This article will teach you how to use cans of single-component closed-cell expanding polyurethane foam—AKA Great Stuff—to air seal gaps, cracks, and joints in your house. As discussed in the previous article on the properties of spray foam, your goal when using those cans should be to air seal, not insulate, your home. If you’re confused about the difference, spend a few minutes reading about air sealing vs. insulating.
Finally, before you get started, make sure that spray foam is really the best choice for your project. In the previous article, I downgraded Great Stuff to “Really Good Stuff” because I think it has become people’s go-to for too many projects, and it has some real downsides in terms of application—which this article will go into. If you landed on this page from a search engine, you might want to think about using caulk for your project instead.
The Ideal Spray Foam Seal
If you read the article about how to use caulk for air sealing, you’ll recall that the ideal caulk joint has a nice hourglass shape in a 2:1 width:depth ratio, which allows the seal to stretch and rebound with minimal stress at the points of adhesion.
The ideal spray foam seal is a little simpler: a gap that’s well filled.
In contrast to liquid-applied sealants (that’s “caulk” to you), I am not aware of any relevant technical standard that describes the geometry of a perfect spray foam seal. I suspect that’s because you have very little control over the shape that spray foam expands into after applying.
I also suspect that shape just isn’t as important as depth to spray foam performance. As discussed in the previous article, spray foam is a little bit elastic, but it is insanely adhesive. The more contact your foam has with the materials you’re trying to seal (i.e., the deeper you fill your gap), the more durable your bond should be. And if the relatively rigid foam does fail in cohesion (say, a crack in the middle due to movement), a deeper seal should minimize the likelihood that air will actually find its way through.
If anyone is aware of a technical standard that I’ve missed, or can provide relevant testing data, please don’t hesitate to reach out here or in the comments.
What Size Gaps Can You Spray-Foam?
Cans of spray foam will answer this question by giving you the upper limit. This makes sense, because spray foams are all about expansion, and big gaps are where they really shine:
- Red cans (Great Stuff and Loctite brands) are for gaps up to 1″ wide
- Black cans (Great Stuff and Loctite brands) are for gaps up to 3″ wide
Above 3″, you should probably be looking at making your gaps smaller before sealing them. (For example, if you’ve got an enormous opening around a pipe penetration, you should patch your wall or ceiling material prior to sealing.)
But what about the lower limit of spray foams? What’s the smallest gap you should seal using the red cans?
To answer this, let’s go back to the two major joint types discussed in the article on how to caulk. Below I’ve sketched out what’s happening in a few different scenarios at the moment of application. This is the part of applying spray foam that you have some control over—after that, you have to cross your fingers and hope it expands the way you want it to!
As you can probably tell immediately, the size of the straw is a critical factor. On the left is the ideal scenario: a butt joint between two materials in the same plane, with a gap wide enough that you can insert the spray foam nozzle. This means a gap that’s greater than about 1/4″, depending on the can you bought. In a wide butt joint, you can be sure that the spray foam is going to expand in two directions—deeper into the joint, and out the opening—pressing itself into the sides of the joint and creating a great adhesive seal along the whole joint.
On the right is the second-best scenario: a fillet joint between two materials at right angles to each other. I’ve sketched it so that the gap is not wide enough to insert the spray foam nozzle. (If it were wide enough, you could treat it like a butt joint and be in scenario 1.) In this situation, your goal is to dispense the spray foam in such a way that it either:
- Expands into the gap. This will only work for gaps above a certain size, and is essentially impossible to verify.
- Expands to adhere to the material on both sides, essentially forming a bridge seal. This can be difficult to accomplish, especially when working overhead. And even with this approach, it can be difficult to verify that you haven’t left a “bypass” somewhere that air can get around.
Finally, the worst scenario is a butt joint that is too tight to insert the straw (smaller than about 1/4″). In this case, you’d either be spraying foam on top and hoping that it expands into the opening, or spraying enough foam that you hope to completely bridge the gap by adhering to both sides. These are both pretty terrible options.
So to summarize the lower limit for spray foams, it really depends on the nozzle and joint type:
- For butt and fillet joints, spray foam is a good option anytime you can fully insert the straw.
- For fillet joints where you can’t insert the straw, spray foam can be a good option if you can be sure of good adhesion across the joint. Otherwise, you’re probably better off using caulk.
- For butt joints where you can’t insert the straw, use caulk.
What You’ll Need
For a typical air sealing session using spray foam in your attic or basement, you’ll need the following tools and supplies. (Note that this list includes affiliate links, but none for spray foam itself. I can’t in good conscience tell you to buy spray foam online. The prices are many times higher than what you’ll pay in the store, likely due to hazardous materials shipping.)
- The right type of spray foam: red, black, or blue can. (Do not use spray foam for high-temperature applications!)
- Personal protective equipment (PPE):
- Long-sleeved clothes you don’t mind ruining. Spray foam is hard to remove from your skin, and virtually impossible to remove from clothing.
- Latex gloves. I use Diamond Grip gloves because they’re easy to take on and off, even if you’re sweaty. They’re kind of expensive, so I try to take them off carefully when I’m done and reuse them.
