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Old 09-16-2018, 12:30 PM   #61
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Re: Cathedral Ceiling Redo


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Originally Posted by arcticranger View Post
Quick update, I have started this project finally. Bud, I completely overlooked this statement by you:

"With spacers to hold the first layer of rigid away from the roof deck that becomes your baffle"

OK got it (bonk) the 2" foam IS the baffle. Should that XPS channel run all the way to the ridge vent?
Yes.............
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Old 09-16-2018, 12:50 PM   #62
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Re: Cathedral Ceiling Redo


The baffle is there to keep the insulation away from the the roof deck so if insulation goes all the way to the ridge so should the baffle.

Bud
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Old 09-18-2018, 04:34 PM   #63
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Re: Cathedral Ceiling Redo


So I've got my 2" XPS foam with 2" air channel enclosing 3/4" holes, foamed tight. I should probably have gone with caulk but I lost that detail.

I was surprised it was only 13' from floor to peak.

Now I've got to figure out the best filler for the remaining 6.5 inches. 3 more layers of rigid will run $300 as the first layer took 3 full sheets of 4x8 XPS at $100. Filling it with R23 Roxul will be about $150, I could go R30 Roxul but I'd have to add an inch to the rafters. So with the 2" foam being R10 my options are:

air gap
4 layers of 2" XPS
-----------------
R40 at $400


air gap
1 layer of 2" XPS
5.5" of Roxul R23
-------------------
R33 at $250


air gap
1 layer of 2" XPS
7.5" of Roxul R30
------------------
R40 at $280


I do have a good deal of 1", 1.5" and 2" XPS as well as a LOT of 1 and 2" foil faced polyiso sitting around but I am not sure if puzzling it all together is a good idea. Also I've been avoiding the foil faced rigid as I don't want to create a vapor barrier.

I can also add a layer of 1" XPS over the inside joists, that would only be $60 or so.

And there will be a ceiling fan up there.

Further thoughts?

Last edited by arcticranger; 09-18-2018 at 04:43 PM.
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Old 09-19-2018, 01:36 PM   #64
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Re: Cathedral Ceiling Redo


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Originally Posted by Bud9051 View Post
The baffle is there to keep the insulation away from the the roof deck so if insulation goes all the way to the ridge so should the baffle.

Bud
and ps to Bud thank for the great advice, and to the others who chipped in. I do think this was the best solution to my tricky cathedral ceiling issue.

(I can already see the real estate agent's eyes rolling as I wax poetic about my fancy air channel)
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Old 09-24-2018, 02:21 PM   #65
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Re: Cathedral Ceiling Redo


A few questions if anyone is still out there.

What's the purpose of air sealing the rigid foam that created my air channel? Is it about maintaining a certain pressure? My worker did it pretty well but not perfect. I may tape the seams tomorrow.

I used 2" of XPS for my baffle. Some guy on the greenbuilding site said 2" has a perm rating of .5 and will block drying of room moisture via the air channel. He said I should have used 1" rigid and a 1" channel. I didn't pursue that as those greenbuilder guys seem to go OCD on everything. But I would appreciate any feedback on it.
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Old 09-24-2018, 03:12 PM   #66
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Re: Cathedral Ceiling Redo


Doesn't make sense for that advice to come from GBA, maybe another site using the greenbuilding name. Send me a PM with the link to those comments so I can see who said that.

2" is the desired channel and 1" is considered absolute minimum. As for 2" rigid not having a high enough permeance to dry to the outside, it's not supposed to. The objective is to prevent moisture from reaching the cold roof deck and if any gets through the ventilation is supposed to carry it away. 2" is also thick enough that it's inside surface will remain above the dew point. 1" becomes questionable.

The benefit of air sealing those channels is to prevent air leakage from the house carrying humidity up where it can find a cold surface.

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Old 10-18-2018, 01:25 PM   #67
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Re: Cathedral Ceiling Redo


My semi-vented cathedral ceiling has been done for a month, the layers from top to bottom are

roof sheathing
2" air channel with 3/4 holes as Bud recommended
2" solid foam (XPS)
6" Roxul

I've been holding off on closing it up to get some wiring done up there like a ceiling fan. Today that was completed.

