This article will teach you how to make sure your caulked joints, gaps, and cracks stay sealed for as long as possible. When you’re air sealing your home (or caulking anything else around your house!), the last thing you want is for your work to go to waste. And unlike the caulk around your kitchen sink, you probably won’t see most of your air seals often enough to notice if they’ve failed.
The last article explained why you need to select the right material for the most durable seal—but your technique is the other half of the equation. You may have done caulking projects around the house before, but unless you’re a professional contractor there’s a good chance this article includes some tips you don’t know.
So let’s start with the goal in mind: what does an ideal caulked joint look like?
The Shape of a Perfect Caulk Joint
If you can believe it, the authoritative source ASTM C1193 — Standard Guide for Use of Joint Sealants contains more than 30 pages of information about sealants and “joint design.” New commercial construction relies heavily on sealants for things like window glazing and expansion joints in masonry, so this stuff is actually pretty important. For our purposes, I’ll boil down for you the critical parts you should know for air sealing—and other caulking jobs around the house.
Big picture, the guide defines two major types of joints, which I’ve sketched for you below:
On the left, you have a caulked gap between two elements in the same plane, which is called a butt joint. On the right, you have a caulked gap between two elements at right angles to each other, which is called a fillet joint.
Caulk works best when it’s applied within a butt joint as shown on the left, but obviously you won’t always have that option. In the image on the right, the gap is too small to apply caulk so that it adheres to the two sides within the gap, like a butt joint—instead it adheres to the material outside the gap, sort of straddling it.
By the same principle, there’s a third type of joint called a bridge joint which looks like a butt joint but the caulk is basically smeared over the top of the gap. This is the worst of the three options, but sometimes it’s all you can do.
Most importantly, notice that in the butt joint, the caulk is sort of hourglass shaped. Now notice how the fillet joint is similar, only stretched a little funny because of the angle. This is the ideal caulk profile for maximum durability—an hourglass shape with a width:depth ratio of 2:1. (That is, your seal should be twice as wide as it is deep.) According to ASTM, this hourglass shape “shifts the peak stresses away from the adhesion surface and into the middle of the sealant joint” during joint movement. In other words, the hourglass shape will help keep your seal from failing.
As you probably know, you’re going to tool the caulk using your finger to give the seal its curvature on the outside face. But what’s going to give it the curvature at the underside of the joint?
Backer Rod: The Secret to Durable Seals
The answer is a product that—unlike caulk—actually is sold in the weatherization section of the big box stores, only under the super misleading name of “caulk saver.” Professionals call it backer rod. The point of the product isn’t for you to use less caulk when filling a large joint (though it will do that for you), it’s to help you establish the ideal hourglass joint profile shown above.
Polyurethane foam backer rod is available in a range of sizes. The photo above shows a range from 5/8″ down to 3/8″. I recommend you buy a couple packs of each size (affiliate link) to have on hand—it’s cheap stuff. Don’t get too hung up on the exact widths, just keep in mind that you’ll always need the backer rod to be a little wider than the gap you’re filling so that you can wedge it in place. That’s what the gray ovals represent in the sketches above—round backer rod that’s been squished because it was pushed down into the joint.
You don’t always need to apply backer rod—and you often won’t be able to, because the joint you’re sealing will be too small. The 100% silicone caulk I recommend says to use backer rod for gaps over 1/2″, but you can easily get backer rod in a 3/8″ width that will fit joints in the 1/4″ range. Use backer rod any time it fits—it’s worth the peace of mind that your seal will be as close to permanent as possible.
What Size Gaps Can You Caulk?
The ASTM standard referenced above implies that some construction sealants can be used in joints up to about 2″ wide. But if you’ve got gaps that wide in your house, you’ve got problems bigger than air sealing. On the other end of the spectrum, ASTM doesn’t recommend sealing gaps smaller than 1/4″, presumably because they are concerned about achieving the ideal joint profile, which is difficult in tiny joints.
