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Wall Assembly Selection

I am working on construction drawings for a speculative home I plan to build in Kansas City, Kansas (Climate Zone 4). It is a small (18' x 34') two story home with a simple pitched roof over a full basement. The wall assembly of most new construction around here is siding (this can be a variety of things from very inexpensive sheet goods to cementitious products, very rarely masonry) over building wrap over 7/16" osb over 2x4 framing @ 16" o.c.

I am looking at 2x6 framing @ 24" o.c. (smart framing alone has been met with confused looks from local subs and suppliers). I am probably looking at cementitious lap siding for the skin. Zip sheathing is beginning to show up on job sites in the metro, I believe mostly due to the fact that the builder can avoid the building wrap. I am leaning to this product for the sheathing as it seems like it should provide a decent, somewhat idiot proof air sealing solution as well as structure and wrb. However, I have concerns about the longevity of the tape joints and the longevity of osb in general in this climate. I plan on blown in cellulose for cavity insulation.

I am concerned about condensation on the interior side of the sheathing and this assembly's ability to dry properly (cementitious siding over Zip sheathing over 2x6 framing @ 24" o.c. with blown in cellulose). In an ideal world I would like to add an inch of rigid insulation (possibly Zip's R Sheathing) and a rain screen siding installation. However the cost of this system and the battle that will be fought to get it installed properly in this market is slowly pushing it out of the realm of possibility.

I suppose the gist of my question is whether the assembly I plan to use is ultimately going to produce a mold sandwich? After much research I feel (if it can adequately dry) that it may be the right balance of performance and economy in this market and climate at this time.

Any thoughts would be appreciated.

Asked by Matt Van Becelaere
Posted Thu, 12/19/2013 - 12:18

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2 Answers

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1.
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Matt,
No, your wall won't be particularly risky. But it won't be very well insulated, either. It will achieve about R-20 in the center of the stud bays, and significantly less if you consider the thermal bridging through the studs.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 12/19/2013 - 12:32

2.
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Martin,

Thanks for the quick response. I appreciate the concerns with thermal bridging and relatively low r value (however it is much better than the status quo in the area). As I said I would love to provide a better solution including perimeter rigid insulation, but am concerned about the cost and quality level of the installation in this market. I am still considering it and will certainly employ it if there is enough profitability on the first project.

In that vein, do you have a particular preference between the Zip r product and a standard rigid insulation over wrb over osb sheathing board? I know the Zip product is relatively new and was curious about its reputation thus far.

Thanks again.

Answered by Matt Van Becelaere
Posted Thu, 12/19/2013 - 13:34

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