I am almost to the point of installing insulation in my new house. I am the GC. I have 2x4 walls with R7.5 ThermalStar LCI-SS exterior sheathing/foam. The foam seams are taped. For the 2x4 cavity I am planning fiberglass batts because, as I understand once you limit the air movement through the wall and add exterior foam, the insulation material in the wall shares similar performance no matter what the material (batt, dense pack, etc). I have found unfaced R-19 batts to be more cost effective (and available) than unfaced R-13. JLC has had some columns regarding the compression of fiberglass batts, essentially an R19 batt gives R13/R14 performance in 2x4 wall. I am going to use Membrain on all the interior walls. is there any reason not to use the R19 when it is more cost effective and provides similar or better performance? Thanks! Scott

Can you get damp sprayed cellulose installed in your area? I just had 2x4 walls sprayed at a cost of $0.60/sq ft. That was labor and materials. Compared to fiberglass batts, the cellulose adds a bit of mass to the wall plus it buffers moisture. Plus I didn't have to handle a bunch of fiberglass batts.

Yes, and the pricing I have seen is about the same as what you mentioned. However that is still 2x the cost of the fiberglass. I believe that is another positive with the R19 batts- more mass in the wall vs the R13. Certainly the cellulose would be a lot more mass in the wall which would be a benefit. I will take another look at that.

I'd carefully follow the instructions here so that the Membrain will act as an interior side air barrier. Otherwise, fiberglass is susceptible to air entering and leaving on the same side of the wall (depositing moisture in the process).

An R19 batt is the same weight per square foot as an R13, and performs at R13 when compressed to 3.5" in a 2x4 cavity (and R18 when compressed to 5.5" in a 2x6 cavity.) It won't outperform an R13, but it won't be any worse, and it doesn't have more mass- it's identical. (The amount of thermal mass in fiberglass is pretty inconsequential anyway, well below that of cellulose at any customary installed density.) Here are a couple of representative compression charts for batts:

http://www.greenbuildingadvisor.com/sites/default/files/Compressing%20fiberglass.JPG

http://www.tmsoundproofing.com/products/owenscorning/R30,5.JPG

Sometimes you can get "contractor roll" kraft faced R13s on the super-cheap side, usually cheaper than unfaced R19s.

The key to getting performance out of batts requires two things:

1: The air tightness of the wall: Prior to insulating, caulk the sheathing to the framing in every stud bay, between doubled up top plates, and between the bottom plate. Seal around any electrical or plumbing pentrations of studs or plates with can foam. Use air-tight electrical boxes, or pre-seal the with the appropriate tapes or can-foam.

2: The completeness of the cavity fill: Cut the lengths a half-inch long, never short, and tuck the batts all around the edges & corners, then lightly tug them out to where they're just proud of the stud edges. Split the batts rather than compressing them to accomodate wiring & plumbing and use a batt knife to sculpt out spaces for electrical boxes.

If there are no gaps or voids to convect around the batt, minor air leakage on the interior side isn't going to change the thermal performance very much. At R13 type density fiberglass has some air retardency (unlike R11s or a fully expanded R19), but you do want to make it reasonably air tight. Caulking the edges of wallboard as it goes up and sealing the penetrations is cheaper than MemBrain.

With R7.5 on the exterior of a 2x4 wall you don't need an interior side vapor retarder (other than latex paint on wallboard), unless you're in US climate zone 7 or 8:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

Mind you, R13+ R7.5 c.i. doesn't quite meet code for thermal performance in climate zone 6 or higher:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

In zone 5 or lower the MemBrain really isn't going to change either the thermal or hygric performance very much at all. It won't hurt, but it's not going to do a whole lot.

Thank you Dana. We are climate zone 5. I am already planning on that caulking and should see how effective we are with that for the first blower door test before we install the insulation. I'll talk to our building inspector regarding no interior vapor barrier. I am finding with my house we are in new territory for building practices in this part of the country, and this is only a (sort-of) attempt at Pretty Good House, not even close to net-zero or PH.

If this is in the US, show them the letter of the IRC regarding using Class-III interior side vapor retarders (like standard latex paint on wallboard). See TABLE R702.7.1 in either the IRC 2012 or 2015- it hasn't changed:

http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%207.html

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

With R7.5 on the exterior you meet the "Insulated sheathing with R-value ≥ 5 over 2 × 4 wall" allowance with a decent amount of margin.

In Canada the inspectors don't always buy off on that unless the foam-R is sufficient to keep the sheathing above a presumptive 4C dew point at the 99% outside design temp based on their interpretation of the NBC. That would be just a bit ridiculous, and most would accept it at IRC type levels. But for those who don't, MemBrain usually fixes the "inspector problem" more resiliently than 6 mil polyethylene, even if it's doing basically nothing for solving a building-science problem.