Which frame wall is more efficient: 2 by 6 or 2 by 4 staggered stud?
Claire Anderson
| Posted in Energy Efficiency and Durability on
I’m wondering if folks have whole-wall R-value estimates, etc., on 2 by 6 vs. 2 by 4 staggered stud construction. I didn’t see this as an option on ORNL’s site.
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http://www.buildingscience.com/documents/reports/RR-0903_High-R_Value_Walls.pdf/view?topic=/doctypes/researchreport
Claire,
Unlike furnaces or boilers, walls are not rated for their efficiency, so it's impossible to say that one wall system is more "efficient" than another.
John — thanks for the excellent link. According to the report, "High-R Walls Case Study Analysis," a 2x6 wall sheathed with OSB and insulated with R-19 fiberglass has a whole-wall R-value of 13.7, while a double-stud 9.5-in.-thick wall insulated with cellulose has a whole-wall R-value of 30.1.
Martin,
One thing I noticed about the case study...
They tend to exagerate the poor performance of the 16 inch on center assemblies.
25 percent framing factor is on the very high side...
Framing factor can be more influenced by the number of windows than the stud spacing.
I am not suggesting 16 oc .. just pointing out a little bias.
John,
Framing factors have been rising.
"As a result, the amount of structural components is increasing. The most current study performed for California Energy Commission (Carpenter 2003) demonstrated that framing (fraction of the opaque wall area represented by solid wood used for framing) for residential walls is close to 27%."
http://www.ornl.gov/sci/roofs+walls/NewRValue.pdf
"According to a 2002 report, framing factors up to 27% can be found in residential walls in California in 2001. A similar study performed by ASHRAE in 2003 found an average 25% framing factor for US homes."
"Couple Secrets About How Framing is Affecting the Thermal Performance of Wood and Steel-Framed Walls" by Jan Kosny, David Yarbrough, and Phillip Childs.
Comparing apples to apples: a conventional 2x6 16" oc wall frame with 23% (conservative) framing factor, dense-pack cellulose, with drywall interior, CDX sheathing and wood siding exterior, would have a whole-wall R-value of 17.3.
A staggered 2x4 stud wall 12" oc (24" oc exterior and interior, offset 12") on 2x6 plates (same thickness as above), with a 20% framing factor, would have a whole-wall R-value of 21.2, which is a 22.5% improvement over the standard construction.
With careful header, corner and partition take-off details, the staggered-stud wall could have a framing factor closer to the 16% of OVE framing, for an R-value of 21.7, or 25% improvement over conventional. Careful detailing can also result in a more resource-efficient wall system as well, using less overall framing lumber than a conventional wall.
While the double-stud wall is more labor-intensive than a conventional wall, the energy payback will be considerable over the life of the house.
Comparing apples to pomegranates, the double-stud wall can be made as thick as you'd like, with incrementally greater resource and energy savings. Splitting the plates and setting band joinst on the interior wall will dramatically increase the thermal efficiency of the envelope. And large volumes of using dense-pack cellulose will also dramatically improve the hygric performance of the envelope, maintain a more constant relative humidity in the living space, and dissipate occasional moisture loads more quickly (assuming no vapor barriers, but only vapor retarders in the assembly).