However, when you’re air sealing your house you’re going to be caulking a lot of joints smaller than 1/4″ wide. That’s because caulk is usually your only good option for jobs that small. Your major alternative, spray foam, just isn’t great at filling narrow gaps. So while your smallest caulked joints may not have the ideal shape—they might be more like bridge joints than butt joints—they are the best you can do. Depending on how important the location is in your house, you’ll probably be caulking joints as small as 1/8″ down to 1/16″ wide.
At the upper end, you’ll mostly be limited by the commonly available sizes of backer rod. The largest size you’ll usually find as a consumer is 3/4″, so the widest joints you’d want to caulk would be a little smaller than that, say maybe 1/2″-5/8″. Fortunately, these larger gaps are where spray foam really shines, as we’ll discuss in the next article. In the range around 1/2″ where both products work well, you’ll choose between caulk and spray foam based on other factors.
What You’ll Need
For a typical caulking job on a joint that won’t be seen, you’ll need the following tools and supplies (note that this list includes affiliate links):
- A good caulk gun.
- The right caulk for the job. In most cases, this will be 100% silicone caulk.
- Something to cut the tube’s tip. Lots of people use a utility knife, but I use a pair of dykes.
- Something to pierce the caulk tube’s inner seal. A good caulk gun will have this, or you can use a long nail or piece of wire.
- Backer rod just wider than the joint you’re filling, if possible.
- A pair of latex gloves. If you use your bare fingers to tool silicone caulk, you’ll hate yourself afterwords. I love Diamond Grip gloves because they’re super easy to take on and off, even if you’re sweaty. They’re on the expensive side, so I try to take them off carefully when I’m done and reuse them.
- A large, stiff piece of scrap cardboard to wipe your fingers on.
- Maybe a small vacuum, if you’re sealing a dirty area. I recommend that every owner of an old home have a small HEPA vac (I invested in couple DEWALT batteries years ago, so I have the most experience with their power tools. But several manufacturers make 1/2- and 1-gallon HEPA vacuums).
If you’re caulking a joint that you will see and you need a clear, sharp line on one or both edges of the joint, you’ll also need some painter’s tape. And just in case you accidentally make a mess, you should have a rag and a can of mineral spirits nearby.
How to Apply Caulk
You should always follow the manufacturer’s directions for whatever product you’re using. But the following steps cover most caulks, and we have more space to go into detail here than exists on the back of the tube! (Note that the steps below describe how to apply caulk to joints, gaps, and cracks that have never been sealed before. We’ll cover how to deal with old failed seals in another article.)
I know this looks like a lot of steps, but I promise you that once you’ve got some practice this whole list can take you less than 10 minutes, depending on the joint.
- Prepare the surface. Your adhesive bond is only as good as the surface you apply the caulk to, and if you apply caulk to loose dust and dirt—well, don’t expect a durable seal. A quick vacuum plus a wipe with a slightly damp rag is usually going to be enough—use more rags if your first one comes up filthy. Be sure to let the surface dry completely, otherwise your caulk will bubble.
- Insert backer rod, if it fits. Just choose the size that is a little wider than your joint. You can cut it to length, or just tear it with your fingers. Push it into the gap with your fingers or a blunt tool like a ruler (so you don’t pierce it) until the depth of the joint is about half the width, like in the illustrations above. Don’t go nuts making it perfect.
- Apply painter’s tape. If the joint will be visible and you need a clean edge, apply the tape where you want the edges of your joint to be. Use the illustrations at the top of the article for reference.
- Cut the tip of your caulk tube. Cut it at an angle, like the markings on the tip illustrate. Start a little smaller than you think you need—you can always make the opening bigger. You’ll learn to eyeball it with practice. But in general, you can work with a tube that’s cut a little small, while a tube that’s cut too big will waste caulk and make a huge mess.
- Pierce the inner seal. Use the poker on your caulk gun, or a nail or a piece of wire. But make sure you’re using something narrower than the hole you just cut, otherwise you’ll blow out the tip and have a bigger opening than you want. Sometimes you have to pierce the inner seal a couple times.
- Put the caulk tube in the gun. If it won’t fit, make sure you’ve got the pressure rod pulled all the way back.
