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How do I properly use Azek sheet goods as exterior trim?

Three years ago I had my 90 year old house redone. Recently we discovered a leaking door in an addition that was built onto the house. The addition was timber framed 16" on center using 1/2 inch exterior grade plywood as sheathing. 6 inches of spray foam insulation was used on all exterior walls the interior was finished with 1/2" drywall. Exterior walls were wrapped with Tyvek and 3/8" Azek was nailed to the sheathing seams were glued and perimiter was caulked. When we removed the Azek to repair the leak we found black mold on the interior surface of some of the Tyvek, we also found black mold on some of the sheathing and patches of rotted plywood under the Tyvek. Is this wall system properly designed? Can the sheathing properly handle the high humidity levels I encounter where I live?

Asked by Lawrence Reich
Posted Jun 11, 2014 6:55 PM ET


2 Answers

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I'm not sure from your description whether you used Azek for siding of just for trim.

It's hard to speculate without knowing more, but it's possible that your contractor used open-cell spray foam, which is vapor-permeable. This type of foam allows interior moisture to diffuse to the exterior of the wall assembly. Your plywood sheathing is cold during the winter, so it is a condensing surface where interior moisture can accumulate. The Azek is a vapor barrier, so it doesn't allow the wall assembly to dry to the exterior.

If the situation is as I have described, it shows the importance of using materials that can dry to the exterior. A ventilated rainscreen can go a long ways toward addressing this type of problem. For more information, see All About Rainscreens.

Answered by Martin Holladay
Posted Jun 12, 2014 5:04 AM ET


I use Azek, IQM and Royal PVC trim for windows, doors, frieze boards, corners, mud/splash boards, and facia. We have 3 weather sides on the south side of the Great Lakes, so wind, rain, snow come from SSW all away around to ENE. We never nail these products, we use predrilled countersunk screws. We have had one west side gable end facia Axek plank (3/4” X 12’) crack, on a mid plank cut line. It was slightly force fit and so cracked after several temperature and sun cycles. The Azek expands and contracts lengthwise as much as 3/8” so we scarf the joints and use color matched industrial grade PVC caulk on the seams to cover the expansion joints – but I see no reason to back glue any of these planks to the sheathing or the base planks (the PVC is not structural) – just the butt ends. Let it move.

Roof lines, drainage plains (WRB’s), flashing and drain holes are used to keep the trim and walls dry. And from taking aboard water on the backside of siding and trim. It’s important to use an assembly that allows any vulnerable locations (exposed to sideways driven rain for example) to shed during and evaporate after the weather. Back gluing 100% impermeable PVC lumber to sheathing or any based lumber will not allow it to dry (whether the source of moisture is exterior or interior).

Your sentence about gluing is confusing.

" ...Azek was nailed to the sheathing seams were glued..."

Looks like a word or two are missing. How can you glue to the sheathing if you a have a WRB? If you explain this detail, you can get better analysis here.

Answered by flitch plate
Posted Jun 12, 2014 8:42 AM ET

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