I just took out some fiberglass insulation from a couple of my walls. On the plywood sheathing behind most of insulation, there is a layer of frost. I don’t think I have any leaks or holes in the sheathing, and I have a good layer of housewrap and (fiber cement) siding over the sheathing.
What does the presence of frost mean, and what should I do about it?
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Replies
It means you don't have a satisfactory vapor barrier (eg, plastic) on the INSIDE.
happy?
Does the kraft paper on the insulation count as a vapor barrier?
Up to now, I've had the insulation stuffed into the wall cavities until I get through my electrical and framing inspections. I know that I wasn't very careful about making sure that I had everything sealed, but it seems like to some extent I'll always have the same problem around electrical outlets, etc.
> Does the kraft paper on the insulation count as a vapor barrier?It doesn't count very much, depending on where you live. And to be effective the tabs need to be stapled to the studs.Ask your BI what is required for a vapor barrier where you live.
If ignorance is bliss why aren't more people
happy?
It would be helpful if you updated your profile to show where you live.In a heating climate, you need a VB on the inside to keep the moisture out, but if you heat and cool about equal amounts, you'll run into problems with a inside VB in the summer. The VB thing is kind of a can of worms anyway, but without a location and climate, it's impossible to say what the right solution is.zak
OK, I'll update my profile, but I live right outside of Chicago.
This is worth taking a look at: http://buildingscience.com/housesthatwork/cold/chicago.htmIn his house, the sheathing is a vapor barrier, and the insulation dries to the inside. But in the article, he says the opposite is the norm: "In cold climates, walls are generally designed to dry to the exterior, with the vapor permeability of the exterior of the wall being 5 times more permeable than the interior; or, they are designed with insulating sheathing in order to control the temperature of the condensing surfaces."zak
Edited 2/20/2006 3:09 pm by zak
Yeah, you should have a vapor barrier on the inside. Talk to your building inspector's office to see what they require, and inquire about "usual and accepted" practices where you are.A plastic VB is the best approach, and some BIs may require that you slit the facing of the FG if you install plastic (though it's probably not really required).
If ignorance is bliss why aren't more people
happy?