Caulk vs. Spray Foam for Air Sealing

This article outlines the pros and cons of caulk and spray foam so that you can make smart choices about which to use for different jobs when air sealing your house. This article is also a summary of the information contained in the previous few articles, which cover the following topics in more detail:

For those of you in a hurry, I’ll tell you at the top that I stand by 100% silicone caulk (affiliate link) as the best overall choice for most DIY air sealing projects. You can use it indoors and outdoors in both visible and invisible locations, it creates an air and water barrier, and it creates as close to a permanent, flexible seal as you’ll get for the joint sizes the average homeowner will encounter most often. It’s also much easier with caulk than spray foam to avoid losing money on wasted materials.

For those of you with a little more time on your hands, read on for more.

Let’s start with what an architect or engineer might call “design considerations”—picking the right material based on performance characteristics. Then we’ll move on to the subject of actually installing the stuff, AKA “means and methods,” which those same professionals love to not think about—but which is equally relevant when you’re a DIYer.

You should think about the following performance-based factors when choosing between caulk and spray foam. (Note that unless otherwise specified, when this article says “caulk” it refers to 100% silicone caulk. For more on the different types of caulk, click here.)

Joint Size

The size of the gap, crack, or joint that you’re trying to seal is probably the single most definitive factor in choosing between caulk and spray foam.

Key takeaways: Caulk is your only good option for joints below 3/16″, and spray foam is your most practical option for durable seals above 5/8″. That leaves a range in the middle, from about 3/16″ to about 5/8″, when both products are viable options—meaning you’ll need to take into account the other factors below.

Service Temperature

Service temperature—that is, the temperature range that your seal will be exposed to during normal daily and seasonal changes—is another clear deciding factor. (This section includes affiliate links.)

  • 100% silicone caulk’s service temperature range varies depending on the product, but can be quite substantial. GE’s 100% silicone caulk is rated up to 400 °F, making it a safe choice for use near some types of light fixtures, for example. And when you need a truly high-temperature seal, such as near a chimney or furnace flue, a specialized silicone caulk is your only good option: Rectorseal Hi-Temp Silicone has a service temperature of up to 650 °F.
  • Spray foam is fine for most applications within your house, but they have a much lower tolerance for heat than silicone caulks. All of the spray foams you’ll find in a can at the hardware store can ignite at just 240 °F, including the product labeled Fireblock. Don’t panic about this—ignition temperature is different from “flame spread rate,” and spray foams extinguish themselves quickly if the heat source is removed. But the point is, they are not designed for high-temperature applications.

I didn’t include low temperatures above because they are unlikely to be relevant to most air sealing projects, but spray foam is rated down to -200 °F, and GE’s silicone is rated down to -40 °F. I suppose if you live in Alaska, you may need to take this into consideration! On the other hand, every house likely has at least one or two locations that require a high-temperature sealant.

In summary, caulk is the clear winner when high temperatures are a consideration—you just have to make sure you buy the right kind.

(Note: Caulks labeled as Fire Barriers or Fire Seals usually do not have a high service temperature! Rather, they are designed to expand under extreme heat to stop the spread of fire. That is, they can handle heat under unusual circumstances, but not “usual” ones. Always read your labels carefully.)

Durability

Part of the key to making durable seals is proper installation, which is covered in the how-to articles for both caulk and spray foam. But let’s say you did everything correctly, and installed two perfect 1/2″ wide butt joints: one with 100% silicone caulk and backer rod, and one with spray foam. Which would be the most durable seal?

It’s a great question that’s hard to answer. In the absence of all sources of deterioration—say, in a dry location that experiences no real movement due to house occupants or fluctuations in temperature and humidity—you might expect both seals to last essentially forever.

But notice how many qualifiers I had to put in that last sentence to get around the idea of movement. In reality, your house is consistently moving. Wood in particular is swelling and shrinking with seasonal changes in humidity, and metals with changes to temperature, but every single component of your house is in some way responsive to changes in temperature or humidity. And then there’s you and everyone who lives with you, constantly making the parts of your house bend and shift as you go about your lives.

For those reasons, all else being equal, I think it’s fair when talking about air seals to say that durability is elasticity. (For a quick refresher on elasticity, cohesion, and adhesion, all of which are mentioned below, click here.)

  • 100% silicone caulk is highly elastic, so much so that you virtually never have to worry about cohesive failure. The bigger concern is adhesive failure—i.e., the caulk separating from the materials you’re sealing—which is why surface preparation and the use of backer rod to create an hourglass profile is so important.
  • Spray foam is also somewhat elastic, but nowhere near as much so as silicone caulk. However, it is highly adhesive. So in theory, too much movement could result in the foam cracking in the middle, but it is unlikely to pull away at the sides.

