Understanding How PVC Trim Moves

In “Shrinkage: How PVC Trim Moves,” an installment in ProTradeCraft.com’s building science audio series “7 Minutes of BS,” David Parker, director of exterior applications with AZEK Building Products, explains that PVC and wood move for different reasons: “PVC moves thermally, whereas a wood-based product moves with moisture. So, wood-based products typically swell, and they swell within their width and a little bit in their length. The difference with PVC is that PVC does not swell or move in its width, it expands and contracts with its length.”

The problem with PVC trim installs is that builders and remodelers who try to allow for that movement get it wrong, even though they’re trying to do it right.

“What people typically run into with PVC,” Parker says, “is that they install it with gaps.” This seems to make sense because, unless it’s a ridiculously hot day, the PVC will expand in the summer and shrink back down in the winter. “That the first season, it does exactly that,” Parker says. “It expands and they may have very few gaps at the ends of their boards that summer, and then that next winter it contracts, as would be expected with PVC—and most building products, for that matter.”

The surprise comes the second summer, when the PVC never comes back to full length again.

What happened?

“As with any plastic that goes through a heat cycle,” Parker explains, "there are fibers within the PVC product that relax, and that relaxation … causes it to shrink slightly.” It will expand again as the temperature rises, but after that first year it will always shrink back to a smaller length than it was when it was installed.

So What Now?

The first part of the solution is proper installation.

“One of the misconceptions is that for every 18 feet, you need a ⅛-inch gap at the ends when it’s cold outside,” Parker says. “We actually don’t want you to do that.” Instead, plan for the fact that the piece of PVC you are working may eventually end up a bit shorter than it is the day you install it. The key is proper fastening, proper treatment of field joints, and allowing for gaps at the ends of boards.

With PVC skirt boards, for example, gluing the scarf joints and driving fasteners on both sides will prevent a gap from opening up in that initial expansion-contraction cycle. The fasteners will keep the board from buckling as it expands, and the boards will stay tightly joined when they “relax.”

At the ends, use a detail that allows for some movement; ideally, it would also hide any gaps that develop or at least render them inconspicuous. Or with a rainscreen system, plan for a small seasonal gap, which may look better than a caulked joint.

Pay Attention to the Paint for PVC Trim

The second part of the solution has to do with painting PVC. Dark colors absorb more heat than light colors, which causes more heat build up. “The higher the heat-build, the more shrinkage you could have down the road,” Parker says.

Can you paint PVC trim? Yes, but pay attention to the LRV or Light Reflectance Value of the paint. As the name implies, it’s a measure of how much sunlight―and, therefore, heat―a particular paint color reflects. You can usually find it on paint chips or color swatches on the paint manufacturer’s website.

“As long as that LRV is above 55,” Parker says, “with 100% acrylic latex paint, you’re good to go, and you won’t get excessive heat build.” For lighter colors with LRV above 55, you can use standard high-quality acrylic latex exterior paint.

It’s still possible to use very dark colors, but you need to use paint specially formulated with heat-reflective or solar-reflective technologies. Parker notes that many manufacturers have a “vinyl-safe color palette” that has been tested and measured for heat-build.