Continuous exterior insulation rule of thumb
paula_builds
| Posted in General Questions on
I recall reading a rule of thumb about a maximum beneficial thickness of the continuous exterior insulation relative to the thickness of the wall cavity insulation. Am I remembering correctly?
My walls are R23, 2×6 framing. Code requests R5 which I can achieve with 1.25 inches of Rockwool comfortboard (my preferred material). However if I increased to R6 am I just wasting money? (1.5 inches thick). Is there a limit to the benefit of additional thickness?
I seem to recall that there was a point of diminishing returns. My region is Zone 4, mild, wet, east of Cascades in Oregon.
Thanks!
Paula
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Replies
Paula,
Martin's blog will answer all your questions: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
As suggested in the article Malcolm linked, the rule of thumb thickness is used for managing moisture issues—more so with foam than mineral wool.
I don't think there is an easy rule of thumb to determine the threshold for diminishing returns. It depends on too many factors: What are your ultimate goals, how long a 'payback' period are you willing to invest for, what are you heating/cooling your house with (and to what degree), how good are other components of your build (such as windows) etc.
That said, someone familiar with using exterior insulation in your climate zone and who has a sense for heating costs in your area may have a decent sense of what is 'reasonable' (that's not me).
You might find this article interesting:
https://www.greenbuildingadvisor.com/article/how-much-insulation-is-too-much