GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Is code min really enough for unvented roofs?

DamionL | Posted in General Questions on

In my readings of exterior insulation requirements for condensation control, I noticed the interior assumption is 70F @ 40RH. 

However, is this really a realistic specification? Generally aren’t temps and humidity higher at the interior roof line due to moisture and heat stratification? It seems like the code didn’t really account for much of a safety margin, and when you consider clear night sky cooling of the roof, will code minimum really save your roof if you’re not diligent about interior RH as well?
I’m guessing the answer to this is the importance of an interior ceiling air barrier as well as the roof deck air barrier, maybe the codes may update to account for this one day…or am I missing something?

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. Expert Member
    Michael Maines | | #1

    For a given sample of air, raising the temperature will lower the relative humidity. Absolute humidity doesn't change much throughout a house so if it is warmer near the ridge, the relative humidity will be lower, with the same vapor pressure. In a well-sealed, well-insulated house, there is little stratification anyway.

    1. Expert Member
      DCcontrarian | | #3

      The reason roofs are so much more troublesome than other exterior assemblies is that humidity rises. Water molecules are lighter than air.

      1. Expert Member
        Michael Maines | | #4
        1. Expert Member
          MALCOLM TAYLOR | | #5


          Leaving that debate aside, compared to most walls, roofs also have both the stack effect and that they are covered with an exterior impermeable surface working against them.

  2. crawfordesquire | | #2

    what climate zone?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |