Plywood ceiling - covering sprayfoam insulation instead of drywall

[laura vianello]

Hello, Framing is done (knee walls, etc) for my attic. We are ready for insulation (closed cell sprayfoam is the only option because the rafters are shallow). Contractor is asking a lot for plastering, then I need to paint, etc etc. So what about 1/2 plywood to cover the sloped ceiling? The code required min 1/2", so it is ok, the ceiling is not longer than 8' and the rafter are about 16' a part (+ or -...it is a old house). It looks like full 4x8 sheet can do the job. I can cat in correct wide panels and attached on the rafters but...what about the seam? would it warp? Which Ply should I go for? Anybody who has done it upthere? If this is not the right place for my post please let me know and I will move it.
Thanks

[John K Jordan]

Hello, Framing is done (knee walls, etc) for my attic. We are ready for insulation (closed cell sprayfoam is the only option because the rafters are shallow). Contractor is asking a lot for plastering, then I need to paint, etc etc. So what about 1/2 plywood to cover the sloped ceiling? The code required min 1/2", so it is ok, the ceiling is not longer than 8' and the rafter are about 16' a part (+ or -...it is a old house). It looks like full 4x8 sheet can do the job. I can cat in correct wide panels and attached on the rafters but...what about the seam? would it warp? Which Ply should I go for? Anybody who has done it upthere? If this is not the right place for my post please let me know and I will move it.
Thanks

Laura, I don't know your intended use for your attic, but I used 1/2" radiata pine for the ceiling of my shop. My ceiling is flat and the ply was not fastened directly to the rafters as in your situation but to the bottom chord of the trusses (on 24" centers). Since I was working by myself I used a panel jack to hold each one up and fastened with screws. I think I used 4 screws per truss. If I had uneven spacing I would probably have scabbed a 2x4 onto each bottom chord.

This is a shop so I don't care much about the way it looks. I didn't even paint it yet and I've seen no problems after 3 years. I used screws on the walls and the ceiling partly so I could remove panels in case of future access needs. (I have wiring, dust collector ducts, and compressed air lines in the space above the ceiling.

JKJ

[Paul F Franklin]

Are you sure this will meet code? Foam needs a thermal barrier and at least in my area, 1/2 plywood would not be sufficient....

If it is ok there, then I would consider using tongue and groove plywood; that should prevent any issues at the seams. You other option would be to use h clips like those used for roof sheathing. But they would be visible, although I suppose if painted they wouldn't be very noticeable.

[Todd Mason-Darnell]

Code usually requires a 30 minute resistant fire rated barrier between an occupied space and an attic. Unfortunately, plywood will not make the cut.

[Wayne Lomman]

For an attic ceiling, lightly arris the edges of the sheets where they butt together. This hides any slight height/thickness mismatch. Screw fix the sheets in place. Make sure all edges of the ply are fully supported. Its good if you can join the sheets on existing timbers. If not, fit battens to the back of the joins and screw fix the ply on both sides of each joint. Using ply has the added bonus of providing additional bracing to the roof structure. Check your building code but I don't think the fire rating issue applies - it is an attic ceiling, not a ceiling between attic and living space. Also, thermal properties of ply are not worse than 1/2" plasterboard. Cheers

[Robert Engel]

How about drywall covered with panelling?

If your rafters are 16" OC, you should have too much problem with longitudinal seams, but remember, plywood can be irregular.

It all depends on what you're doing with the attic.

I would look into something like T111. You will still have the issue of butt joints.

[laura vianello]

Thanks to all for your help!!! The attic will be officially storage...a nice storage with a desk too. I will not be able to recap the money of this investiment because it is not "leasable space". However, the attic will be painted with flame retardant coat on one side where the A/C unit is and the rest will be drywall and plaster. Because it will be my office (10 hours a day) we installed 2 skylights and I need a pleasant environment. When you are a remote worker you want to be sure your office is cozy otherwise you get depressed! I love the plywood nordic style, in addition I can work with plywood myself without paying for a contractor. I will have tons of drawers in the eaves and we have 3 48" long closets for shirts and other clothes. I was thinking to do plaster only on the high side of the roof covering the rafters. Collar beams will be exposed. Knee walls have been framed and I was thinking to use Mdf paneling (feak shiplap) where I have no door or on the gable walls. I would love T&g but way to expensive. I can add subfloor ply 8" panels on top of Bluebeard but I need to understand the cost. I hope it helps. I have tons of questions on knee walls built-in but I might post a new thread.