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Over-roofing with wood-fiber-insulation and deciding on the vapor profile

Tyler Keniston | Posted in Green Building Techniques on

Hi all,

Working on a roof retrofit using exterior insulation.  Along the lines of what’s described here:https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing

Major difference is we’re planning to use rigid wood-fiber insulation like Steico, Gutex, or similar: https://www.steico.com/en/products/wood-fibre-insulation/steicotherm-dry/overview/
We’ll also over vent with vertical 2×4’s on the flat.

This is in anticipation of the upcoming release of GO-Lab Timber HP products in the state we live (Maine): https://golab.us/
The idea is to begin implementing details that use these products.

The main question here is regarding the properties of the air/vapor control applied at the roof sheathing plane. Most rigid foam details use a high-blocking vapor retarder adhered to the sheathing plane, or the foam provides that itself. Note the sheathing in this case is horizontal board, so it’s not an air barrier or smart vapor retarder like plywood sheathing would be.

As far as I can tell, most details with exterior wood-fiber call for vapor open membranes in this location. Wood-fiber insulation is itself quite vapor open. I believe the insulation can handle some moisture (better than OSB for example) but it is still a wood product. So there is a part of me that feels that membrane should have higher vapor retarding properties than the really open products.
See the following text from 475 smart enclosure: 
“In Tier 3, exterior retrofits, the air barrier wrapped on the existing sheathing boards will also be the vapor control layer as needed. Because the wood structure will generally be wrapped in insulation board, there are more options for vapor control – from smart retarder INTELLO X to vapor open self-adhered ADHERO and stapled SOLITEX MENTO 1000. “

So they essentially say it can be open or smart. 
Any thoughts on this? Just go open? The only concern with open would be moisture loading of the wood insulation, and maybe that’s just not a problem. 

(The plan is to get the right R-value ratio exterior of this membrane (51% zone 6a) to control dew point, but is that fully necessary if the membrane is vapor open?)

You may be wondering about interior finish and it’s vapor/air properties. It’s a mixed bag, and therefore we were hoping to rely on the sheathing-plane membrane to do the work. It’s a 1.5 story cape. Will need to pull insulation off floor of attic triangle.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Tyler,

    This site has a lot of details on proper installation of wood fiber insulation: http://www.woodfiberinsulation.com/wood-fiber-insulation-therm.html.

  2. Expert Member
    Michael Maines | | #2

    Tyler, I'm at least as excited as you for GO Lab's insulation to be available. There are various formulations but most of them have nothing to add rot resistance so in some ways it's actually more sensitive to moisture than OSB, which has toxic glues and sometimes waxes to resist rot.

    What I have done and recommend with vapor-open, air permeable insulation above the roof deck (mineral wool, in my case, but same idea) to provide enough insulation above the roof deck for code-compliant condensation resistance--the same amount you would need for exterior foam insulation. In that case the roof deck can be impermeable.

    If you want to use less than a code-compliant amount of exterior insulation, just as with walls, if the assembly is vapor-open it's ok, but some wetting will occur so you need to be sure it can dry in both directions. In that case I would use a vapor-permeable underlayment such as Pro Clima Mento.

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