EXTERIOR WALLS: What’s better…2 x 4, or 2 x 6 construction
With plans for new construction come spring-utilizing 4″ polyiso on exterior of sheathing, and factoring in window assemblies thickness, is it beneficial to save on forest resources using 2 x 4 walls instead of 2 x 6 construction, and maybe completely forgo ANY insulation material( cellulose) within same 2X wall? This is climate zone 5-upstate n.y.. No plumbing in exterior walls
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No, it's better to forgo the petrochemical foam and the hermetically-sealed "picnic cooler" shell and use more renewable forest resources (which are also a long-term carbon sink while in the house) such as double-wall wood framing and cellulose insulation.
Steve, In a 20 Ft wall, with 2 x 4 at 16 " OC, you will use 16 studs, with a nominal cross sectional area of 128 Square inches or real area of 84 Square inches. In the same 20 Ft wall, with 2 x 6 at 24" OC, you will use 11 studs with a nominal cross section area of 132 Square inches or 90.75 Square inches of actual lumber. There is not much difference in the amount of lumber, unless you use the 2 x 6 at 16" OC. Of course, the top and bottom plates will skew these numbers a bit. I would suggest using the 2 x 6 lumber at 24" because there is more insulation value, in both depth and surface area, and fewer pieces of lumber for thermal bridging. You will help the environment far more by saving fossil fuel over the life cycle of the house than by being concerned about a couple of pieces of lumber. I agree with Robert's suggestion of double stud walls, or use his Larsen Truss method. Merry Christmas - Jim
Jim's numbers are misleading because they ignore not only the single bottom and double top plates but also all the jacks, kings, cripples, headers, partition take-offs and corner studs.
The typical new house, framed on 16" centers, has a 25% framing factor. That means that 25% of each wall is solid wood. Framing on 24" centers reduces that factor to 22%, while the best Advanced Framing might get it down to 16%.
So, if the reduction in framing area is 12% (3/25) but the increase in framing depth is 57% (2/3.5) going from 2x4 16" oc to 2x6 24" oc, then you're using 38% more wood with the 2x6 wall. But you will reduce the thermal bridging losses by 50% because of the extra depth of the wood combined with less area.
Steve: You should also look at the cost of the lumber. I can buy two 2x4s for less than one 2x6. Plastic is way too expensive for my needs, but it does have advantages over framing. The pay back time will be longer with it; run some numbers and see what happens w/ your area's data. I'd double stud and cellulose the place. BTW: I am a DIY guy, not an expert. I built a double stud wall in '80 and have been delighted ever since.