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Shrinkage: (How PVC Trim Moves)—7 Minutes of BS (Building Science)

PVC trim expands and contracts with temperature. But it never expands as much as it does that first time, so install it tight and fasten it right
July 11, 2022

Today's guest is David Parker, Director of Exterior applications with AZEK Building Products who explains how PVC trim moves, how to predict that movement, and how to control it with installation techniques.

Well, that was the agreement, Dave, but here, I’ll define it and if you could expand on it, that’d be swell.

Shrinkage is movement in the negative direction as a result of some kind of environmental condition like moisture or temperature.

Alright, well, let's start by talking about movement and the differences between wood and PVC. One of the things that guys don’t understand is that 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.

While both wood and PVC move seasonally, and both get bigger in the summer and smaller in the winter, the mechanisms driving their movement are different.

Summer is more humid, so wood expands.

Summer is also hotter, so PVC elongates.

Problems happen when people try to predict the future movement by altering their installation process...

What people typically run into with PVC is that they install it with gaps, so it’s going to expand in the summer and it’s going to shrink back down in the wintertime. And what they run into is that the first season it does exactly that. It expands and in that summer and then in the winter it contracts, as would be expected with any PVC—and most building products, for that matter—and then the next summer it never comes back to full length again.

And what happens is that, with any plastic material, there are fibers in any PVC product that relax, and when that relaxation in the PVC causes it to shrink…



Yeah, right.

Understanding how PVC moves is key to understanding how to control PVC expansion and contraction

Just because the PVC fibers relax, doesn't mean the people around them are relaxed, though. Because people install it wrong, even though they're trying to do it right. And because they install it wrong, their customers get unhappy. And then they get unhappy. And the PVC trim company gets unhappy. And it is a bummer all around.

So it matters how you install it if you want to get it right.

We can control the among of movement, the amount of shrinkage, and the size of gaps via how we install the product.

We can begin by measuring and fastening it correctly to help control seasonal movement.

The second part of it, where people make a lot of mistakes is painting. With PVC products, the higher the heat-build, the more shrinkage you could have down the road.

So when we talk about why does this matter and how do we control this through installation? And if we are going to paint PVC something other than white, what is the right paint to use?

Even if you install it perfectly, the painter can mess you up. Typical painters, right?

The key to controlling PVC movement is in the fastening schedule

So plan your strategy to minimize temperature fluctuations, and bring the painter into the tent.

The first part of it is installation. One of the misconceptions is that for every 18 feet, you need a 1/8-inch at the ends when it;’s cold outside.

Don't do that.

We actually don't want you to do that.


What we want to do, is understand that when we’re installing the product, we have to manage and understand where there could potentially be a ⅛-inch gap at the end of a longer run, and we want to plan the installation accordingly.

What he’s saying is that there will be small gaps eventually which will appear in the winter when your customers are not sitting on the deck, staring at the side of their house.

So that those gaps will land on a vertical seam, so if we’re talking about band boards at the bottom of a wall, we want to glue anything in the field tightly, and then when we get to the ends where the band board is butting into the trim, that we’re allowing those to move slightly with the changes in the seasons.

It’s important to manage the expansion and contraction at the end of the runs.

Gaps look normal when they appear at joints, like where a band board meets a corner board. They look terrible where they’re unexpected—like in the middle of the band board.

So it’s important to glue the scarf joints in the field, fasten the board like crazy, and let the movement occur at the ends.

One of the great things about PVC is that if we fasten the boards correctly, we can control the movement and also control the amount of shrinkage in the product by fastening it correctly.

So when that product gets heated up, through the first cycle, it actually relaxes into place with the fasteners holding it.

You can paint PVC trim dark, but you must use the right paint

The other thing about it is the color they want you to paint it. Dark colors absorb more heat than light colors, so if you don’t use the right kind of paint, you can get into hot water.

When painting PVC, we always want to look at the LRV

Light Reflectance Value. How much sunlight, and therefore heat, that color reflects. You can usually find it on paint chips or color swatches on the paint company website.

As long as that LRV is above 55, with 100% acrylic latex paint, you’re good to go., you won’t get excessive heat build.

So, for lighter colors with LRV above 55, you can use standard high-quality acrylic latex exterior paint.

That doesn’t knock darker colors out, it just means you need to use different paint.

Sherwin-Williams has a vinyl-safe color, palette. That color palette’s got darker colors with lower LRVs. Those colors have been tested and they’ve measured the heat-build and they understand it’s a safe heat build for PVC products.

Benjamin Moore also has a vinyl-safe palette, as do other major paint manufacturers, like PPG

And the other place you can go with painting PVC, is there are multiple prefinishers in the market across the country. That use heat-reflective or solar-reflective technologies in their paints.

These paints can significantly reduce the heat-build, particularly in dark custom-type colors. There are even some technologies today that you can paint PVC black.

Again, painting is really important to minimize the amount of heat-build which minimizes the amount of movement, minimizes the amount of shrinkage, and really what you end up with is manageable expansion and contraction, manageable movement—via proper installation and proper paint should you choose to go with a darker color.

So the Dark side of PVC trim is really just special paints that reduce heat buildup. But control begins with installation. Install the ends tightly, understanding that shrinkage will occur there. Don't glue the ends, but do glue the scarf joints.

Fasten it like crazy, or at least according to the manufacturer's specifications, because fasteners prevent the trim from bowing when it expands. Small gaps will occur during winter and they will mostly close up in summer. This will be a normal repetition.

And that becomes the normal movement of that PVC over time. So it's really just that first season that we need to get through.

Just like we got through this latest 7 minutes of BS. We’d like to thank AZEK for lending us Dave Parker for a little while and thank you for listening in. Remember, you get paid for what you do and what you know. Now you know more. 7 Minutes of BS is a production of the SGC Horizon Media Network.

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