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Old 07-18-2018, 06:20 PM   #1
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Cathedral Ceiling Redo


I've just stripped a vaulted ceiling down to 11" rafters and roof deck. Because of a venting limitation I am considering a hot roof which I have read would require an airtight insulation assembly

I'm considering stacked and sealed 2 or 3" polyiso for this reason and wondering how stacked solid foam matches up against fluffy stuff like an equivalent amount of Roxul.

What is the actual difference in R performance between fluffy and solid and why is solid foam stacking so rare?
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Old 07-18-2018, 06:46 PM   #2
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Re: Cathedral Ceiling Redo


What are the venting limitations?
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Old 07-18-2018, 07:27 PM   #3
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Re: Cathedral Ceiling Redo


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Originally Posted by Nealtw View Post
What are the venting limitations?

The room with the vaulted ceiling has only 2 outside soffits. An 11 foot wide floor to ceiling fireplace occupies the wall between them.
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Old 07-18-2018, 07:32 PM   #4
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Re: Cathedral Ceiling Redo


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Originally Posted by arcticranger View Post
The room with the vaulted ceiling has only 2 outside soffits. An 11 foot wide floor to ceiling fireplace occupies the wall between them.
Is there a cricket on the roof to divert the water around the chimney?
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Old 07-18-2018, 08:17 PM   #5
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Re: Cathedral Ceiling Redo


Local codes can vary but as far as I know you can't use fiberglass or Roxul insulation (air permeable) directly against the bottom of the roof deck. Air gets in and finds a cold surface and you have mold and rot.
http://www.greenbuildingadvisor.com/...hedral-ceiling

If you can fill in your location of tell us your climate zone we can be more specific.

Bud
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Old 07-18-2018, 09:56 PM   #6
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Re: Cathedral Ceiling Redo


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Originally Posted by arcticranger View Post

What is the actual difference in R performance between fluffy and solid and why is solid foam stacking so rare?
https://www.todayshomeowner.com/insulation-r-value/

It is significant which allows you to get more r value in less space with foam. I stacked foam board insulation in my cathedral ceiling to hit the required r value in a 2x10 rafter bay and still have room for venting..

The reason for not doing it is cost.

Last edited by ryansdiydad; 07-18-2018 at 09:59 PM.
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Old 07-19-2018, 05:41 AM   #7
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Re: Cathedral Ceiling Redo


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Originally Posted by Bud9051 View Post
Local codes can vary but as far as I know you can't use fiberglass or Roxul insulation (air permeable) directly against the bottom of the roof deck. Air gets in and finds a cold surface and you have mold and rot.
http://www.greenbuildingadvisor.com/...hedral-ceiling

If you can fill in your location of tell us your climate zone we can be more specific.

Bud

I'm in Sullivan County NY, it's zone 6. Both hot and cold.
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Old 07-19-2018, 05:46 AM   #8
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Re: Cathedral Ceiling Redo


Quote:
Originally Posted by ryansdiydad View Post
https://www.todayshomeowner.com/insulation-r-value/

It is significant which allows you to get more r value in less space with foam. I stacked foam board insulation in my cathedral ceiling to hit the required r value in a 2x10 rafter bay and still have room for venting..

The reason for not doing it is cost.



Yes it will cost for sure. Can you describe your assembly from roof deck down and what solid foam product you used?
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Old 07-19-2018, 10:03 AM   #9
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Re: Cathedral Ceiling Redo


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Originally Posted by arcticranger View Post
Yes it will cost for sure. Can you describe your assembly from roof deck down and what solid foam product you used?
So it goes

- Decking
- Baffles to ensure I didn't push the foam boards up against the decking and cut off air flow..
- 4 layers of foam board with spray foam applied with each layer to fill in any gaps
- Drywall

I am pretty sure I used these boards and I did reach out to the manufacturer to confirm it was okay to stack these..

https://www.menards.com/main/buildin...9869921&ipos=7

Given the R-value required the only other way to hit the mark was spray foam and the cost for that was way higher than my just doing the boards myself.

Passed inspection the architect just needed to note the use of foam boards in an "as built" letter for the city.

