Moisture barrier if spray foamed?

9 years ago

Hello all,
I'm not sure where this question should be posed so I started here.

Here's the skinny:
Modern home built in 2010
Flat roof (obviously some slope)
Spray foam insulation for all walls except 3 walls in basement which are ICFs

We have experienced some ceiling cracking that no one can find a culprit for (not the normal settling)--they've decided it is in the middle of the home where the center metal beam is where the weight is being pulled. Not sure I am with them, but ok.

We had the crack fixed by a great drywall company and when they opened up the ceiling, they were surprised to find the plastic moisture barrier with spray foam.

Is this common practice? Could it be bad for the "breathability" of the home??

Any insight would be much appreciated!
Heather

Comments (7)

renovator8
9 years ago
Where is the barrier, what is it, what is the foam, what is the thickness, where is it and what is the climate?

It is unusual for a vapor retarder to be on the warm side of a roof-ceiling assembly but you have not told us anything about the design.

As for the cracks start with the nature of the roof structure and the loading (snow?).
Annie Deighnaugh
9 years ago
It depends if it is closed cell or open cell...my understanding is closed cell is waterproof and needs no vapor barrier...not so for open cell.
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Brian_Knight
9 years ago
Oh boy! This is a tricky issue that could get quite technical. Renovator and Annie asked the necessary questions and its difficult to answer without knowing the details.

A better term for plastic vapor barrier is vapor retarder and if you search "breathable walls" you might see a link on eliminating that terminology in our discussions of walls and buildings. Hopefully your home features some type of mechanical ventilation introducing outdoor air for Indoor Air Quality.

2" Closed Cell meets the vapor retarder definitions while Open cell does not in any usable thicknesses. That doesnt mean you want to include a polyethelyne vapor retarder in your assembly. Generally, plastic vapor retarders are only recommended in the coldest of climates.

Here is a link that might be useful: BSC link, Understanding Vapor Barriers
satchel14
Original Author
9 years ago
Sorry for the lack of info--I was afraid of getting too wordy :)

Open cell insulation
6" of foam in walls
8" minimum in the ceiling

The vapor barrier is on the inside of all drywall. Masked off with the red construction tape.

I live in Minnesota where it is humid and we have a lot of snow.

And NO, the cracks are not from snow load. We had 22 inches in a weekend our first winter and nothing. The house is basically a rectangle box. The center beam that supports the house is in the direct middle of the house. This is where the cracks (one on each side of the beam--aka, each side of drywall covering it) first occurred. We had a drywalled in and he installed a control joint in that space to alleviate the push and pull.....now we have one 15' away, so.....

They seem to have happened in the fall and spring when the humidity values change and so when the drywalled mentioned that he hadn't seen plastic done on spray foam before, I wanted to check more.

Thank you for your help so far guys!
H
Annie Deighnaugh
9 years ago
Understand that I know nothing, but it seems to me that cracks on either side of the central beam need more than just drywalling. I'd want to make sure the building was structurally sound. A buddy bought a house and found that the building wasn't sound as beams on the first floor did not line up with the posts in the basement. A GF is in a 20 yr old condo unit and just had to have her and every model that was the same shored up because of structural deficiencies in the design that was allowing the cathedral ceiling to come down and the walls to separate. (Thank goodness one resident was friends with an architect who noticed the building was failing!) It happens. I hope it's not happening to you, but it can't hurt to check.
satchel14
Original Author
9 years ago
Hi Annie,
I certainly hope that isn't happening and I honestly don't think it is. When the cracks first started happening, the architect firm was brought in, a drywall guy from the drywall company, the builder, an engineer, a guy from the trust manufacturer and even a paint guy from Benjamin Moore. They all concluded it was most likely the point in the house where the push pull is happening.

My point to all of this is, it happens in spring and fall (meaning, the cracks reappear) and that is usually when the humidity and air pressure changes. Add that to the drywalled that we hired mentioned he hadn't seen a vapor barrier with spray foam before and my mind went wandering. I sought out my best resource, HERE, to see if anyone had heard anything differently.

Thanks again!
Brian_Knight
9 years ago
I guess my main question is why do you only have R29 (R3.6*8") in your MN roof? Thats way below code for your area. Not picking just want others to be aware of this issue.

I think you are far enough north to be ok with the plastic vapor retarder although it would have been better to omit it and use a vapor retarder paint on the drywall.