Maryville, TN
1250sq ft single story

Just added bathroom and laundry room with 2x4 outside walls. I'm looking at adding 1 or 1 1/2 inches of unfaced foam board on the exterior and then 1 inch of spray foam against the foam board on the interior and then unfaced fiberglass batte and then sheet rock. Will this cause any moisture problems?

Also, I'm looking at re-siding my existing house with the same method, except the 1 inch of spray foam would be against the sheet rock then the fiberglass batte and then the 1 or 1 1/2 inches of unfaced foam board on the exterior. Again, will this cause any moisture problems?

I'm looking at doing this in phases, which would mean ordering DIY spray foam kit like Tiger Foam, etc. Thanks for the help!

Edit:

After reading some more tonight, I'm starting to think about going with an unfaced r15 fg batt with no spray foam, which would allow inward drying and also some outward drying through the 1" foam board.

I would love to do cellulose, but I can't do it all at one time, so I would need to batts and no supplier I called knows of cellulose batts.

The total permeance of the foam-board + 1" of closed cell spray foam will still be no lower than ~0.75 perms, making it at-worst a class-II vapor retarder. Wintertime average temps in the Knoxville area are high enough that the average interior-side temp of the foam will be WELL above the dew point of the interior air, so no interior side vapor retarders would be called for. It'll be able to dry toward the interior just fine during the cooling season, and won't condense in the during the heating season- I'd say you're good to go with the original stackup.

With 1" of exterior foam an R15 batts you'd still an need interior-side vapor retarder to be completely safe from condensation during the heating season, since the average temp of the interior side of the foam would then be close to the dew point of the interior air. Average January exterior temps in Knoxville are ~36F. 1" of XPS foam board would be ~ R5, or 1/5 of the total center-cavity R, and during cold snap weeks you'd get some condensation in the fiber layer. If you're going with unfaced R15 batts, stick with 1.5" of XPS foam for R7.5, which would be 1/3 of the total center-cavity R, giving you a lot more margin against condensation in the wall during winter, and would give you close to class-II vapor retardency during the cooling season. You could also go with 1.5" of foil-faced iso for ~ R10, which would give you 40% of the total R in foam, reducing your wintertime condensing conditions in the wall to under 5% of the heating season hours, and totally eliminating summertime condensing conditions with class-I vapor retardency on the exterior.

Were it me, I'd do 1.5" of iso & wet-spray dense-packed cellulose as hygric buffer, but the thermal performance difference between R15 batts & spray cellulose would be small, if the batt installer is fairly meticulous around wiring & plumbing. Making the foam-board & sheathing as air-tight as possible would be a priority detail. Dense-packing dry cellulose from the exterior after the wallboard goes up, but before the foam board is installed is another option- a reasonable DIY project even with a box-store rental blower. (You'd have to make up your own dense-packing tubes.) But R15 batts are about as good as it gets if you have to go with batts, and it's not bad.

Thanks for the great explanation.

Let me now throw in another variable or two.  I work with an architect who suggested that doing 1/2" plywood on the outside of the foam would give me a much better nailing surface for the hardiplank.  When he did his house he had to nail the siding into the studs and he ended up with a lot of wasted material as well as nails every 16" rather than a smaller spacing between fasteners.  I have 1" kraft faced rock wool batts in the existing house which I could augment with r13 unfaced batts and then the 1" foam board.  In the new part of the house I would do kraft faced r15 batts with the 1" foam board.  This would provide the interior vapor retarder and I would be diligent (now that I understand so much more from reading this site) in the batt installation for a good r value.

Posted By sweetlew on 01 Apr 2010 08:46 PM
Thanks for the great explanation.

Let me now throw in another variable or two.  I work with an architect who suggested that doing 1/2" plywood on the outside of the foam would give me a much better nailing surface for the hardiplank.  When he did his house he had to nail the siding into the studs and he ended up with a lot of wasted material as well as nails every 16" rather than a smaller spacing between fasteners.  I have 1" kraft faced rock wool batts in the existing house which I could augment with r13 unfaced batts and then the 1" foam board.  In the new part of the house I would do kraft faced r15 batts with the 1" foam board.  This would provide the interior vapor retarder and I would be diligent (now that I understand so much more from reading this site) in the batt installation for a good r value.

Rather than half-inch ply, use vertical furring strips through-screwed to the studs, leaving a rainscreen gap allows the Hardi to dry to both sides, and relieves the intense vapor drive that occurs when the sun shines on rain or dew-wetted siding.  Both the siding and the wall assembly stay drier that way.  Any housewrap/felt layer (both of which are highly permeable) go between the furring and foam, not butted against the siding. (I've read some reports of material incompatiblity issues with cement-fiber and housewraps, but not felts.)  Any plywood or furring that comes in contact with the Hardiplank needs to be installed with Hardi-tolerant fastener materials (not just any steel will do.)

Posted By sweetlew on 01 Apr 2010 08:46 PM
Thanks for the great explanation.

Let me now throw in another variable or two.  I work with an architect who suggested that doing 1/2" plywood on the outside of the foam would give me a much better nailing surface for the hardiplank.  When he did his house he had to nail the siding into the studs and he ended up with a lot of wasted material as well as nails every 16" rather than a smaller spacing between fasteners.  I have 1" kraft faced rock wool batts in the existing house which I could augment with r13 unfaced batts and then the 1" foam board.  In the new part of the house I would do kraft faced r15 batts with the 1" foam board.  This would provide the interior vapor retarder and I would be diligent (now that I understand so much more from reading this site) in the batt installation for a good r value.

1/2" sheathing is not a suitable nailbase for Hardie Plank - or really any siding for that matter.   Professional siders nail to the studs.  With the 1" foam sheathing you are talking about use firring strips.