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Cape Cod wall insulating and air sealing. Seal the T&G? Rock wool and Intello?

Henry8im | Posted in Energy Efficiency and Durability on


I could use a little help please. I’m a remodeling carpenter in CT. I’m actually doing some work on my own place. I’ve done my best to search for and read multiple articles and blogs concerning air-sealing and insulating.  I haven’t come across anything addressing the specific situation that I’m looking at but it’s possible that I missed it.  I have a 1950’s Cape in zone 5A, (CT shoreline). I have the opportunity to open up 90+% of the exterior walls from the inside to insulate and install fresh sheetrock.  I cannot touch the exterior cladding (which is original grooved red cedar shakes on backerboard and then unfortunately a vinyl clapboard siding without a foam layer installed over that.) I believe there is a tar paper layer on the sheathing.  Budget is a major issue of course.  The walls are 2×4 and the sheathing is the problem, 1×6 T&G. My plan was to install Roxul R15 comfort batts in the 2×4 wall bays and then use either Membrain or Intello plus sheeting over that on the inside of the studs. So, a couple of questions.  
       Of course I’ll seal any major penetrations in the wall bays such as wiring, etc. but beyond that is it worth doing anything else because of the tongue and groove? Can’t caulk every joint!
       Can’t put a shallow layer of foam board in 1st.
       I considered cutting some cardboard (or RamBoard) and installing that loosely (can’t put anything tight against the T&G because the siding nails will penetrate it )in each bay and caulking the perimeter. Is it worth it?
       Closed cell spray foam is out of the question, money and green issue.
       I considered dense pack cellulose or spray fiberglass but I get a better R value with the mineral wool. Also, again, more expensive, I can do batts myself. Or is there enough ‘air sealing’ quality to dense pack that using it along with Intello as the netting layer (overkill?) will more than make up for a slight loss of R.
       Any preference Membrain vs Intello?
I know that I can’t get it perfect.  What I would like is to make the best (intelligent and cost effective) improvements that I can while the opportunity is there. If I do this every time work is needed then I can, in a piecemeal fashion, hopefully make a difference in the comfort level and energy usage of the house.

Thanks all,

Tom H.

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  1. Peter Yost | | #1

    Hi Henry -

    The only way to get an air control layer given the constraints you detail above is to detail your interior membrane as an air barrier. You CAN detail Membrain as an air barrier, but it is in my opinion very challenging. I think it is worth using a more robust membrane such as Intello as your vapor and air control layer; it's tough stuff and can be taped to get it airtight.

    Cellulose insulation is more airtight than fiberglass, but 50 times too air-permeable to be considered as your dedicated air control layer.

    You could detail your new gypsum wallboard as your air control layer:


    1. Henry8im | | #2


      Thanks for chiming in. I'm going with the Intello, I was leaning that way to begin with but your confirmation on it being a more durable membrane is right on. I'll do my best trying to detail the airtightness of the membrane layer and then probably seal the sheetrock layer as a backup. I just didn't want to either waste time/money trying to do something in the exposed wall bays or worse, do something harmful. I mentioned the cardboard layer in the bays to stop any wind-washing because I read a post where I believe Martin mentioned it to hold insulation in a roof retrofit and it wouldn't cause harm.

      We actually run into similar circumstances often because of the type of remodeling we do. Kitchens, baths, interior ref-fits. It's always a challenge to convince some of these guys not to just haphazardly throw some fiberglass batt in an exposed wall and call it a day. We also have a town here where the inspector still make the insulation contractor staple up poly over unfaced fg batts in ceilings.

      Thanks again.

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