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Rockwool Comfortboard vapor/thickness question

user-7701484 | Posted in Green Building Techniques on

I am building a 10’x14′ passive solar shed on a 8″ thick insulated concrete pad. Plan to use this as a guest house, seed starting area, and studying place (I have a 1 year old and toddler, currently studying to be a nurse practitioner and need quiet space). I built my own very small house recently with a wall assembly that included 2×4 framing with intello plus and 2″ comfortboard on the exterior, so I am familiar with the material.

Here’s my question: 1 1/2″ thick comfortboard is readily available at Lowes in the smaller quantities I need (lumberyards can only special order entire pallets). I want to frame my shed with 2×4 walls/1/2″ thick CDX plywood and the comfortboard on the exterior. At 1 1/2″ the R-value is 6 and falls short of the IRC table for exterior rigid foam. Do the same rules apply for a vapor permeable product like comfortboard? I used intello plus on my house so I didn’t have to worry about outward vapor diffusion in winter. I’m not going for extreme high R-values for this shed and it’s possible I may leave the stud bays unfinished for at least a few years.

I have health concerns about rigid foam and want to avoid it. Should I just keep it simple and insulate the stud bays and use intello on the interior?  I’m very open minded about the build including totally rethinking the wall assembly.  Keeping costs as low as possible is a priority here. Thank you for your opinions!

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  1. Expert Member
    Michael Maines | | #1

    user-7701484, because mineral wool is vapor-open, as long as you have a rain screen gap you shouldn't have any problems. The mineral wool will keep the sheathing warmer (and thus dryer) than the sheathing on millions of homes that don't have any exterior insulation. It would be better to keep the sheathing above the dewpoint at all times, but most assemblies are resilient enough to handle a small amount of moisture accumulation. Intello (or another variable permeance interior membrane) will help a lot. As will ensuring your assembly has a good air control layer, which can be the sheathing or the Intello (or both).

  2. Expert Member
    Akos | | #2

    If cost is an issue, then I would forget rigid mineral wool and intello.

    Stick to standard batts in the walls. For something that won't get used all the time, energy efficinecy doesn't matter much, in most climates an R10 whole assembly wall gets you 80% of the energy savings.

    Moisture resistance in not an issue with buildings that don't have a high interior moisture source. If you are not living, bathing and cooking in there, there is no chance of condensation in your walls. Rain screen is always a good thing as it will make your siding last much longer.

    Focus on getting the place well air sealed and your water management details right (roof overhangs, gutters, window/door flashing). These are cheap/free and will make the place much more comfortable and durable.

  3. user-7701484 | | #3

    Thank you for the replies! I definitely have a tendency to overthink things! For this shed i will take the simple, insulate between the studs, approach. Appreciate the advice!

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