UPDATE Oct 28th 2015:  I tried the 3M #8067 tape on the fiber faced polyiso and it seemed to stick very well.  However, I talked with a local reclaimed insulation guy and I was able to trade the exterior layer of the fiber-faced stuff for foil faced stuff.  Therefore, I returned the mastic and just taped the foil faced polyiso with the 3M #8067 tape (of course it adhered very well).  I apologize to anyone that is curious about using fiber faced polyiso as an air barrier.

However, for the benefit of the forum, what I will do is set a piece of scrap fiber-faced polyiso aside with a bit of 3M #8067 taped on it and see how well it remains adhered 6 months from now (by comparing it to a piece of tape that has been adhered to it for just a day or so).

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Hello,

I am looking for ways to use reclaimed fiber-faced polyiso as an air barrier on the walls and roof of a small house I am building (this is a small DIY project) in upstate New York.  The reason I am using fiber-faced polyiso is that I was able to acquire it inexpensively.  The 4x8 boards came from the roof of a military installation.  They do not have any cupping and the fiber-facing is not peeling.  On each sheet there is only the occasional small ding and there are 6 holes from long screws that were used to fasten the boards in place (and so these holes will need to be air sealed as well).

First off, the wall assembly is as follows (from inside to outside):
* post & beam frame consisting of 8x8s and 4x4s
* 1" tongue-in-groove white pine, fastened to the frame.  Dried for 1 year
* 4" fiber-faced polyiso, 2 layers of 2" with staggered seams
* 15lb felt paper
* 1" 1x3 vertical strapping and vented rain screen. Strapping is fastened through to the frame with 8" and 10" fastenmaster timberlok screws
* 1" shiplap hemlock siding.  Dried for 1 year

And the roof assembly (inside to outside):
* 1" tongue-in-groove white pine.  Dried for 1 year
* 2x12 rafters filled with fiberglass batt insulation.  The rafters end at the eave (their plumb cuts are flush with the outside face of the top-plate / beam) so they will not penetrate the polyiso
* 5/8 zip system roof deck, taped
* 4" fiber-faced polyiso, 2 layers of 2" with staggered seams
* 30lb felt paper
* pressure treated 2x4 sleepers running parallel and directly above the rafters, fastened through to the rafters with 10" screws.  These are laid on their 3.5" side and doubled up to create a 3" channel for soffit-to-ridge venting, and they are also used to construct overhangs.  They are pressure treated because I am told that condensation in the roof's ventilation space could cause them to rot
* 7/16 zip system roof deck, taped
* standing seam metal roof

The foam on the roof would meet up with the foam on the walls (and the foam on the walls would meet up with foam on the foundation), ideally creating a single uninterrupted air barrier around the entire building envelope.


My issue is: how do I air seal the fiber-faced polyiso?  The worry is that tape will not adhere well to the fiber-facing on the polyiso boards.  I've read about several solutions online, but none of them were fleshed out in detail:

1) Duct mastic or acoustic sealant
     ^ I am not familiar with this product or how to apply it under these circumstances.
2) Minimal expansion foam sealant.  A bead along each edge of the butted up boards and also used to repair imperfections (in my case, the residual screw holes and the occasional ding).  Excess is scraped off.
     ^ If anyone has a particular product they would recommend, this would be very helpful.  I am worried about the foam sealant shrinking over time.
3) Forgo air sealing the fiber-faced polyiso itself and instead add an extra layer of paper-faced foam.  Seal the seams of the paper faced boards with tape as usual.
     ^ This seams to be the safest solution, but it could be significantly more expensive.  And although it is a 10:12 roof, paper faced foam purchased for the roof would need to be able to withstand the roof loads.


So I'm just curious if anyone has any experience or insight to add to this.  The first roof deck is up at the time of writing and so I am just waiting on the proper way to air seal this foam.

Thank you

By having two layers and staggering the seams, you have addressed most air sealing concerns.

Duct mastic in gallon or so tubs is applied with a stiff paint brush. I recently applied quite a bit of it to my new duct system using a cheap 1" nylon bristle paint brush. For your situation I would probably use a 2" brush. If the brush isn't stiff enough you can cut the bristles in half with some scissors.

Dow Great Stuff Window and Door low expansion foam would be a good option for bigger gaps in the foam. It remains pliable after cured and sticks like glue.

3M #8067 (or Siga) is pretty amazing - might be worth a short trial (not a perfect indication of long term performance, but better than no data). But it's also expensive.

Don't expect sealants, mastic or spray foam alone to work well in an edge gap that changes size significantly (as foam sheets do).

Duct mastic over tape seems to work well with almost any facer. Can-foam between the seams doesn't really cut it, but an always-flexible polyurethane caulk might go the distance (or not)

Thank you (everyone) for the help. It's looking like I'll have to use duct mastic over the seams due to potential movement of the underlying sheathing. I know many contractors that say that a Zip System roof deck doesn't need expansion gaps, but since the manufacturer's manual indicates that I should, I assumed there would be movement and added 1/8" gaps anyway.

Posted By Dana1 on 15 Oct 2015 04:42 PM
Duct mastic over tape seems to work well with almost any facer. Can-foam between the seams doesn't really cut it, but an always-flexible polyurethane caulk might go the distance (or not)

Dana1, when you say "Duct mastic over tape seams" do you mean that I should tape the polyiso and then apply duct mastic over it? Or that I should apply duct mastic where the tape would otherwise be used?

A few other questions for anyone:

1) I'm assuming that I should apply the stuff to both edge faces of the 4x8 sheets, butt them up to each other, fasten the polyiso in place, and then apply it over top the seams?

2) Is it OK to use duct mastic in conditions of high humidity (I don't mean rain) or should I wait for a dry and sunny day?

3) How much should I use at each seam? I don't know what the consistency of the material is, but should enough be applied so that when the two sheets are butted up to each other the duct mastic is squeezed out of the seam?

4) Should the duct mastic be fiber-reinforced?

5) Should I air seal the inner sheet of polyiso or the outer sheet? Or both?


Thank you

EDIT: The product I am looking at is Design Polymerics 1030 Duct Sealant, and it says it is "Recommended for sealing connections on flexible duct or fiberglass duct board."  Would the fiberglass facer on the polyiso be similar to fiberglass duct board?

Well I cannot wait any longer to air-seal the polyiso so I've bought the 3M #8067 All Weather tape and the DP 1030 Duct Sealant, and I will try to apply it within the next two days. It has only been a week since I last posed more questions in my previous reply to this thread, but I only have a small window to get this done (the weather isn't likely to cooperate for too much longer and I'm having a metal roof delivered shortly).

I am not confident with the procedure but my interpretation of Dana1's post is that the mastic is used to adhere the tape to the fiber-facing rather than to directly adhere the polyiso edges to each other (as jonr said would not work well). This is my procedure:

1) Lay down the first layer of polyiso and temporarily fasten it with short screws
2) Repair imperfections with canned Great Stuff Window and Door minimally expanding foam, scraping off the excess.
3) Tape all of the seams with 4" wide 3M #8067 All Weather tape
4) Use a stiff paint brush to spread a layer of DP 1030 Duct Sealant over the seams where the tape comes into contact with the fiber-facing (should I lift the tape up slightly and spread the mastic under it too?)
5) Let the sealant dry for several hours and then cover it with a tarp to protect it and the polyiso from the rain