Replies

1. GBA Editor
   Martin Holladay | Nov 08, 2016 02:59pm | #1

   Philip,
   What you call "siding underlayment" is usually called housewrap. Almost all housewrap is vapor-permeable.

   In most cases, you don't want to substitute roofing underlayment for housewrap. If you decide to use a roofing underlayment as housewrap, you should certainly investigate the product's vapor permeability. Many brands of synthetic roofing underlayment are vapor-impermeable. You want a product that is vapor-permeable.

2. Expert Member
   MALCOLM TAYLOR | Nov 08, 2016 03:34pm | #2

   Martin,
   The standing seam metal used as siding will be very vapour impermeable. Given that, does it make much difference whether the house wrap is?

   Philip,
   Are you sure damage is that much of a concern? Standing seam metal roofing is regularly istalled on wood strapping, and it is subject both to higher uniform and point loads than siding.

3. Expert Member
   Michael Maines | Nov 08, 2016 05:49pm | #3

   Philip, the rain screen gap serves two purposes: it allows the siding to dry, which is not a concern in your case. It also allows the rest of the wall assembly to dry, which is where your assembly falls short.

   Hunter lists your panel as less than 1 perm. It's not clear what the facing is on the polyiso, but assuming it's foil, you are proposing sandwiching plywood between two impermeable surfaces, which is a terrible idea. In a perfect world it could be fine, but if there was ever a leak or even an access point for water vapor, it would have no way of drying out. Installing a proper, vapor-permeable housewrap (there are many to choose from) and using a rain screen gap will make for a much more forgiving wall assembly.

4. GBA Editor
   Martin Holladay | Nov 09, 2016 06:22am | #4

   Philip,
   This is an unusual wall. It will certainly not allow any outward drying. But the R-value of the rigid foam layer (R-15.9, according to Hunter) is adequate to pass the theoretical threshold for safety (36% of the total R-value of your wall's total R-value needs to come from the rigid foam layer, according to the guidelines in this article: Combining Exterior Rigid Foam With Fluffy Insulation).

   You've got:
   Total wall R-value: R-42.7
   Rigid foam layer: R-15.9
   Percentage of total wall R-value that comes from the rigid foam: 37%

   The fly in the ointment is cold weather performance of the polyiso. If you derate the polyiso to R-4 per inch (a conservative assessment), then your rigid foam layer is at R-12, and the rigid foam layer's contribution to total wall R-value (R-40.8) drops to 29%. That's too low. (For more information on this issue, see Cold-Weather Performance of Polyisocyanurate.)

   Either thicker rigid foam, or the provision of a ventilated rainscreen gap, will make this wall safer.

5. 
   Jon_R | Nov 09, 2016 11:54am | #5

   I would do both - upgrade the foam (to reduce condensation at the OSB/Zip layer) and a rainscreen to allow the plywood to dry well to the exterior. Perhaps one of the mesh products for the rainscreen gap (since it offers continuous support).

   > with windows flashed to this layer {Zip}

   So water is directed between the low perm zip and the low perm polyiso?

6. Expert Member
   Dana Dorsett | Nov 09, 2016 02:27pm | #6

   The flashing should NOT be directed between the nailbase panel and the ZIP, since there is no drain channel. The water needs to be directed somewhere that will allow gravity to remove the bulk moisture relatively quickly. That would most likely be on the exterior side of a WRB installed on the exterior side of the nailbase panel.

   A mesh type underlayment on the metal siding should work, but a true rainscreen would be better. The OSB facer of the nailbase will be running MUCH colder (= higher average moisture content) than the ZIP, and it's also subjected to exterior moisture drives.

   The polyiso dosen't need to be fully derated to R4/inch from a dew point control perspective. It's performance may be that low at your 99% outside design temperature, but in most zone 6 locations your average January temperature will be above +10F, and your average temperature over the 10 coldest weeks will likely be 20F or higher. The mean temp through the foam will average above 30F. Most polyiso would average R5/inch or so under those conditions, and even the worst performers would be more than R4/inch, and rapidly increasing to R5/inch or higher by the time the mean temp through the foam layer rose to 50F. In practice it means the moisture accumulating season starts later than implied by a steady-state R4/inch model, and the drying season starts much sooner. Derating to R4/inch would be excessively conservative for your climate zone from a dew point control perspective. It should perform at least as well as a presumptive R4.5/inch, but it may be even better than R5/inch, depending on the blowing agents and processes used.

   Also note, the hygric buffering capacity of dense packed cellulose is pretty good, so you'll usually be OK even if the exterior R is pretty marginal.

   All that said, going thicker gives you more margin- it's better to not have to count on hygric buffering of cellulose or better than lowest-performance polyiso.