I posted this message in another forum, but didn't get an answer so I thought I'd try here. My apologies for the duplicate post...

I am redoing a ceiling and am curious whether my tongue and groove planks are sufficient for a 15-minute thermal barrier? According to the table in this link tongue and groove planks 15/32" are adequate....

http://www.foam-tech.com/theory/firebarriers.htm

Mine are 3/4", but some boards have small open knots. My plan was to first install 5mm luan against the ceiling joists to provide a solid surface for the cellulose insulation, and then follow up with the knotty pine. To accomplish a thermal barrier, however, would I be better off using 1/2 drywall instead of the 5mm luan? Or is my knotty pine, along with the luan adequate, even though the knotty pine contains some holes?

Thanks,
Rob

I think what you want to do will be fine. You'll want a continuous sheet between you and the cellulose to keep it contained. Cellulose is fire treated, so it isn't going to be viewed the same as expanded plastic foam insulation. I would paint the bottom of the lauan black so you don't see anything through the open knots.

Thanks for the reply, and good idea about the black paint!

I agree with "Maintenance6"