I see a lot of you have used knotty pine panels and I have a couple of questions.  I have 3/4" thk by 5" T&G panels for the interior of a small house I'm building in Southeast Texas.  My inital plan was to install sheetrock on the walls and ceilings before putting up the panels to help seal the interior.  I have 2x4 stud walls, wrapped with 1/2" plywood sheathing, then totally wrapped with 3/4" RMax (R-5), underneath 1/2 x 10" log siding on the outside.  I wish now I would have spent the extra money and put in 2x6 walls but it's too late for that now.

My question is, would it be OK to install 1/2" or 3/4" unfaced foam (blu) board on the inside (under the knowtty pine) instead of drywall?  The thought  being that this would add a little more R-value and could still be sealed.  I know that wouldn't help on the fire rating but I'm in the country and have no codes, other than good common sense, to follow.

Any thoughts or ideas are appreciated
Earnie

Where are you located, what's your climate?
Air conditioning?
Full time residence?
Are the wall cavities already insulated?
If not, what's planned for the wall cavity insulation?
Are the windows and doors framed for 2x4 walls? Or do you have to build jambs for them anyways?

Don,

I'm building outside of Conroe, Texas, about 40 miles north of Houston.  The climate is hot and humid.  I have not installed it yet but I'll be putting batt insulation in the wall cavities.  It will be a full time residence for the little lady and I.  And yes Sir, AC is a definite must in these here parts!!  The windows and doors are already installed but I'm using a "D" trim around them so wall thickness will not matter there.

Earnie

Hot, humid and A/C is why I asked.

It sounds like you have sealed the exterior walls very well. That's good.

Under those conditions, hot, humid and A/C, it is not recommended to use a low vapor permeable interior surface. That means things like polyethylene or vinyl wall coverings are really bad ideas. As well interior wall vapor barriers including sealed foam sheets are bad ideas. The reason is that in this climate and A/C the building walls will dry to the interior. Interior wall sealing will prevent this and one of the results could be mold.

One way to add additional insulation on the interior would be to add 2x2 furring strips horizontally across the walls. That would have the additional advantage of creating a thermal insulation break on the vertical studs. The horizontal strips would make it difficult to install horizontal wall boards though.   Where horizontal wall boards are desired 2x2 could be added to the studs for greater insulation value.

With the exterior walls well sealed already you might want to look into blown in cellulose wall insulation. It takes professional installation but fills the cavities better than fibreglass batts. Another method to obtain the best wall insulation would be to have spray in foam installed. That can be done DIY but is messy. It is also probably the most costly when installed professionally.

Building Science has many articles on building in different climates. Here's one on hot-humid...
http://www.buildingscience.com/documents/profiles/etw-houston-profile?topic=doctypes/enclosures-that-work/etw-building-profiles

If you are doing batts yourself, be aware that squishing a thick batt into a space smaller than the insulation was designed for is a bad idea. That will reduce the effective R-value.

For some strange reason I've never received a good answer for, many building products suppliers carry an R-19 fiberglass batt that is used a lot in 2x6 walls. What I don't understand is that the manufacturers own info states that it requires 6 1/4" space. The manufacturer also has an R-21 batt that is rated to fit in a 5 1/2 " cavity. When the R-19 is forced into the standard 2x6 cavity it becomes R-18 according to the same manufacturers info.

For a 3 1/2" thickness they sell both an R-13 and an R-15, both rated at 3 1/2 inches.

Thanks alot for all the info Mountain Don!!

I've thought about going back in with the 2x2s but was trying to get a few more Rs without doing so.  As sheetrock offers no insulation value, that where my idea of using a non-vapor tight foam board comes in.

Are all rigid foam panels sealed?  Should I just stick with the drywall underneath the pine boards?  I'm determined to put something behind the boards because if though they are T&G, I don't want any change of air leakage.

I saw your reference about hot, humid climates when you posted to someone else a few months back.  Sure wish I'd seen that earlier on in the project!!!

I don't know about other brands, but Owens Corning states this on their webpage

http://insulation.owenscorning.com/homeowners/insulation-products/foamular-150.aspx

FOAMULAR® 150 rigid foam insulation for exterior walls contains hundreds of millions of densely packed air cells to provide exceptional thermal performance. It's also virtually impervious to moisture, preventing loss of R-value due to moisture penetration......