i’ve got a second floor rec room with cathedral ceiling, r-30 fiberglass in the rafter cavity propervent the entire way from soffit to ridge vent there is a 5′ crawl space at each side v-match boards on the interior. the roof in question faces north and condensation is forming as frost on the inside of the rafter space . Now this wouldn’t be a big deal but with a big tempature swing the frost melts and collects and runs through the insulation into the room and down the inside wall of the garage below which is a heated space as well ( woodstove ). I don’t know if this is enough info but any ideas wood be great. hopfully i can figure this out without ripping the inside apart completely
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Sounds like the mistake is the Propavent the whole way up. You're not getting any ventillation of the attic space that way.
Also, it's not clear what you have for a vapor barrier.
It is a cathedral ceiling so there is no attic space to be vented
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There is the "crawl space", whatever that is.The description leaves one wanting, to say the least.
If your view never changes you're following the wrong leader
My guess is there's no vapor barrier above the v-matched board ceiling. Am I right?
If so, you need to tear down the ceiling, install a 6mil poly VB and re-install.
Solar & Super-Insulated Healthy Homes
Riversong,
Assuming that the ceiling is as he described --boards on the ceiling as the interior finish, with the boards nailed to the rafters -- then vapor diffusion has nothing to do with it. This is not a "vapor barrier" problem. It's an air barrier problem. It can be solved by removing the boards and installing any solid material under the boards -- either drywall or 6-mil poly -- and then reinstalling the boards. It's a classic scenario, this problem.
Here's the proof that it is an air barrier problem, not a vapor barrier problem: if you think it is a vapor barrier problem, try solving it by covering the boards with 4 layers of polyurethane. Now you have a great vapor retarder -- you've stopped diffusion. The problem is, you haven't stopped air movement through the cracks between the boards. That's how the moisture is entering the rafter bays -- it's piggybacking with exfiltrating air.
This is not a "vapor barrier" problem. It's an air barrier problem.
I couldn't agree more, and I've gone into this in length in other threads. 99% of building envelope moisture problems in a heating climate are due to exfiltration of moist air - 1% is due to vapor diffusion (in actual full-scale tests at Univ of Ill).
The proper name for the 6mil poly that most builders use is air/vapor barrier - I was just trying to keep it simple - and the air/vapor barrier must be immediately inside the insulated cavity and must be continous throughout the envelope.
I don't use poly at all. I use the air-tight drywall system, which serves the same purpose and IMHO is a simpler and more effective way to get a continuous air/vapor barrier.
But, in the case of the OP, it seems that what's missing is the barrier membrane above the wood ceiling and possibly the knee walls as well. Additionally, there might be air/moisture leaking into the kneewall area from the ceiling below or from ductwork.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 1/31/2008 1:08 pm ET by Riversong
Robert,What do you think of trying to retrofit DP cells and skipping the VB in this instance? I've been of the mind that, while not perfect, DP cells stops enough of the air movement as to be an effectively good enough air barrier. Airtight drywall is really only there to keep the inspectors happy.Has there been any testing on how much air exfiltration DP cells can mitigate? Steve
He's got a problem of how to gain access, I believe.
If your view never changes you're following the wrong leader
please elaborate , what is a dp cell, and would that eliminate the tearing out of the v-match boards.
Dense pack cellulose.
If your view never changes you're following the wrong leader
thanks dan idon't believe that will help me in this situation though
You need to do somne studying to understand how denspacked cellulose works then.
It would help you a lot.
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What do you think of trying to retrofit DP cells and skipping the VB in this instance?
"Cellulose insulation has been installed in thousands of homes and buildings successfully since 1946 without vapor barriers. The hygroscopic nature of the cellulose means that moisture can slowly diffuse through the material without accumulation or damage."
- Technical Manager, Bill Hulstrunk at [email protected]
Has there been any testing on how much air exfiltration DP cells can mitigate?
The following test seems to indicate no air movement by temperature gradient though even loose-fill cellulose.
"Fiberglass dry-applied loose-fill insulation was installed at R-19 in the Large Scale Climate Simulator at Oak Ridge National Laboratory. The R-value of the fiberglass insulation was measured at various attic cold air temperatures. With the metering chamber temperature held at a constant 70oF, measurable convective heat loss began at an attic temperature of 30oF. A 40% to 50% loss of R-value occurred at cold temperatures. An identical test was run in the Large Scale Climate Simulator at Oak Ridge using dry-applied loose-fill cellulose insulation, also installed at R-19. The measured R-value of the cellulose insulation actually increased from R-18 at 40oF to R-20.3 at -18oF."
- CIMA (Cellulose Insulation Manufacturers Association)
You don't say whether there are soffit vents or not. with the vent channels to carry moisture out, that and the ridge vent should work, if the ridge vent is open
The other thing unclear to me - is this crawl space behind a kneewall? is there a continuous tight vapour barrier in the entire ceiling including in this space? That would stop moisture from even getting into the space you are trying to vent.
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I'm sorry if my discription was not enough for some, but it seems some of you have hit it right on the head there isn't any air barrior and that has to be my issue. I have finished rooms like this ,insulation wise , but installed sheetrock over it not boards and haven't had any issues. It sounds like the 6 mil poly is going to be my answer. I am going to pull the insulation out of the crawl space portion of the rafters near the soffit end and insulate the kneewall to see if that helps for the rest of the winter months then tackle the problem in the spring. Any other suggestions would be great thanks for your input.