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What would you do?

BruceAF | Posted in GBA Pro Help on

Here’s the question: All things being equal, would your recommend to insulate a 2×6 exterior wall with R20 closed cell spray foam insulation at approximately 3″ leaving the remaining wall empty or fill the wall with cellulose (or other fluffy type) insulation. For sake of argument, lets stipulate the following:

-From the exterior: wood siding with WRB, Huber zip wall taped, 2×6 frame, board and plaster
-Wall is reasonably air sealed at the exterior sheathing
-Zone 5
-Code R20

Please provide you answer and reason why.


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  1. GBA Editor
    Martin Holladay | | #1

    Since you are asking the question on a green building web site, the answer is clear: the cellulose is preferable for environmental reasons. The closed-cell spray foam uses a blowing agent with a very high global warming potential -- in other words, it does more harm than good.

    A secondary reason: because of thermal bridging through the studs, which neither type of insulation addresses in your planned assembly, there is very little R-value to be gained by switching from cellulose to closed-cell spray foam.

    If you want to beef up the R-value of your wall with foam insulation, you should install a continuous layer of rigid foam on the exterior side of your wall sheathing. When done that way, the foam addresses thermal bridging through the studs.

  2. Dana1 | | #2

    The 3" of closed cell foam with the rest fill with fluff a 25% framing fraction adds about R1 over what it would be in an all-fluff. You could buy the same performance with all-fluff cavity fill and 1/4" of perforated fan-fold XPS siding underlayment.

    If you don't add the fluff, the R-value of the framing fraction drops to less than R4, compared to R6.6 with a full cavity fill, since the thermal path through the framing is now only 3" instead of 5.5". That means that the thermal performance of R20 cavity fill with 3" of closed cell foam is less than that of a full cavity fill of R20 fluff.

    Even though it's 25% more foam to put rigid foam on the exterior, the lower installed cost of exterior foam would allow the same or even higher foam-R for the same money as the 3" of stud-bridged foam, but it would not be thermally bridged, since it s continous layer over the exterior of the sheathing.

    So the question is, would you rather have an R16-ish whole-wall performance with the closed cell foam, or R30+ with exterior foam for the money?

    If you don't want a fatter wall, drop to 2x4 fluff-wall, and add 2" of exterior foam for about an R20 whole-wall R, a performance improvement over any variation of closed cell foam in 2x6 framing (even a full 5.5" of closed cell foam), despite the lower center-cavity R. It would also be much cheaper.

  3. BruceAF | | #3

    Martin and Dana - thanks for you input. A couple of things.

    - No opportunity for exterior continuous insulation
    - I understand the environmental consequences of foam but I am not asking to factor that at this point
    - I understand the framing fraction as well.

    What I am trying to arrive at is the consequence of not filling the cavity. I discussed this with a colleague and we pondered the following:

    -With the ccSPF the thermal boundary would be at the face of the insulation and not the wall board. If you draw a red line to designate the thermal boundary it would follow the face of the insulation and the side and face of the 2.5" of stud that is still exposed, in theory, adding and additional 5" of thermal boundary per stud bay or approximately 30% more surface area. Dana - i think this is what you are stating above, correct?
    -The additional 5" would presumable be between R3 and R5.5. This seems like a big penalty.
    -Also with the ccSPF not filling the cavity, the 2.5" remaining unfilled space would/could cause thermal convection looping adding to the transfer of heat thru the boundary.
    -I have not modeled this, but I would assume the surface temperature of the drywall would be cooler and possible add comfort in my house :-)
    -Filling the cavity with fluffy stuff would result in the same R value and less thermal boundary.
    -I guess I need to run these numbers.
    -Also not sure if there would be any moisture issues, though I suspect not.

    So, am I over thinking this?

    Thanks for your input and for listening.

    Regards - Bruce

  4. Expert Member
    Dana Dorsett | | #4

    The much higher conductivity of the stud and the greater surface area of having the 2.5" of stud sides exposed to indoor temperatures makes it a heat sink- it's actually slightly worse than if you had a 3" deep stud with only the 1.5" edge showing.

    The temperature striping at the drywall layer would be less if you left the 2.5" cavity unfilled since the interior side stud-edge in contact with it would be warmer. But the average temperature of the drywall during cold weather would be lower, due to the higher net heat loss. Even with convection you'd get some miniscule benefit out of the extra air films, but not nearly enough. It meets code on a center-cavity-R basis per TABLE N1102.1.1- it's legal to build that way, but it would fail miserably on a U-factor basis (U0.057) per TABLE N1102.1.3:

    At only 3" of R6.7/inch cc foam you're looking at about R10.4 whole-wall, R11.2 if counting the air films, which barely better than a 2x4/R13 wall (R9.5 whole-wall, R10.4 if counting the air films.)

    If you fill the 2.5" cavity with compressed R13s or split R21-R23s they would perform at about R10 (center cavity), increasing performance to about R16.5 whole-wall (~R18.2 with air films). That's a very REAL boost in thermal performance, and well worth the trouble/cost.

    Empty: ~R10.4 whole-wall, only meets code on a technicality (center cavity R only)

    Filled with fluff: ~R16.5 whole-wall meets code on either basis with a bit of margin.

    In some jurisdictions an empty 2.5" cavity would have to be fire-blocked at mid-level on each floor to meet fire safety code.

    Are you seriously thinking of leaving it empty?

  5. BruceAF | | #5

    Dana - I am arguing against this application of ccSPF but others think it is a great approach. They seem to be enamored by the perceived benefits of ccSPF such as the higher R-value and air sealing capabilities as well as moisture control (air impermeable). I believe that filling the cavity with cellulose would be more beneficial, more economical, and as effective than half filling with foam.

    thanks for your input....verifies my thoughts.


  6. Expert Member
    Dana Dorsett | | #6

    BTW: 3" is a lousy depth for ccSPF, since it can't be safely applied in a single lift. It's a fire hazard during curing at anything over the manufacturers' recommended 2". But 3" or more is necessary to hit the technical code-min center-cavity R.

    At 3" or more ccSPF adds significant structural capacity, and in hurricane zones that can be another rationale for applying it to roof decks, but not walls.

    For the foam-enamored, open cell foam has comparable (even superior) air sealing characteristics as closed cell foam, can be safely applied in 5.5" lifts, uses about half the polymer per R, uses water rather than HFCs as a blowing agent, and is about 2/3 the cost per R. Open cell foam is also far more vapor-open, which can be both good or bad depending on the application. For studwall wall cavities in colder climates the vapor diffusion issue can be real, but it's far cheaper and more resilient to fix that with a smart vapor retarder rather than closed cell foam. In the realm of foam insulation it doesn't get any lower environmental impact than ocSPF.

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