If you’ve read any other articles on Green Old Home, you probably already know that I think 100% silicone caulk is the single most useful product for do-it-yourself weatherization—despite the fact that you won’t find it in the weatherization aisle of most big box stores. But what about everybody’s other favorite sealant, the cans of spray foam that you often will find in those aisles?
Single-component closed-cell expanding polyurethane foam—the most popular brand of which is Dupont’s Great Stuff—definitely has its place in making your old home more efficient. Plenty of gaps, cracks, and joints in my house are sealed with Great Stuff.
However, in my experience, spray foam has become the go-to for a lot of homeowners, landlords, and contractors even in situations where it’s not really the best option. And I think a lot of that has to do with some basic confusion about what you’re trying to achieve when you use these foam products.
Air Sealing vs. Insulating with Spray Foam
If you haven’t already, I encourage you to take a couple minutes to read about the importance of air sealing your house. As that article explains, there is a crucial distinction between air flow through gaps, cracks, and joints in your house—what you probably know as “drafts”—and heat flow through the materials themselves. Air sealing addresses the gaps, while insulation is what slows down the flow through solid materials. And if your old home doesn’t have many seals or much insulation, adding air seals is almost always a more important place to start.
So when you’re using spray foam, are you air sealing or insulating? No one talks about the R-value of caulk because it’s simply not a material you use to insulate—it’s clear when you’re using it that you’re blocking air flow through gaps. On the other hand, spray foam is a little trickier because people are aware that foam insulation is a thing.
Yes, spray foam products can be used to both air seal and insulate. In some new construction and gut-jobs of old buildings, you’ll see entire walls covered in the stuff—and it counts as an air barrier and insulation, if it’s applied thick enough.
Setting aside the question of whether you should ever do this to your house, you probably should not do this yourself. And you absolutely should not try to insulate your old home with cans of Great Stuff.
First, there are many types of foam products, and the types contractors use are usually different—and more cost-effective for a large-scale project. Second, Great Stuff explicitly tells you not to inject it into voids (which is what you will be doing if you try to pump your walls full of it) #1) because flammable gasses can build up in the cavity and #2) because the foam could damage your walls if it expands too much in a constrained space. So forget about the fact that stores will sell these cans as spray foam “insulation.” Don’t do it.
On the other hand, cans of spray foam are a good and cost effective way of air sealing many gaps, cracks, and joints. Are you also “insulating” those gaps, as the can advertises? Sure you are. But if the rest of your wall or ceiling isn’t insulated, it just isn’t super helpful that you’ve insulated a couple square inches. The important thing is that you’re controlling air flow.
TL; DR: When you’re using cans of spray foam, your goal is to air seal. Not to insulate.
Important Properties of Spray Foam
As discussed in the article on caulk, the most important goal in air sealing is for your seals to last. Especially with spray foam, you’ll probably be sealing joints that you won’t see in your daily life—so if your seals fail, you probably won’t realize it in order to fix them.
That article introduced the concepts of adhesion, cohesion, and elasticity. As a refresher, adhesion is how well a sealant sticks to other materials; cohesion is how well a sealant stays intact internally; and elasticity is how well a sealant can expand and contract to accommodate movement in your house’s materials while returning to its original shape.
I’ll be following up with an article comparing the properties of silicone caulk and spray foam, but in general: silicone caulk has better elasticity and therefore cohesion. If you’re working with joint sizes that you can caulk, and you know that the joint will experience a lot of movement, silicone caulk is probably the safer choice.
However, anyone who has ever used spray foam knows that spray foam has incredible adhesion. Get it on your hands once and you’ll never want to do it again. I have had spray foam residue on my winter jacket for almost five years now, despite repeated washings. This adhesion makes polyurethane foam easy to apply, including to surfaces that haven’t been well prepared. It also allows you to use foam to hold other air sealing and insulating materials in place, such as rigid foam board insulation. If your spray foam air seals fail, it will almost certainly not be because the foam separated from the materials you applied it to!
The other key property of spray foam is its ability to expand to fill larger gaps. Different products offer various levels of expansion, but all of them excel above the width where silicone caulk starts to get harder to install around 1/2″-5/8″. Sure, silicone is more permanently flexible—but if your gap is 1″ wide and you haven’t gone to the trouble to buy contractor-sized backer rod, spray foam is really your only good option.
Types of Spray Foams
The section above on air sealing vs. insulating mentioned that there are different types of foam out there—some of which are used primarily by professional contractors, and some you’ll find on the shelf at your local hardware store. All of the products you’ll find readily available in cans are single-component, closed-cell, expanding polyurethane foams. However, even Dupont sells about 12 different types of Great Stuff!
To simplify, there are the three main types you’ll want to focus on for air sealing projects:
- Foams for gaps up to 1″ wide. For both the Great Stuff and Loctite brands, this is the red can.
- Foams for gaps from 1″ to 3″ wide. For both the Great Stuff and Loctite brands, this is the black can.
- Foams for around window and door frames. These formulations expand less than the standard red cans so that you don’t accidentally make your window or door openings so tight that you can’t operate them. For both the Great Stuff and Loctite brands, this is the blue can.
Those are pretty much the only types you need to worry about (and they’re essentially the only types offered by Loctite).
What About Fireblock Foam?
I have no idea why this product exists. Dupont advertises that their Fireblock product works by providing “providing an airtight barrier that impedes the spread of smoke and flames.” Well, all of their polyurethane foams provide an airtight barrier! And a quick test with a lighter will show you that all cured polyurethane foams, including Fireblock, will ignite (please don’t try this inside your house)—and then extinguish themselves if you remove the heat source. According to Dupont, all of their foams are combustible at just 240 °F.
This is relevant to your home air sealing concerns: the Fireblock product, like all other canned spray foams, is not rated for use in high-temperature applications such as around furnace flues! Unfortunately, the marketing could lead you to believe that it is. You will need a high-temperature caulk for those seals.
There may be some narrow building code requirement that the fire-block foam meets that the standard foams don’t, but for your home air sealing projects, I can’t think why you would use this stuff. (As always, if I’m wrong—please educate me here or in the comments!)
Types of Spray Foam Dispensers
Finally, it’s worth talking about the strange world of spray foam dispensers. I say “strange” because different retailers sell different sized cans with different tips. How this came about, I don’t know.
The key thing to know about spray foam cans is that they are much harder to reuse than caulk tubes. Once the foam cures in the dispensing straw, there’s virtually nothing you can do. And if you’re on a mission to make your old home more green and efficient, the idea of wasting money and materials by throwing out 75% full cans of spray foam should really bother you.
There are a couple different solutions to this problem floating around out there. I am most familiar with the Great Stuff dispensers. Here is the situation at the two national home improvement retailers as of September 2021:
Home Depot sells:
- 12 oz. cans with a non-restartable straw
- 12 oz. cans with a new straw called a “smart dispenser”
- 16 oz. cans with a “quick-stop” straw, which has a yellow cylinder around the tip that you can slide back and forth to open or close the tip
Lowes sells:
- 12 oz. cans with a non-restartable straw
- 12 oz. cans with a restartable straw, which works by bending the straw backwards to plug it onto a nub at the base of the straw
- 12 oz. cans with the new “smart dispenser”
If you live in a part of the country with a Menards, it looks to me as though they sell cans with the “smart dispenser” but neither of the other two options.
(Incidentally, I can’t recommend buying spray foam from any online retailers, because it sells for exorbitant prices presumably due to hazardous materials shipping.)
So what should you buy? Here’s how I would think about it:
16 oz. cans with the quick-stop straw are clearly the best value for the money; red cans are under $4. The nozzle closer works well for preserving the can, and you can be reasonably confident that if you take precautions to close and clean the tip after working, you’ll be able to use most or all of the 16 oz. Just don’t wait too many weeks between projects. The biggest drawback is the wide closing mechanism on this model: if your projects will require you to stick the tip into 1/4″ gaps, the wider nozzle just won’t do it.
12 oz. cans with the restartable straw also work pretty well to preserve the contents of the can. These aren’t as good a deal as the 16 oz. cans, but the tip of the nozzle is the normal size and you can use it for smaller openings. They’re also a good choice if you’re not sure you’ll get through the 16 oz. can. The worst thing I can say about these is that once or twice I’ve had the straw pop off of the little nub that’s supposed to plug it, flinging sticky spray foam. After that I’ve always kept my unfinished cans in a plastic bag.
12 oz. cans with the normal straw are frankly not a great option if you hate waste. Unless you are very efficient and know you have many, many linear feet of gaps to foam in your basement or attic, you will have a hard time finishing these. It’s always good practice to try to plan for a spray foam “session” rather than opening up a can to seal one or two small things, but even then I would lean towards one of the resealable options.
The only option I haven’t tried is the new “smart dispenser” straw. Some reviews online make me a little skeptical that they work well, and the smart dispenser just about doubles the cost of a can. But for the sake of science, I’ll check it out and report back.
Finally, I don’t think it makes sense for the average DIYer to invest in a foam dispensing gun. They are expensive, not immune to clogs themselves, and then you’re locked into buying 24 oz. cans of foam that will almost certainly be too much for you. It can be hard enough to use up all the foam in smaller cans, depending on how quickly you move through your air sealing projects. Think carefully about your own house, your “project schedule,” and the money math before investing.
OK—now that you know more than you ever wanted to about spray foam, let’s talk about how and when to apply it.