Roxul attic insulation – will my 2×4 ceiling joists 16oc support it?
Hi I’m considering using Roxul Comfort batts to insulate my attic floor. My attic floor has joists that are 2×4 and 16 inch on center. There are 26 rafter bays in the attic floor, each 340 inches long, from outside of top plate to outside of top plate. The approx sq footage of my attic is around 975 sq ft.
To get to R38 for my area in PA outside of Philadelphia Roxul suggests two layers, the first being R15…they go between the 2×4 joists.The R15 batts are 3 1/2″ high by the way, so their top surfaces end up flush with the tops of the joists. The second layer of R23 Roxul is then laid perpendicular to that first layer and those will end up sitting on the tops of the ceiling joists obviously.
SInce I have 2×4 joists and Roxul is a little heavier than standard fiberglass insulation does anyone know if my ceiling, with its 2×4 joists, will support the weight? I know there are a lot of unknowns that you don’t know , like the layout of the walls that support the ceiling etc. However the longest span room is the dining room and that is about 14 ft of span there all the other rooms are smaller. My house is about 28 feet 4″ deep front to back (hence the 340 inches).
I’ve aclculated the entire weight of Roxul that I might put up there to be about 1400 lbs.
Should I be concerned at all? I’ve called Roxul and they said that it wil be fine except, like I said before, they don’t know the structural lay out of my house.
Also these Roxul batts do not have a vapor retarder Kraft facing. If I use these, what should I put down as a vapor retarder before placing the batts between the joists? Roxul said some paints can count as a vapor retarder, a paint that I would apply to the ceiling in the rooms below, is that OK?
Can someone who has experience with this insulation give me advice on this?
Thanks in advance to all.
Replies
2x4 ceiling joists can't clear span 28'.
Are these roof trusses, or are there a couple of bearing walls in there?
Yeah, most likely you have trusses (with those maddening diagonal braces you have to crawl through to get around). In that case the ceiling (if built to normal standards) is plenty strong to support the slight additional weight of Roxul.
Roxul and 2x4
Thanks. There are roof trusses up there and these 2x4's are attached to them out near the top plates and also about 50 inches back from the top plate at those diagonal braces.
This weekend however I decided to go with FG just to be safe because there already is evidence of cracking in the ceiling in one of the rooms and someone actually fastened a 2x6 "strongback " perpendicular to those 2x4s sometime in the past. That room has 16ft span, bearing on a loadbearing wall. Plus Hd was running a sale on insulation (expiring yesterday). I would've prefered using the Roxul.
One question though: While up there in the attic I noticed a gap (see my attached pdf drawing) between the top course of exterior brick and the top plate running the length of the building. Should I leave that open or cover it? I am air sealing this attic too and was wondering if that gap needed to be closed off. If I close it, will that impede any drying that may need to occur if water gets past the brick and into the space between the celotex and the brick? My house was built in early fifties and for some reason has no weep holes at the bottom exteriors of the brick wall.
I'm also battling a mice problem and hoped that by closing that avenue of entrance into the attic I could stop them too.
Thanks