Flash and Batt?

[DFB]

Looking for some quick insulating advice here job probably starts tomorrow lol or surely when I'm gone and will be done by the time I come back again from my weekend sales work.

Gawd I just love surprises.

Down at the farm we're doing additional R Value by insulating an unheated attic space OVER both a newly closed cell spray foamed room below and currently unisulated winter heated (gas modine) garage bay.

The new room is a produce cooler and will have high humidity and avg temp of around +35F

2" of foam isn't enough to keep it cold they way they want. We had already experienced this with another cooler room one I built for them 7 years ago. That ended up with loose fill insulation in the joist bays and a layer of unfaced batts crossed over that. And the foam is on the underside of the plywood ceiling just like this one.


So I figure the 2" of closed cell spray foam under the plywood ceiling generally acts like a vapor barrier and then just unfaced fiberglass batts or rock wool are installed in the attic joist bays.

But...I came back to work today after being gone since Friday and my Boss had went ahead and purchased Kraft faced fiberglass insulation over the weekend and him and some of his minimum wage minions are wanting to install it with Kraft up so they can staple it to tops of joists.

I'm thinking that move would create a vapor barrier on both sides of the fiberglass and could eventually trap moisture over the cooler area.

So part of the entire attic space 15' will be over the garage bay's plywood ceiling and the other 30' of the attic space is over the cooler.

Now the kraft facing down over the heated garage space would be the vapor barrier over the heated space and also installed that way shouldn't be any problem over the cooler space either and don't see any need for slitting or removing it either.

Opinions please...

 

[DFB]

Nobody???

 

[DC73]

Kraft paper is a Class II vapor retarder. It can breathe somewhat. I found one blurb online from a building scientist that said "The paper is vapor closed unless the wall cavity becomes wet, at which point the paper becomes vapor open to allow drying."

Owns Corning said this in one of their technical bulletins about Kraft paper: "Testing shows its permeance increases significantly as relative humidity increases, thus improving its ability to allow excess moisture flow."

So, their plan is no different from a house that gets insulating foam board on the exterior with Kraft faced fiberglass in the walls with the paper on top of the interior face of the studs.

So, it might not matter - especially if you're in a dry climate. If you are in an extremely humid climate, I'd think your reasoning is valid that moisture could get trapped which might be a reason to think about putting the paper next to the foam layer.

DC

 

[DFB]

Thks for the response

Ya we are in an extremely humid environment during the summers

Just to close this off my boss installed it today with his minion help R30 batts in a 2 x6 bay Kraft side up, no staples

After climbing the ladder up to loft after they were done and watching it expand a good 4 inches above the ceiling joist cavity, I'm not really worried about moisture entrapment