If your goal is to make your home more comfortable and energy-efficient, you will inevitably find yourself in the weatherization section of one of the big box stores. You know, the aisle where they sell weather stripping, door sweeps, and sheet plastic window covers. And that stuff’s all great.
But frustratingly, those aisles usually don’t include the single most powerful product for do-it-yourself weatherization: caulk.
This article will explain the properties of caulk that make it so useful for air sealing, talk about the different types of caulk, and recommend the best types of caulk for different applications in your home. But first, let’s clear up some terminology.
Caulk vs. Sealant
For the purposes of air sealing, you can thing of sealants as a general category of products that includes both caulk and spray foam. So all caulks are sealants, but not all sealants are caulks. Simple, right?
Well, sometimes you will hear people differentiate caulk and sealant based on their elasticity—that is, their ability to repeatedly expand and contract. By that definition, caulk is something painters use to fill gaps and cracks that aren’t critically important and that don’t experience much movement. That definition reserves the term sealant for joints that need to be air- or water-tight, and experience cyclical movement—say, exterior joints around doors or windows, or joints in concrete. Probably for that reason, sealant seems to be the preferred term among architects, engineers, and specifiers.
However, the authority on this stuff, ASTM International, defines caulk and sealant interchangeably. This is more than you wanted to know, but in ASTM C717 – Standard Terminology of Building Seals and Sealants the definition of the noun “caulk” is: “see sealant.” The definition of the verb “caulk” is: “to install or apply a sealant…”
Look, all of this sort of misses the point anyway. It’s much more important to focus on the material the caulk or sealant is made out of. If it’s called 100% silicone caulk, it’s the “100% silicone” part that’s important. But to avoid confusion, Green Old Home generally uses “caulk” to refer to anything that comes out of a tube—after all, you never hear anybody talking about a “sealant gun”!
Important Properties of Caulk
When you’re air sealing your home, you want your seals to last. Especially because some of the joints, gaps, and cracks you seal might become inaccessible later (say, if you’ve installed insulation), you really want to make sure you don’t have to worry about your seals failing. The name of the game is durability.
So what makes a a caulked joint durable? Well, some of it comes down to execution—i.e., how you prepare and caulk the joint. But the material properties are important too.
Keep in mind that your old home is always moving. Wood swells and shrinks with changes in humidity. Different materials expand and contract at different rates due to temperature changes. Your structure bends and flexes as you walk around. That’s why elasticity is important in the world of sealants, as mentioned above.
It’s also useful to understand two other terms: adhesion and cohesion. Adhesion is how well a material sticks to something else. Cohesion is how well a material holds together internally. You want your sealant joints to do both.
Do you have cracked caulk in your house, maybe from an old paint job? If the cracks run through the middle of the caulk, that’s a failure of cohesion, likely due to insufficient elasticity—as we’ll discuss below, painter’s caulk isn’t very flexible. If you have cracks where it looks like caulk is pulling away from the materials where it was installed (say, around a window or door), that’s a failure of adhesion. That can also be due to insufficient elasticity (the caulk couldn’t stretch enough to keep up with expansion of surrounding materials), but it can also be due to poor surface preparation. We’ll talk about that in the next article.
In summary, when air sealing you’ll want to make sure that the products you’re using have good adhesion, cohesion, and elasticity. Products will often advertise adhesion and elasticity (or flexibility)—and the cohesion is sort of implied because if the caulk sticks to the surrounding materials and is highly elastic, you can expect the caulk to hold together internally.
Types of Caulk
If you head to the aisle where all the tubes of caulk are standing in their little rows and you look closely at the labels, you’ll probably see all of the following:
- Acrylic latex
- Siliconized acrylic latex (or “acrylic latex plus silicone”)
- 100% Silicone
- Polyurethane
If you’re in a store that caters more to professionals, you might see other types like butyl. But around your home, it’s OK to focus on just the common types listed above. If you’re looking at a tube of caulk and you can’t tell which of these categories it fits into, don’t waste your money. Know what you’re buying.
Acrylic latex is the cheapest option, and it’s what most painters use—because it’s paintable! Many other caulks aren’t. Acrylic latex isn’t waterproof and is the least elastic of caulks.
Siliconized acrylic latex is an acrylic caulk that has a little bit of silicone added to give it some of the properties of a silicone caulk—namely, water resistance and increased elasticity. These are usually still paintable, and still pretty cheap. (Which probably tells you how little silicone is actually added…)
100% silicone caulk isn’t paintable, and it can be 2-3 times as expensive as acrylic latex. However, it has a few major advantages:
- It is completely waterproof (which is why it is often used for kitchens and bathrooms).
- It is highly elastic, meaning it will expand and contract with the joints in your house without degrading.
- It has a broad service temperature range—up to 400 °F.
- It can last for decades, if applied properly.
Polyurethane caulk is also completely waterproof and highly elastic. However, it has a shorter service life than silicone. Compared to silicone caulk, its greatest advantage is that it adheres better to porous masonry materials.
The Best Caulks for Air Sealing
It may have been obvious because of all the positive bullet points above, but I recommend that you use 100% silicone caulk for most of the air sealing projects in your house.
Yes, 100% silicone can run you $6-10 a tube. However, if you put a value on your time—and you absolutely should—then the certainty that your seals will last for many, many years is well worth it. You only need acrylic latex painter’s caulk to last as long as your paint job. You want your air seals to be as close to permanent as possible. In the next article, we’ll talk more about how to make sure you get your money’s worth out of silicone caulk by not wasting material and by using proper technique.
Here are the products that I use and recommend (this list includes affiliate links):
- For most jobs: GE 100% Silicone Caulk. When you’re in the basement or attic and won’t see it later, I recommend white if you’re sealing wood, or whatever color contrasts with the material you’re sealing. That will make it easier for you to see imperfections in your seal before the caulk cures.
- For joints that will be visible: GE Paintable Silicone Caulk. Yes, a paintable silicone! They’re very careful not to say that this product is 100% silicone, but it has similar performance characteristics while still taking a paint finish. Thanks to this product, there’s no excuse for not sealing gaps because you don’t like the the available colors. But make sure you do paint it—otherwise this stuff yellows, as it is not intended to be exposed.
- For joints involving masonry (brick, concrete, or stone): Sikaflex Construction Sealant, a paintable polyurethane caulk from a manufacturer a lot of professionals use. (It’s often in a different part of the store than the latex and silicone caulks.)
- For joints in high-temperature locations, such as around furnace flues and chimneys: Rectorseal Hi-Temp Silicone has a service temperature of up to 650 °F. Unfortunately, you may not find high temperature caulks in your hardware store. (Make sure not to buy an “intumescent” fire caulk, which expands when exposed to heat! The service temps of those products are actually pretty low—usually under 200 °F—because they’re designed to deal with heat in an emergency, not on a daily basis.)
Go ahead and get yourself a couple tubes to start, along with a nice caulk gun. (Already have an old caulk gun? Treat yourself to one that doesn’t suck. I promise you won’t regret it.)
Next up, we’ll talk about proper caulking technique for creating good, durable seals.