So which type of sealant handles movement better? The answer has to be silicone caulk—provided of course that you are able to install it properly.*

For joints where you expect a lot of movement, you should do everything you can to ensure a proper silicone seal. On the other hand, if you won’t be able to do it properly—for example, if you’re working in a location where you can’t remove dirt from a gap—you’re probably better off relying on the adhesion of spray foam and crossing your fingers.

Water and Sun Resistance

Most of your air seals won’t be directly exposed to sun or liquid water, but when they are—for example, when filling gaps on the exterior—you’ll need to take sun and water resistance into consideration.

  • 100% silicone caulk is completely waterproof, which is what makes it the sealant of choice for sinks and bathtubs. And according to GE, exposure to ultraviolet light will not cause their silicone to crack.
  • Spray foam is water-resistant, but not waterproof. And you may be familiar with the lovely orange-brown color it turns when exposed to ultraviolet light. Further, according to Dupont, if left exposed and unpainted, their foams “will eventually crumble.”

If your seals will be exposed to sunlight or liquid water, caulk is the clear choice.

Summarizing the performance-based considerations above, silicone caulk is your only good option for joints below 3/16″ wide, while spray foam is your most practical option above about 5/8″. In the middle range where both are options, silicone caulk’s performance characteristics are generally superior—if you are able to install it properly. So let’s move into a discussion of installation considerations.

Access Issues

Access issues are probably right up there with joint size in terms of deciding for you whether you’ll be using caulk or spray foam for air sealing. After all, if you can’t get the tube of caulk to the joint, you’re going to have trouble!

  • 100% silicone caulk is usually (and best) applied using a caulk gun. In tight spaces, the gun might be too wide for you to reach the gap or crack you’re trying to fill. Technically you might be able to apply the caulk using your fingers or some similar method, but I wouldn’t trust that you’re adequately filling the joint and achieving the proper profile. And if you can’t reach the gap with your caulk gun, there’s a good chance you couldn’t adequately clean the surface either. These warning signs that your joint won’t be durable.
  • Spray foam cans come with a variety of dispenser types, but most of them have some sort of flexible straw. In addition to being flexible, the straws are usually about 5″ long, which allows you to reach spaces that your caulk gun just won’t.

For sealing joints in tight spaces, cans of spray foam with flexible straws are often your only good option. Because gaps in tight spaces are often very hard to prepare (i.e., clean) adequately for a durable caulk seal, this is another point in favor of spray foam for hard-to-access air seals.

Required PPE

Personal protective equipment (PPE) isn’t just for people on construction sites. No, you don’t have to wear a reflective yellow shirt when you’re air sealing your own attic, but if you don’t take certain precautions when air sealing I can promise you that you’ll come to regret it. (Note that this list includes affiliate links.)

  • 100% silicone caulk will require you to use latex gloves. Silicone is very sticky, and if you try to tool a joint with your bare fingers, you’ll never do it again. Also, different silicones cure in different ways and release different chemicals in the process, but you may want to consider doing the research to fit yourself for a respirator with an organic vapor cartridge.
  • Spray foam will also require you to wear gloves and consider a respirator. But because cans of spray foam are harder to control than caulk, and because spray foam is extremely sticky, you’ll also need to wear safety glasses, long sleeved-clothing that you don’t mind ruining, and maybe even some sort of hair protection if you’ll be working overhead.

As discussed below, both caulk and spray foam can be pretty messy. But because of spray foam’s extreme adhesion and those same flexible straws that make it easy to apply, it’s even easier to accidentally get foam all over you. Caulk is the clear winner if you’re not interested in “suiting up” every time you start air sealing.

Surface Preparation

Any adhesive material will stick better to surfaces that are clean. If a surface has loose dust and dirt on it, the material will stick to that and not to what you were intending. This is why surface prep—cleaning—is a best practice for both caulk and spray foam. However, it is not equally important for both sealants.

  • 100% silicone caulk can have great adhesion. However, it is necessary to ensure that the surface is free of—as it says on the back of the tube—dirt, grease, moisture, soap residue, and old caulk. (Silicone particularly doesn’t like to stick to old silicone, but that’s likely more relevant for your bathtub than for your attic.) A quick vacuum followed by a wipe-down with a slightly damp rag is often all that’s necessary; just make sure to let the surface dry before applying the caulk. However, there are times when you simply may not be able to achieve a well-prepared joint in your basement or attic.
  • Spray foam has excellent adhesion and quite simply does not require as much surface preparation.

