Exterior Caulking Tips

A Painter’s Guide to Getting it Right

Despite the fact that caulking formulations have never been better, real-world caulking performance hasn’t kept pace with this increased potential – especially in exterior applications. Why is this? One of the major reasons for exterior caulking failure is poorly managed expansion/contraction issues.

Caulking and Joint Movement

If every caulkable joint were completely stable all the time, then successful exterior caulking would be simple. But that’s not the case. Caulkable joints, especially exterior ones, almost always expand and contract substantially with changes in temperature (and sometimes humidity), especially where different materials flank the joint. And unless you make allowances for this inevitable movement, your caulking will fail.

The first thing to understand in your quest for reliable caulking performance is also the strangest. The narrower the caulkable gap, the more difficult it is to get a long-lasting seal! Wider really is better, up to a point. Here’s why.

Let’s say you fill a 3/32”-wide gap between a wooden door frame and the surrounding bricks with polyurethane caulking. Even though this is one of the most flexible products on the market – able to expand and contract more than 50 per cent from its cured size without cracking apart – the joint will almost certainly fail in short order. The reason is that this 3/32-inch gap could easily get as large as 3/16-inch wide as the wood and brick move away from each other. The gap could also shrink to less than 1/16-inch during hot, humid weather when the wood expands. This amounts to a percentage change in joint width approaching 100 per cent – way beyond what average caulkings can handle. But the picture changes substantially if you start with, say, a 3/8-inch wide gap between brick and wood. In this case, the amount of absolute movement of materials on each side of the joint stays the same – about 3/32-inch. But as a percentage of the total width of the larger, 3/8-inch caulking bead, 3/32-inch now represents only 25 per cent – well within specs for premium polyurethane caulk.

Backer Rod and How to Use It

Although wide joints are good because they don’t overload the elasticity of caulking, they can also gobble up caulking way too fast if they’re too deep. That’s one reason there’s something called backer rod. These lengths of flexible foam — usually cylindrical in cross-section — are designed to be stuffed into gaps before caulking is applied. You’ll typically find backer rod in diameters ranging from 1/4” to 7/8”, though it does get as large as 4” in diameter. Uncompressed backer rod should be 25 per cent larger than the gap it fills (or the next size up), and stuffed far enough in so the depth of the gap is half of its width, down to a minimum of 1/4-inch deep and a maximum of 1/2-inch deep. Why the depth limitations? Besides reducing the amount of caulking you’ll use, limiting caulking depth with backer rod has to do with flexibility again. When the depth of a bead of caulking exceeds its width, it becomes more difficult for the product to stretch as much as it’s rated to. So, use backer rod if the gap is deep.

Bond Breaker Tape

This is all fine, as far as it goes, but how often do you have the luxury of determining the width of joint you need to caulk? As a painter, almost never. That’s why the people wearing white lab coats invented something called bond breaker tape. This thick, self-sticking tape, often made of polyethylene, is applied over or across under-width joints. The purpose of a bond breaker is to prevent adhesion of caulking in a specific area. It spreads the joint movement over a wider area of caulking, resulting in a less crack-prone joint.

Beware the Three-Way Bond

Something called a three-way bond is another thing that can cause caulking to fail. When caulking is installed so it sticks to both sides of a groove and to the bottom – three way adhesion – the resiliency of the caulking is greatly reduced. Adhesion at the bottom of the groove prevents full elasticity by impeding the ability of the cured caulking to get thinner as it stretches with increased gap width. When this happens, something has to give, and it’s usually the bond between the caulking and the side of the gap being filled.

Using backer rod or bond breaker tape is the best way to prevent a three-way bond. The main thing is that the bottom of a caulkable gap should never be touched by caulking. If the gap is deeper than 1/4- inch, or half the width of the gap, then backer rod should be stuffed into place. This both breaks any potential bottom bond (caulking never sticks firmly to backer rod), and it fills excess gap depth so you don’t use tons of caulking. Even if the gap is no deeper than necessary, bond breaker tape should be installed along the bottom to stop bonding there.

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Dave Wenners has been a residential and light commercial painter since 1988, but lately he’s been bragging about a tighter…

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