Last edited by ryansdiydad; 07-19-2018 at 10:07 AM.
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Old 07-20-2018, 12:05 PM   #10
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Re: Cathedral Ceiling Redo


Quote:
Originally Posted by ryansdiydad View Post
So it goes

- Decking
- Baffles to ensure I didn't push the foam boards up against the decking and cut off air flow..
- 4 layers of foam board with spray foam applied with each layer to fill in any gaps
- Drywall

I am pretty sure I used these boards and I did reach out to the manufacturer to confirm it was okay to stack these..

https://www.menards.com/main/buildin...9869921&ipos=7

Given the R-value required the only other way to hit the mark was spray foam and the cost for that was way higher than my just doing the boards myself.

Passed inspection the architect just needed to note the use of foam boards in an "as built" letter for the city.
Good info. That's pretty much what I will be doing, it's called cut and cobble. By "baffles" do you mean you have rafter vents in each bay or just spacers?

The greenbuilder article Bud9051 referenced does say you can't do cut and cobble in an unvented cathedral ceiling and that's my problem as I only have one soffit on each side of my fireplace.

Is there's a way to vent rafter cavities that don't have a soffit available? Do all rafter pairs have to have soffits?
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Old 07-20-2018, 12:25 PM   #11
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Re: Cathedral Ceiling Redo


@arcticranger, "The greenbuilder article Bud9051 referenced does say you can't do cut and cobble in an unvented cathedral ceiling" To save me from having to re-read the entire article can you post a quote for that statement.

As far as i know piecing together sections of rigid insulation can be used for either vented or unvented assemblies as long as you follow all of the other guidelines.

Yes, there are ways to vent rafter cavities that do not have soffits, as in under shingle vents or edge vents (or other approaches).

Bud
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Old 07-20-2018, 01:11 PM   #12
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Re: Cathedral Ceiling Redo


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Originally Posted by Bud9051 View Post
To save me from having to re-read the entire article can you post a quote for that statement.

Bud

Here's the quote.

---------------------------------------------------------------------

What about the cut-and-cobble method?


Cut-and-cobble is an insulation method used by some homeowners, but never by insulation contractors. It involves cutting rigid insulation into narrow rectangles, and inserting the rectangles between rafters or studs. In most cases, the perimeter of the each rectangle of rigid foam is sealed with canned spray foam or caulk.


When it comes to cathedral ceilings, here's the rule: the cut-and-cobble method can be used for vented cathedral ceilings, but not for unvented cathedral ceilings. (There have been several reports of moisture problems in unvented cut-and-cobble cathedral ceilings.)"
-----------------------------------------------------------------------
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Old 07-20-2018, 01:51 PM   #13
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Re: Cathedral Ceiling Redo


Did you discover any issues or signs of issues when you opened up the ceiling?

At some point someone, maybe Bud, provided a calculation for the amount of venting required for an addition on my house. You maybe have enough to leave it vented? Have you looked in to a calculation for your place?

The baffles are just spacers used to keep insulation away from the decking..

Last edited by ryansdiydad; 07-20-2018 at 01:53 PM.
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Old 07-20-2018, 01:53 PM   #14
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Re: Cathedral Ceiling Redo


Just FYI that same author Martin Holladay warns against cut and cobble for vented OR unvented ceilings and cites various disasters. He then provides instructions for doing both. In the same article!


http://www.greenbuildingadvisor.com/...ble-insulation
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Old 07-20-2018, 01:59 PM   #15
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Re: Cathedral Ceiling Redo


I don't have a specific example but the industry has been for years fitting rigid foam into rafter cavities on unvented assemblies. The "There have been several reports of moisture problems in unvented cut-and-cobble cathedral ceilings." just tells me those ceilings were not air sealed. "Cut and Cobble" makes it sound easy but the devil is in the detail and from doing a bit of cobbling I can say it takes patience.

One tip is to give the can foam you use enough space so you can get it in place to do its air sealing. Rigid foam cut to "almost fit" means a thin gap where the foam can only cover the outside and not get into where it can seal the entire space between the foam and the framing or other foam.

In fairness in my work I used as many full length pieces as possible and always staggered seams.

You will have to make your choice but my take on that statement was more of a caution as opposed to a definite never.

But I definitely prefer vented assemblies.

Bud
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