Foam Insulation Cost Analysis

by Josh on September 30, 2009 · 11 comments

in Ideas,Projects

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.


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?

{ 10 comments… read them below or add one } September 30, 2009 at 9:00 am

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.


reuben Collins September 30, 2009 at 9:31 am

Nice analysis. I like this a lot.


KristinB January 17, 2010 at 11:00 pm

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.


Chris December 13, 2011 at 7:30 pm

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 ….


Josh December 13, 2011 at 9:29 pm

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?


sheryl June 30, 2012 at 9:28 pm

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



Toby February 2, 2014 at 6:23 pm

When I initially commented I clicked the “Notify me when new comments are added”
checkbox and now each time a comment is added I get four emails with the same comment.
Is there any way you can remove people from that service?
Appreciate it!


Josh February 11, 2014 at 3:34 pm

I think I have the problem fixed, Toby. Thanks for letting me know!


Tom November 17, 2014 at 5:36 pm

Not to make anyone’s head explode, but a true cost must include the value of the square footage that the insulation occupies. If a home is worth $120 a square foot, then an extra 2″ of insulation is 1/6 of a square foot or $12 in lost floor space per foot of perimeter.


Garrett July 22, 2015 at 12:59 am

I ran this calculation for a closed cell spray foam insulation bid. The thickness was 2 inches on the basement block wall for the full height. My cost/ft^2*R-value was $0.24 which is 13.33 times higher than the cost of do-it yourself batts with R-13. Since moisture and air transmission are primary concerns for this house I believe closed cell spray foam is the way to go… but this bid seems very high. Thoughts?


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