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Old 10-09-2009, 05:09 AM   #1
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Can foamboard insulation be used in living areas, vaulted ceiling?


Hello
I see foamboard insulation like Dow Corning Super TUFF-R suggested for use on exterior walls or interior basement walls.

Can I use it for interior vaulted ceilings, cut to fit between the rafter cavities?

I need the high R-Value of foamboard because I only got 4.25" rafter depth.

Is it ok with code ? If spray foam can be used inside then rigid foamboard should be ok right? An issue might be fire hazard as foamboard will give toxic vapors when burnt.

1/2 inch or 3/8 inch sheetrock should take care of that ?

Super Tuff-R is yellow Polyisocyanuate foamboard with facings on either side, 1 side being foil faced.


One other question.
I have read from online resources on Polyisocyanurate insulation that it can be got with R-8 per inch, but I haven't seen any brand of foamboard insulation approaching that R-Value. The closest is Super Tuff R with R-6.5 per inch.

Anyone know what brand has the highest R-Value per inch?


Last edited by Spud; 10-09-2009 at 05:13 AM.
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Old 10-09-2009, 09:26 AM   #2
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Can foamboard insulation be used in living areas, vaulted ceiling?


The fire code issues are going to be your major problem.

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Old 10-09-2009, 10:26 AM   #3
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Can foamboard insulation be used in living areas, vaulted ceiling?


As long as it is covered with drywall you should be OK. The drywall acts as a fire barrier.
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Old 10-09-2009, 11:54 AM   #4
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Can foamboard insulation be used in living areas, vaulted ceiling?


Was the ceiling original, 2x4's cathedral? A 2x4 at 16"o.c. only spans 8'11" with no snow load. At 24"o.c., 7'10" span w. no snow.

Did someone make this into a sloped ceiling that was flat before with ceiling (rafter ties) members that held the walls in check? Are you going to ventilate above the foam (cold roof), or not (hot roof)? Is there any way to furr down the rafters with additional material to use another type insulation?
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Old 10-09-2009, 02:56 PM   #5
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Can foamboard insulation be used in living areas, vaulted ceiling?


Quote:
Originally Posted by GBR in WA View Post
Was the ceiling original, 2x4's cathedral? A 2x4 at 16"o.c. only spans 8'11" with no snow load. At 24"o.c., 7'10" span w. no snow.

Did someone make this into a sloped ceiling that was flat before with ceiling (rafter ties) members that held the walls in check? Are you going to ventilate above the foam (cold roof), or not (hot roof)? Is there any way to furr down the rafters with additional material to use another type insulation?
Be safe, Gary
Hi
The house is a sinle story ranch house built some time in the 50s or 60s. The rafters are 1.5" x 5.25" . The distance between centers is 14.75".

The ceiling has always been a sloped ceiling. It has a flat false ceiling now.

I have rectangular soffit vents which I am going to replace with continuous soffit vents . At the moment the roof has box vents , I am going to be adding ridge vents. The inside of the roof below the roof deck will have a 1" air cavity and vent baffles then insulation then radiant barrier and 1" air cavity then sheetrock.

I really only have 4" for insulation plus another inch for radiant barrier air cavity.
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Old 10-09-2009, 09:14 PM   #6
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Can foamboard insulation be used in living areas, vaulted ceiling?


Hold the continuous soffit vents towards the fascia board, the closer the better. As the wind brings snow or rain towards the house it follows the wall up and easily gets into the soffit vent if installed closer to the wall.

Be sure to use ridge vents with baffles, a superior product to create a suction over the ridge: http://www.oikos.com/esb/30/atticvent.html

The 1" air space at the bottom of rafters would not do much for the radiant barrier as it needs the dead air space to work as an insulator against the heat, other than a vapor barrier effect, read up on those, mainly for warmer climates: http://www.ornl.gov/sci/roofs+walls/radiant/index.html
Be safe, Gary
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Old 10-10-2009, 12:33 AM   #7
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Can foamboard insulation be used in living areas, vaulted ceiling?


