MAIL: Insulation Typo?

Raising a SIPs wall.
Custom-cut structural insulated panels go up fast, and insulate well with little air leakage or thermal bridging
Raising a SIPs wall.

I read with interest the cover story in HP156 Everyone involved in that project is to be commended for doing the best for Mother Earth. One thing caught my attention, however. The cross-sectional drawing on page 39 shows the R-value of a nonbearing 2-by-4 wall filled with loose-fill cellulose as R-24. My best guess, or maybe my hope, is that this is simply an editing error.

As a charter subscriber to both Home Power and Fine Homebuilding, I have read many similar articles and researched cellulose insulation extensively for use in my own home. Cellulose increases in R-value as density goes up, to a point. To the best of my understanding, R-4 per inch is the highest value. So, with a 3.5-inch cavity, taking the absolute best-case scenario, you could maybe achieve R-14 (3.5 x 4 = 14). Loose-fill cellulose is more in the R-2.5 to R-3 range, and that is only achievable in an open cavity. Judging from the picture on page 40, I would say the stud spacing is 24 inches on center, and framing members bring down the overall R-value average, sometimes between 15% and 20%. I would be surprised if the 2-by-4 wall is R-10. If the author stands by the R-24 assertion, I would be interested to learn how that number was arrived at. Incidentally, the same drawing shows an 8.25-inch SIP wall at R-24 as well. I realize it’s a different type of insulation, but this is far more plausible.

Steve Borgatti • via email

The wall assembly for this home is actually a 61/2-inch SIP panel (R-24)—not an 81/4-inch panel as depicted—with a 13/8-inch airspace. The 2-by-4 wall inside the panel is faced with 5/8-inch plasterboard, yielding an overall wall thickness of 12 inches. According to your calculations, this would make the total nominal R-value 44. Using Oakridge National Testing Laboratory’s whole-wall R-values (,  the 2-by-4 wall plus the airspace would yield R-19.88. With the SIP R-value of 22.04, that gives a whole-wall R-value of 41.92.

One of the features of this double-wall system is that the inner wall can be positioned to add as much space as you need to achieve your target R-value, using the insulation of your choice.


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