Hi, Thanks all for the good advice regarding a stick built house in a previous question I posted here. I'm currently comparing the two wall assemblies listed below for a new house south central PA (zone 6), ~2,200 sq ft.

To me it seems:
#1: better insulation value & air sealing but higher material cost and labor
#2: less cost but lose a small amt. of interior sq. ft.

Option 1: (this is the one I had been planning on going to go with...)
  Vinyl siding 
  2” XPS taped
  Zip sheathing taped
  2x6 studs
  blown cellulous (DIY)
  airtight drywall (DIY)

Option 2: (...but then I learned about a Mooney wall, and it looks so DIY friendly and less costly)
  Vinyl siding
  Zip sheathing taped
  2x6 studs
  1.5” horizontal ‘studs’ 24" oc on interior (DIY) & then blown cellulous (DIY) in thicker cavity 
  airtight drywall (DIY)

I would be interested in any comments or suggestions. Also, two other questions for those in the know:

- I've heard criticism of Zip system for walls and roof, saying that the tape WILL eventually fail. Jeez, is that really a risk? This product seems well reviewed for it's benefits (ease of use, air sealing) but future tape failure would be a deal-breaker negative. How much merit should I put in such criticism?

- I've read that it is important to get enough XPS on the exterior to make sure that I don't get condensation inside the wall cavity. In my area, I think 2" XPS just barely makes it. I could go with 1.5" iso (possibly in two 3/4" layers - but yikes on labor costs!). Anyway, in Option 2 there is NO exterior foam at all (which BTW is typical in houses I see built 'round here) - so how does that "condensation inside the wall cavity" issue work with a wall with no exterior foam?

Thanks much for your time and effort!

Your option 1 (2" exterior XPS) will give you slightly better whole wall R value. That layer on the outside adds R10 to the roughly R21 of cellulose in the cavity, and it eliminates a lot more of the thermal bridging of the studs. For a 1.5" wide slice of wall where there is a stud, you'd have about R1 for sheathing and drywall, about R6 for the stud, and R10 foam, for a total of R17. Center of cavity R would be about R32. This assumes dense packed cellulose giving R3.8/inch. For 16" stud spacing, whole wall R (ignoring top and bottom wall plates) would be over 29.

Option 2 adds only about R5-6 on the inside, and eliminates less of the thermal bridging of the studs. Center of cavity would provide R1 for sheathing/drywall, almost R27 for the cellulose (7"), for a total of almost 28. A slice including a stud between horizontal strapping would give you R1 plus 6.6 for the wood plus 5.7 for the 1.5" of cellulose over it, for a total of R13. The horizontal strip including the strapping (assuming 2x4 studs ripped lengthwise, for 1.7" wide strips) would have R1 + R21 +R2 or a total R24. The 1.5 by 1.7" wood overlaps would give you R9. The whole wall R would be 25.

Adding foam to the outside will be more expensive, and the 2" foam layer adds complications around windows, although installing the windows and getting the flashing right has been detailed elsewhere enough times.

As to moisture management, I assume your drywall will be airtight, with sealant or gasketing where it contacts the framing, the framing is sealed at floor and ceiling, and you use electrical boxes designed for airtight sealing (the AirFoil boxes come to mind; I just used those, and I like them). In other words, you take steps to keep interior air from leaking into and through the cavity, then all you need for vapor retarder in either case is a 1 perm primer paint on the drywall. The absolute rate of water vapor diffusion through the primed drywall will be very low in winter. Part of that moisture will pass on through the sheathing (a vapor retarder) and either through the exterior foam (another vapor retarder) or just to the outside without foam. The rest will be absorbed safely by the wood framing and cellulose, then passed on to the outside or back in to the inside later as things warm up. You do not want to have a poly vapor barrier (or vinyl wallpaper) on the inside of the wall assembly.

As to water diffusion rate, I did a calculation once to see for myself. With a 1 perm vapor retarder, and normal inside air, and assuming all the water diffusing through that 1 perm retarder is absorbed by the sheathing and framing only, it would take about 8 months of subzero dewpoint outside air to raise the moisture content of the wood from 10 to 19%, the limit for wood being considered "dry." In zone 6, winter doesn't last that long. Diffusion is a very slow process. Transport by air movement is much faster, and water leaks past improper flashing is faster still.

Now, if your wall is not sealed so as to prevent convective flow of inside air into the cavity, then you do need to worry about how much foam you need outside to prevent condensation at the sheathing by keeping it warm. Exterior foam or not, you don't want interior air leaking into the cavity. Air leakage is a heat loss. Finally, assuming you are shooting for a very tight house, don't forget mechanical ventilation, presumably through an HRV if you are concerned about heat loss.

Thanks so much for your very detailed and yet easy to understand response. I have read your response several times and did not respond sooner as I was considering what I should do on our upcoming build. I want to produce a good quality house with an eye toward energy efficiency in its future operation. But I need to do this within our budget and also with the constraints of what I think we can get accomplished on site. Following are just some of my thoughts on where I’m currently headed, in case you or others might be interested. Mostly I just wanted you to know, Dick, that I really appreciated your time and effort.

Your analysis for a 16" stud spacing, whole wall R (ignoring top and bottom wall plates) of ~29 for Option 1 and ~25 for Option 2 is interesting to me. The R values are not as different as I might have imagined, and nudges me away from using the exterior foam.

Also, your explanation of the water vapor issue related to the risk (or not) of condensation in the wall cavity really helped. If I understand you correctly, our existing plan to implement as best we can (diligent DIY effort) airtight drywall (gaskets, caulk, special wall outlets, no ceiling can lights) makes the foam or not on the exterior more about whole wall R value and less about protecting against condensation inside the wall. So that is good news also, in that it further nudges me away from exterior foam.

You said: “…Diffusion is a very slow process. Transport by air movement is much faster, and water leaks past improper flashing is faster still.”

I agree! And this is the real concern for me, on our project, in terms of using exterior foam. I’m concerned that if for any of a number of reasons we don’t get proper detailing on the wall openings, then in a few years I may have the worst case situation – water leaks. If the framers are not that familiar with detailing ext. rigid foam (rarely used 'round here, esp. in any thickness), I believe it may be an expensive (more labor) and possibly not very effective approach for me, as an owner builder. This is just my opinion for my build – for others it may be a great solution. I feel I must be careful to pick an approach that will yield decent energy efficiency (of course, that's a relative thing!) AND that we (the framing crew and me) can get accomplished with good quality in a timely manner. Therefore, I’m thinking a Zip wall system as air barrier and possibly an additional WRB (tyvek? - not sure I trust zip tape for forever). Also, this will be a fairly traditional window/door install from the POV of the framer and seems attractive in its “doable-ness”. Then my husband and I can DIY the interior details (air barrier work, extra framing for the Mooney wall, ADA details). I’m reserving judgment on DIY vs hire-out of dense blow cellulous pending quotes when we get to that point in the project.

The only down side that I can see is the loss of a few inches of space on the perimeter walls. I think I can live with this though.

So, that’s the long and short of it at present, for my particular situation. Once again, thanks for your help.

P.S. I agree the HVAC systems needs to consider some type ventilation for fresh air.

Hey,

I would use a 2x4 structural wall with a second non structural internal 2x4 wall seperated by as much space as you want insulation. Filled with cellulose to the R-Value you want. This approach is less expensive than inches of foam. 18 inches of cellulose also acts as a vapor barrier. Walls that partially breathes on each side are healthier. I have built zero energy homes since the late 70s, if you are still in the design phase I could help you eliminate your heating and cooling bills at little to no additional cost by smart design. You can reach me at http://www.zehtalk.com/ or email at [email protected].

Good luck

Brian