Comments on: Insulating Cathedral Ceiling with Foam Board

By: Todd

Todd — Tue, 07 Aug 2012 23:39:28 +0000

Angelo – No real moisture issues but you really won’t have enough R value for this part of the country. You really need to shoot for R38 as a bare minimum and more like R50 if you can. 2×8 rafters make it VERY difficult to do. I’d definitely consider spray foam if possible.

By: Angelo

Angelo — Mon, 06 Aug 2012 17:17:38 +0000

I would like to do the same in 2 different areas. I live in Eastern Massachusetts. In 1 area I was looking at using 1″ rigid foam with foil facing on both sides against the roof deck in the same manner you used but then filling the rest of the cavity with R19 batt insulation then 1/2″ drywall. The rafters are 2×8 so I figured R19 with the 1″ Foam board leaving an air channel between the rigid foam board and roof decking to vent air from soffit to ridge vent. Would I need to still add a vapor barrier on the inside across the rafters before drywall? does this cause moisture to be trapped between this vapor barrier and the rigid foam?

In the 2nd area I would be using it on the Attic Floor with the Rigid foam against the drywall Ceiling and filling the rest with batt insulation then plywood for storage. I would leave the attic as unconditioned space but hopefully increase the R value and reflect Radiant heat gains in the summer.

By: Todd

Todd — Fri, 06 Jul 2012 23:46:33 +0000

I would shop around. A real building supply store should carry thicker options up to 2″. Foil is only really necessary on the top, but it also really helps create a great vapor barrier. If you can’t tape, I’d try to use spray foam or caulking to seal the joints as best you can. Nothing is ever perfect, but I’d try hard to seal things as best you can.

BTW>…..you’re very welcome. I hope you come back often.

By: Max

Max — Mon, 02 Jul 2012 00:04:08 +0000

I measured the space this morning and the space between the ceiling and the roof is 5 inches high. I went to the local home improvement store and the thickest polyiso they had was 3/4 with a single foil faced side.

1. Should I try to shop around town to find thicker polyiso board or just cut multiple boards and stack them in the space?

2. Do I need a foil faced side on both sides or just the side facing the roof?

I will be cutting the foam board to size so it fits snugly in between the rafters. I will not be about to tape the seals like you did because there is no room. I am excited about getting the airflow back!

Thank you again for the great website and information. It was around 100 degrees again today and the forecast is it will stay this temp for about two more months.

By: Todd

Todd — Sun, 01 Jul 2012 11:44:26 +0000

Max – Thanks for the nice compliment. You certainly have a tough situation. I’d definitely recommend getting the air flow back. The next question is, how much space do you have? I’d recommend at least 2 to 4 inches of foil faced foam, but you really need to leave probably 2 inches of air space.

This will certainly help. Is it perfect? no….is it your best option without tearing down the ceiling..probably.

Good luck.

By: Max

Max — Sun, 01 Jul 2012 05:24:17 +0000

Todd,

Great website!I live in Texas and it is hot! I have a two story house that gets hot upstairs. The previous owner stuffed fiberglass batts tight against the roof above the second story kneewalls. They are blocking the air flow from the soffit vents and the whirlybird on the top of the roof. If I remove the fiberglass batts and use foil polyiso to insulate this area:

1. How much air space do I need to leave between the polyiso and the roof?

2. The cathedral ceiling space in question between the top of the kneewall and the flat 2nd floor bedroom ceiling is only about 4 feet. Will the foam board be effective if it is cut tight to the rafter width and layed on top of the drywall. (the slope of the ceiling is about 45 degrees.

Thanks for your suggestions.