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

Burying the dew point

Hatherly | Posted in General Questions on

I need to cite an official source for my understanding that in Climate Zone 5, exterior rigid foam insulation needs to be R 7.5 at least to bury the dew point within the insulation. Can someone help?

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. homedesign | | #1

    Amanda, Have you seen this Blog?
    Be sure when you read the first sentence to notice THE BIG IF

    If you plan to install exterior rigid foam.....

    Some people choose NOT to use foam.

  2. Hatherly | | #2

    Thanks, John. I agree with you about the big IF. I am writing a white paper for our city about the inadvisability of adding rigid foam to the exterior of historic adobes ( they can melt away behind the foam unless the details are absolutely perfect) but I needed that information to show how difficult it is to ensure it can be done correctly. People will taper the foam around our bull-nose windows and then you go below the R 7.5.

  3. GBA Editor
    Martin Holladay | | #3

    The question you raise is probably more complicated that you are implying. The cited guidelines in my article on calculating the minimum thickness of foam sheathing apply to new construction -- specifically, wood-framed buildings with 2x4 or 2x6 walls with batt or blown-in insulation in the stud cavities.

    They do no apply to historic buildings or adobe buildings. I am not an expert in adobe buildings, but I think you are probably overstating the dangers of exterior foam installations.

  4. Hatherly | | #4

    Martin, We have seen many cases of old adobes crumbling away behind even something as seemingly benign as cement stucco, let alone rigid foam. I have photos of a recently built house from 2005 that is adobe with rigid foam on the exterior and when part of the wall was removed for an addition, even this more recent adobe was a mess. Traditionally adobe was finished with a sacrificial plaster exterior which would be redone every few years to help preserve the adobe beneath it. The old historic churches are now being redone this way after preservationists have found that the stucco exteriors were hiding adobes that were basically falling apart behind them. Many historic buildings have had to be extensively rebuilt.

    Rigid foam on the exterior of homes is being recommended by builders to help with energy savings. In the historic district we are really trying to discourage this with adobe structures. Unless the details are perfect (and when are they ever?) and there are no existing moisture issues in the building, it is not a good idea.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for the further explanation. I defer to your expertise on matters of adobe.

    However, I'd just like to repeat that my guidelines on the minimum R-value of foam sheathing on frame buildings is irrelevant to the discussion.

  6. Hatherly | | #6

    Martin, my thought process on that, which may well be incorrect, is that if you have a situation where moisture gets into a wall or water vapor passes from the inside towards the outside, but then condenses on the back side of the foam because it is not warm enough, the adobes where the mosture is will start to disintegrate. In the past people would put fairly benign finishes on the interior but now people are also using things like Diamond Finish which is less permeable. So the moisture in the wall cannot easily permeate out to the inside or the outside once it condenses. Is that thought process correct?

  7. GBA Editor
    Martin Holladay | | #7

    Adobe is hygroscopic, so any moisture accumulation will be distributed through the adobe. It's not going to appear as water droplets or puddles. If diffusion can happen from the interior outward, that should mean that it can also happen from the adobe inward. In other words, I assume these walls can dry to the interior.

    But assumptions have gotten builders (and scientists) into trouble often. So, as I said, I'll defer to the adobe specialists who have examined a lot of adobe buildings.

  8. Hatherly | | #8

    Thanks for that explanation, Martin. I think the moisture often comes from bad exterior detailing or, in older adobes, wicking up from a rubble foundation. Then it gets trapped by the interior finish and the exterior foam, but, as you pointed out earlier, I am mixing my building science up as the depth of the foam isn't the issue here.

  9. gtmtx | | #9


    I would be helpful if you posted photos of the installations you are referring too, at least for us non-adobe professionals.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |