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Vapor permeability of polyiso

Attached is a picture of foil-faced polyiso which as a bunch of holes in it. The holes were made with a wall paper removal tool which puts holes in the paper to allow liquid stripper to get behind the paper. I welcome comments on this approach to see of foil faced polyiso could be modified to allow vapor permeability thru the sheet if holes were done on both sides.

I was given 22 brand new sheets. In there current impermeable form I cannot use them. I need a product which will allow drying of a cathedral ceiling to the inside of the home . I can hang onto them for a future project but I would rather use them up now if possible.


Polyiso holes.jpg181.83 KB
Asked by Dirk Gently
Posted Tue, 03/18/2014 - 21:04
Edited Wed, 03/19/2014 - 09:02


10 Answers

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Dirk, IMO you are miss using the foam and not improving the perm of the product.

Why does your ceiling have to dry to the interior. Build it so it can't gain moisture. What climate? What details for the ceiling assembly?

Dana... next... the amazing perm calculating bro.

Answered by aj builder, Upstate NY Zone 6a
Posted Tue, 03/18/2014 - 21:38

Helpful? 0

I build cathedral ceilings with a layer of the same foam with all seams taped to keep interior heat and moisture in the home. This assembly has worked here for decades.

Answered by aj builder, Upstate NY Zone 6a
Posted Tue, 03/18/2014 - 21:40

Helpful? 0

That won't get you any improvement in vapor permeability. The foil is still at least 99% intact, so it's at least 99% effective as a vapor barrier. It's also still 100% effective as a radiant barrier.

I would sell that foam on craigslist or something like that, and get mineral wool panels if you need rigid that passes vapor.

Answered by David Meiland
Posted Tue, 03/18/2014 - 23:36

Helpful? 0

David Meiland is right. The foil facing on the sheet of polyiso shown in the photo is still a vapor barrier.

AJ Builder is also right (if I understand his point correctly): Depending on the details you plan to use, it's possible that your cathedral ceiling assembly will still work, even if you include foil-faced foam. For more information on this topic, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 03/19/2014 - 05:17
Edited Wed, 03/19/2014 - 05:19.

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Thanks David that pretty much answered my question.
I was thinking if the same number of holes were in an normal vapor would be considered useless.
The reason for the drying to the interior is that much of the cathedral ceiling is low slope dormers which have are covered 100% with Ice shield and asphalt shingles. The open cell foam will need to dry to the interior. Zone 5.
Because of the 3/12 slope and also a valley, venting is not really an option for this remodel.
Looks like I will be getting reclaimed felt facers to boost my r-value/reduce thermal bridging.

Answered by Dirk Gently
Posted Wed, 03/19/2014 - 06:12

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You wrote, "I was thinking if the same number of holes were in an normal vapor would be considered useless."

I think that you are confusing an air barrier with a vapor barrier. For more information on the difference, see Do I Need a Vapor Retarder?

If a piece of polyethylene has a great many holes -- say, holes amounting to 5% of the area of the sheet -- it is still a perfectly acceptable vapor retarder. But it isn't an air barrier.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 03/19/2014 - 06:45

Helpful? 0

Vapor diffusion is a surface area x permeance deal, but with enough perforations the foil isn't much of a vapor barrier.

Vapor permeable radiant barriers with grids of tiny holes on roughly a half-inch grid have a measured permeance greater than 10 perms (eg: 14 perms, not even a class-III vapor retarder!) To get above 1-perm with two facers you'd probably have to use a 2" grid on both facers with that tool, but it would get there. The polyiso itself is greater than 25 perms @ 1".

Polyethylene sheeting punctured on a grid that constitutes even 1% (let alone 5%) of the total area it would also probably be greater than 10 perms if done on a tight grid as in the perforated foil RB. Air is EXTREMELY vapor permeable.

Answered by Dana Dorsett
Posted Wed, 03/19/2014 - 13:58

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1" unfaced polyiso perm is 2.5-4. with the punctured foil that would drop to much less than 1 IMO


other basic roof polyiso foams that are faced with other than foil, glassed felt etc... perm .5

EPS 1" 5 perm, 2" 2.5 perm, 3" 1.67 perm, 4" 1.25 perm

You need a perfect air barrier. And low indoor moisture. Or you need to closed cell spray foam which needs to be done right during a mild day with no burning sun, overcast perfect. You need the roof to be all the same temperature so the spray temperatures are similar. Less chance of problems. Not a fan of spray foam but done right is great.

No spray foam option; I would trust myself to install your foil foam perfect enough to be your air barrier. For me the ceiling would have to include a vent channel and high density fiberglass batts. Then all is pretty much bullet proof.


Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 03/19/2014 - 15:08
Edited Wed, 03/19/2014 - 15:12.

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I appreciate your input. Do you use a 1" vent channel or a 2" channel?
Your point about even roof temperature is also a great one.
I see the permeability of 1" poliso in your link....I am not sure where Dana got his number from. I will have to do some more research into that to see if there is data to back his claim.

The breathabilty of the perforated foil is the main goal. I remember using some type of paper/foil which was perforated under vinyl siding back in the 80's. and that is what gave me the idea.

My original plan was 1" channel then dense pack cellulouse AND "cut n cobble" rigid foam into the area where the valley is (it is not very big). I was to follow that with 2" rigid foam on the underside of rafters to reduce thermal bridging. BUT the more I learned about lower slope roofs (a large portion of my project is 3/12 and 2/12 shed dormers) The less I felt comfortable with vented route. Both of those roofs are E or W facing.
ALL of the insulation contractors I had look at the project wanted to use open cell foam on the one advised dense pack (indeed they talked me out of it) OR closed cell foam (CCF).
I may be open to CCF. I am a believer of Riversongs advice on not trapping sheathing between 2 impermeable surfaces (ice shield and CCF). Is Riversongs line of thought still valid?

A note about insulation contractors: I have a small amount of open cell on underside of roof deck (about 7 linear feet @8" thick) in my cape/knee wall areas. NO drywall over it. It was painted with fire retardant paint and I was told that was all I needed by the contractor in 2010. I know of 2 others who have that same setup (more disasters waiting to happen). Given what I know now about OC foam needing drywall and paint.....NONE of those contractors have much credibility with me now....

Answered by Dirk Gently
Posted Wed, 03/19/2014 - 16:10

Helpful? 0

AJ: OOPS! It Looks like missed the decimal point- I need an editor. I know from reading way too many specs that 2lb & 3lb per cubic foot roofing polyiso can be as low as 1.5-2 perms @ 1" without facers, and you can actually buy it that way (eg: ) But nearly all foil-faced goods sold these days is 1.5lbs which is quite a bit more permeable. It's hard to find hard data or specs on unfaced 1.5lb polyiso is never actually sold that way. I was trying to be conservative with 2.5 perms. The holes made by that stripper tool are far larger than those in vapor-permeable radiant barrier, my unstated WAG on it was that if you run a 2" grid of holes you'd get the permeance up to about 5, and if you put two ~5-perm facers on a 2.5 perm core the total permance is north of 1. If you put two 10 perm facers on a 2.5 perm core it's in the 2 perm range. At the size of those holes it might get there too, even if you assumed 2.5 perms for the foam. If it's actually 4-perm foam (within the range of the BSC estimates) you'd be over 2.

Your EPS permeance estimates are only applicable to 1.0lb nominal density "Type-I" EPS. At higher density it has lower permeance. At 1.5 density (Type-II) EPS is typically ~3 perms @ 1" range, Type IX 2lbs nominal density goods run around 2 perms @ 1". Here are the specs across a range of densities from one manufacturer:

Nick: In-re exposed open cell foam- it's worth spray-applying "vapor-barrier" latex on it, which according to BSC data comes in at about 5 perms in that application (not the ~0.5 perms it reaches when applied to wallboard.) At 5 perms it's the rough equivalent of standard latex on gypsum, and though you'll have some moisture cycling in the roof deck it's not a problem as long as the interior humidity is kept to 35%RH @ 70F (it can be a higher RH at lower indoor temps- consult a pschrometric chart.)

Answered by Dana Dorsett
Posted Wed, 03/19/2014 - 17:49

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