- Put on your latex gloves.
- Squeeze some caulk out onto your scrap cardboard to get a sense of the bead size and flow rate at that hand pressure.
- Apply caulk to the joint. Start caulking at an end of the joint so that you’ll be pulling the gun towards you. Squeeze at a constant pressure and pull steadily. This is something you just have to practice. But a few tips:
- In general, you’ll have better results if you move a little more quickly than you might think.
- If you notice an area that you missed or that has a gap, apply a dab of caulk to that area before the next step. Don’t assume that you’ll fill all those imperfections naturally when tooling.
- Depending your caulk gun, you might need to press the release trigger on the back after each “pull” to keep it from dripping.
- Tool the joint. “Tool” is a funny word, because you’re just going to use your finger. Once you’ve completely filled the joint with the gun, run your finger steadily from one end of the joint to the other to create the curved profile in the illustrations at the beginning of article. Again, practice is key, but a few more tips here:
- Make sure your glove is pulled tight to your finger.
- Make sure your finger is angled so that it’s making contact with the material at each edge of the joint—i.e., the two points where you want the caulk to stop.
- If you’ve applied too much, you’ll start building up a “snowball” of extra caulk as you’re tooling. As soon as you notice this, stop and clean your finger off on your scrap cardboard. Otherwise you’ll smear caulk everywhere.
- If you applied tape, remove it. Don’t wait until the caulk cures.
- If you really made a mess, clean up with a rag and mineral spirits.
- Stop touching the caulk. For silicones, you’ve only got around 5 minutes of working time before it starts to cure. After that, leave it alone—and absolutely don’t add more caulk! If you really goofed up, just wait until it cures and you can peel the whole joint out and start over.
As they say, practice makes perfect. Definitely make sure your first air sealing job with silicone caulk is in a space like an attic or basement where no one will see it but you. As discussed in the articles on how to air seal, you’ve got enough work to do in those spaces that by the time you need to caulk something visible in your living room, you’ll have your technique down.
Tips for Avoiding Common Problems with Caulk
Old caulk tube won’t dispense. Chances are, your caulk has cured inside the tube and is blocking the tip. With silicone, you might be able to squeeze the hardened caulk through the tip and pull it out with your fingers or a pair of pliers. You might also be able to sort of “drag” it out with the tip of an awl, nail, or similar tool, though you will almost certainly widen the tip of the tube in the process.
You may not be able to prevent this completely, but in the future make sure to seal your caulk tubes after each session. If your tube came with a cap, but it back on right away to keep the remaining caulk from curing in the tube. (Save those caps when you finish a tube, because many don’t come with them and they’re super handy.) If you don’t have a cap, wrap some painter’s tape around the opening at the tip.
Caulk forms bubbles while curing. This can happen when you apply silicone caulks to surfaces that aren’t completely dry. It’s not necessarily a problem if it looks like you still have a good seal, but in the future make sure to let the materials dry, especially if you’ve just cleaned them as part of surface prep.
Caulk profile gets messed up after removing painter’s tape. If you smear caulk too far onto the tape when tooling the joint, you’ll pull off too much caulk when you remove the tape and ruin your perfect curved profile. (This is more likely to happen if you apply too much in the first place.) Ideally, you want your tooling finger to make contact exactly at the edge of the tape on each side of the joint, so that the caulk tapers down to zero thickness at those two points. That way when you pull the tape up, you’re not pulling up a part of your joint.
Caulk won’t cure at all. The cure time of your caulk will vary based on joint size and environmental conditions. But if your caulk is still tacky or liquidy after 24 hours, check to make sure the tube isn’t expired. I’ve learned the hard way that expiration dates on silicone caulk are real. No, they don’t go bad the day after the date printed on the tube, but use with caution after that. I always recommend keeping a couple tubes of silicone caulk on hand, but don’t stock up on more than you’re confident you’ll use in the next year or so.
Next up, we’ll talk about your other major option for air sealing gaps, cracks, and joints in your old home: spray foam.