I'd like to ask for a last bit of advice on closing this ceiling up. Should I go with sheetrock or is metal or some other material ok? The most important thing of course is maximizing the insulation I have done. I'm considering galvanized steel corrugated roof panels as an interior ceiling, mostly because they are light and go with the industrial look of the space. And maybe they can help reflect heat?

Last edited by arcticranger; 10-18-2018 at 01:28 PM.
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Old 10-18-2018, 02:57 PM   #68
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Re: Cathedral Ceiling Redo


I've never worked with or read about using galvanized steel corrugated roof panels on an interior ceiling. The only concern that comes to mind would be a perfect air seal. Metal expands and contracts with temperature and screws will often end up in elongated holes. The screws with rubber washers might work but unknown how long.

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Old 10-18-2018, 03:19 PM   #69
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Re: Cathedral Ceiling Redo


Drywall and wall paper for the look.
https://www.wayfair.ca/decor-pillows...RoCwPkQAvD_BwE
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Old 10-19-2018, 11:07 AM   #70
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Re: Cathedral Ceiling Redo


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Originally Posted by Bud9051 View Post
I've never worked with or read about using galvanized steel corrugated roof panels on an interior ceiling. The only concern that comes to mind would be a perfect air seal. Metal expands and contracts with temperature and screws will often end up in elongated holes. The screws with rubber washers might work but unknown how long.

Bud

Yes all true. Ever heard of using Tyvek on an interior wall? I'd have to cover it but it is

a) an air barrier
b) not a vapor barrier.

If that was tight it might allow for more creative finish options than drywall. Just thinking.
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Old 10-19-2018, 01:49 PM   #71
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Re: Cathedral Ceiling Redo


http://www.dupont.co.uk/products-and...for-walls.html
Highly permeable to water vapour – can be fixed directly to insulation (Sd-value: 0.01m)
That would not be what you want for a ceiling you want to keep dry.
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Old 10-19-2018, 02:06 PM   #72
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Re: Cathedral Ceiling Redo


Tyvek was specifically designed to not be a vapor barrier and it was not intended to serve as an air barrier. It provides a drain plane to shed water while allowing drying to the outside. As for the air barrier, it can help, but an air barrier also needs to be rigid. Tyvek alone would billow in and out with air pressure pumping air in and out as it does.

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Old 10-19-2018, 02:53 PM   #73
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Re: Cathedral Ceiling Redo


The sheet metal ceiling is just an idea at this point. I found another guy on GBA who actually did it. Interestingly it was on a cathedral ceiling with the same vented + rigid foam + air channel gap setup that Bud gave me. The 3rd entry by Don J describes it

https://www.greenbuildingadvisor.com...ing-wall-panel

My idea was to staple the Tyvek to the rafters, flush against the Roxul, with the metal sheeting directly over it. The Tyvek would be my air barrier and my 2" XPS would be my moisture barrier.

But aside from the difficulty of finding anyone who can do this well, I doubt I could get this assembly as tight as sheetrock. It's kind of amazing in 2018 that there isn't another technology lighter and easier to install than drywall.
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Old 10-19-2018, 03:07 PM   #74
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Re: Cathedral Ceiling Redo


Quote:
Originally Posted by Nealtw View Post
http://www.dupont.co.uk/products-and...for-walls.html
Highly permeable to water vapour can be fixed directly to insulation (Sd-value: 0.01m)
That would not be what you want for a ceiling you want to keep dry.

If I add sheetrock this would be my assembly. I assume some moisture would exit through via the XPS to the air channel and the rest I don't know, back out via the sheetrock?

roof sheathing
2" air channel with 3/4 holes as Bud recommended
2" solid foam (XPS)
6" Roxul
sheetrock

Those wallpapers really are cool.
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Old 10-19-2018, 03:17 PM   #75
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Re: Cathedral Ceiling Redo


Quote:
Originally Posted by arcticranger View Post
If I add sheetrock this would be my assembly. I assume some moisture would exit through via the XPS to the air channel and the rest I don't know, back out via the sheetrock?

roof sheathing
2" air channel with 3/4 holes as Bud recommended
2" solid foam (XPS)
6" Roxul
sheetrock

Those wallpapers really are cool.
You don't want any moist air from the house getting in there. If it were here it would get a 6 mill poly before the drywall,
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