There are no fire code issues that would prohibit the use of foam in this application. As long as it is covered with a code-approved ignition barrier you're good to go. 1/2" sheetrock is an acceptable barrier.
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Old 10-10-2009, 12:54 AM   #8
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Can foamboard insulation be used in living areas, vaulted ceiling?


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Originally Posted by thekctermite View Post
There are no fire code issues that would prohibit the use of foam in this application. As long as it is covered with a code-approved ignition barrier you're good to go. 1/2" sheetrock is an acceptable barrier.
Thanks, that is great news.
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Old 10-12-2009, 08:02 PM   #9
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Can foamboard insulation be used in living areas, vaulted ceiling?


I have a follow up question.

Can I use wood planks instead of sheetrock, to go over the rafters? Or would this not meet the fire code?

I probably won't do planks as I am going for wood floors and don't want to have too much wood effect, but in the bathroom I am going tile floor and would like to have wood planks for the vaulted ceiling.

Roof with wood planks, like this roof.


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Old 10-12-2009, 08:17 PM   #10
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Can foamboard insulation be used in living areas, vaulted ceiling?


NOt sure if wood alone would be OK. It might. But you could do drywall with a cheap tape job and wood over it.
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Old 10-12-2009, 08:47 PM   #11
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Can foamboard insulation be used in living areas, vaulted ceiling?


I wonder if I could use Fire retardent slip sheets , or fire retardent coating to the underside of the wood planks.



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Old 10-12-2009, 10:12 PM   #12
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Can foamboard insulation be used in living areas, vaulted ceiling?


I don't know if you need the retardant on wood as they install wood paneling. I would ask your local B.D. as they would have final say. It has to do with the rating for flame spread and smoke developing class of the type of covering. It may be limited to 10% of the aggregate area and ceiling. Anything less than 1/4" needs drywall behind it, though plywood (1/4") and fiber board are used.
Be safe, Gary
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Old 10-12-2009, 10:24 PM   #13
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Can foamboard insulation be used in living areas, vaulted ceiling?


This is what the ceiling/roof in my garage looks like . The rest of the house is sheetrock attached under the joists and about 1 inch of cellulose and 4 inches of fiberglass. The Joists in the garage are on every 4th rafter.


I really like the look of the wood panelling above the rafters, thats why was wondering if I can use wood panelling over the rafters.

GBR in WA

What is B.D. ?
Building Department?
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Old 10-13-2009, 01:28 AM   #14
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Can foamboard insulation be used in living areas, vaulted ceiling?


Correct, sorry, I was in a hurry. You know you have to leave the ceiling joists (rafter ties) every 4' alone, correct? Could you post another picture of the top of the valley rafter taken from under the garage door opener? It just doesn't look right at the tie-in of the ridge board.
Does it lose heat easily and quickly in the house? That sounds like minimal insulation for your location....
Be safe, Gary
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Old 10-13-2009, 01:48 AM   #15
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Can foamboard insulation be used in living areas, vaulted ceiling?


Quote:
Originally Posted by GBR in WA View Post
Correct, sorry, I was in a hurry. You know you have to leave the ceiling joists (rafter ties) every 4' alone, correct? Could you post another picture of the top of the valley rafter taken from under the garage door opener? It just doesn't look right at the tie-in of the ridge board.
Does it lose heat easily and quickly in the house? That sounds like minimal insulation for your location....
Be safe, Gary
What is the "top of the valley rafter"? Is that the spot on the Ridge beam that all the Rafters meet at? Will take a picture tomorrow.

This is another picture I took earlier.

I haven't really lived there , no one has in about 1.5 years so not sure on how quickly the heat escapes. I do recall dad saying how expensive heating was.

The roof design is below.


Last edited by Spud; 10-13-2009 at 02:02 AM.
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