Benefits of exterior foam vs interior foam
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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