Poly Iso rigid foam and house wrap? - Fine Homebuilding (2025)

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patmando| Posted in onApril 24, 2009 06:45am

I’m preparing to install 1†foil faced poly iso foam insulation over ½â€ OSB. After reading Rick Arnold’s article where he states that taping the seams will create a “secondary air barrier and secondary drainage plainâ€, and comparing it to Building Sciences’ “Guide to Insulated Sheathingâ€, in which they rate this approach as being the least desireable approach, I’m confused. The Building Sciences article seems to advise that house wrap be applied over the foam, except where rainfall is limited. Is there any consensus as to whether or not poly-iso with taped seams is sufficient? I live in north central Oregon with about 15†inches of rain and snow in the winter but with hot dry summers.

Replies

1. 

   mike_maines |Apr 24, 2009 07:54pm |#1

   Pat, this is one of those areas of construction where consensus is difficult to find. There is a lot of information in the Breaktime archives (try "Advanced Search"), and you can get more information at http://www.greenbuildingadvisor.com.

   With your relatively dry climate I might be comfortable with just taped seams, but I always prefer the shingle-lapped drainage plain that housewrap affords, instead of relying on adhesives to keep the water out.

   The other concern I have is if you are using a vapor barrier (such as foil-faced foam) on the outside of the building, you need to:

   1. Make sure it's thick enough to prevent condensation on the interior surface on all except the very coldest days of the year. That thickness will vary depending on your area's climate and whatyou have forinsulation inside the wall cavity.

   2. DO NOT install a vapor barrier on the inside wall of the house. The wall will need to be able to dry to the inside on occasion. Sealing wall penetrations and using latex wall paint will slow the movement down, but allow drying to occur.

1. See AlsoTop 9 Foil Faced House WrapsHow To Install Foam Sheathing: Thickness, Vapor Barrier & Housewrap? - Building AdvisorWhere, When, And How To Use Foil-Faced Insulation - HVACseer.com

   coldbuilder |May 02, 2009 10:15am |#2

   here in alaska we use XPS with taped seams. and then house wrap as a drainage plain, and then 3/4 inch strips of AWW fastentd to framing members. They serve as backer for siding and they create a convective loop that helps pull water vapors that condense in your wall cavity to the outside. works like a charm.

1. 

   mike_maines |May 02, 2009 09:54pm |#3

   What's AWW?

   How thick is your XPS?

1. 

   coldbuilder |May 02, 2009 10:22pm |#4

   AWW is All Weather Wood, and standard here is 2"XPS if not 6"EPS. 2" is standard for retro apps."A small leak can sink a great ship."-Benjamin Franklin-

1. 

   1hotwire |May 02, 2009 11:07pm |#5

   Can you tell me what size / type of nails or screws you are using and what is the installed spacing of each fastener? What type of siding are you using? Do you have any links or photos of the process? I've been thinking of residing my house with Hardie fiber cement lap siding, and would like to add a thick layer of foam board insulation underneath. Thanks,Kenny

   Edited 5/2/2009 4:56 pm by 1hotwire

2. 

   coldbuilder |May 03, 2009 12:31am |#6

   well we use button nails or cap nails to fasten the foam to the sheeting (16" oc). and 4" outdoor screws to fasten the strips of AWW ply (12" oc). I have never done hardi siding with this app. I know you can get away with vynl, metal, cedar lap, T1-11. no problem. not sure for any likns of pics right now try cchrc.org"A small leak can sink a great ship."-Benjamin Franklin-

   Edited 5/2/2009 5:36 pm ET by coldbuilder

3. 

   mike_maines |May 04, 2009 05:58pm |#7

   AWW is All Weather Wood,

   Never heard that term, is it the same thing as pressure treated (CCA, ACQ, etc.)?

   and standard here is 2"XPS if not 6"EPS. 2" is standard for retro apps.

   Moisture will diffuse through EPS, and very slowly through 2" of XPS, but I don'tunderstand how convection would have anything to do with your wallsbeing dry. What am I missing?

4. 

   Clewless1 |May 05, 2009 03:51pm |#8

   AWW has been used for many years ... yes same thing as pressure treated, I believe. Back in the 70s, the plywood association had an "all weather wood" foundation wall (which I've used before). There MAY be a difference between AWW and pressure treated, but I don't think there is. Maybe todays AWW is not really for e.g. direct contact w/ earth, etc.

5. 

   mike_maines |May 05, 2009 07:32pm |#9

   Looks like it's a west coast product: https://www.allweatherwood.com/product-list.htm

6. 

   Clewless1 |May 06, 2009 04:14am |#11

   It's a Brand ... of treated wood products.

7. 

   mike_maines |May 06, 2009 05:22pm |#12

   Yup, on the west coast. Not available on the east coast which is why I've never heard of it.

2. 

   cargin |May 05, 2009 08:54pm |#10

   pat

   I would avoid using foil faced iso. There are other iso boards that are not a vapor barrier. Or use XPS. I don't like a vapor barrier on the outside and you didn't say if the inside had a vapor barrier.

   I like the house wrap on the outside of my foam. Just an extra layer of protection to keep everything dry. It's a small added cost.

   Rich

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