- Maybe something to cover your hair, if you’re working overhead.
- Because of the fumes, you should consider wearing a respirator with an organic vapor cartridge, especially in a poorly ventilated area. (It will also help protect your face from spray foam!) There are lots of other good reasons for DIYers in old homes to own a respirator, just make sure to get one that fits you properly.
- Rags or paper towels and a can of acetone (or traditional nail polish remover). If you want to maximize the chances you can reuse the can and also minimize the mess you make, you’ll want to clean up the nozzle and any drips with acetone.
- Maybe a small vacuum, if you’re sealing a particularly dirty area. Every owner of an old home should have a small HEPA vac (I bought a couple DEWALT batteries years ago, so I have the most experience with their power tools. But other manufacturers also make 1/2- and 1-gallon HEPA vacuums).
Finally, although you probably shouldn’t be using spray foam in any location that will be visible in your daily life, there may be situations where you will want to trim the excess foam. For example, if you’ve foamed a gap that will have trim installed over it. The secret weapon (OK, they do tell you on the back of the can) for cutting foam cleanly is a long, slightly flexible serrated knife such as a bread knife. I think you’ll find an offset model is particularly useful for keeping your knuckles out of the way.
How to Apply Spray Foam
Make sure to follow the manufacturer’s directions on the can that you bought. However, the following instructions should generally apply to most spray foams:
- Put on your PPE. Eye protection, gloves, long sleeves, maybe a respirator and something to cover your hair. Do it now before you forget.
- Protect nearby surfaces. If for some reason you are applying spray foam in a living area of your house (caulk is usually better for this), you’ll probably want to put down some sheet plastic or similar to protect floors, carpet, etc.
- Prepare the joint. This is probably a less critical step for spray foam than for caulk due to the crazy adhesive power of spray foam, but you should still take a minute to vacuum any loose dirt from in and around the gaps you’re sealing.
- Shake the hell out of the can. The better you mix up the product, the better it will dispense and the more you will get out of the can. Shake for at least 60 seconds. (A friend of mine tapes the can to a Sawzall and lets it do the shaking. I’m not recommending you do that, just throwing it out there.)
- Screw on the dispensing tip. Make sure it’s installed all the way, otherwise you can have a blowout of foam at the base.
- Squeeze gently. The can will dispense air for a little bit—resist the urge to squeeze harder before you’ve seen the foam come out. Get a sense of the flow rate before you apply more force.
- Apply foam to the joint. Starting at the far end of the joint, squeeze steadily and pull the can slowly back towards you. Keep in mind that the foam has to stick to something. Think about the two major joint types in the sketch at the top of the article and ask yourself where you need the foam to adhere. Dupont tells you to fill joints “about halfway.” This is good advice for butt joints, where good adhesion is guaranteed. For fillet joints, make sure you’ve made good contact with both faces of the adjacent material. Don’t rely on expansion to do this for you later.
- Stop squeezing the trigger a few seconds before you’re finished with the joint. Foam will continue to dispense after you release the pressure. You’ll get the hang of how long.
- Clean off the tip with a rag or paper towel before continuing. If you start dispensing again with foam already on the tip, you’ll have trouble controlling the application.
- When finished, follow instructions for ensuring reuse of the can, if applicable. Hopefully you bought a can you can reuse. This will always involve wiping down the nozzle (preferably with acetone). Then you’ll either slide the nozzle closed, bend the straw back to the nub, or do nothing at all, depending on which type of dispenser you have. If you bought a can with a plain old straw, Dupont says you’ve got 2 hours before it’s all wasted.
- Clean up any drips on materials or clothing. Do this with acetone before the foam cures, or you’re out of luck.
- After the foam cures, trim if desired. Cure time depends on the foam you used and the size of your seal, but usually you can safely trim after about 8 hours. (The surface will feel cured after about 15 minutes, but don’t be fooled: the inside will still be extremely sticky.)
In some ways, spray foam is easier to apply than caulk. But it’s also easier to accidentally make a huge mess, and waste a ton of material. Fortunately, you should be applying spray foam in areas of your house where you can afford to goof up!
Tips for Avoiding Common Problems with Spray Foam
Fortunately, there’s not too much else to think about when it comes to spray foam. We’ve already addressed the most common issues:
Spray foam won’t dispense. The product has almost certainly cured inside the nozzle. Yes, this can happen even with the reusable cans. Sometimes you can clear a clog in the tip of the nozzle with a narrow nail or piece of wire, but usually you can’t. You may be able to get the product to dispense if you remove the nozzle and spray directly from the can, but you’ll have a very hard time controlling the application and I can’t recommend doing this to create durable air seals.
Spray foam leaks out of the base of the nozzle. Check to make sure the nozzle is screwed on all the way. If it’s installed correctly, the nozzle is probably clogged and the foam is forcing its way out any way it can find!
Spray foam has a funny consistency or won’t cure. If the foam isn’t behaving normally—for example, it’s kind of runny—you probably didn’t mix it up well enough. Keep shaking.
Pretty simple, right? Now that we’ve covered all the basics of using caulk and spray foam to air seal your house, we’ll finish out this series by summarizing the major pros and cons of each.