Posted By SkyHeating on 24 Apr 2013 01:24 AM
Thank you Dana, that is exactly the answer I was looking for! I guess my next question is, should I look into ICF or are my floor plans not conducive being a 3 story home?

The costs of going ICF are usually substantially higher than stick-built high-R foam-overs of equal annual-energy use performance.  With ICF you end up with a more structurally sound home in general, but in seismic zones (not that Portland is within view of any active volcanoes ) maybe not- concrete walls crack rather than flex. 

If you're holding the line at R22 whole-wall, an ICF would thermally outperform an R22 w-w stick built, but not the performance of an R30+ low-mass wall.

Here's a recent bit o' bloggery about the goop-science approach to air sealing, as well as an older one.

There is nothing forcing the fiber into seams/cracks/holes

You are under estimating the pressure created by fairly dense and fast moving cellulose running into a stationary wall. But data (like A/B comparisons) on this issue and caulk vs films would be interesting.

A couple of things to consider are the best air sealing possible and reducing thermal bridging. Thermal bridging in a 2x6 wall can reduce R value from R23 to R17. Spray foam will not stop thermal bridging. I would look at a double stud wall or using rigid insulation on the exterior. Unity Homes uses laminated Ijoists to reduce thermal bridging.
http://bensonwood.com/lifestyle/insulation.cfm

Unity Home also uses a double strip gasket for air sealing top and bottom plates.

A couple of things to consider are the best air sealing possible and reducing thermal bridging. Thermal bridging in a 2x6 wall can reduce R value from R23 to R17. Spray foam will not stop thermal bridging. I would look at a double stud wall or using rigid insulation on the exterior. Unity Homes uses laminated Ijoists to reduce thermal bridging.
http://bensonwood.com/lifestyle/insulation.cfm

Unity Home also uses a double strip gasket for air sealing top and bottom plates.

Roberth: At a typical 16" o.c. type ~25% framing fraction the whole-wall R of a 2x6 wall with R23 rock wool is only R15, not R17, and that's counting another ~R1 for gypsum + sheathing. To get it all the way up to R17 requires an 18% framing fraction, which requires employing a lot of OVE/Advanced Framing technique.

Dana the point being is that thermal bridging plays a big role in the whole wall performance. For those that are new to energy efficiency there are nuances that have a big impact on performance. At the top of the list you can put air sealing and thermal bridging. I know it is not new to you but those learning it is something new. At one point I thought spray foam was such a great thing but I no longer feel the same way. with the high price and potential for installer error I think there are better alternative. To me flash and batt is a bad idea.

"...the point being is that thermal bridging plays a big role in the whole wall performance. "

Of course it does!

I was just pointing out that your whole-wall numbers were on the significantly optimistic side of reality for all but the most pored-over and optimized-for-minimal bridging framing. Using real-world typ numbers for framing fractions yields a sufficient lower R to matter.

So a double stud wall sounds expensive, thats like framing the house twice right? I understand thats surely better than spray foam but for the cost I can't expect it to return on the money out of pocket like I can with spray foam or damp applied cellulose and 2" XPS on the exterior of the home being taped.

Does XPS create the thermal bridge needed?

I've done both. 2x6 & XPS works, especially if you tape the foam joints, but you can't get the higher R values with single walls. And no, it's not twice the cost since the inside 2x4 doesn't need headers or doubles or anything structural It's holding the insulation and drywall, not the floor or roof. It may be hard to envision the difference between R30 and R40, but the house does work differently and better. The higher R value aids in keeping the interior temps consistent and lowers the heating cost and makes it work easier. not easy to explain. The bigger factor is that when you get the insulation values high enough and the infiltration low enough, you can utilize inexpensive and unducted minisplit heat pumps for heating and cooling. The breakthrough with Passive House came when they realized that the higher expense of insulating was offset by the savings on mechanical systems. We're not talking Passive House values here, but the same applies. I built a house a few years ago (2x6&XPS) which had a $25,000 geothermal system without AC. the much larger(double stud) house I just finished had $8000 worth of minisplits and has heat and AC. You can build a lot of walls or add lots of insulation for $17,000.

RE: caulking the stud bays: foam any opening between framing with spray foam and tape the exterior of the sheathing. You can use ZIP sheathing and tape or OSB and the European tapes like SIGA. Both work well and last. You won't get the same life out of the commonly available American tapes. Much better air seal than with caulking alone.

Also consider an additional air barrier film on the interior side. Taped not stapled.