If you’re sealing an area that’s particularly dusty and dirty, you should do your best to remove that loose material before applying either caulk or spray foam. However, when push comes to shove, surface prep is simply less important for spray foam.

Messiness

Trust me, you can make a huge mess with both caulk and spray foam—especially when you’re using either for the first time. But while you can learn to control both materials better with practice, there are some inherent qualities of caulk and spray foam to consider.

  • 100% silicone caulk is usually messiest during tooling, i.e., when you run your finger along it to achieve the proper shape. However, if you apply it carefully and in just the right amount, remember to squeeze the release trigger on your caulk gun when finished (if applicable), and keep a piece of scrap cardboard handy to wipe your finger on, you can keep the mess to a minimum. With a little bit of practice, you should be able to caulk overhead without worrying about protecting the room you’re in.
  • Spray foam can be messy from the moment you start or re-start the flow and continue making a mess after you’ve finished squeezing the trigger. Practice can make the flow of spray foam more predictable, but you should always be prepared for surprises—particularly if you’ve restarted a can from a previous session. Between the potential for blowouts at the base of the nozzle and the tendency of the flexible straw to fling sticky foam, you basically have to wear all the PPE listed previously. You also need to protect your floors and anything else around if for some reason you’re applying spray foam in a finished part of your house, particularly when working overhead.

In summary, when silicone caulk is messy, it’s usually your fault. In contrast, spray foam can make a huge mess even if you’ve done everything right. Maybe this isn’t important in your basement or attic, but if you’re working in a finished part of your house—especially if you’re working overhead—you’ll probably want to opt for caulk.

Waste

Wasting money and materials should be a huge concern for anyone trying to make their home greener and more efficient.

  • 100% silicone caulk tubes can be sealed between uses with a cap or with some painter’s tape to preserve the tube for reuse. The caulk can still cure a little bit in the tube, but it is often possible to unclog. Another consideration is that expiration dates on silicone caulk are real, and if you wait too long past those dates you run the risk of the product losing its ability to cure.
  • Spray foam’s reusability depends on what type of can you bought. You only get one use out of a can with a normal straw, and there is more in one can than the average DIYer is going to use unless you’ve got several hours of sealing to do. The reusable cans can work fine if you’re going to resume air sealing within a few weeks, but in my experience none of them restart as reliably as a tube of caulk.

To my way of thinking, spray foam is significantly more wasteful for the average DIYer. I try to be as efficient as possible by planning my projects in advance, but I still end up throwing out half-cans several times a year because I thought I’d come back to them in time and didn’t. On the other hand, I’ve restarted tubes of silicone caulk that were upwards of a year old.

I could see a counterargument that spray foam is less wasteful because Dupont advertises, for example, that a 24 oz. can of Great Stuff Pro is equivalent to up to 22 quart tubes of caulk. I suppose if your definition of waste is limited purely to dollars, you could argue that Great Stuff can seal more gaps per dollar spent and that therefore caulk is “wasteful.”

You can make your own judgment. I just hate throwing out unused plastic products that I know are just going to end up in a landfill, especially when I know I could have planned better to avoid it.

Summarizing the above installation considerations, caulk and spray foam each definitely have their pros and cons. Caulk can involve a little more preparation in order to install properly and can be harder to install in tight locations, but it also makes less mess, requires less PPE, and usually results in less wasted material.

Conclusion?

In case you couldn’t tell from the length of this article, it depends. I still think 100% silicone caulk is the most versatile option for most typical air sealing jobs, but you need to be aware of the advantages and limits of spray foam too.

Finally, I encourage you to think about all of the above factors and choose the right material for each application. But I also know that sometimes you’re going to grab the can or tube that’s open and available—to minimize waste, or just save yourself a trip downstairs—rather than spend 10 minutes weighing the pros and cons of caulk vs. spray foam. And that’s totally fine.

The point is for you to be aware of the choices and trade-offs you’re making. Maybe that caulk you applied at midnight, when you were too tired to prepare any more joints properly, is worth taking a look at in a year or two to see how it held up!

*Long footnote about comparing the physical properties of these products: Unfortunately, doing a quantitative comparison of properties like elasticity between silicone caulk and polyurethane foam is more difficult than it should be. If you read the product data, most of the ASTM testing reported for caulk and spray foam is different: silicone caulk is tested using methods specifically for flexible sealants, while Great Stuff is tested using methods specifically for insulation and rigid plastics—this despite the fact that cans of spray foam are much more practical as sealant! If anybody actually knows how to intelligently translate between the different ASTMs, please feel free to comment or contact Green Old Home.