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Old 11-28-2017, 11:44 AM   #1
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Vapour Permeability of 3 mm (1/8") hardboard


Hi all,

I will be putting in site-built ventilation baffles from soffit to ridge vent on a cathedral ceiling.

Rafters are 2x4s, so space is at a premium to get needed R-31. Instead of 1/4" or 1/2" OSB as most would recommend, I'd like to use 3 mm hardboard on top of 2x2s ripped to 1".

https://www.homedepot.ca/en/home/p.3...000117636.html

The 4x8 panels will cut in half nicely with a table saw and fit between my 24" rafters.

Will 3mm harboard be think it'll be rigid enough? Alsoit needs to be vapour permeable (so that I am not creating un-dryable space above the interior vapour barrier). I know OSB has decent vapour permeance, but what about hardboard?

I can't seem to find vapour permeability stats anywhere for it.
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Old 11-28-2017, 02:52 PM   #2
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Re: Vapour Permeability of 3 mm (1/8") hardboard


Well, it would allow some diffusion but since no one is going to use it in a vapor control application I doubt the mfg will test and publish a number. Basically it is thin and slightly permeable so will allow drying to the outside. As always the key to moisture control is eliminating air leakage, from the inside in this case.

Not familiar with your minimum insulation requirements but never seen an R-31 in the north.

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Old 11-28-2017, 02:58 PM   #3
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Re: Vapour Permeability of 3 mm (1/8") hardboard


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Not familiar with your minimum insulation requirements but never seen an R-31 in the north.

Bud
My reading of Ontario code is that R-60 is required for ceilings below attic space, but in "ceilings without attic space" (ie cathedral ceilings) only R-31 is required.

http://insulation.owenscorning.ca/pr..._Jan_01_17.pdf

Seemed low to me too, but hard to read the code any different.
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Old 11-28-2017, 03:23 PM   #4
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Re: Vapour Permeability of 3 mm (1/8") hardboard


Why not just staple air chutes the whole way, I have not seen anyone build anything like described.
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Old 11-28-2017, 03:32 PM   #5
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Re: Vapour Permeability of 3 mm (1/8") hardboard


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Why not just staple air chutes the whole way, I have not seen anyone build anything like described.
I've heard they are flimsy, hard to air seal, and depending on the material may trap moisture.

This article describes those issues and recommends site built:

http://www.greenbuildingadvisor.com/...-baffles-roofs

I know there are good products out there, but all that seems to be available in my area is something called Durovent.

https://www.lowes.ca/insulation-acce..._g1190132.html

I suppose the edges could be air sealed with a bead of spray foam, and I know thin polystyrene foam is somewhat vapour permeable ... but do you think it might be too flimsy?

Based on my rafter size I will be compacting 3" mineral wool into about 2.5" of space - would that be too much pressure and crush the Durovent, or do you think it'd hold up?
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Old 11-28-2017, 03:57 PM   #6
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Re: Vapour Permeability of 3 mm (1/8") hardboard


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Originally Posted by jmetrail View Post
I've heard they are flimsy, hard to air seal, and depending on the material may trap moisture.

This article describes those issues and recommends site built:

http://www.greenbuildingadvisor.com/...-baffles-roofs

I know there are good products out there, but all that seems to be available in my area is something called Durovent.

https://www.lowes.ca/insulation-acce..._g1190132.html

I suppose the edges could be air sealed with a bead of spray foam, and I know thin polystyrene foam is somewhat vapour permeable ... but do you think it might be too flimsy?

Based on my rafter size I will be compacting 3" mineral wool into about 2.5" of space - would that be too much pressure and crush the Durovent, or do you think it'd hold up?
Neither link got me to where you wanted it to go.
Air leak. The idea is to have free air flow above the insulation. If you were putting a 6" insulation in a 2x10 you don't add anything. So there is no need to worry about that.
The biggest problem with any insulation is the bridge from warm to cold that is made with the rafter adding to the rafter so you can nail something up, would be making the bridge bigger.
You will not be compacting safe and sound, you might compact fiberglass.
Either way they are most effective when used as intended.
I would nail 2x3s to the bottom of the rafters, use chutes off the shelf and 5 1/2 inches of insulation.

The idea of material absorbing water is bogus as the whole idea is air movement to dry to keep things dry.
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Old 11-28-2017, 04:16 PM   #7
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Re: Vapour Permeability of 3 mm (1/8") hardboard


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I would nail 2x3s to the bottom of the rafters, use chutes off the shelf and 5 1/2 inches of insulation.
That's only give me 5" - are you saying that mineral wool will not be able to compact by 1/2" like fiber glass would?
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Old 11-28-2017, 05:22 PM   #8
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Re: Vapour Permeability of 3 mm (1/8") hardboard


Rigid foam, whether pre-done board or spray is the best for this application.

If the other side is going to be impermeable roof, even if you use vapour permeable sheathing, it won't have good drying potential.

No vapour barrier is perfect, there will be some leakage.

Hence using foam, provides a complete thermal break with no danger of trapping moisture. This is if it's installed right.
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Old 11-28-2017, 05:31 PM   #9
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I have considered going unvented but decided against for a variety of reasons.

