I’d like to upgrade the insulation in my attic to R50 – R60.
I currently have about 10″ of badly installed rock wool on 1/2 inch ordinary drywall ceiling nailed to trusses spaced on 24 inch centers.
I’ve found a couple of references that state that the weight of cellulose and rock wool will _at_some_level_ cause the ceiling to sag, and that therefore the only choice is fiberglass, which is difficult to install well around trusses.
As I intend to remove the rock wool to seal and install a vapor barrier, I’d be starting from scratch with new insulation.
My tentative plan is to use polyisocyanurate board in the thin areas near the eaves and under the crawl-way and then several courses of unfaced fiberglass batts everywhere else.
I’d rather use cellulose, but the weight problem sounds like a show-stopper. If it’s real and not fiberglass industry propaganda.
Or possibly to fill in the bottom 2x4s of the trusses with cellulose. then lay some sort of support system across the 2x4s to support another 15 inches of so of cellulose. Maybe lath and tyvek…
Or…
Any comments, ideas, humor will be appreciated.
Replies
Or possibly to fill in the bottom 2x4s of the trusses with cellulose. then lay some sort of support system across the 2x4s to support another 15 inches of so of cellulose. Maybe lath and tyvek...
Sounds feasible, sounds cheapest, sounds easiest too.
Joe H
There are two ways that moisture enters the attic from the house: (1) air leakage upward by stack or chimney effect (warm air rises) through unsealed areas and (2) moisture diffusion through permeable materials such as drywall. Air leakage is responsible for 98-99% of the moisture that enters the attic so seal the ceiling and don't worry about the other 1-2%....the attic venting will take care of that.
Don't remove the rockwool to install a vapour barrier....it's a waste of a good material if there's no pest dung in it. Spread it around to be even and level. Airseal at the sealing level....all wiring, plumbing and other penetrations, check around chimney chase (if interior chimney) and seal with fireproof materials. Weatherstrip and insulate attic hatch. Consider adding bath fan/s venting to the exterior, not soffits, and controlled by hand crank/electronic timer.
Blow lightweight fiberglass (cellulose is my favourite but in this case you need a light product) over the rockwool. It will form around the trusses, etc as will cellulose.
Interesting. My attic has 10 in. of cellulose and the drywall is staying up just fine. As it was dense packed below a plywood floor, its density is something between 3 lbs and 3.5 lbs per cubic foot. Blown loose, I think cellulose settles to something like 2.5 lbs per cubic foot. I've never heard of drywall not supporting cellulose.
Also, as the other posters said, air seal, and leave the mineral wool.
Andy Engel
Senior editor, Fine Woodworking magazine
Never try to teach a pig to sing. It wastes your time and annoys the pig.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
None of this matters in geological time.
In this case he mentions that the 1/2" drywall is fastened on 24' centers. I was being overly cautious with the ceiling. I stopped blowing fiberglass and went soley to cellulose in 1983.
I missed the 2 ft. centers. Might be worth comparing the density of blown glass and blown paper. It's possible that there'd be enough weight to create a problem, but I don't know. Say you did a foot of cellulose, and it's at 2.5 lbs. per cubic foot. A 4x 8 sheet of drywall would hold 80 lbs. of cellulose then. Assuming 5 nails per truss, that's 25 nails per sheet. The drywall is what, 40 lbs? So there would be 25 nails holding 120 lbs. Round up to make the math easier, and each nail is responsible for 5 lbs. Seems doable, but it might bow the rock. I bet that USG or some other manufacturer addresses this problem.Andy Engel
Senior editor, Fine Woodworking magazine
Never try to teach a pig to sing. It wastes your time and annoys the pig.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
None of this matters in geological time.