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

Multiple vapor barrier approaches in one house

skidmorebay | Posted in Energy Efficiency and Durability on

I am building a small house in Southeast Alaska (mild, wet maritime climate). The walls will have R21 fiberglass batt insulation and 1 ½” of interior polyiso foam. The R-Max foam will also act as a vapor barrier.
The house has a small rectangular bay window/window seat, 5′ wide, 8′ tall, projecting 2′ out from the building on cantilever joists. I would like to skip the interior foam in the bay to conserve the seating space. Would it be asking for trouble to use 2″ of exterior XPS foam just on the bay window? The bay window would be expected to dry to the inside, while the rest of the building envelope is expected to dry to the outside.
Could mixing vapor control approaches in one structure lead to problems?

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

    Your area of Alaska is in Climate Zone 7. If you want to install exterior rigid foam on the walls of your bay window, the walls will only be safe from moisture problems if the R-value of the rigid foam is at least R-10 for 2x4 walls or R-15 for 2x6 walls. If you follow those guidelines, your plan will work.

    Here is a link to an article that explains the principles behind my advice: Calculating the Minimum Thickness of Rigid Foam.

  2. Expert Member
    Dana Dorsett | | #2

    Marine zone 7 does not need as much exterior foam to be moisture safe as zone 7A or 7B. It's zone 7 due to the number of heating degree-days, but the heating season is 12 months of the year. The critical dew point control factor is average temp at the sheathing in winter. At any given exterior-R the average temp at the sheathing will be MUCH higher in coastal AL than in zone 7A, northern MN or ND. The binned hourly mean outdoor temp in January for Juneau is about +25F, comparable to many zone 5 locations.

    Ketchikan is even warmer:

    Compare those to wintertime temps to Chicago, zone 5A:

    But the mean temp in in Juneau for July is about 55F, which is much lower than most zone 5A/B locations, which is how it adds up to enough heating degree days to qualify as zone 7 (rather than zone 5).

    From a dew point control point of view you would probably do just fine with R5-R8 exterior foam in most SE-AK locations, but taking to R10 or more wouldn't be insane from a life cycle fuel cost point of view.

  3. skidmorebay | | #3

    Thanks for the great info. Our climate is essentially the same as Juneau, but I'll probably go with 4" of XPS since I'm only treating a small area of the house.

  4. Expert Member
    Dana Dorsett | | #4

    The 4" of XPS will eventually perform like 4" of EPS after it loses it's climate-damaging HFC blowing agents. Going with 4" of EPS would be greener over it's lifecycle, since it's blown with a much more benign pentane, often reclaimed and burned at the factory for process heat, reducing it's impacts even further.

    EPS has stable performance over decades, whereas XPS of equal density starts out somewhat higher performance due to characteristics of the retained blowing agents, but that benefit eventually goes away, sometimes pretty quickly:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |