Insulating open beam ceiling

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Old 09-10-18, 05:55 PM
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Insulating open beam ceiling

I'm along the coast of Los Angeles. It never gets really hot or really cold here. Ceiling is open beam. Not a whole lot of pitch to the roof either. It's a hot mop asphalt roof with white rocks on top.

I wanted to stick some rigid insulation up there between the beams with some paneling finish.

https://www.lowes.com/pd/Armstrong-C...Tiles/50246697

My question is do I HAVE to leave a few inches of space between the insulation and the roof? And do I have to have some air vents in there? If so what is the purpose? To get moisture out? Wouldn't a vapor barrier (paint or plastic) eliminate the need for vent space?
 
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Old 09-10-18, 06:14 PM
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No, you do not need an airspace if you will not have a radiant barrier. No you do not need ventilation, but you must airseal the edges very well to prevent moisture problems. See this recent similar thread: https://www.doityourself.com/forum/i...nsulation.html
 
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Old 09-10-18, 06:42 PM
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What is the "radiant barrier"? Would that be the aluminum side of the insulation that blocks heat from rising out of the room?

By "airsealing the edges" would that mean applying caulking where the plastic barrier meets the wood beams? Do I HAVE to put plastic up there between the insulation and the wood paneling?

On a separate but related question, a few years ago I stuck rigid insulation between the open beams of another room of my house with NO space at the top (where the insulation meets the roof), then finished with drywall and painted it. Do you think I could have any problems inside there (mold and what not)?

I also have a 2nd room where I did the same thing only I added plastic sheeting before applying the drywall.
 
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Old 09-10-18, 07:25 PM
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Airsealing the edges can be done with foam adhesive or spray foam. What plastic barrier? You don't need one. Thermax is double sided foil... a radiant / reflective surface.

related question... maybe, maybe not... no way to tell.
 
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Old 09-11-18, 02:42 PM
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For my other two rooms that I already insulated, I'm wondering if there's problems there. I cut the rigid insulation to squeeze into place between the beams. But I didn't use any spray foam or adhesive to fill in small cracks between insulation. But does a VAPOR barrier (plastic) do the same job as a radiant barrier by keeping moisture from getting in there? And wouldn't the paint that I applied to the drywall serve as a vapor barrier?
 
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