Foam board over fibreglass batts?

[AldeanFan]

I have the opportunity to get a great deal on some 3" thick foam sheets.

My garage 20x22 attached garage has r12 batts in the attic, the ceiling is drywalled with vaporizer barrier. I was planning to add insulation to the attic, is it ok to put foam sheets over the pink fibreglass?

 

[DC73]

Hmmm. Foam sheets that thick are likely to be a good vapor barrier. With the existing vapor barrier you have, the fiberglass will become enclosed within two vapor barriers. It might work if you could guarantee no moisture vapor could ever end up between the two barriers. If not, you could end up with a moisture sandwich (read mold). You should run this by some experts before taking the leap.

You might find an article on BuildingScience.com (see the Information tab and then search). Or pose your question in the Q&A forum at GreenBuildingAdvisors.com. Those guys should be able to help.

I've blown cellulose loose fill insulation over fiberglass batts before and it works very well. Inexpensive too and HD or Lowes will loan you the blowing machine with a 10 bag purchase.

Good luck.

DC

 

[jvitez]

EPS or XPS? EPS will not behave like a vapour barrier.

 

[St-rider]

You don't want to compress the r12 batts.

 

[Kogashuko]

You could put it under the trusses under the roof deck. Probably with that much space between you would be fine.

 

[Denwood]

Don't put it over the fiberglass. Put it under, or face your ceiling with it, adding a drywall fire barrier after.

We have 9500 sq/ft done with a furred airspace assembly. Our walls are lined on the inside with SM, then a radiant reflective vapor barrier, horizontal furring and drywall. It's crazy efficient.

Depending on climate, the only way to put it over your batts is to ensure the foam is thick enough to bring the dew point out of the glass. In my temp zone for example, that would be about 5". Folks are finding this out the hard way adding foam to the outside of their houses..and creating a mold issue. In these cases the building science would suggest it's ok to add foam on the outside (where it can trap moisture) as long as you add enough foam thickness.

 

[Nickmm]

I've read into this and decided against it. I asked the same question of the forum, exactly, in Dec I think. I was also just at building science reading on this subject in regards to roofing.
http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

You'll note they do have a section under unvented roofs where they use 2" closed foam over R-58 fiberglass. I think the issue you will run into is the R-12 you have in the walls not being sufficient. If you run the Foam sheets, you'd want to completely seal between sheets w/spray foam.

Either way, you can do it if you want, just be sure you understand your heating/cooling zone, and note that another option is buying it and putting it on the outside of the building instead, more work, sure, but more common.

Good luck. I decided to go with either spraying the roof or doing the 6" of sheet on the roof to get my extra insulation in the garage. Its 30x20.

 

[rburke65]

I stuffed my stud cavities and truss cavities with EPS then covered with the pink Fiberglas with kraft paper.

 

[NUTTSGT]

EPS or XPS? EPS will not behave like a vapour barrier.

That's why it's important to know what type of foam board the OP is getting. I used some EPS over my attic fiberglass and wasn't worried about it creating a vapor issue.