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Exterior insulation mistakes

rhl_ | Posted in General Questions on

lets suppose you screw up insulating a flat roof from the exterior in climate zone 4A. Let’s suppose 1.5” of polyiso is installed to the exterior. For a non derated value of R-9.75. Let’s further suppose it’s very costly to fix this problem (because lots of solar panels are installed on top). What is the maximum amount of fluffy stuff you can install to the interior? Further the joists are 2×6.

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  1. Expert Member
    Dana Dorsett | | #1

    In zone 4A going by the IRC chapter 7 prescriptives, (R15 exterior, R49 total) about 30% of the total R needs to be on the exterior of the fluffy stuff so with R9.75 x 70/30 you're looking at ~R23 on the interior for an ~R33 total R at center cavity.

    If it's a low slope roof (less than 2:12) the roof deck would run warmer than a higher roof pitch due to less convection cooling, which gives it a bit more margin. I'd be comfortable painting the underside of the roof deck with half-perm latex and installing R23 rock wool or 1.8lb blown fiberglass under that. YMMV. If you're nervous, a WUFI simulation would tell you how close you are to the boundary- you'd likely find that the half-perm paint isn't really necessary.

    If there isn't going to be any ceiling gypsum, stapling ~5-perm perforated aluminized fabric type radiant barrier would hold the batts in place and slow the rate of moisture accumulation at the paint/fiber boundary. If there IS going to be gypsum board on the underside, put the half perm paint on the gypsum, not the roof deck.

    If you want to take it closer to the code min performance, 2" of HFO blown closed cell foam on the underside of the roof deck (about R14, between 0.5-1 perm) and an R15 rock wool batt gets you R39-ish at center cavity, but the R9.75 thermal break over the rafters still buys you something. It probably still won't make code on a U-factor basis, but it'll be a lot closer than with just R23 fluff + semi-permeable radiant barrier.

  2. Expert Member
    Michael Maines | | #2

    Ryan, to walk you through the steps: in your zone, the IRC says you need R-49 in your roof ( It also says that the minimum amount of exterior insulation is R-15 ( From a dewpoint control viewpoint, what matters is the ratio of exterior R-value to interior R-value. So you need at least 31% of the R-value to be on the exterior.

    Using your value of R-9.75 for the polyiso, your maximum total insulation should be R-31.8, leaving about R-22 for fluffy (i.e., air permeable) insulation.

    But since we are talking about the serious issue of moisture accumulation (and resulting damage) if it's not done correctly, I don't know why you would use a false value for polyiso. R-5.0 to R-5.5/in are more reasonable values for polyiso a few years old in cold weather. I'd use a value of R-8 or so for the polyiso, putting the total allowable fluffy stuff at R-18.

  3. GBA Editor
    Martin Holladay | | #3

    This is one situation where you might consider the dreaded "foam sandwich." If the roof sheathing is dry, you can safely install closed-cell spray foam on the interior -- and in that way you could make the foam portion of your roof assembly thicker (by adding the R-value of the exterior rigid foam to the R-value of the interior closed-cell spray foam -- and using that R-value to calculate your permissible fluffy layer).

  4. Jon_R | | #4

    Ignoring code, some other things that effect the risk of moisture problems:

    a) air sealing of the interior side
    b) smart retarder (like Intello)
    c) the fluffy insulation used (cellulose is better)
    d) indoor humidity (less is better)
    e) roof color (darker warms more if open to sun)
    f) the perms of the exterior side (even when < 1)
    g) sheathing used (plywood is better than OSB)

    I'm sure you can get it to work, but you would need WUFI to know how.

  5. rhl_ | | #5

    I am generally pretty spooked about spraying foam. I’m more accepting of the rigid foam boards. So living with below code values isn’t the end of the world. With 2x6s and non foam insulation at R-4 per inch the best we could achieve is R-24. So even if it’s only R-22 or R-23 rockwool we haven’t lost too much from the mistake. We just didn’t get all the way to code from exterior insulation..

    While we are on this topic is there any articles here which discuss how to hire a foam contractor for people nervous about foam contractors?

    Also, the pitch is about 5 degrees? So I think that’s more like 2:24 or something

  6. rhl_ | | #6

    Also, questions for Dana: you didn’t recommend dense pack cellulose? Why?

    Can you provide some examples of this paint product? Can you paint a ceiling any color after that?

    Also for Martin:

    How do you know if the sheathing is dry enough? What is dry enough and how do you measure that accurately ?

    1. Expert Member
      Dana Dorsett | | #7

      >"Dana: you didn’t recommend dense pack cellulose? Why?"

      Because at 5.5" (the actual dimension of a milled 2x6) cellulose is about R20, and the question was"

      "What is the maximum amount of fluffy stuff you can install to the interior?"

      The maximum is more like R23, not R20. If it's a full dimension rough sawn 2x6 you'd get about R22, which is still not the maximum.

      At R20 with R9.75 up top you'd be at nearly 33% foam, plenty of dew point margin. At R22 it would be 30.7%, which is "good nuff".

      >"Can you provide some examples of this paint product? "

      There are others (google can be your friend...)

      >"Can you paint a ceiling any color after that?"

