spray foam on house wrap?

[gogolf0401]

So I added a house wrap like tyvek on the outside of my pole building before the steel was put up. I was thinking of doing an inch or so of spray foam on the inside before putting up fiberglass roll insulation just to seal things up nice.
As far as I know house wrap is meant to keep water and air from penetrating the building, but still allow water vapor to escape the inside walls to the outside. This is to prevent mold/rot in the walls I think. I figure if I add the spray foam directly to the house wrap on the inside that I will end up diminishing the purpose of the house wrap since the foam will act as a water/air barrier and not allow water vapor on the inside of the walls to escape.

I know it sounds silly as why would I have water vapor on the inside of the walls if I have house wrap plus spray foam keeping water out.. but I imagine condensation would form just because of the inside to outside temperature difference.

Thoughts?

 

[soapii]

I have no issues with my house or garage with both spray foam and housewrap.

--Joe

 

[MBfreak]

If outside is colder than inside,( Which is ALWAYS the case at the 60th latitude where I live) the only vapor barrier is to be on the inside of the insulation ( yours will be humidity permeable I suppose?)

An outside ( cold side) vapor barrier will trap inside moisture in the permeable insulation leading to much reduced insulation and also risk of fungus growth.
As to having two vapor barriers, I have no experience. Maybe some southern gentleman can step up to the plate?

Best regards

Ola

 

[gogolf0401]

If outside is colder than inside,( Which is ALWAYS the case at the 60th latitude where I live) the only vapor barrier is to be on the inside of the insulation ( yours will be humidity permeable I suppose?)

An outside ( cold side) vapor barrier will trap inside moisture in the permeable insulation leading to much reduced insulation and also risk of fungus growth.
As to having two vapor barriers, I have no experience. Maybe some southern gentleman can step up to the plate?

Best regards

Ola

I was planning to add poly over the insulation as well.

Sent from my SM-G900V using Tapatalk

 

[kwoody51]

I was planning to add poly over the insulation as well.

Sent from my SM-G900V using Tapatalk

You don't want to do this.

Your vapor/ water has to have route to escape. If you have house wrap, spray foam (like another vapor barrier), batts and and then poly you've created a sandwich for the mositure and it will have no means to exit. With this method you have effectively stopped air and vapor from traveling into or out of the wall cavity. Generally this is a good thing but condensation can occur in the wall cavity and you want a means for this vapor to be diffused to dryer space, not stay and grow mildew/mold.

building science has some good articles on insulation and vapor barriers and the do's and don't.

 

[BruceMc]

I know it sounds silly as why would I have water vapor on the inside of the walls if I have house wrap plus spray foam keeping water out.. but I imagine condensation would form just because of the inside to outside temperature difference.

Thoughts?

The trick is to have enough insulation to move the dew point outside of the impermeable layer. There is a temperature gradient in the wall, and when moist air comes in contact with fiberglass insulation at the dewpoint, water will condense out and collect. Then you get into a vicious cycle - the more moisture, the less the wall insulates and the dew point keeps moving towards the inside. The traditional inside vapor is an attempt to keep moist air away from the dewpoint, but there is a trend to provide sufficient insulation to the outside of the whole wall assembly so that there is no surface for moisture to condense on. An example:

http://www.cchrc.org/sites/default/files/docs/REMOTE_Walls.pdf

 

[MBfreak]

Bruce MC.
Thanks for the interesting article. If it works in Alaska it should work here in Sweden too.
However, it is completely different from any approved method here nowadays
There was a Swedish system that used a supposedly impermeable spray-on layer that doubled as outside layer and color. It was used on quite a few residential midrises. After a couple of winter/summer cycles it started to fall of and revealed severe fungus growth behind.
Several mid-sized building companies refused to bid on jobs specing the new method. "One little problem and we will go bust" was their thinking.

Not in any way related to the design in your link except " the idea". It cost the builders here a very large bundle to put right, and I guess closed the window for rethinking on the subject for a good many years.

Ola