Add foam insulation to cathedral ceiling on interior side then drywall over
My large master bedroom has a cathedral ceiling with blown in fiberglass insulation at R30. I want to increase the insulation level, unfortunately there is only 3-5 inches of space between the existing insulation and the roof sheathing and that is needed for airflow and venting.
Can I add a 2″ layer of rigid foam board on the interior side of the ceiling, over the existing dryall, then add a layer of drywall over the foam to finish it off without causing any issues? From an aesthetics perspective the 2″ lower ceiling height will not be objectionable because the ceiling level is elevated already.
I am hoping to add the insulation without having to drop the existing drywall and insulation to reframe the ceiling line. The room is about 20′ x 28′ so it will be quite a project.
Thanks,
Replies
The basic concept will work just fine. There are probably ways to screw it up, but you'd have to try pretty hard.
XPS foam over drywall
I don't see why not other that the fact the foam board will act as a vapor barrier. As long as you dont have another vapor barrier back there you should be fine. Those building science people don't like multiple vapor barriers. Take time to tape the joints of foam boards and air seal the perimeter edges
same same
I did the same thing years ago in my attic.
When I built, I had FG batts in the vented rafter bays with 6-mil poly over it all. In winter the attic was pretty darn cold, in summer it was pretty darn hot.
I added 2" of foil-faced ployiso on the faces of the rafters. When I hung the sheets, I gapped the sheets about 1/4" or so from each other. It doesn't matter if the ends of the sheets break over a rafter or in the middle of a rafter bay.
I then took a sharpie marker and peeking through those 1/4" gaps, I made a tick mark on the foil face to show the center of the rafter edges behind the polyiso.
I used canned foam to foam the 1/4" gaps. Air tight.
I ran 3/4" thick furring strips horizontally across the ceiling, perpendicular to the rafters. The sharpie tick marks helped to locate the rafter edges so the screws hit the rafters.
Hung the drywall off the furring strips with conventional 1-1/8" screws. It's a lot easier to hang drywall with 1-1/8" or 1-1/4" screws than with 4-1/2" screws. No need to worry about misses, foam compression, later foam expansion that pops a screw, etc. Furring strips are worth the effort.
I did the polyiso in the middle of the summer. The day I started the attic temp was 127 degrees. A couple of days later it was 77 degrees. Same outdoor weather.
Foam versus FG? Foam wins. Polyiso, or even XPS. Either will work wonders.
Just make it nice and air tight. No can lights, no penetrations through the foam at all, except for the screws.
I want to say thanks for the replies!
I had not considered the idea of using the firing strips. I can see how that would simplify the process. Now that I know I am not going to create any moisture control issues, I will contact my drywall contractor and run the idea past them.