2x4 or 2x6 Walls?

The question is whether to frame your house with 2x4 or 2x6 walls. The idea is that you will get a much improved insulation package with a 2x6 wall. With a better insulation value you get a payback with a lower utility bill. You may be surprised by this anecdotal information.
In the preparing stages of building a house for a client, the question he brought up was "Should we frame with 2x6 walls." That was a good question so I looked at the difference in cost to frame, seal, and insulate the exterior walls with 2x6 over 2x4 and the payback period for this investment as measured by the reduced energy consumption, i.e., power bill.

By the Numbers
House: 3600 heated sqft
Fenestration: 15%
Window NFRC: U-Factor 0.27, SHGC 0.24
Thermal Break: ½” DOW SIS Board

I relied upon my licensed engineer and HVAC contractor to generate the heating/cooling load or BTU balance difference between a 2x4 and 2x6 wall to determine tonnage differences. In this house the cooling load went from 3.76 to 3.94 tons or 0.18 tons difference. Since we where using a 4 ton geothermal unit any way there was no savings gained by the 2x6 wall to be had in reducing the HVAC sizing.

Next we compared the cost difference to insulate. I use blown cellulose insulation and the difference in the cost was $1260 more for the extra insulation to fill the 2x6 wall.

U.S. Greenfiber Llc 40Sqft Cell Insulation Ins541ld Cellulose InsulationNext was the framing package. We changed the plates and studs to build the exterior to 2x6 which increased the lumber package by about $1340.

The final step is to take the energy savings per year that you get with the 2x6 wall over the 2x4 wall and divide the total cost increase to get our payback period. Based on the energy load difference and projected energy reduction to maintain the indoor climate we came up with about a $50 per year savings. So, here is the break down:

Total extra cost for 2x6 wall: $2600
Annual savings: $50
Payback: 52 years

Needless to say we did not frame with 2x6 walls. The investment just didn't make sense. I should point out one more item. If you missed it, we are cooling this house at a rate of 900 sqft per ton (3600sqft/4tons).

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