I am replacing my 80 year-old rotten wood front porch columns with new 4 x 4 western red cedar posts on top of galvanized steel standoffs bolted to the concrete porch. The structural 4 x 4 posts will then be wrapped with AZEK PVC composite 1 x trim boards for appearance and weather protection. The non-tapered columns will measure 5.5″ x 5.5″ on the outside so there is about 1/4″ air space surrounding the 4 x 4’s inside. Should these columns be ventilated? I live in Virginia where humidity can be high.
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When you say 1x trim boards I think you mean 3/4" net. Why not just make the columns 5x5 with no space? Or buy Azek in full 1" dimension? No need to vent this. How did you intend to shim these anyway?
The two columns being replaced on the porch need to match the two half-columns against the house which are 5.5" wide. The AZEK boards are 3/4" thick and will be shimmed just like the old wood columns were shimmed. The old columns had small wood shims between the 4 X 4's and the column covering at the top and the bottom of the column.
I did similar for columns on my last house. I'm in Oregon - lots of moisture. I ran the wrapping boards 1/4" short of the ceiling, then put trim around them, again leaving a 1/4 gap. It looked fine visually and allowed for a small amount of venting. Was it required? I don't know. Was it a good precaution? I think so. I also left a gap at the bottom, where they came down onto stone pillars. That was more to keep the wood from the masonry, but it also allowed for some venting.
I've used Azak on my deck columns and Boral on my front porch columns. The Azek shrank in the winter here in Cincinnati way more than the Boral. I've resorted to caulking the gaps in the Azek. Not pretty.