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Benefits of exterior foam vs interior foam

fall50 | Posted in General Questions on

I have read as of late several posts regarding the appropriate placement, if any, of using foam specfically XPS.

Robert has said

“If you’d like to use foam board instead, then the better place for it is on the interior. Use 2″ of XPS, with seams taped and edges foamed, and that will become your air barrier, vapor retarder, and thermal bridge limiter. But it will not limit drying to the outside the way exterior foam board will.”

of course I believe it would be fair to say Roberts perfered method is

“is to thicken the wall assembly, either with cross-hatching or a double wall, in order to improve the thermal efficiency and decrease thermal bridging, control air exfiltration with a well-sealed drywall layer, and allow the wall to breathe to the outside to eliminate any moisture that (inevitably) finds its way in.

Exterior foam Robert argues also has its draw backs becauae of the ‘tested perm values for the various foams (and the inverse relationship between thickness and permeance), and the basic physics that stipulates that a reduction in permeance equals a reduction in drying capacity, while an increase in R-value equals a reduction in the heat flux that fuels drying”

Couple this with the fact that some exterior claddings will most likely need a rainsreen which is another added expense.

This isn’t to say that its shouldn’t be used but to me its a fairly balanced position of the benefits of using interior foam over exterior

I would like specfically to read more about the benefits of interior foam vs exterior foam. I know in previous posts it has been questioned whether this position has been written about elsewhere about or if links exists where someone like myself could read more information. Does BSC have a detail showing foam on the interior other than the SPF’s where they actually use a rigid foam board like XPS EPS or Poly ISO? More questions to come

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  1. user-869687 | | #1

    Note that if you search the GBA Product Guide you will not find any XPS products, as the high global warming potential of that material makes it hard to justify using. The one exception still in favor is for below grade. It takes many decades of energy savings by the insulation to recoup the energy invested making the stuff, plus the global warming effect of the blowing gas. Some EPS and polyiso products remain listed. Some will say it's hard to justify a design for new construction using plastic foam above grade, because there are alternative design strategies that address the drawbacks.

    As for locating a "bonus" layer of insulation to the interior or exterior, a design priority should be to protect the structural frame for longevity, which means bringing the structure more into the conditioned volume by insulating the exterior.

  2. GBA Editor
    Martin Holladay | | #2

    M S,
    While many builders in New England used interior rigid foam in the 1980s, and think the verdict is now in that exterior foam makes much more sense than interior foam -- chiefly because of the ease of insulating rim joists and partition intersections. Exterior foam covers everything; interior foam requires many transitions and fussy details.

    I anticipate the Robert Riversong will disagree. Do your own research if you're not sure who to believe.

  3. fall50 | | #3

    Ok Martin.....Can you expand more when you say "interior foam requires many transitions and fussy details" I understand the partition wall intersection point.... But an above grade wall has 2 sides. So I'm trying to think of what interior details fits the bill of fussy that you wouldn't have to also do on the exterior. Your +2 wall partitions and electrical outlets. But what else? I'm not looking to start heated debate as to who is right or wrong rather a well rounded discussions of the benefits/drawbacks of each approach.

    As you point out Martin I'm looking to start this research by asking the question. if you have another thread you can direct me towards that address some of Roberts points would be great.. In the several I have read I have not yet seen a direct response to some of his points, rather broad generalizations that foam is "successfully used and advocated by many building professionals etc"

    I'm also digesting TJ's point out about XPS not being a truly green product as whats involved in its construction....Fair point as I'm on "green" forum...

  4. GBA Editor
    Martin Holladay | | #4

    M S,
    You need a continuous air barrier and a continuous thermal barrier. When you transition from walls to rim joists, this transition can be tricky with interior rigid foam. I'm not saying it's insoluable; there are many ways to do it. You just have to think it through.

    With exterior foam insulation, you just run the insulation past the rim joists.

    As you mentioned, there are also electrical boxes and partition intersections on the interior.

  5. Riversong | | #5


    First, I should point out that my recommendation for interior foam is only for heating climates, in which the dominant moisture drive is toward the outside (unless you have a reservoir cladding such as brick or stucco).

    And you forgot to mentions that, using the foam as interior air/vapor/thermal barrier allows the use of the less permeable but less ecologically damaging polyiso board; but since an impermeable material on the outside of a cold-climate house is an invitation to disaster, "outsulation" requires the less "green" XPS.

    The chief complaint about interior rigid foam board as thermal break and air/vapor barrier is, as Martin suggests, the difficulty of creating a continuous barrier around floor assemblies and partitions. There are ways to accomplish this, and I'm not sure they're any more fussy than the challenges of proper integration of flashing and WRB with strapped exterior foam.

    In my view, the primary reason that exterior insulation has become commonplace is that it is quick and easy (relatively) - and builders will almost always prefer quick to good.

    My primary concern about it is that it relies on what Dr. Joe calls the "perfect wall". He means it's the perfect way to build, but I understand that as requiring a level of initial and lifetime perfection that is unreasonable to expect or achieve. A wall with very limited drying to the exterior is a durable wall only if it remains perfect and never leaks.

    Thomas accepts the notion that "bringing the structure more into the conditioned volume" will promote durability (the "warm sheathing" approach). I prefer a wall assembly that can tolerate the inevitable wetting, store and redistribute the moisture and dry out over an assembly that doesn't need to dry because it theoretically will never get wet.

    As long as we continue to use wood to build shelter, we should treat the wood like the living thing it was and allow it to breathe, not strangle it within a hermetically sealed container.

    Then there are the whole range of issues around how healthy a shelter can be if it is built like a spaceship rather than a log cabin.

  6. Riversong | | #6

    I should add, though, that I'm not a proponent of using petrochemical foams in any quantity in any new construction, except below grade.

    There are far better ways to accomplish the same goals using more ecologically-appropriate materials and creating a more healthy house.

    I've used interior foam in renovations, where the options are limited by the existing structure. My preference for interior foam over exterior is not an endorsement of the use of foam insulation.

    The primary interruptions of the air/vapor/thermal/weather barrier - doors and windows - are problems for both interior and exterior approaches.

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