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Old 12-12-2011, 03:10 PM   #1
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Foam board over batts


There is another post with a similar question going on but I did not want to hijack it, so here is my own.

Long story short I have a 2x6 rafter cathedral ceiling. The best route was to use rafter vents, batts, and then foam board over the rafters. I installed unfaced insulation after being told the foam board I was buying had a vapor barrier. It is polysheild sheathing/underlayment. The
Expanded Polystyrene (EPS) rigid insulation type. It has a foil like face on one side. After speaking with the manf. they said this does not act as a vapor barrier but I shouldn't install one...

This doesn't make sense to me. I was planning on gluing and taping the edges and thought that was the vapor barrier. If itís not why would I not install one over the boards?

I plan on running strapping and then drywall on top of the foam to finish the room.

Also located in MA.

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Old 12-12-2011, 05:23 PM   #2
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Foam board over batts


Foil facing most certainly is a vapor barrier (Class I vapor retarder).

Tape the seams and make it air tight and your are fine.

Do not install another vapor barrier over the foam. Face the foam into the space, tape and seal the seams, spray foam any larger gaps.

Make the ceiling air tight as well.

The necessity of a vapor barrier is often overstated and overvalued in many regions of the country/world. Air tightness, lack there of, is far more detrimental to moisture control.

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Old 12-13-2011, 10:32 AM   #3
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Foam board over batts


So here is the catch.. the words foil LIKE. It's not actually foil, its a plastic type of material that looks like foil.

According to the website "a reflective metalicized film facer"

Speaking with several more people at the manf. they had no clue how to answer me till I got a typical sales persons answer.

He said that its a type of plastic that hasn't been tested as a vapor barrier so cannot be called a vapor barrier, and that there was no way to know if ANY system would actually work till it was installed and inplace... I don't think just installing and ripping back out is typical method to find the right materials for an application.


At this point I had to move forward so its up, seams glued and taped so I guess time will tell.


I saw the link you posted about that in the other thread and have kept that in mind. I am guessing the key there is to keep nice tight seams where they drywall will but against each other?
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Old 12-13-2011, 01:12 PM   #4
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Foam board over batts


Foil or foil like, it is likely going to be a Class I vapor retarder (less than 0.1 perm) with any plastic or foil like covering.
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