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Dow Tuff-R vs, Sturdy-R

I am planning on wrapping a home with rigid foam before I apply exterior siding, actually cement fiber. I will use a air space between the foam and the siding via strapping.

My question is whether to use a foil faced product like Tuff-R (in the interest of getting a great air infiltration barrier) or something with a bit more perm like Study R (fiberglass faced) I am wondering if I should worry about any water getting behind the foam (or from the inside of the wall) and the foil faced product trapping it, where as the Sturdy-R has a perm rating of 3, Doesn't need to be a DOW product specifically. I guess I'd use a vapor barrier on the outside of the Sturdy-r such as felt or some other house wrap such as Tyvek. I like the idea of the Tuff-R as it serves well as a air infiltration barrier and it tape sticks to it well at the seams.

I plan to use about an 1" thickness and I think this should keep the dew point outside the interior wall.( assumed R-13 in wall.

I haven't investigated the price point of the Tuff-R vs the Sturdy either but most places i know don't carry it, so it would end up being a special order or I would end up buying a lift of it

Thanks

Asked by Steve Greenberg
Posted Wed, 05/27/2009 - 21:33

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4 Answers

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1.
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I like using taped tuff-r directly under a rain screen shiny side out to maximize the radiant barrier quality of the foil by leveraging the air space of the rain screen. However I need to know your climate to help with the dew point question. If you're in a cold climate, one inch may not be sufficient to keep the warm side of the foam above the dew point.

There's a chart in here somewhere (John Brooks please chime in with that chart's location from Building Science Corp) but you may well want to run a layer of cheap 3/4" foam board first and go over it with the tuff-R foil faced poly-iso board with staggered seams. You only need to tape the exterior layer though.

Answered by Michael Chandler, GBA Advisor
Posted Thu, 05/28/2009 - 00:11

2.
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Michael, ( or anyone else who would like to chime in)

The project I'll be working on, actually two of them, will be in the Boston area. From what I've been told, 1" of foam should keep the dew point outside the wall cavity.

I'm more concerned, or more of the question is, using the Sturdy R vs the Tuff. The Tuff R has the foil facing which is going to really help with the air infiltration, I think the Sturdy R with the fiberglass less so. The mechanical engineer who has been helping me a bit is concerned about trapped water between the sheathing and the foam (vapor or liquid from a leak) which is why he suggested the Sturdy R as it would allow drying.

What's your thinking, or better yet practice, on this? I know I'm going to have to be careful about flashing details to keep water to the outside of the foam in either case, but I'd love to be able to use the foil face. I guess if I do go with the fiberglass face, house wrap would do a bit more for the air infiltration, but not as much as the foil with taped seams.

Thanks again.

Answered by Steve Greenberg
Posted Thu, 05/28/2009 - 10:51

3.
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Steve,
The Building Science website is fantastic and full of great info....and it is constantly growing and being updated.
There are also books.
If you do not own a copy already .. I would suggest an EEBA (Joe Lstiburek's) Builder's Guide to (your climate)
There are guidlines and theory concerning condensation potential in walls and roofs.
I would be cautious about attempting to create a reliable air barrier with rigid insulation and House Wrap.
If you were Joe Lstiburek .. you could make it work.
If you were Joe Schmoe I would be cautious.
Outsulation is a good thing .. just not the best primary air barrier.
At this point .. I prefer spray foam because I think it is more user friendly...not cheap...but more user friendly.

Answered by John Brooks
Posted Thu, 05/28/2009 - 16:32

4.
Helpful? 0

I think for the Boston area you should have 1/3 r-value inside to 2/3 r-value outside to prevent the sheathing from reaching the dew point. That is according to the Building Science website.

Answered by Gerard Michney
Posted Tue, 06/09/2009 - 18:10

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