Making Sense of Caulks and Sealants

Caulks and sealants. They all look like they do the same thing, don't they? But in reality they're all quite different, which can make choosing the right one difficult and confusing. And all that flashy packaging that promises you the world makes the task even more difficult.

But the right information can make your decision and your repairs a lot easier. And the repairs will last a lot longer, too.

You can find most of this information on the back of the products themselves. But you'll also need information you won't find on the packaging, no matter how hard you look.

Caulk or Sealant?

So what's the difference between a "caulk" and a "sealant"? None, really. Manufacturers tend to use "caulk" (an old boat-building term) for their base-level products, and "sealant" (a home building term) for their high-performance products. But the two terms generally mean the same thing -- a product that seals gaps in building materials to keep out air and moisture.

But each type of caulk/sealant has a unique set of qualities, and it's important to choose the right one for the job. If you don't, you could have a disaster on your hands.

Why Caulk Fails

Caulking failures fall into one of three categories: adhesive, cohesive or substrate. For instance, the caulk can fail to bond to the base (substrate), the caulk can tear, or the base can break.

But when a caulked joint fails, it's usually because either the base wasn't prepared properly or the wrong sealant was used.

How to Choose the Right Caulk

The first step is to examine the joint and work out what traits the caulk needs to have. Is there considerable movement? Is the gap fairly wide -- say, more than half an inch at the midpoint? (If it is, you really shouldn't fill it with caulk.)

What materials are you sealing together? For a gap between a vinyl window and wood clapboard you'll need a caulk that adheres well to both materials. It also needs to be flexible and water-resistant. If you have a gap between glass or tile you can use silicone, provided you're not trying to seal them to wood.

Where is the actual gap? If you're sealing gaps around a sink or elsewhere in the kitchen or bathroom, you'll need caulk that's mildew-resistant. Otherwise, the mildew could degrade and eventually break down the product. These days, most caulks are mould and mildew resistant, but they can still stain once they're cured.

Need a special colour? No problem. You'll find a wide selection of coloured caulk to match your paint, tile, grout, and other masonry materials. Can't find the right colour? Just mix your own.

Caulk and the Elements

Temperature, humidity and moisture can greatly affect how you apply sealants and how they perform once cured. For example, water-based caulks can stand up to the weather once they've cured, provided they've had a few days of perfect weather to set.

Forget about polyurethanes -- they don't work in the cold weather because they become thick and extremely sticky. Opt for silicone instead.

Water-Based Caulks

This type of caulk now works just as well as silicone, and is by far the easiest type to work with. These varieties (e.g. vinyl and acrylic latex) are thin and smooth, so they're easy to apply and tool. There's no harsh odour; they're non-toxic, and you can clean them up easily with water.

While water-based products come in everything from painter's caulk to elastomeric sealant, they work with most common building materials, and can be used just about anywhere. Some are flexible. Others are weather resistant (ideal for exterior joints) or designed specifically for interior joints. And most water-based sealants can be easily painted over to give you look you're after. Just make sure you choose the right one for the job.

Latex and other water-based sealants are often the best choice when working on interior projects. Latex caulk is ideal for sealing interior window and door casings, as well as baseboards and mouldings. But for the outsides and other exterior joins, look for high-performance water-based caulks.

You can apply water-based products to moist surfaces, but you need to do it under ideal conditions. You'll also need some warm, dry weather, so the product can cure properly. Otherwise, the moisture will just wash the caulk away. And keep an eye on the humidity -- the more humid it is, the longer the products will take to cure.

Water-based spray foam sealants are another option. They're soft and spongy, so they're not as durable as polyurethanes. They're not water resistant either, and have a slightly lower R-value than polyurethanes (R-4). But they expand less, so they're often easier for people to use.

Polyurethane Sealants

Ideal for high-traffic areas, polyurethane sealants are the only type that can withstand abrasion. Unfortunately, they're also stringy, odorous, toxic and a health hazard. So, despite being paintable, flexible, and weather resistant, they're really unsuitable for most jobs. Thankfully, you can choose from dozens of substitutes that will work in most residential building situations.

So, where would you use polyurethane? You may use it to fill gaps in a floor, driveway or garage, but make sure you wear the proper protective gear (respirator, rubber gloves, etc.) when applying it.

As mentioned earlier, polyurethane spray foams have a slightly higher R-value than water-based varieties (up to 4.5 per inch). They cure hard, but aren't quite as easy to apply as their water-based counterparts. (Polyurethane spray foam cures as it expands.) However, it is available in low, moderate, and high expanding formulas. (The low grade expands by up to 300% before curing.)

Silicone Caulk

Having been around for more than fifty years, silicone sealants have been developed for almost every situation. One reason they're so popular is they're the best option for non-porous materials such as metal, glass, tile, porcelain and areas exposed to cold weather. (It stays soft and flexible.)

You can clean up with a little soap and water, and they're non-toxic (even though some have a strong smell). Silicone is also inorganic, so it's mould-, mildew- and UV-resistant. You can apply it at almost any temperature, and it doesn't need a long curing time before it can resist weather and precipitation.

Unfortunately, silicone does have some downsides: It's tricky to gun and tool, doesn't adhere to wood very well, and once it's damaged, it loses its strength and tears easily. It also doesn't adhere to cured silicone, so you'll need to strip the old silicone away before reapplying it.

Synthetic Rubber Sealant

Synthetic rubber and modified-silicone polymer sealants seem to do it all. They're flexible, and return to their original form after being stretched, making them ideal for exterior joints that expand and contract. It doesn't take much pressure to get them to work either, so they're less likely to break their seal when moved. (They have a low modulus.)

These types of sealants adhere to most materials, resist mildew, and can be applied in wet and cold weather.

But don't use solvent-based rubber caulks indoors. They're flammable until cured, and contain a high amount of volatile organic compounds (VOCs). They also shrink considerably as they cure.

Synthetic rubber is one of the clearest caulks available on the market today, but can be painted easily with water-based paints. This makes them a popular choice for log homes, wood siding, wood roofs, and joints that move frequently.

Butyl Caulk

Need to fill a gap somewhere that gets a lot of water? Good news! Butyls caulks are water resistant. They stretch like chewing gum, but do NOT go back to their original shape or size. In fact, they look and behave a lot like tar (read: extremely messy). And Butyl doesn't harden, which is a good thing. Just remember that so you're not waiting for something that will never happen.

Butyl sealants are the most water-resistant type on the market, and because they never harden, they're perfect for joints where two materials overlap (shear joint), such as roof flashing and foundations located below grade.

Now you know exactly what sealant you need for your next project. So get to it!