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.
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.
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?