Expert: Dave Johnson - 7/27/2008

Question
QUESTION: Hi Dave, thanks for your work in this forum.

Question re: building a 2000 sqft 1.5 story 3-season cottage for Maine coast (strong winds, high moisture, cool temps).  This is to be a rustic "camp" with walls open to the studs, (true unshaved 2x4s) exposing the exterior plank sheathing,  shingled with cedar.  This cottage is to be constructed like 100 years ago.  The client intends use in spring and fall, with low temps 40's-50's, heating with woodstove & FP.  

We are hoping to insulate walls from the exterior, using 1" rigid foam panels (or other??) on top of the sheathing and under the shingles (with strapping). Roof will not be open, will have recycled cotton insulation in the cavity.

Have you any experience with this?  We want to assure walls breathe well.  Any concern about moisture, the shingles warping or cupping more than usual when nailed to strapping?  Suggestions, ideas?  Anything "green" is great!  Thank you.

ANSWER: Hi John,
I have personally done a similar thing. We found that the space between the shingles and insulation became over run with insects and an occasional mouse nest.(here in Maine)
We wound up stripping the entire thing down to the foil faced rigid foam and sheathing with 1/2" cdx. Not especially green, but it was way ahead of the original.
As far as the walls breathing, there hasn't been a problem, probably because the inside is exposed...
Good luck, and I hope I've helped...if not, mail me again...

Dave

---------- FOLLOW-UP ----------

QUESTION: Dave, Thanks for your reply with good info.  Very helpful. And you're in Maine?  Interesting- I picked the right guy to ask.  The site is near Lubec.  I'm not the builder, but the "designer", so please forgive my ignorance.  (I'm helping the builder with some specs, and he's never done this ext. insulation before).  I can see how that space would be attractive to insects, etc.  Good advice.

Q. Just to clarify-- you removed the strapping and put cdx plywood sheathing directly on top of the foam (and then shingled the sheathing)?  If I got it right, it sounds like a good solution.  

In addition to between the cdx and shingles, would you add barriers (Typar or ??) between the plank sheathing and rigid foam, or between the foam and cdx, or both, or neither?

Also, do you think 1" is sufficient R-value for spring/fall?

Thanks again, Dave.

John (Portland and Belfast)


Answer
Hi John,
Yes, we put the cdx directly on the insulation, then shingled.
No need for typar,etc. the insulation should be taped tho...
Yes, I think 1" is ok, should be easy enough to heat at that time of year.
(personally, I think the "house wraps" are overrated... needed only when doing a remodel on a building that has open sheathing...old boards etc.)

Dave