I will have plenty of soffit venting and a ridge vent. Shouldnt that air movement mean there will be plenty of drying potential?
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Old 11-28-2017, 05:39 PM   #10
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Re: Vapour Permeability of 3 mm (1/8") hardboard


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That's only give me 5" - are you saying that mineral wool will not be able to compact by 1/2" like fiber glass would?
So add another half inch to the rafters.
Don't think mineral wool would compress much.
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Old 11-28-2017, 07:00 PM   #11
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Re: Vapour Permeability of 3 mm (1/8") hardboard


Let's go back to the r-value for a minute. Two issues.
1. Even if the code as you read it says r-31 you need to verify with your local authority that you have the right number. I couldn't find their definition of "ceilings without attic space" and saw the same number as you did. But when calculating total heat loss, due to the increased surface area of a cathedral ceiling they really need more than when insulation is at the attic floor. Attic floor was listed at r-60 so makes no sense to allow r-31 in the rafters.
2. Now, if r-31 turns out to be your number, how are you adding depth to accomplish that? Starting with 2x4's and providing a 2" air channel leaves 1.5" less the chute thickness. You would need to add 2x8's to the existing 2x4's to get your depth. How you plan on achieving that should also be discussed with your local authority.

Also, Roxul doesn't compress much and whatever number you come up with should be achieved by adding layers at mfg designed thickness.

Once we know what number you are headed for we can make suggestions.

Bud
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Old 11-29-2017, 07:29 AM   #12
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Re: Vapour Permeability of 3 mm (1/8") hardboard


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Originally Posted by Bud9051 View Post
Let's go back to the r-value for a minute. Two issues.
1. Even if the code as you read it says r-31 you need to verify with your local authority that you have the right number. I couldn't find their definition of "ceilings without attic space" and saw the same number as you did. But when calculating total heat loss, due to the increased surface area of a cathedral ceiling they really need more than when insulation is at the attic floor. Attic floor was listed at r-60 so makes no sense to allow r-31 in the rafters.
2. Now, if r-31 turns out to be your number, how are you adding depth to accomplish that? Starting with 2x4's and providing a 2" air channel leaves 1.5" less the chute thickness. You would need to add 2x8's to the existing 2x4's to get your depth. How you plan on achieving that should also be discussed with your local authority.

Also, Roxul doesn't compress much and whatever number you come up with should be achieved by adding layers at mfg designed thickness.

Once we know what number you are headed for we can make suggestions.

Bud
Thanks for the reply.

The R-31 for "ceilings without attic space" are in the tables on page 30 and 32.

http://insulation.owenscorning.ca/pr..._Jan_01_17.pdf

I suspect they recognize the practical limitations of putting R-60 in cathedral ceilings, and have allowed a lower R value.

Most jurisdictions in Ontario don't require a permit to add or replace insulation. I also cannot fathom a cathedral ceiling not qualifying as a "ceiling without attic space"

But you have a good point - whether I require a permit or not, it couldn't hurt to inquire with the building dep't to ensure I have the right interpretation of the code.

As for getting to R-31, the plan is likely to be:

-1.5" air channel
-scab out rafters with 2x4s, giving me 5 1/2" depth - fill with R22 roxul
-add 1" layer polyiso - R6
-strap with 1x4s, put 3/4" polyiso in between strapping - R4.5

That'll get me a total R-32.5

Another alternative would be:

-1" air channel
-fill with 2.5" roxul for R10 (would have to compress 3" safe 'n sound 1/2")
-3" layer of polyiso for R18
-strap with 1x4s, put 3/4" polyiso in between strapping - R4.5
-3/4" strapping would be nailed through 3" foam with 5" ring-shank nails (1.25" rafter penetration should be plenty?)

That'll get me a total R- 32.5, with the advantage of not having to scab out the rafters. It also save me some living space - would only extrude from rafters 3.75" vs. 5.25" for the other configuration.

I don't love the idea of trying to compress Roxul (even by 1/2"). I think there exists 2.5" fibre glass batts out there, but they are only R-8, which will leave me R-0.5 short.
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Old 11-29-2017, 09:02 AM   #13
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Re: Vapour Permeability of 3 mm (1/8") hardboard


I have elected to trim some Roxul to a lesser thickness and did so by building a simple box to hold the batts in place. Then, like sawing the lady in half, I ran my handsaw down through a slot i had provides and "tada" I had neatly trimmed the batt to the desired thickness. The more you have to do the more it justifies the box approach.

Codes aside, that will be a nice assembly, either one.

Bud
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Old 11-29-2017, 12:17 PM   #14
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Re: Vapour Permeability of 3 mm (1/8") hardboard


Good call on talking to the building dep't.

They said that it is advisable to get a permit in this instance.

They confirmed that R-31 is required for cathedral ceilings.

Further, they said that because I am essentially adding to R-0 (this is an old lathe and plaster home with no insulation), they will not be super strict about hitting the R-31.

I am still going to try for R-31, but am now not too worried if I end up shy of that.
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Old 11-29-2017, 01:10 PM   #15
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Re: Vapour Permeability of 3 mm (1/8") hardboard


If you want to finish with 1" foam board, skip the strapping, they make long drywall screws.
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