I have been in the energy business for the last 12 years or so in one capacity or another. I have been an HVAC installer, insulation company owner, HERS Rater, BPI Building Analyst trainer and energy auditor. I have no affiliation with any product manufacturer and I am not getting paid for this series. So here we go…
To determine the cost effectiveness of 2x6 construction
Explore if there is a path to code compliance with 2x4 construction.
I just recently modeled a 2200 sq/ft house with a 2-car garage, let’s call it the McCabe Residence, in the energy modeling software REM Design. I also calculated the cost difference to build that house with 2x6 instead of 2x4 construction. The house is a 2 story with an attached garage and has 9’ walls and an unfinished basement located in Michigan which is climate zone 5A, and Michigan has adopted IECC 2015 with some minor adjustments.
As of 9/21/18- Lowes lumber was $6.98 for a 2x6x9 and $4.36 for a 2x4x9 for a total difference of $2.62. According to a local insulation company, the cost difference between installing R-15 to R-21 is $.25 sq/ft. The owner of the insulation company, who is also a builder, notes that the cost to extend doors and windows to accommodate a 2x6 wall is substantial because everything that is standard is made for 2x4. So, I called Pella windows and selected a standard 3x4 vinyl window and the price difference was $25 per window and $50 per standard door.
Increased Cost for 2x6 Construction
The total difference in cost to build your house with 2x6 exterior construction vs 2x4 is $2,149.90. That equates to $.98 per sq/ft of living space. As I was researching this project I ran across a similar study on a 2,350 sq/ft house with similar details and the author came up with a total cost increase of $1,951.56 or $.83 per sq/ft of living space. Very similar! That study can be found at http://info.turnerandsonhomes.com/blog/cost-to-build-a-house-2x6-walls. After running this model through REMDesign replacing all the R-15 walls with R-21 walls, the total energy savings are $41 per year. That is a simple payback of 52 years.
If you are a developer building 2,200 sq/ft houses, saving $2,141.90 per house can add up!
There are plenty of other benefits from building with 2x6 construction, energy just isn’t one of them.
Energy Code Compliance
Now let’s talk about the code. In REM Design there is a built-in module that lets you easily determine if you pass the Michigan Energy Code. This design using R-15 wall insulation passes the Michigan Energy Code…What? I know that seems crazy because the prescriptive code states you have to install R-20 in the walls. Ok, so let’s talk about the ways you can pass the code in Michigan. There are 4 ways to pass the code:
Prescriptive- Do exactly what the code recommends
Average U value- Beef up insulation in some areas so you can skimp in others as long as the average U-Vale is the same or better than a reference house.
Performance- This method is based entirely on energy use. Using approved software, make your proposed design house use the same or less energy than a reference house.
ERI (Energy Rating Index)- Have a HERS Rater do a rating on the house and score a 52-55 (depending on climate zone) or better.
Walls Options and Pricing:
Prescriptive-In order to meet the code prescriptively (just do what the code says) you have to have a wall cavity R- value of 20 or you can have a cavity R-value of 13 plus R-5 continuous. Let’s look at these 2 options:
$3.00 sq/ft- 2x4 construction and use 3” of closed cell foam in the cavity.
$1.95 sq/ft- 2x4 construction, 1” exterior foam board and R-13 cavity.
$1.00 sq/ft- Insulate with R-21 fibrous material
Average U value- I have tried several times in ResCheck and could never get it to pass with 2x4 construction.
Performance- I have passed using several different home designs.
$.75 sq/ft- 2x4 construction, R-15 fiber
ERI- There is a requirement when using the ERI method that mandates you follow the 2009 prescriptive code, no exceptions, which requires R-20 wall cavity or R-5 continuous and R-13 cavity.
The McCabe Residence passed the Energy Code Using the Performance path with the approved software REM Design. The software is designed to block using energy savings from heating and cooling equipment, trust me I tried. So, the main reason this design passed is the cooling load was lower than the increase in the heating load, not by much, but it was lower.
Let’s look at some insulation options relative to a baseline of 2x4 with R-15 batt:
The cost to build with 2x4 prescriptively is extremely high, adding up to $6,264 in costs.
The cost to build with 2x6 is also high and at a minimum adds $2,149.90 to the cost of the build, even more if you upgrade your insulation.
The energy savings from building with 2x6 are minimal and don’t outweigh the costs, saving about $.02 per sq/ft living space or $41 per year.
Building designs can pass the Michigan Energy Code with 2x4 construction.
Why can’t we build with 2x4? Well, you can...with the right compliance method.
This study was conducted to reveal the truth about the energy savings from 2x6 construction. OK so the payback was 52 years...there are other reasons to build with 2x6. Sound attenuation is a big one, a more even heating and cooling experience, the look of deep sills and structural stability. A lot of information out there is geared toward the energy efficiency benefits of building with 2x6, well we can’t say that anymore.
The next series in this installment is “Insulation Pricing- A Complete Guide”. Please subscribe to get updates when the next blog is published. All of our content is relevant, we publish about 1 per month and you can un-subscribe at anytime.