Wet insulation, to wrap or not to wrap..

[AdironDoc]

As I was working on my cabin project a couple of weeks back I noticed the back of my fiberglass batts were wet. Wood planks have not gone up yet, but exterior walls are finished with MDF, tarpaper, board and batten, and sealed with Sikkens. It's my understanding that the vapor barrier backing on the insulation is usually all that's called for. Nevertheless, after a few days of firing up the wood stove to heat while we were working, I noticed all the windows are dripping with condensation, and when I reached behind, the exterior facing insulation felt wet.

My guess is that, like a house, the warm humid inside air condenses against the colder exterior wall but that the craft paper backing is a fair enough barrier to prevent much airflow. My concern is that unlike a house which is kept warm for the most part, cabins are regularly cycled between temp matching the outside winter air, then warmed, only to be left to cool again, and again, for perhaps hundreds of cycles. This must certainly create a unique situation and make moisture a greater challenge. I'm considering using a roll of 4 mil 10ft plastic sheeting I got and cover the walls before the pine planks go up. Since this would differ from routing building practices, I wonder how you all felt about whether it's necessary or even helpful vs harmful and second, whether I should do it while the current moisture is still within the walls.

Wishing my all my friends here a blessed holiday,

Doc

[MountainDon]

1. I hope the reference to MDF is an error, that you meant OSB or something. MDF does have some places where it is a good material to use, but exterior wall sheathing is not one of them.

2. I wonder where all the interior moisture is coming from? We have never had any condensation on our windows in our cabin. Not once, not even when we keep a pot of water on the wood stove during the winter. We do get condensation on the outer storm door glass if we leave the inner exterior door open, but I expect that as the storm door is only single pane. The windows are vinyl framed, dual pane, low-e; not even a drop of condensation on the frames. Our climate is probably drier than yours though. My eyeglasses though get an instant hit of condensation when I go in from the cold exterior. The humidity level was around 50% when we were there last with 80 to 99% outside when it was snowing.

3. The kraft paper facing is properly called a vapor retarder IIRC. Since you are in a cold climate the installation of a true vapor barrier (poly sheet) would likely be a help. But search the building science website for loads of info and recommendations of vapor barriers and differing climates first.

If you were doing drywall vapor barriers can be created with the application of a couple of coats of special paint. With wood planking on the interior a good poly barrier is likely called for in you climate.

4. Since finishing the walls on inside and outside I have cut three holes through the walls for various things. No traces of any moisture was found within the wall cavity. Our cabin goes through many heat and cool off cycles and we continue through the winter months.

[AdironDoc]

Thanks, Don. You're right about my error, OSB, not MDF. The moisture could be a result of all the logs and interior wood not having been as dry as I thought, but more likely, it's moisture from the basement. As you recall, my PWF has crushed gravel over a plastic sheeting. Before the roof went on, there were some days of heavy rains and I'm sure plenty of water ended up in that gravel. When I'm up next, I'll keep a dehumidifier running and maybe leave the doors and windows open a few days.

I think that unless there is a good reason not to, I'll be putting up the plastic sheeting. Was worried that I may trap the existing moisture inside the wall forever but I'll first give it some time to dry out.

Cheers

[MountainDon]

The use of building paper, not Tyvek or similar, should aid in any moisture being "wicked" out to the exterior.

[Arky217]

If you were doing drywall vapor barriers can be created with the application of a couple of coats of special paint.

MtnDon,

Could you elaborate on just what type of paint you're referring to as opposed to typical indoor latex.

Thanks,
Arky

[MountainDon]

Most paint manufacturers offer primers with enhanced perm ratings. (the lower the number the better) I have used Benjamin Moore's Super Spec Latex Vapor Barrier Primer Sealer 260. (had to look at the can). Perm of less than 0.5 whereas standard latex primer is rated about 7.0.  Kraft paper is rated at 1.0

[Arky217]

Most paint manufacturers offer primers with enhanced perm ratings. (the lower the number the better) I have used Benjamin Moore's Super Spec Latex Vapor Barrier Primer Sealer 260. (had to look at the can). Perm of less than 0.5 whereas standard latex primer is rated about 7.0.  Kraft paper is rated at 1.0

Thanks MtnDon,

My wall, from outside to inside, is vinyl siding/Zip wall sheathing (membrane coated)/unfaced Roxul insulation batt/drywall.

For this type of wall construction, and since the Zip panel is a type of OSB, meaning that the wall should be able to dry inwardly if it needs to, I wonder if I should use the 0.5 rated primer or should I use just the regular 7.0 rated primer ?

Thanks,
Arky

[MountainDon]

Hmmm...

Since Huber states:  Specially engineered to allow moisture vapor to travel through the panel, allowing walls to breathe and dry out.  it would seem that whether or not to use an intyerior vapor barrier is still going to be a decision based on the type of climate. Cold vs hot'n'humid for example. I didn't look any further than the ZIP brochure I had so am not sure if Huber has any info on that.