      Yup! Use any vendor's interior latex/ latex-acrylic paint, not alkyd.

      >"How do you know if the sheathing is dry enough? What is dry enough and how do you measure that accurately ?"

      A 2 prong wood moisture meter is a good enough instrument. Anything under 18% is OK, but 12% or lower is better.

      With a 0.5:12 pitch there is effectively no convection cooling of the roof deck- it will run warmer than steeper pitched roofs on sunny days and a higher overall average temperature, which means it can withstand a cheating the IRC prescriptive by a few percent. The PV array up top also adds shading from the nightime radiational cooling affects over the portion of the roof affected, further raising the average temperature. If the PV is tilted the PV will be convection cooled to something a bit cooler than the air temperature, not the nighttime sky's much cooler radiation temperature, but the roof temperature will be warmer than the PV at night.

  7. rhl_ | | #8

    Reading all of this, but I wanted to quickly comment. It’s an 87 year old house. The beams really are 6”.

    I see, so mineral wool it is, or if a discussion with the boss/contractor gets us to 2” of HFO foam plus rockwool..

    The panels are indeed tilted, the highest part of the roof is the south side, and the lowest corner of the panels is the south - west side, the panels are tilted like 2.5 degrees south and 2.5 degrees west (because a tree is in the way on the south side), there is maybe half a foot clearance on the lowest point and something like 32” (if I recall correctly) on the tallest beam. The earlier mentioned angles are off of the flat axis, i.e one diagonal of the rectangular array is like a 10 degree angle with the roof line. I guess the angles don’t matter terribly compared to the actual distances..

    Also to fill in the details — the vast majority of this roof is covered by panels, nearly all of it.

  8. rhl_ | | #9

    Two more questions: if I don’t want to use spray foam but do want to improve on the R value above R-23 could I use more polyiso to the underside of the sheathing ? Like two 1” boards with staggered seams ?

    If you take the rule bending to the limit: why not just fill all of the cavities with rigid foam? Then the ratio constraint essentially doesn’t apply?

    1. GBA Editor
      Martin Holladay | | #10

      Q. "Why not just fill all of the cavities with rigid foam?"

      A. You are suggesting the cut-and-cobble approach. When that approach is used in unvented roof assemblies, it is associated with moisture accumulation and rot, because it's hard to keep all the seams airtight with changes in humidity and temperature and with changes in snow loads on the roof. For more information, see "Cut-and-Cobble Insulation."

      1. rhl_ | | #11

        Yes ok, but, my larger point. Let’s supposed I sprayed the cavities full of foam...

        1. Expert Member
          Dana Dorsett | | #12

          Six inches of HFO blown foam would get you to about R50-R51 total (at about $8 per square foot, in my area), but at ~0.2-0.3 perms would pretty much block all drying toward the interior. Above 3" the foam itself is structural, but you still don't want the roof deck to rot.

          For the money it's hard to make a financial case for anything more than what's needed for dew point control. At 2" all closed cell HFO blown foam is under 1 perm, a class-II vapor retarder, but at the vapor-open end of class-II, which is exactly where you want it to be for protecting the roof deck from winter accumulation without blocking drying too much. At 2" the cost should be under $3 per square foot.

        2. GBA Editor
          Martin Holladay | | #13

          Q. "Let’s suppose I sprayed the cavities full of foam..."

          A. As I noted in my Comment #3, you could do that.

          1. rhl_ | | #14

            I see. Very good.

  9. Expert Member
    Dana Dorsett | | #15

    From a lifecycle environmental cost point of view 6" of 2lb polyurethane isn't necessarily "worth" the offset in energy use vs. doing it with 2" of 2lb polyurethane + 4.0" of rock wool or 1.8lb fiberglass. (It's really 5.75" of polyurethane- can't do the full 6.0" because it's very difficult to trim flush with the rafter edges.)

    In antique buildings with full dimension framing and often random or non standard spacing batts often take a lot of trimming. If there is going to be ceiling gypsum there 1.8 lbs fiberglass installed after the gypsum is up, but prior to painting is pretty straightforward, and would deliver about R17 @ 4.0" at that density. ( It may be cheaper to use damp-sprayed JM Spider at 1.8lbs prior to installing the gypsum.) That would give you about R30-R31 if combined with 2.0" of HFO blown foam, with ~R10 up top it would be R40-ish at center cavity but would outperform R40 between joists due to the thermal break on the rafters. If the roof decking is also 2" thick plank it might even squeak into full thermal code compliance on a U-factor basis. (It only needs to be under U0.026, which is R38.5 "whole assembly", where the thermal bridging and the thermal performance of all layers including air films is factored in.)

    If you do the full U-factor and energy-use math on it you'll find that the difference between the 2" foam + 4" fiberglass and 5.75" of HFO blown foam is quite small. But the difference in cost, (both environmental and financial) for that additional 3.75" of foam is large.

  10. Expert Member
    Peter Yost | | #16

    Hi Ryan -

    In your climate (CZ4A), I would consider whole-house dehumidification to control even wintertime interior relative humidity and keep that load low year round, say 30 - 40%. If you have a good air control layer, and you are code-compliant with your insulation package, you are all set.

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