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Zip-R and Polyisocyanate: is cold weather really a concern?

richmass62 | Posted in Energy Efficiency and Durability on

Hi I am learning more about insulating with Zip-R6 and Polyiso as I am have ordered some of the materials and as we get ready to install some it in a week on our 1930 home.

I have two kinds of materials going up:  Zip r-6 sheathing that has a 1″ insulation layer and also reclaimed fiber faced (thin cardboard covered) 2″ Polyiso. I also have some brand new 1″ polyiso.

1) question about degradation in R Value

All of these panels including Zip have polyiso and I just learned that polyiso doesn’t do as well in cold temperatures. Is that true for all Polyiso? I have:

a) 15 year old polyiso taken off an old roof

b) brand new polyiso used by roofers

c) Huber’s special formula for polyiso used in Zip-R6

Has anyone seen anything that explains whether the makers of chemical makeup of polyiso (including the formula used by Huber’s product) has improved to eliminate this cold weather degradation problem?


2) question about gaps on adjacent 4′ x 8′ panels

I see that the Zip install instructions say that the panels can be butted against each other on the LONG side because the panels have some kind of a built in expansion gap on that axis. However if you are putting the panels against each other on the short side Huber likes you to have a 1/8″ gap.

As for the reclaimed poly iso, I have seen people using a 1/8 to 3/16 gap so that there is room to seal the gap with spray foam and also take care of any imperfections and dents. But I will probably forego any real gap more than 1/16, and then go with some kind of weather resistant barrier, to better protect the cardboard on the outside of the polyiso.

Not only would that keep wind and mist off of the product, it might trap some of my home’s heat and (since it is polyiso) this would even improve the performance of the insulation layer. So, I am looking at the Grace Vycor Env-s wrap. I can get it fairly cheaply compared to something like Blueskin or WRB.

3) final question: polyiso in multiple layers

There are a couple of places where I might want to layer zip r-6 on top of 1 inch polyiso, making a homemade zip-r12. Is there a reason not to do this?  Of course this put one of the fiber faces of the poly iso inside a polyiso sandwich. I doubt much moisture would end up there but maybe I am naive? 


thanks in advance for your help!

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  1. Expert Member
    BILL WICHERS | | #1

    For #1, it's not usually much of an issue. If you want to 'derate' polyiso for cold weather, it's usually considered to be about R5/inch. The issue is that the blowing agents used condense inside the little cells in the polyiso, and that reduces the effective R value of the panel (that's the theory anyway). Very old polyiso didn't really have this problem, and new polyiso is a little better than some that was made in the more recent past. I wouldn't worry too much about this, just derate to R5/inch if you want to be extra careful, and use that derated value for any R value rations for interior to exterior insulation.

    For #2, for polyiso panels alone, I sometimes inject canned foam into the gap with one of the small plastic tips for the foam gun. Usuaully I just but the panels closely together and tap the seams. The panels tend to shrink with age, expansion isn't usually as much of an issue. I suppose if you put the panels up in very cold weather, you might have a little bit of expansion when the seasons change and temperatures increase, but I've never seen a problem with buckling, so I don't think it's much of a concern. It's the OSB part of the Zip-R panel that makes the gap more important, just like it is with plywood sheathing.

    For #3, the issue isn't the insulation or moisture trapping, the issue here is that the nails you use to secure the OSB part of the Zip-R panel now pass through a lot more foam, so Huber's shear specs are no longer valid. This isn't an issue if you're just using Zip-R in a non-structural application, but if you're using the Zip-R as part of a shear wall, to provide racking resistance to a studwall, you MUST follow Huber's nailing schedule. If you deviate from Huber's nailing schedule, which includes layering as you're describing, you void the structural rating of the panel due to the shear issues with the nails.


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