Cost Effectiveness of Different Wall Assemblies
Hello. I’m building a workshop (36’x32′) in Climate Zone 4A (southern middle TN). It’ll have a slab foundation and a vented, scissor-trussed attic. It will also have heat and AC.
I’ve found lots of great info on this website regarding different types of wall assemblies with above average insulation but I’m struggling to find articles that quantify the energy savings compared to the additional cost of constructing higher performing walls.
I’m considering: (1) 2×4 wall w/fiberglass batts, (2) 2×6 wall w/fiberglass batts, (3) 2×4 wall + R5 ext. rigid foam, (4) 2×4 or 2×6 w/dense pack cellulose and (5) 2×4 wall with Zip system R-6 sheathing.
I found a GBA article “Exterior Insulation with 2×4 Walls versus 2×6 Walls With Cavity Insulation Only”. It gives a detailed analysis of the energy savings of “advanced walls” (2×4 with rigid foam and 2×6) compared to “normal” 2×4 walls. To summarize the article, a 2×4 + R5 wall reduces the heating/cooling load by 32% compared to 2×4 wall with no ext. insulation.
How much money does that save per year? I realize it depends on your local energy cost but an estimate would be really, really nice. Is there a 10 year payback or is it a 50 year payback?
Any thoughts on what is the most “cost effective” wall assembly for my climate? Thank you for your help.
P.S. I downloaded the BEopt software and I’m running a few simulations now. If I’m using the program correctly, it’s showing a difference in annual energy cost ranging from $360 to $485 (for 2×6 24″oc + R5 wall compared to 2×4 wall with no extra insulation). Seems like the difference should be larger? Does this line up with other people’s experiences (using the software and much more importantly – in real life)?
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Replies
I'll give your post a bump. FWIW. Energy savings and ROI are important considerations, but many people are trying to create an indoor space that is comfortable. Comfort is probably less of a concern in a workshop.
You're on the right track using BeOpt to compare the different assemblies. In milder climate zones like yours, you're likely to hit the point of diminishing returns fairly quickly. This study is 10 years old, so a lot of the economic considerations may have changed, but it's a useful starting point if you're looking for general rules of thumb: https://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view download the full PDF and take a look at the table on page 10.
The recommendation for climate zone 4 (for a house, not a workshop) is that a true r-value of R-25 is a good target for cost-effective, high-performance walls. That's something along the lines of 2x6 @ 24" OC w/ cellulose + R-5 continuous insulation, or 2x8 @ 24" OC w/ cellulose and no continuous insulation.
For a workshop, I bet 2x6 @ 24" OC w/ cellulose and no continuous insulation is a good place to stop.
Regarding the total annual energy savings, it doesn't shock me that they aren't huge. However, the cost of changing from 2x4 @ 16" OC to 2x6 @ 24" OC should also not be huge. The 2x6 studs will cost more than the 2x4 studs, but there are fewer of them, so that should balance out the cost. You'll pay a little more for the insulation in the thicker wall, but not a whole lot in the grand scheme of things.
What type of work are you doing in the "workshop"? If it's going to have heat and AC, I'd follow residential guidelines, especially if there's potential for it to get re-purposed in the future. At the very least, use 2x6 to start, even if you don't use exterior insulation now (I would if you can afford it).
Not included in your list, butI feel a stagger stud 2x4 wall is very cost effective. Relatively deep insulation, very little thermal bridging and no exterior insulation to make finishing hard.
2x6 24"o.c. with fiberglass properly installed. 1" rigid foil faced xps. Cheap and easy for a great wall. Fiberglass over cellulose in the walls because you can install the batts yourself, unless you happen to have an appropriate dense packing machine. Rent a cellulose machine and loose fill the attic trusses with cellulose . Done....