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How do I get rid of condensation in a cathedral ceiling?

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I just built a new home and I am having a problem with condensation dripping through the cathedral ceiling. The roof was constructed with 2x8 rafters, sheating and shingles. When I insulated the ceiling I used R-19 insulation thinking I would have something close to a two inch air space, that was not the case.

The insulation expanded and filled the cavity till no air could get through roof structure. I have plenty of soffit vents and a cobra ridge vent installed on the house but with the air cut off I guess it would't matter what kind of ventilation you had. I have pondered on this problem  for a few days and without having to tear out my new ceilings here is what I came up with.

With a little attic space on both the front and back of the house I could go in and pull the ceiling insulation down allowing me to see up towards the ridge, the only thing I could think of was that I would have to get something stuck through there to hold the insulation down and allow air to flow through. After trying several different things I came up with vinyl siding.

I could slide the siding over the insulation with ease and the weight of it held it down leaving me about a 2 inch air gap. With part of my ceiling removed I could see through to the soffit vents fairly easily. So I guess my question is do you think this will solve my problem or do I need something else?
asked in Attic Area by do (140 points)
edited by darekrudy
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1 Answer

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Hi Do,
This may or may not work. By placing a siding panel on top of the insulation you’re compressing it and lowering insulation’s thermal value. Decompressed insulation has air packets between the fibers which work as an insulator. 
Second problem is that by placing a siding panel on top of the insulation you’re adding a second moisture barrier / vapor retarder. Whatever the amount of warm air that was penetrating the ceiling, R-19 insulation layer, and condensing on the roof’s decking will be now condensing on the siding panels surface, soaking the insulation, and dripping onto the ceiling’s drywall.
One of the attic or cathedral ceiling insulation rules is to have only a single vapor retarder installed on the side of insulation facing the house / conditioned space. Never trap moisture absorbing insulation between two vapor retarders.
You can try your idea it if it doesn’t involve much labor and investment. In case it doesn’t perform well you can always take down the drywall, staple vent chutes between the rafters, and re-install insulation. A lot of work but I’m not aware of any other quick solutions to your problem…
answered by darekrudy (21,730 points)