to your particular home.

Products for Air Sealing

The materials you’ll use to air seal your house fall into two major categories:

  • Weatherstripping for stuff that moves. Usually that means doors and windows.
  • Sealants (including caulk and spray foam) for stuff that doesn’t move. That’s pretty much everything else in your house.

There’s a lot to talk about within each of those categories, as well as plenty of specialty products designed for one particular use. We’ll talk more about your options for each situation in your house. But if you’re dying to know, you’re probably going to want to use 100% silicone caulk for the greatest number of jobs. I always have a couple tubes ready to go in different colors. (I’m going to talk about silicone caulk so much that you’re going to think GE is paying me to do it, but I swear they’re not—though that is an Amazon affiliate link up there.)

Finally, don’t forget that air sealing just means stopping airflow—it does not mean using a particular product. So use your head. If you’ve got a giant hole in the middle of your wall or ceiling, you’re going to want to patch that “in-kind” using drywall, or plaster, or wood, or whatever the rest of the wall or ceiling is made out of.

Where to Start Air Sealing

When I started getting serious about air sealing my home, I took a look at this graphic from the U.S. Department of Energy—and felt completely overwhelmed. Not only does it list 19 separate “trouble spots,” but some of them include multiple elements: #17 is “rim joists, sill plate, foundation, floor”! I can tell somebody spent a lot of time trying to put together a comprehensive list. But where on earth was I supposed to start? What’s most important?

The key to figuring this out in your own old home is to take the time to understand the concepts I keep talking about. If you took my advice and walked around your house to identify your thermal envelope, you probably already figured out that it’s more important to air seal, say, the wall between your house and your unheated garage than it is to air seal your garage doors themselves. And if you spent some more time thinking about stack effect, you probably also realize that it’s more important to focus on the top and bottom of your house than the stuff in the middle.

Sure, you’ll get to all of those “trouble spots” someday, but why not make sure that first tube of caulk pays for itself many times over in your next heating or cooling season? To do that, here’s how I would think about prioritizing air sealing projects, in order:

  1. Big holes in critical locations.
  2. Big holes in other locations.
  3. Small holes in critical locations.
  4. Small holes in other locations.

Depending on how you define “big” and “small” and “critical location,” you might argue for switching #2 and #3. (And if you do, let me know in the comments!)

Here are what I would consider critical locations in most old homes:

  • The top of your thermal envelope (usually the uppermost ceiling of each section of your house). If you cut off stack effect’s preferred exit, you’ll also cut down on unwanted makeup air infiltration.
  • The bottom of your thermal envelope. Depending on your house, this could include your basement ceiling, your basement or crawlspace windows, and/or the above-ground portion of your foundation. This will cut down on infiltration in the heating season, and should help you retain your conditioned air in the cooling season when “reverse” stack effect is pushing it downward.
  • Your ductwork, anywhere it runs through an unconditioned space like a basement or attic. When your conditioned air is leaking out before it reaches the rooms you live in, that’s a pretty critical leak.
  • The area near your thermostat. If your thermostat is being triggered by, say, a drafty door or window nearby, it’s going to turn your system on unnecessarily often, wasting money and energy.

In terms of “big” vs. “small” holes: don’t overthink it. Use your senses—and some common sense. If a hole is large enough that you see it when you go about your everyday life, that should probably count as a big hole. If you have a particularly drafty window or door, or a room that is noticeably less comfortable than others for no clear reason, you’re probably dealing with a big hole. Don’t make the mistake of getting all your priorities from a diagram when you should be paying attention to your house: if I had only listened to my wife about how drafty our living room was, I would have sealed probably the largest air leaks in our house years earlier.

Finally, don’t forget that seemingly small holes can add up to big ones. Do some quick math. If you have an 1/8″ gap around all four sides of your back door, that’s something like 20′ x 1/8″ = 30 square inches. I would call a hole the size of your hand a “big hole.” Similarly, if the perimeter of a 4″ ceiling junction box (what your light fixtures are probably mounted to) has an 1/8″ gap around it, that’s only a little over 1.5 square inches, plus another maybe 0.5 square inches of gaps inside the box… but how many ceiling boxes do you have?

Now that we’ve talked about your big-picture priorities, in the second part of this article we’re going to dig into the specific projects you can expect to run into when you start air sealing your old home.

But first, one important note:

You Should Know about Backdrafts

Any good advice you find about air sealing your home is going to come with a warning about backdrafts, and it would be irresponsible of me not to mention them. Here’s the idea: if you’ve got a gas furnace or water heater, or another combustion appliance, it’s exhausting the hot air from combustion somehow—which means new air has to take its place. And most old mechanical systems don’t have a fresh air intake, so that means the air is coming from all the gaps in your house.

So… what if you seal your house so well that your appliance can’t draw more air in? In theory, the device will fail to exhaust properly and you could end up with carbon monoxide building up in your home. That could kill you.

Unless 100% of your appliance are electric, you should be aware of this concept and use your best judgment. If you want, you can hire someone to do something called a “worst-case depressurization test” where they will run all of your exhausting devices at once (clothes dryer, kitchen and bath fans, etc.) to see if your appliance can keep up.

I am not going to tell you not to worry about this at all.

However, I do strongly encourage you not to let it prevent you from getting started air sealing. If you own an old home, you very likely have a long way to go before you need to start thinking about your house being “too tight.” Various sources, including this article over at Green Building Advisor and the Illinois Home Weatherization Assistance Program,* seem to agree that you’re probably only going to be able to reduce about 20-36% of your house’s air leaks unless you undergo extensive renovations. (And if you are able to achieve more than that, it’s likely because your house was extra leaky to begin with!) But again, use your best judgment based on what you understand about your own home, and if you feel uncomfortable—call an expert. Just don’t make excuses to do nothing and keep your house running inefficiently.

As a minimum safeguard, however, you should already have carbon monoxide detectors on every floor of your house. I have a combination of these carbon monoxide/smoke detectors mounted to my ceilings, and these dedicated plug-in detectors on alternating floors (those are affiliate links).

I’ll leave you with this: One of our detectors once alerted us to a leak that turned out to be from our attached neighbor’s rusted flue pipe. They did not have any CO detectors in their home and could have been killed. Best $20 I ever spent.

*The IHWAP field manual says that the average home in Illinois has a leakage rate of 3900 CFM (cubic feet per minute) in the blower door test prior to being weatherized by the program. The program’s target for a home in that range is a 36% reduction in leakage.