Replies

1. GBA Editor
   Martin Holladay | Oct 27, 2017 12:43pm | #1

   Brooklyn,
   The correct (short) answer is that you want more than 1/2 inch of rigid foam. The reasons are explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

   Once you've read this article, you need to ask your contractor: "Why is it, exactly, that you are telling me that I can't have thicker rigid foam?"

   The longer answer is that you can get away with 1/2-inch rigid foam, but only if you are sure that the rigid foam is somewhat vapor-permeable -- in other words, that it doesn't have a polyethylene or foil facing. Ideally, you would need to know the brand name of the product so that you could research its vapor permeance. All of that said, 1/2 inch of rigid foam will only provide an R-value of about R-2 or R-2.5 -- not much.

2. 
   BrooklynW | Oct 27, 2017 01:11pm | #2

   The article that you cite indicates a minimum 2.5 R value for zone 4 (which I believe is correct for NYC).
   If I were to use Dow Super Tuff-R polyiso, 1/2in this is supposed to be R 3.3.

   Would this product satifsy the requirements? It has one side reflective foil. Would it be ok to have this side face outward?

   In your opinion, would it be better to just forget about the foam if the 1/2in restriction is required?

   Thanks for your feedback.

3. 
   user-2310254 | Oct 27, 2017 01:37pm | #3

   You can have any amount of foam in climate zone 4 (as long as it is not Marine Zone 4). Are your studs 2X4 or 2X6?

4. GBA Editor
   Martin Holladay | Oct 27, 2017 01:48pm | #4

   Brooklyn,
   I stand corrected -- sorry to lead you astray. You're right that Climate Zone 4 has lenient requirements. In fact, as Steve correctly notes, you can use any thickness foam you want in Zone 4.

   So the issue becomes simply one of how much R-value you want. Thicker foam obviously gives you more R-value, but 1/2-inch foam (of any type, with any facer, including foil or polyethylene) will work.

5. Expert Member
   Dana Dorsett | Oct 27, 2017 03:00pm | #5

   A typical 16" on center 2x4 framed house with fiber insulation comes in at about R10 "whole wall".

   Adding a half inch of polyiso raises that to R13.3, which is a very real performance boost, cutting the conducted heat losses through the walls by 25%. With the 3/4" rainscreen gap between the foil facer and siding it's R14+ whole-wall, yielding about a 30% reduction in heat flow.

   Foil faced polyiso is also fairly easy to detail as a air barrier, reducing infiltration losses.

   So, is a 25-30% performance boost "worth it"? (I think so- don't you?)

   Regarding cold temperature performance: At NYC average winter temperatures you don't really need to derate the polyiso much. Even though on the coldest hours of the coldest days it won't perform to spec, on the average winter day it will. With R13 fluff in the cavities the polyiso is ~1/4 the total center cavity R. The average January day in Brooklyn has a mean outdoor temperature of about 35F:

   https://weatherspark.com/y/23912/Average-Weather-in-New-York-City-New-York-United-States-Year-Round

   So with a 70F interior the warm side of the polyiso would be about 9F warmer than the outdoors or ~44F, making the mean temp through the foam layer 38-40F. Most polyiso is still delivering ~ R5.5/inch (or about R2.75 for half-inch) at that temperature:

   https://www.greenbuildingadvisor.com/sites/default/files/Polyiso%20penalty%20-%20BSC%201.jpg

   But most of the heating season it'll be warmer, delivering something like R3 (not counting the boost from the foil facer & air gap), if not the full R3.3.

1. 
   lmosakow | Feb 07, 2024 09:31am | #6

   Dana, I have a similar situation here with a proposed assembly informed by many of your posts. Could I ask for your input on it in this thread? https://www.greenbuildingadvisor.com/question/zone-4a-wall-assembly-for-old-house-foam-sandwich-risk?discussion=edit