GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Justification for Full-Cavity Insulation

andybooch | Posted in General Questions on

Hey gang.  In many examples, I’ve seen a trend towards filling only a portion of a wood frame exterior cavity with foam insulations – lets call it closed cell, leaving an open cavity behind drywall.  We’re in zone 5 Mass

Instinctively, it doesn’t seem like a good idea to have an open cavity behind drywall, even when there is full contact to the outside Air Barrier (in these cases doubling as a vapor barrier).  I’m pouring through codes and BS articles and maybe I’m just missing it but can’t find a code driven justification to fill the cavity.   thanks!

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. charlie_sullivan | | #1

    It would be rare that you'd get code-compliant insulation levels without filling the cavity. That would be the only code issue, as far as I know.

    Code aside, it's a very expensive way to get worse insulation performance than you'd get filling the cavity with something other than foam. You are using a very expensive insulation material where you don't get the full benefit of it--the thermal bridging of the studs "short circuits" the R-value of the insulation.

    1. andybooch | | #5

      thanks Charlie, and I agree - see Dougs note on foam obsession. at R6.5/Inch, closed cell gets to R49 in 8" or so and R21 in 3.25" of a 2x6 wall

  2. user-723121 | | #2

    A quality built wall will not use foam in the construction. The obsession with spray foam I do not understand. For standard insulation, fiberglass, cellulose, rock wool, the framing cavity must be completely filled with insulation for maximum performance. The insulation must be enclosed on all sides to avoid convection in the stud space.

  3. andybooch | | #3

    I agree Doug, it's an odd addiction. And a far odder marketing victory that foam has enjoyed!

  4. Expert Member
    BILL WICHERS | | #4

    The reason for partial fills with closed cell spray foam is due to the difficult trimming it flush. You need a flat wall for interior finishes (drywall), so the insulation can't go past the edges of the studs. That's especially a concern for non-compressible insulation like most spray foam. Open cell spray foam is softer, and much easier to trip, so open cell is typically "overfilled", and then trimmed flat with a special trimming tool Closed cell is usually installed as a partial fill so that trimming is not required or at least minimized.

    The end result of a partial fill with any type of insulation is that you don't get the full theoretical R value performance since you didn't completely fill the available space with insulating material. With closed cell spray foam, the typical ~3" fill in a 2x4 studwall will still get you around R18 or so, so you're still good from a code standpoint, but you won't get the max of R21+ that a full 3.5" fill of the same material would be capable of.

    In terms of structure and mositure risk, the open gap between the back of the drywall and the spray foam layer isn't really much of a problem. The spray foam is an excellent air seal -- it's only real advantage over other types of insulation when used in a wall -- so you don't have issues with air leaks and convection currents. You typically have a pretty good R value with the base material too, so you're coming out ahead on total R value even with the partial fill when compared to something like a typical R11-R15 fluffy batt.

    In practice though, you're usually better off putting in batts or dense pack cellulose in exterior walls and air sealing the "usual" way with caulk and canned foam. Put the materials cost savings towards some exterior rigid foam and you'll come out way ahead in terms of overall wall performance. Spray foam should be considered a niche product, used only in the few applications where it's really the best option.

    In my own home I have spray foam in a few walls, maybe a total of 20-25 lineal feet of wall. Why? Because the spray foam crew was already onsite to do an unvented cathedral ceiling, which is one of those niche applications where spray foam is really the best option. It was $100 or so extra (this was a few years ago) to do the relatively small amount of wall while the crew was already onsite, so I had them do those walls to save a step on the project. I would not have used spray foam otherwise, and I've used mineral wool and exterior rigid foam everywhere else in the house.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |