What to do if ZIP insulation is too thin in climate zone 5?
Hi,
Just reading about condensation issues with exterior rigid foam construction and think I maybe getting into a problem so would love some advice. I’m in Zone 5, Boulder CO.
Addition is being built with 1/2 ZIP rigid foam exterior panels, R3? Then 2×6 walls but with 2×4’s offset in it. This part of the addition is done.
Interior wall insulation is going to be cellulose.
Was planning on using acrylic air sealing to get ACH50 to <1.0 and using an ERV.
And using Tescon Vana Premium OSB & Plywood & Concrete multi-surface air sealing tape which I believe keeps the water out but allows vapor to leave the house, like a rain jacket.
Hope I didn’t get into too much trouble! Any suggestions on how to prevent rot, mold etc?
Thanks,
Shane
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Replies
Shane,
If it's not too late, change the specifications for the thickness of the Zip R sheathing.
If it's too late, read these two articles:
"The Exterior Rigid Foam is Too Thin!"
"Rethinking the Rules on Minimum Foam Thickness"
Doesn't having the sheathing on the outside of the foam, as you do with ZIp=R, mitigate some of the potential for problems by removing the main surface on which moisture accumulates?
hI Malcolm -
I think you are asking if the first condensing surface is the interior surface of rigid plastic insulation board instead of wood-based structural sheathing, isn't that less concerning?
On the one hand, this is true; the rigid foam insulation does not really "care" if it sees condensed water, but it also has about zero potential to absorb/safely bear some condensed water.
It also means that any condensed water that can't be safely absorbed by the wood sheathing runs down to accumulate at the wall's bottom plate.
I think the long and short of it is: ANY water that condenses inside a building assembly is something to avoid, with various building assembly configurations having pros and cons for the way in which they handle vapor and liquid water.
For example, ZIP-R having the structural sheathing to the exterior makes the structural sheathing colder. Does that make it less likely to dry should it get wet than if it were sheltered/warmed by the the rigid insulation? And further, if the ZIP-R is part of a ventilated rainscreen assembly, isn't it better for the structural sheathing to be out in contact with this ventilated space instead of blocked from this space by rigid insulation?
Just makes you realize how unique the "perfect" wall is and how just about any diversion from that configuration has pluses and minuses.s
Peter
Thanks everyone for your feedback. Will read those articles once my subscription turns on, seems like it takes several days after paying which is odd...
>"Then 2×6 walls but with 2×4’s offset in it."
I'm not totally clear on what that meas- is this is staggered stud 2x4s on 2x6 plates?
Assuming the ZIP-R is already installed and a 5.5" cavity depth, flash inch of closed cell foam would bring the total R exterior to the cellulose to R9-R10, and the 4.5" of cellulose would be R17-ish +, which would have very decent dew point margin for zone 5, and would even work in zone 6.
Ok so from what I gather the affordable thing to do is make the drywall side as air tight as possible to reduce moisture transfer to the sheathing. Air tight outlets, caulking etc.
And to have a rain guard under the wood siding.
Not to use polyethylene as AC is used in this house in the summer.
How does that sound? Since not reasonable to remove the Zip?
Oh and will the cellulose insulation get soggy, use some open cell spray foam?