Insulating a post and beam construction roof/cathedral ceiling

insulationroof

My house (built 1972) has a cathedral ceiling with post-and-beam construction in the main room. The ceiling temperature can get as high as 90F, or even greater on a very hot sunny summer day, as measured from the floor with an IR thermometer ("gun"). The roof doesn't look to be very thick, probably less a foot in total? I'm not sure what insulation is in the roof, if any. This room has quite a few floor registers, however, it is still much colder in the winter and hotter in the summer than adjacent bedrooms (~5F). I was wondering if there is anything that can be done to improve the efficiency of heating/cooling this room. Can more insulation be blown in between the pine ceiling and the roof?

I live in sunny eastern Washington, hot dry summers 100F+, sometimes cold but not too bad winters, 10-20F lows.

I don't need a new roof yet, the current asphalt shingles look fine so I don't want to do something as drastic as replace the roof to add more insulation but it would be good to know that is an option for the future.

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Best Answer

The tongue-and-groove wood planks conceal insulation above them, probably fiberglass batts between the rafters. As you've observed, they are clearly insufficient, because the room is too hot. There are two safe ways to improve the situation:

  1. Add more insulation underneath the existing ceiling wood, then install a vapor retarder (retarder, not barrier. A smart membrane, not poly sheeting), then cover that up with drywall.

  2. Remove the roofing materials and install 4-8 inches of rigid foam insulation board above the roof decking, then new roof sheathing over that, then new roofing materials.

  3. Build a metal roof (white or bare metal) over the existing shingle roof, using eave-to-ridge 2x4s to build ventilation channels under the new roofing. Such a roof will reject the vast majority of the sun's rays, which is the primary driver of heat gain.

Since you don't want to mess with the roofing, that leaves option 1. If you don't want to ruin the look of the ceiling with the tongue-and-groove boards, you're out of options and will have to live with the situation.

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