GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Building Science

The Ratio Rule for Hybrid Roof Insulation

Ensuring the right combination of R-values above and below a roof deck

Hybrid roof assemblies with two kinds of insulation should follow the ratio rule [Photo by Energy Vanguard]

If you want in-depth coverage of the intricacies of insulating roofs, whether the insulation is on top of the roof deck, on the underside, or both, Martin Holladay has lots of information for you here at GBA. As I’ve been writing a book on this stuff lately, I’ve been working hard to get at the essence of the principles of building science and their applications, and that includes these hybrid roof assemblies. One of the terms that’s tossed around in discussions of this topic is “ratio rule,” so today I’ll give you just the basic facts about it. If you want to go deeper, I’ve got some links for you at the bottom of this article.

The ratio rule I’m discussing here applies to hybrid roof assemblies with two kinds of insulation, some of it above the roof deck and some below. (It could also apply to roofs with two kinds of insulation beneath the roof deck, with the upper one being air impermeable and the lower one being air permeable.) Typically, the insulation above the roof deck is rigid foam, but it could be mineral wool or some other type. The diagram below shows the configuration. The ratio rule tells you what percentage of the total R-value of insulation you need to put above the roof deck.

The ratio rule applies to hybrid roof insulation with rigid insulation above the roof deck and air permeable insulation below
The ratio rule applies to hybrid roof insulation with rigid insulation above the roof deck and air-permeable insulation below. Although the model code (2018 IRC) doesn’t call it out, you should use the rule for air-impermeable, vapor-permeable insulation (i.e., open-cell spray foam) below the roof deck, too.

The reason for the ratio rule is that you can’t just put any combination of R-values above and below the roof deck. Well, you probably can in warmer climates (zones 1 to 3), but the colder the climate, the more you have to pay attention to the ratio of above- and below-deck insulation. Why? Because of the temperature of the roof deck in winter.

If you put all the insulation on top of the roof deck, you have no problem because the the roof deck stays nice and warm. If you put all the insulation below the roof deck (and do it properly), again, you have no problem. The roof deck stays cold, but you keep the water vapor from inside the house away from the roof deck.

So, insulation on top raises the temperature of the roof deck and insulation below lowers the temperature. The science behind the ratio rule is to keep the roof-deck temperature high enough that it doesn’t accumulate enough water to cause moisture problems. Yes, there is actually science behind the ratio rule. But the numbers they settled on in the model code are a combination of science and politics.

Table of R-values for above and below roof insulation and the ratio needed if you use more insulation
Table of R-values for above- and below-roof insulation and the ratio needed if you use more insulation. (Values in table adapted from 2018 International Residential Code, Table N1102.1.2 and Table R806.5)

The table above shows the ratios needed. If you’re doing the the code-minimum insulation for a roof (the middle column, Rtotal), you don’t need to worry about the percentage. Just make sure you have the minimum R-value required (second column, Ra) above the roof deck. The percentages (last column) apply when you’re doing an insulated roof with above-code total R-value.

One more note about this kind of hybrid assembly: The model code around this issue is about air-permeable insulation (e.g., fiberglass, mineral wool, cellulose), but the same science holds for air-impermeable, vapor-permeable insulation. That is, open-cell spray foam, as shown in the lead photo above.

That’s probably the shortest explanation of this topic you’re going to find anywhere. If you want to go deeper, my first recommendation is Joe Lstiburek’s article, “Hybrid Assemblies”. If you want even more (and are a GBA Prime member), Martin Holladay has covered this topic extensively in several articles. A good starting place would be “Combining Exterior Rigid Foam With Fluffy Insulation”“. (And be sure to check out the comments, especially anything from Dana Dorsett.)

____________________________________________________________

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.

8 Comments

  1. Expert Member
    Malcolm Taylor | | #1

    " Typically, the insulation above the roof deck is rigid foam, but it could be mineral wool or some other type. "

    So is this simply about the ratios relative to the roof deck, and the air-and vapour permeability of the above deck insulation doesn't matter?

  2. Jon R | | #2

    R-total and the derived ratios vary with the year of the code and the method used (R49 isn't always correct for Z4-8) . With above sheathing insulation, avoid the ambiguity by using this:

    b. Alternatively, sufficient continuous insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.

    1. Expert Member
      Malcolm Taylor | | #3

      Thanks Jon,

      I bring it up partly because the discussions on GBA about exterior insulation on walls have yielded differing opinions on whether the type of insulation matters. If I recall correctly, Martin answered a question about that by saying the thickness of insulation outboard of the wall sheathing wasn't important if mineral wool boards were used, while Ben Bogie said he still thinks the ratios apply as the insulation is dense enough to slow down drying.

  3. Jon R | | #4

    > air-and vapour permeability of the above deck insulation doesn't matter

    Practically, it's unlikely to matter. But say you had a vent channel above the above deck insulation and permeable underlayment. Then above sheathing mineral wool would produce an assembly that has a) warm structural sheathing and b) significant ability to dry outward. This is more resilient than say foil faced foam that only has a). On the other hand, "more resilient" usually isn't needed.

    Similar logic applies to walls. Apply enough exterior foam and the ability to dry outwards is a usually unnecessary supplement. But I'd take the free extra insurance of say unfaced EPS vs foil faced.

    1. Expert Member
      Dana Dorsett | | #5

      +1 (What Jon said!!)

      When using vapor & air permeable insulation above the roof deck the average winter temperature of the roof deck is sufficiently above the presumptive average indoor dew points and it's moisture content stays low as long as the R-ratio is preserved, and it is adequately protected from bulk water from above. That makes it useful to have a vent channel above the permeable exterior insulation as drying path for any bulk water/rain penetration that gets by the roofing layer.

      1. Jon R | | #6

        > bulk water/rain penetration that gets by the roofing layer

        Which is probably a question of when not if. If this happens, different designs will vary in terms of repair-ability.

        1. Expert Member
          Malcolm Taylor | | #7

          Jon,

          I differ from you , Martin, and Dana in that I have enough confidence in the metal roofs I install that I don't anticipate them ever leaking - although I don't design my roof assemblies based on that assumption.

      2. GBA Editor
        Allison A. Bailes III, PhD | | #8

        Dana, my understanding is that the amount of insulation on top of the roof deck should be sufficient to keep the deck temperature above indoor dew point regardless of whether the insulation on top is air or vapor permeable. Are you suggesting it's different?

Log in or create an account to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |