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Insulating the underside of the ceiling between rafters

Posted by wolfgang80 (My Page) on
Mon, Oct 29, 12 at 20:03

We have a one-story house with a near flat roof close to the beach in Southern California. Winter temps drop into the 40s. Our ceilings are pushed up to the rafters, so what you see inside are the 4x6 beams with 1x8 shiplap boards running across the tops. On top of the 1x8s is the roofing material.
The ceilings get quite warm in the summer, but that isn't really a problem because opening the windows will bring in plenty of cooler air. The problem is during the winter time. We'd like to insulate the ceiling but will not do so if it means covering the wood beams with drywall.
Our idea is to put 1" rigid foam insulation between the rafters up against the shiplap boards. Then we'd cover the insulation with 1/2" drywall, leaving us with 4" of beam exposed. It won't look as nice as the current ceiling but if it means the house will be warmer in the winter, we'd do it.
Our questions are:
1) Will an inch of rigid foam insulation do anything?
2) Is it okay to have the insulation pressed against the underside of the roof?
3) Can you think of any alternative solutions that would provide some insulation while preserving the exposed wood beams?

Thanks so much for your help.


Follow-Up Postings:

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RE: Insulating the underside of the ceiling between rafters

Insulation for a low sloped roof normally goes on top of this kind of roof deck and a roofing material like a synthetic fully adhered single ply membrane is installed over it.

If you don't want to do that, one inch of extruded polystyrene foam board will add an R value of 5. That's not a lot of insulation but it will be a huge improvement over what you have now.


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RE: Insulating the underside of the ceiling between rafters

will add an R value of 5

That's between the 4x6s.

Interestingly, the 4x6s alone have an R value of 6.88.

How about adding insulation to the walls?

If you add insulation to the roof, consider going up to the Energy Star recommended R25-R38 for southern California.

But before adding any insulation, tighten up the house. Loss of heated air through cracks, chimneys, vents and the like can account for 16% of heat loss.

Here is a link that might be useful: ZIP Code Insulation Calculator


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RE: Insulating the underside of the ceiling between rafters

Thanks for your responses. We've redone most of the house except for the roof, which is fairly new. In the process, we've put tyvek on the exterior and R-13 fiberglass in all of cavities of the exterior walls. All of the windows and doors are new and have dual pane, low E glass, so the rest of the house is about as tight as it can get. The roof is the one area that we'd like to address for heat loss.


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RE: Insulating the underside of the ceiling between rafters

I'd put the insulation on top of the existing roof-- 4-6" of rigid foam. Making it look good on the inside will probably cost more than replacing the roof and 1" of foam doesn't really give you very much insulation.

Speaking as a New Englander though, if your lows are only in the 40's a sweater would probably work as well for a lot less cost and hassle. How often do you even have to turn the heat on?


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RE: Insulating the underside of the ceiling between rafters

All of the windows and doors are new and have dual pane, low E glass

Some installers are still omitting proper sealing around retrofit windows, i.e., low pressure single component spray foam, such as Great Stuff, Handi-Seal etc. If you don't know if they used it, you can use smoke pens, pencils or puffers to detect air leaks.


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RE: Insulating the underside of the ceiling between rafters

I will address the openings around the windows and the doors and I think we'll take RW's advice and kick the can down the road on the ceiling insulation until we are ready to replace the roof.

Thanks again. I appreciate your help.


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RE: Insulating the underside of the ceiling between rafters

I second putting the foam sheathing on the roof
when re-roofing. stopping thermal conductivity
of framing members is a plus.

best of luck.


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