GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Most efficient use of 3/4″ polyiso on exterior (polyiso is foil faced on both sides)

victorbrownjr | Posted in Energy Efficiency and Durability on

My home is standard built- 2×3’s 16″ oc w/craft faced fiberglass ins not very well installed( gaps,etc.). I have enough 3/4″ f f polyiso to cover it. FH’s recent articles indicate Roxul batts would be best way to insulate, but I already have the foam.

I’m trying to build the most efficient way. Question is, what’s the best way to utilize existing materials 1) 2 3/4″ sheets together 2) one 3/4″ dead air space between the two sheets 3) two 3/4″ dead air spaces, one at the sheathing & other between the sheets? I’ll be taping sheets w/ foil tape to create air tight integrity and having a 3/4″ space behind the siding with air gaps at top & bottom. The dead air space(s) would be sandwiched w/foam at top & bottom.

OR , is there another way you might suggest I approach this to maximize my materials to get most efficient results?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    We need to know your location or climate zone to provide you good advice.

    First, are you sure that your house has 2x3 studs? It's far more common to have 2x4 studs. (So-called 2x4s are 3.5 inches deep.)

    Installing two layers of rigid foam on the exterior side of your wall sheathing (presumably, OSB or plywood, although you didn't mention sheathing) is very common. You don't want to introduce any air spaces between the polyiso layers; nor do you want to introduce any air spaces between the polyiso and the sheathing. You should stagger the polyiso seams for the second layer of polyiso -- this will reduce the chance of air leaks and improve the performance of the assembly.

    Two layers of 3/4-inch polyiso will have an R-value of about R-9 or R-9.75, but the thermal performance of the rigid foam will be less in cold temperatures -- call it R-6.5 or R-7 during the winter in a cold climate.

    Here are links to two articles you should read:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    If you read the second of these two articles, you'll discover that your plan to install two layers of 3/4-inch polyiso will work if you live in Climate Zone 5 or anywhere warmer. It's a little marginal for Climate Zone 6, and definitely not thick enough for Climate Zone 7 or 8.

  2. victorbrownjr | | #2

    After checking & rechecking,I still missed the mistake in first line-sorry, it's correct 2x4... My location is near the top of zone 4, Bowie,Md ( right in line w/Annapolis,Md ) My wall sheathing is gypson board with T-111 siding. I was going to go right on top of that since it presented a flat surface & could use both those for added insulation. I had already read your articles you mentioned ( including all the referrals in each article except where GBA wouldn't let me ). I wanted to read them again before responding- refresher was good because I picked some added info & knowledge. I guess you answered my question, but I thought I could get some additional value out of dead air space which I always heard or thought would be a good thing.. I guess, now, my ? is : Why is that not a good thing? Trapped moisture that can't get out? Reason I was trying to max what I had to work with , was worrying about in/out ins ratio , keeping moisture out of sheathing & inside. After this second reading , I realized I have enough ins at 1 1/2" to keep condensation out of wall system for my zone.. So , 2 things: 1) why no dead air space 2) am I correct on in/out insulation ratio . After this second reading, I'm thinking of attaching the foil w/Wind-lock plastic buttons & go right on top of foil w/vinyl siding. I know your feeling , but what's your suggestion for my system, regarding WRB, if at all ? Martin , thank you for your input .

  3. user-2310254 | | #3

    Victor. You want to keep the sheathing warm so it will not absorb moisture. You do that by installing the rigid foam. You also want to keep the sheathing dry. You do that taping the seams and installing flashing.

    If you haven't purchased the foam, you can save money by installing one layer of foam instead of two. As Martin noted in "Calculating the Mininimum..." any amount of exterior foam is sufficient for Zones 1 through 4.

    You will want to attach strapping to the foam if you plan to install wood or cement siding. The strapping creates a gap and encourages drying.

    The sheathing will dry to the inside. That's why it's important to have a permeable insulation inside your walls.

  4. victorbrownjr | | #4

    Steve, thanks for your input,,,,,, but I know all of what you referenced.. Please read my reply to Martin & address my questions 1) why no dead air space 2) with my situation, what's the best location for a WRB if any since foil will be under my vinyl siding (not wood or cement siding ) ?

  5. GBA Editor
    Martin Holladay | | #5

    If you can make the air space truly "dead," with perfect air sealing, it will provide a slight benefit. But if you have a small leak in any of these air spaces, you will get worse, not better, performance, because air leakage robs the heat from your building.

    Moreover, the expense associated with creating a dead air space is not worth the small (R-1 or R-2) potential benefit (a benefit which will only be realized if your air sealing work is impeccable and eternal). If you want a thicker wall, install thicker insulation, not air.

    Concerning your question about WRB placement, read this article: Where Does the Housewrap Go?

  6. victorbrownjr | | #6

    Martin, thanks, I know the article,,,, just wanted your experienced input ,( if it was your house ).

  7. Dana1 | | #7

    In zone 4 a mere 3/4" polyiso is sufficient for dew point control on 2x4 construction (you only need R2.5) , but it would take an inch to bring it up to IRC 2015 code min performance, assuming there is R13 in the wall cavities. At 1.5" you're a bit better than code-min.

    If there is a polyethylene vapor barrier on the interior side under the wallboard, or vinyl/foil wallpaper installing foil faced foam creates a moisture trap. That's not particularly dangerous as long as the window flashings extend to the exterior of the foam layer. If the only vapor retarders in the wall are the kraft facers the wall still has plenty of drying capacity toward the interior.

  8. victorbrownjr | | #8

    Thanks for the input.....,but I'm now trying to determine if WRB is needed. I quess it wouldn't hurt to have the redundancy. (Foil will be under vinyl siding ) To that end does pic shrink that much when it has foil wrapped completely around it ( corners, but not ends ) ? If shrinkage is going to be problem in future, guess it wouldn't hurt to have a WRB ?????

  9. user-1072251 | | #9

    The T-1-11 has grooves which can leak cold air behind the foam, short circuiting your insulation. A thick bead of caulk at the bottom of the wall will seal the grooves and plywood irregularities.

  10. victorbrownjr | | #10

    Thanks,,,,, that's a given , but kindly address the WRB ???

  11. GBA Editor
    Martin Holladay | | #11

    A WRB is required by code. You can use a housewrap like Tyvek as a WRB, or (in some cases) you can use rigid foam as a WRB -- but the WRB is not optional.

    If you choose to use rigid foam as your WRB, all of your window and door flashing still needs to be integrated with the WRB.

    For more information on this issue, see Using Rigid Foam As a Water-Resistive Barrier.

  12. victorbrownjr | | #12

    Thanks for your input Martin !

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |