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CZ4 foamed attic, is R-20 enough?

88Clayton | Posted in Energy Efficiency and Durability on

I’m on the southern edge of CZ4.  My spray foam contractor recommended R20 of open cell.  That’s what is currently sprayed on my roof deck.  The code inspector came by today and is insisting on R38, as if it was a conventional vented attic with loose blown fibers.  

My current ACH50 score is 1.7.  

Two things….

1)  Is R20 enough?  All four foam contractors I received bids from were quoting R20 for my house.   My walls are R25 (Zip-R6).  It’s a bit strange to have walls that are better insulated than roof.  Where is the realistic point of diminishing returns?  I know the encapsulation itself has benefits that go beyond just raw R value, even though the inspector doesn’t quite get this, but perhaps I do need more foam thickness before the diminishing returns kick in. 

2)  I’m worried about thermal bridging through roof rafters.  The 5.5” of ocspf isn’t enough to fully cover them.  Is this reason enough to increase thickness?


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  1. Expert Member
    BILL WICHERS | | #1

    Is this foam to be applied directly to the underside of the roof sheathing? If it is, then YOU NEED CLOSED CELL SPRAY FOAM! Open cell has moisture issues here!

    I've seen spray foam contractors say R20 is enough because "spray foam is so awesome" or "it air seals really well". R20 is NOT code minimum R38. You don't get a pass because you used spray foam. You still need at least R38.

    You are correct to be concerned about thermal bridging of the rafters as that robs spray foam of it's performance potential. In some applications, like cathedral ceilings, spray foam is still the best way to go in many cases. You might have some other options in your particular case, but we need more detail about where you're applying it. Is this an unoccupied attic, or a cathedral ceiling over conditioned space? If it's an attic, how deep are the rafters? 2 by what?


    1. 88Clayton | | #2

      It’s open cell on underside of roof deck. Open cell is pretty acceptable around my climate zone. Almost all the homes are getting open cell. I think my rafters are 2x8”. Unoccupied attic.

      1. Jon_R | | #10

        > Open cell is pretty acceptable

        Lstiburek agrees - he says open cell is fine if the space under it is conditioned. But not R20.

    2. 88Clayton | | #3

      Though I could probably use more, I’m willing to bet R20 of open cell on a conditioned attic roof deck is a better performer than the typical R38 house with loose blown on vented attic floor.

      1. RussMill | | #6

        NO IT IS NOT! i can assure younof that. They've been called out on it for YEARS.

        Alot of spray foam guys sell snake oil

        1. 88Clayton | | #8

          The typical houses getting passed are 5.0-7.0 with little if any air sealing of penetrations and gaps on the attic floor. I agree that I need more insulation on my lid.

  2. 300TTto545 | | #4

    "willing to bet" doesn't really matter. The inspector won't allow it.
    I am on southern end of CZ4.
    I have R-78 in the attic and have lesser walls than you. All depends on your goals of course but your thermal bridging is terrible. My ACH is 1.9 (before we finished a few things so probably 1.7).
    The foam companies argument is typical and tired.
    R value is R value. My ACH is nearly the same as yours. You want to bet that my R-78 blown fiberglass performs less well than your R-20 foam?
    What does your energy rater say? Are you in NC?

    1. 88Clayton | | #9

      My rafters are actually blown but it’s probably only a couple inches and 5-6” between each one. I was wrong to make it sound like wood from the rafters is exposed.

  3. Expert Member
    Peter Engle | | #5

    "I’m willing to bet R20 of open cell on a conditioned attic roof deck is a better performer than the typical R38 house with loose blown on vented attic floor."

    This is a false equivalent, and an old argument by the spray foam industry that has been thoroughly debunked. It counts on old testing done on "typical" fiberglass installations from years ago. Those "typical" installations didn't have any air seals and did not meet current code, even at that time. They wouldn't meet code now, as the code requires air sealing as well as insulation. Your argument essentially argues that your house with spray foam insulation that doesn't meet code is better than an assumed "typical" fiberglass house that doesn't meet code. Since neither one meets code, there's not much point in arguing, is there?

    The building inspector is right. Your roof doesn't meet code. In your jurisdiction, that's illegal and you need more insulation.

    From a purely practical standpoint, R-20 is never the same as R-38. R-38 has half the heat loss of R-20, period.

    1. 88Clayton | | #12

      So a conditioned R38 foam attic performs exactly the same as a code compliant R38 vented attic with loose blown fibers?

      In terms of the actual system as a whole in the real world, I find it hard to believe these are equal. If for any reason, houses like mine have 2nd floor ducts in the attic.

      1. Expert Member
        Dana Dorsett | | #14

        >"So a conditioned R38 foam attic performs exactly the same as a code compliant R38 vented attic with loose blown fibers?"

        If done correctly, yes pretty much- sometimes better. There's less total area with blown fiber on the attic floor, and some highly air retardent fibers such as cellulose have enough thermal mass to make a difference.

        The quality of the air sealing step still matters, but in a house already running ACH 7/50 prior to air sealing and insulating the attic floor isn't going to have thermal performance problems due to excessive air leakage.

        >"In terms of the actual system as a whole in the real world, I find it hard to believe these are equal. If for any reason, houses like mine have 2nd floor ducts in the attic."

        That's a function of having installed the HVAC in the attic, not the performance of the fiber. Installing the mechanicals in the attic forces one to take the more expensive step of insulating at the roof deck. It's often cheaper to build a horizontal utility chase below the attic floor for the ducts, insulating the attic floor above the chase, than it is to insulate at the roof deck. But that's an option that has to be decided as part of the framing design.

        There are several ways to get there insulating at the roof deck at varying levels of cost/risk. R49 is the current code minimum, but R38 is often accepted if getting compliance on a whole house performance basis. R38 of open cell foam-only will fit in 2x12 rafters, but it would need an interior side vapor retarder to control moisture cycling, and NOT a true vapor barrier. To do it without the interior side vapor retarder a minimum of 15% of the total R would have to be closed cell foam at the roof deck with open cell under that. With 2" of HFO blown closed cell foam (R14-ish) and 6.5" of half pound open cell foam it would fit easily in a 2x10 rafter. With 2" of HFO blown foam and R23 rock wool batts it would fit in 2x8s.

        1. 88Clayton | | #16

          It was so much easier to get the HVAC designed efficiently as far as turns and branch runs by putting it in attic. Energy Vanguard did my design. I tried the vented route, but didn’t have enough clearance at roof for air handler location and drip pan because it was too close to eaves and other framing members were causing too many obstructions.

          Can open cell not be sprayed deeper than roof rafters? For instance, 10-11” cavity depth on a 2x8”? If you only had 12” insulation in a 12” rafter is barely covering the inside edge of rafter. What about thermal bridging?

  4. RussMill | | #7

    No one here at greenbuildingadvisor has anything to gain from steeringvyou wrong.

    Believe them when they say what works and what doesn't.

    1. 88Clayton | | #11

      I put more stock in what is written here than what any contractor tells me.

  5. Expert Member
    Dana Dorsett | | #13

    >"Is R20 enough?"

    Uh, no... ABSOLUTELY NOT!

    >" All four foam contractors I received bids from were quoting R20 for my house."

    There are two primary reasaons for that, the most sigificant being:

    1: R20 is about the most that can be installed in a single pass with most half-pound open cell foam. To install more requires a waiting/curing period. If more than that is installed in a single pass it becomes a fire hazard during the curing period (!), and there are often quality issues such as char-void, shrinkage cracks and separation gaps from the framing.

    The lesser but still influential reason is:

    2: In low-energy cost areas R20 is the most that can be installed with a reasonable "pay back" when looking at a net present value of future energy costs perspective.

    Foam guys will give the usual BS about how much more air tight it is, but a competent blown fiber contractor spends as much or more time on the air sealing details than blowing insulation.

    But it doesn't really matter if it's "cost effective" in the nearer term to go for more- prescriptive code minimums on other parts of the energy code don't "pay off" in short years either, but it's easier to make a rationale for R49 cellulose at $1.50 - $2.00 per square foot than R49 of open cell foam at $4.00 - $5.50 per square foot.

    In Zone 4 the current IRC prescriptive is R49- at R20 you're not even half way there.

    To get compliance on a U-factor basis it has to be under U0.026, which is R38.5 "whole assembly". factoring in the thermal bridging of the framing, but also adding in the R-values of the roof deck, roofing, interior & exterior air films. In most houses that could be achieved with 6-7" of continuous rigid polyiso foam board above the structural roof deck, held down with a 5/8" plywood or OSB nailer deck up top through screwed to the strucutral deck with pancake head timber screws. For less money, if the house has 2x6 rafters one could get there with R21-R23 batts between the rafters and 3.5 - 4" of polyiso up top, with plenty of dew point control at the roof deck.

    So, what are your rafter depths, and how far along is this house? (Is the roofing already installed?)

    1. 88Clayton | | #15

      2x8 rafters. The rafters are sprayed but I bet it’s an average of 2-3” on them. Most of the existing depth is between rafters.

      Roof is already on. I really need them to spray another pass of the open cell.

  6. Expert Member
    Dana Dorsett | | #17

    At a 2x8's 7.25" depth a full cavity fill of half-pound foam is still only R27-ish, and you'll still need to install an interior side vapor retarder, such as "vapor barrier latex" on sheet rock, or one of the broad sheet membranes to avoid moisture cycling in summer. To hit R38 with half pound foam takes about 10.5"

    It's possible to just encapsulate the rafters with another pass of open cell, but it's really hard to control or even gauge the depth.

    Are the rafter edges covered? Installing 2x4s perpendicular to the rafters 24" o.c. would bring the total depth to 10.75" , and would put an R13 thermal break over most of the framing fraction once fully filled with open cell foam. Trimming the foam flush with the edges and mounting gypsum board on the 2x4s, and giving it a shot of vapor-barrier latex (which runs about 0.5 perms, not a true moisture-trapping vapor barrier) would meet the R38 local code, and would come close to hitting IRC code performance on a U-factor basis.

    Without the vapor retarder (paint or membrane) the risk of ending up with crazy moisture cycling in the attic is high, and the peak wintertime moisture content of the roof deck on the north side is in the high rot-risk range. See the "Full-depth oc SPF" column in Table 3, and the rows for Kansas City, Seattle, and Boston, all of which were simulated with R38 half-pound foam in zone 4-ish climates:

    At 28% moisture content for 4 weeks+ out of the year the rot risk is getting pretty real. Keeping it under 20% is better, and under 16% is essentially totally safe, which is why you absolutely need an interior side vapor retarder to control the peak moisture content when using open cell foam in your climate.

    1. 88Clayton | | #18

      Thanks for your comments. I am leaving several penetrations unsealed and am pulling air out of the attic with my ERV return side. Wouldnt this help with moisture? This was the recommendation of Allison Bailes.

      1. Expert Member
        Dana Dorsett | | #19

        Drawing air from the attic with the ERV would help somewhat with the summertime moisture cycling, but it would do absolutely squat for the peak roof deck moisture content issue.

        If the 100% of the roof deck was oriented to the south (say one big shed roof) and not shaded by trees the peak moisture content risk is a lot lower, since the sun-warming limits the uptake going into the cold season leading to a lower peak moisture content and begins driving moisture out earlier in the spring season limiting the amount of time it spends in the danger zone.

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