In trying to decide how to frame and insulate our planned home, I am getting conflicting information. The Build America report indicates that 2X6 Advance Framing with 1" Exterior XPS is cheaper and has similar whole wall R-values to 6.5" SIPS. Meanwhile, the Oak Ridge Labs report indicates that 6.5" SIPS (combined with geothermal, solar, etc. that we don't plan on implementing) is 50% more efficient than Build America's benchmark home.

I will list more specific data, but the gist of my question is, "Which is a better bang for the buck? 6.5" SIPS or 24" AF + 1" XPS?"

Here are links to the two reports:

Building America - High R-Value Walls

Oak Ridge - '50% Better than Building America' Scenario 2a in the Building America report, 2X6 Advance Framing @ 24" OC with R19 Fiberglass and 1" XPS exterior foam gets a Whole R-value of 20.2

In the same report, 3.5" EPS SIPS get a whole R-Value of 14.1. I will assume a standard 5.5" EPS SIPS has a whole-wall R-value of about 22 (Acme Panel Whole Wall R-Value Study)

In the Oak Ridge SIPS report, they never cite any actual numbers comparing R-values of these individual components. They just keep adding green features to the list and build a 50% more efficient home. They seem to assume that SIPS are superior to other framing methods. Is this correct? Or are they creating a tighter envelope with less air leaks and fewer air exchanges per hour?

In short, which is more cost effective? SIPS or AF with external XPS sheathing?

Any response will be greatly appreciated!

Mark

I would use a cellulose filled wall, but with more than 1" of rigid foam.

As Jon, mentions you probably need more than 1" of exterior sheathing for most Utah building climates. Also, going with 2' stud spacing can create issues with code departments and finish attachments.

Do you have a source for PU foam panels? You can get better performance with less thickness. Reducing thickness of walls arguably reduces costs per square foot as thicker walls can eat up a surprising amount of floor area.

Costs ultimately come down to the builder. As a builder comfortable with SIPS, we can build with EPS for about the same costs as what it would take to do well-detailed foam sheathing. However, the new ZIP R product is likely to change the scenario and Iam looking forward to using it on our next home for the walls. We will also have more on-site flexibility for our stepped, stem wall foundation. The biggest drawback to me is we are adding 2.5" to our overall wall thickness to obtain equal performance to the 4.5" PU panels that we would have used.