My last post about getting insulation-ready prompted a question about the cost of insulation. But simply posting the price I paid isn’t very useful to a reader. I wanted to offer a response in a way that got away from the specifics of my project and that could be useful for comparison. A fair price comparison for insulation should include both the square footage covered and the insulation value (R-value) of that coverage.
The Grand Unified Formula of Insulation Value
Here is my attempt at a price comparison formula for insulation:
Value = Cost / (Square Footage x R-Value)
To put this formula in practice, here is an example from an add-on to one of my insulation bids:
Cost $425 / (143 ft2 x R19) = $425/2717 = $.156 per R ft2
So by my calculation, the bid represents a cost of 15.6 cents per unit of R-value over each square foot of coverage.
Insulation Bid Comparison
Are you still with me after that bit of math? When I applied this formula to my three insulation bids the results were:
Bid 1: $.165 per R ft2
Bid 2: $.138 per R ft2
Bid 3: $.112 per R ft2
Even before I ran this calculation, I could see that Bid 3– the bid I selected– was the best value. Now I can put an exact number on it. At equivalent insulation depth and coverage, Bid 3 is 19% less than Bid 2 and 32% less than Bid 1. The price variation between bids is in part due to the different formulation of foam specified. Bid 1 used soy-based foam that commands a price premium for its greener pedigree.
Fiberglass Comparison
Just to satisfy my curiosity, I looked up the price for kraft-faced fiberglass batt insulation at one of the big home improvement chains for comparison. Applying my formula, the costs for fiberglass insulation were:
R-13 Batts: $.018 per R ft2
R-19 Batts: $.018 per R ft2
R-30 Batts: $.024 per R ft2
I should note that these are the costs for do-it-yourself installation of the fiberglass, while the spray foam bids above are installed costs.
Conclusion
My formula was really useful for making comparisons between similar insulation bids or between different types of insulation.
At six times the cost of DIY fiberglass, spray foam insulation may look like an extravagance. However, because of foam’s high R-value of 6.5 per inch of depth versus R 3-4 per inch for fiberglass, I can get high R-values without excessively furring out rafters and wall studs. That time and material savings is certainly worth something, as is spray foam’s superior performance as an air sealer.
For me, the price of my selected “best-value” foam insulation bid will be worth it, but you’ll want to make your own calculations. Why don’t you try out my formula and let me know what you think?
I'm Josh and since 2005 I have shared home improvement stories and ideas from my family's Arts & Crafts bungalow in Minneapolis, MN. I'm trying to combine the best of 1923 with the best of the 21st century-- and I hope it won't take another 88 years to do it.
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Now that i did a partial renovation of my home, insulation in the attic is one of my next projects. I might just go with the DIY stuff since money is tight, but this is an important step.
Nice analysis. I like this a lot.
THANK YOU! My husband and I are renovating a bungalow in CA. He’s an engineer and has been pondering what type of ceiling insulation to use for months. This plus several of your other posts on insulation have been super helpful for us.
what is the absolute minimum distance I can have between the wood studs and the concrete wall, prior to having the spray
foam applied. Some spots are only an 1/8″ between the concrete and wood stud. this is due to bowing in the concrete wall as it was poured 30 years ago. I am trying to prevent any moisture penetration ….
Hi Chris! It sounds like you are finishing a basement, right? If so, you’re smart to focus on moisture as an issue– particularly if you’ve had dampness in the past. However, if you are relying on an air space alone to deal with water infiltration, you might not be doing enough. Make sure the yard is graded away from the foundation and that downspouts are clear and connected several feet out from the foundation also. Inside, you should repair any cracks with concrete or caulk. Then, consider using a waterproofing concrete paint like UGL’s Drylok. Depending on the severity of your water issues, you may even need to install perimeter drain tile. If you have done the things above to keep moisture out, the 1/8″ gap between the foundation and your basement framing should be fine. You just want to prevent water from wicking into the studs through direct contact with a wet foundation wall. Does that help?
great info. I also live in the TC. do you have any company recomendations for spray foam insulation. We are tearing off the siding on our home and were considering it on the exterior a closed cell type
Thanks
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