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Old 03-31-2011, 08:52 PM   #1
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insulating cathedral ceiling


Building a new house and trying to determine the best way to insulate the cathedral ceilings. This will be 12-12 pitch using a manufactured truss with knee walls in the loft.

One option is to spray 10 inches of closed cell spray foam insulation onto the under side of the roof sheeting with out an air baffle and no ridge vent, then put the tongue and groove on. (recommended by building inspector and insulation contractor) expensive

Next option is to use an air baffle and ridge vent, put up the tongue and groove pine on the ceiling leaving a gap at the top and then blow insulation down into the cavity. Pushing the hose all the way to the bottom and fill as the hose is removed. (recommended by different insulation contractor) ceapest but can not be certain all the cavity will be filled

Another option is use an air baffle with ridge vent, spray 2-3 inches of closed cell spray foam insulation then use fiberglass insulation and place the tongue and groove on. (my thought on the best of both insulation options) middle of the road and possible best of both worlds

So my question is are any of these good options? Pro's/con's to these. Am I missing something here, are there other options I haven't listed?

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Old 04-02-2011, 09:48 PM   #2
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Find your Zone on the map or closest City below the map; http://publicecodes.citation.com/ico...001_par001.htm

What is required for R-value? http://publicecodes.citation.com/ico..._11_sec002.htm

Gary

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Old 04-03-2011, 10:47 AM   #3
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I would be in zone 5A, walls require R-20 and ceilings R-49. I have a quote of $4000 just for the ceilings with 10 inches of spray foam on 1,200 sq ft. foot print with 12-12 pitch cathedral ceiling. Kinda pricey and that's with out an air baffle. My concern with out the air baffle my shingles won't last as long due to the higher heat they will experience. I see alot of homes in this area getting new roofs put on that are 15 years old or less. Some may have went with cheap shingles but the Architectural shingles typicaly are a 25-30yr and these are being replaced as well.

I like the spray foam cause it will seal all the tiny cracks that fiberglass doesn't but the cost is quite high. That's why I was thinking if I could use an air baffle, 2 inches of spray foam then put up fiberglass. Unless there is a better idea or I'm missing something. Also would I need to use a vapor barrier or just use fiberglass with vapor barrier backing?
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Old 04-03-2011, 11:33 AM   #4
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Spray foam

Question....did YOU tell the foam guy you wanted the cavity filled to 10" or did HE recommend it?

10" sounds excessive...that's like an R 60
I believe the rule of thumb is 1" foam = R6

Typically 4-5" is used in cathedral ceilings.



I'd call another spray foam company and get another price.

Also find out what R value is recommended in your area and get a foam price for that.

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Old 04-03-2011, 01:45 PM   #5
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That was recommended by the insulation contractor who gave me a bid. My area requires R-49 in the ceilings. Is there a maxium thickness that can be sprayed? Would there be a problem if the area between the foam and the dry wall wasn't filled. Say there is 12 inches between the roof sheeting and the dry wall and they spray in 10 inches would the 2 inch void create a problem?
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Old 04-03-2011, 03:49 PM   #6
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If that area won't be accessible at all, I say go all the way with R60. It might hurt the wallet now, but it will pay off in the long run.

I'm just wondering if it's ok to not have any air circulation at all, I guess if there is no place for air to even exist, it's not an issue, I guess. I'm guessing it will come flush with the end of the studs, and drywall is going over that?

While you are there consider all forms of lighting/electrical so you can run that wiring now. Ceiling lighting, fans, etc. "while we're here" usually ends up costing more then what you expected though. :P
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Old 04-03-2011, 04:41 PM   #7
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Quote:
Originally Posted by South Branch Ca View Post
Say there is 12 inches between the roof sheeting and the dry wall and they spray in 10 inches would the 2 inch void create a problem?
No problem...it's done all the time....






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Old 04-04-2011, 10:17 AM   #8
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What about the air baffle between the roof sheeting and the spray foam. Should I use it or not?
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Old 04-04-2011, 10:18 AM   #9
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No baffle needed with closed cell foam
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Old 09-26-2011, 10:44 AM   #10
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In my area 6 3/4 in is R40.
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Old 09-26-2011, 11:41 AM   #11
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Quote:
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In my area 6 3/4 in is R40.
In 2lb foam, that would be correct.

1/2 lb foam will be closer to 20-24 depending on what number you use.
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Old 09-26-2011, 12:07 PM   #12
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Closed cell foam should be installed with maximum of 2" lifts. Lifting more than 2" in a single pass can cause an exothermic reaction within the foam and damage the closed-cells and greatly reduce the effective R-value. People have actually died from "over-spraying" and having the excessive heat from the exothermic reaction cause the foam to burst into flame.
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Old 09-26-2011, 01:04 PM   #13
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The OP seems to be evaluating the merits of a 'vented' vs an 'unvented' roof and seems to have both options open to him...in most cases, a roof will tend be easier either to vent or not to vent depending on the shape and structure of it. With a vaulted ceiling and knee-walls, he has the option of both.

But in Zone 5a, the question of a vapour retarder comes into it; and for convenience, I'd go with an unvented roof, with 7.5" of closed-cell foam, no baffles - but I'd throw in something the OP perhaps hasn't considered: consider a thermal break i.e. a 2" XPS foam board on the inside. Not only will this meet your R49 code, but will provide that thermal break.

Other options for an unvented roof involve adding something to the exterior of the roof, like XPS or iso-panels, but we're not sure you're needing that.
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Old 09-27-2011, 03:39 PM   #14
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My installer says 5 in is plenty(R30) in cathedral as opposed to R 49 in flat roof.
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Old 09-27-2011, 04:22 PM   #15
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Quote:
Originally Posted by ccarlisle View Post
The OP seems to be evaluating the merits of a 'vented' vs an 'unvented' roof and seems to have both options open to him...in most cases, a roof will tend be easier either to vent or not to vent depending on the shape and structure of it. With a vaulted ceiling and knee-walls, he has the option of both.

But in Zone 5a, the question of a vapour retarder comes into it; and for convenience, I'd go with an unvented roof, with 7.5" of closed-cell foam, no baffles - but I'd throw in something the OP perhaps hasn't considered: consider a thermal break i.e. a 2" XPS foam board on the inside. Not only will this meet your R49 code, but will provide that thermal break.

Other options for an unvented roof involve adding something to the exterior of the roof, like XPS or iso-panels, but we're not sure you're needing that.
Be careful when adding 2" XPS to the interior wall face along with partial closed cell spray foam fill. Both are, in the thicknesses mentioned, class II vapor retarders, and when a void left it creates a double barrier assembly with the potential for trapping unwanted moisture.

If 2" XPS or Polyiso foam in used as an interior sheathing beneath the wall board then open cell foam or un-faced batt insulation should be used for the bay fill to ensure any moisture within the wall system can dry to the exterior of the system.

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Last edited by AGWhitehouse; 09-27-2011 at 04:34 PM.
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