Hi, I am a little puzzled by what must be RTFM question.
Working this week on 40 year old rustic Japanese summer
cabin. Uninsulated, no vapor barrier, no underlayment,
very damp open crawlspace year round.
Two rooms have floorboards that are cupped, slightly.
4 inch yellow cedar planks with urethane finish on the
topsides, and 8 inch redwood planks with oil base paint
on top.
I thought cupping was supposed to occur on the wet side
of the board. Seems to me these are cupped the wrong way.
Am I just dyslexic or something?
Replies
I've heard that cupping happens on the bark side, but I know that panels warp when one side gets damp and expands, so your idea of how the board should have cupped makes sense. Maybe was so damp it penetrated all the way through? I'm sure ithers will have more experience to tell you why.
If the finished top side is cupped, then you are seeing what happens when the dampness gets to the underside of the board and expands its surface more than the top.
If you had said the boards are reverse-cupped, i.e., their top centers are high, then I would not know how to respond.
When the moisture level is first increased it will affect the unfinished side and thus it will expand more than the finished side.
But if the ambient moisture level does not change (and there is nothing trying to dry out the other side) then the board will reach a uniform moisture level all the way across. At that point the direction of cupping will be determined by which is the bark side unless it is quater sawn (vertical grain).
A finish does not stop the exchange of moisture levels, it just slows it down.
That must be what it is. The space is never heated or
air conditioned, and is in fact the opposite of
airtight.
I am at a bit of a loss as to how to winterize this
cabin. I hear that some of the surrounding cabins with
vapor barriers (e.g. vinyl flooring) have serious
condensation problems and rot.
True. I've seen it more than once, a house not designed for resistance to heat loss gets retro fitted with a few pieces of insulation and no thought given to ventilation, moisutre, etc, and the whole house goes to h3ll right away.
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Some have advised me to just get a big heater and blast
it with BTUs during the couple of weeks of winter that it's
used.
But the summer sun on the hot tin roof in the open
atrium space is my biggest concern. I am going to try putting some
chunks of XPS foam up there between the rafters and keep an
eye on it. Not going to try and make it airtight, just a little
less leaky.