Can condensation happen inside wood (without a surface)?
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If a rafter has parts that get below dew point, and the rafter is encased in closed cell foam on both sides and above (foam board is on top of the deck on top of the rafter), condensation will happen whether or not there is a surface at / below dew point. Right?
(Theoretical question. I know there should be ventilation or adequate insulation, and I know foam sandwiches are not recommended. I don’t need practical answers to this question, thanks.)
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Replies
The moisture has to have a path in. Closed cell foam is a vapor barrier. To the extent there is leakage, rafters tend to spend a lot more time hot than cold, and when hot the vapor drive out will be greater than the vapor drive in when cold.
"rafters tend to spend a lot more time hot than cold, and when hot the vapor drive out will be greater than the vapor drive in when cold"
North of Toronto, in climate zone 5, the rafters in a cathedral ceiling will in winter have warmth from the room heating, but the cold will be severe and long.
I hope vapor drive will save my me, but I'm guessing the warmth of the rafters from room heat in winter won't be enough to drive out moisture considering the rafters will also be cold from exterior temperatures. (There's no insulation above the roof deck, so no insulation above the rafters.) If the rafters have "sorption" in November or December, I'm guessing vapor drive out won't happen until May. If the rafters suffer from that, if they will still last 15 years, before the ceiling and roof and rafters need replacing, I'll stop worrying. Any guess whether they will last 15 years in those conditions?
Thanks
Even in winter the sun warms the roof. That's what I meant by them spending more time hot than cold.
Closed cell foam is not a vapor barrier, it's a vapor retarder, so some moisture will work its way into the wood even if it's fully encased, though foam slows the process down a lot. When the wood accumulates moisture and the temperature is below the dewpoint, it will "condense"--more accurately, accumulate via absorption and adsorption. That's why the most resilient assemblies have enough insulation on the exterior of the structure to keep it from falling below the dewpoint temperature.
Ecological,
Michael has it right, of course. If the lumber is cold enough, and moisture is available, the moisture content of the lumber will increase. But that isn't called condensation -- it's sorption.