I built an addittion with the in-floor heat,pured gyp-crete to cover all the tubing,we have 2-10 joist,with 9.5in. insulation. we installed tyvek to the joists, then covered with .5in. plywood to keep roadents out. the problem is…. sweating between tyvek and insulation. is there something I can do? this is a building on piers in the north-east.
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I assume you are talking about fiberglass insulation.
Did it have a vapor barrier (the paper facing)??
If so, is the vapor barrier facing up???
If the tyvek and plywood are already up, how did you discover the water???
Rich Beckman
Another day, another tool.
Edited 1/11/2005 9:18 am ET by Rich Beckman
sorry I missed a-day,but yes ihave kraft-faced ins.and it is facing up toward living space. The reason I found the problem was I had to add a gas line for a free standing gas fire place. So I unsrewed a sheet of ply ,get the line through. We left the joist open after the pour for about a week then closed it in.
I'm curious about the intended reason for the tyvek and what sort of space is the room under it all. If a crawl space, is moisture being controled there?
Trying in my mind to distinquish the source of the moisture. Could be some offweepiong rom the gyprockuncured srtill or could be from ground water in crawl getting up to it, or possibly things happened too fast closing it all up and the joists were still damp
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"...what sort of space is the room under it all"LOL!! I read "this is a building on piers" and just assumed it was open at the sides. But of course, that is not a safe assumption. Silly of me.Rich BeckmanAnother day, another tool.
The cold must have all of us indoors today.Yeah, even after you pointed that out, I had to read it three times before I saw that line. Gotta get my atttention span adjusted next week.All I can do so far is guess, and my best guess is that the building got some rain to keep the joists wet, then wetted again with the gyp pour and they were not dry when they got covered over- er - under, trapping moisture.Whether there is tarpaper over the subfloor and under the gyp is a potential issue too. Amnd when the joists got closed in. Imagine if they were insulated and closed before the gyp pour happened...
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Your problem is that the air from inside the home is condensing on the back of the plywood the same as it can do on an exterior wall. Install rigid foam under the floor joists, 1 " would probably enough, tape and seal your joints. You may want to install plywood on top of the rigid to protect it.
how do you know that is what's causing it?
It is only one of many possible reasons.
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I'm am just giving my best guess, but I'm pretty sure the moisture is coming from inside the house and the temp. is at or below the dew point somewhere in the floor cavity and the moisture is condensing on the backside of the plywood. Moisture moves from warm to cold and given the time of year it makes sense to me.
The reasoning is good and the possibility is there. You expresed it so confidently, that i thought I might have missed something. WE need a bit more info from the originator of the thread tho.
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If he installs rigid foam and tapes all joints, as you recommend, he WILL be in trouble in any kind of heating climate. Rigid foam, sealed, is a very good vapor barrier, and will be on exactly the wrong side. VB in a heating climate must be on the inner third (towards the warm side) of any wall (or ceiling or floor).Maybe I'm missing something here. Excuse me if I am. Would not be the first time.Alan
Theoretically true, but in a floor, it doesn't necessarily work that way. radiant heat dries down thru the floor but comnvection currents ( which is what carries the moisture) goes the other way, and the ground under it is going to be moist, though dew points are running around in circles there.
Short of it - i wouldn't really hold to a hard and fast rule for floors as for walls and cielings
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Well, no offense, but that sounds a little voo doo to me.I always thought that when I was stuffing walls (or floors) with insulation, I was stopping convection from occuring.And why would it work differently in a floor as compared to a wall? (or a ceiling)?
Not voo doo, real practical stuffFirst place, stuffed FG insulation has a lot of convection running through it. Y ou need blown in cells or BIBBs tight or foamed in place insulation to stop convection dead. FG batts just slows it down.Then in a cieling or wall the movement is directly to the oputside with heat, radiant and convection, driving it, so you definitely want the VB on the inside to keep the moisture from getting in the cavity in the first place. In the floor you have radiant driving heat down, but convection doing its best to haul cool air up, and moisture with it since the underside is more moist in many situations than the inside is. The currents and forces all compete with each other, so there is no hard and fast rule in real life for the floors as far as which side to place VB on so i would rahter neither.
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In that case, I am betting that the moisture is an original problem. Things were definitely not dry when you closed it up to begin with. You trapped the moisture and it is still trying to dry, but is slowed by the tyvek and ply. Neither of these are a perfect VB, but will slow the vapour transmission.
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