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I'm starting the process of finishing my nearly full height (7'2") basement and am costing out the insulation. Based on my research of what's best below grade, I'm debating between XPS foam panels (DIY) vs. sprayed closed cell (definitely not DIY).

Based on the cost of the panels at HD ($31) vs. professionally sprayed foam ($1/board ft. quoted) it looks like the cost differential for my square footage (~140 lineal ft.) at 2" depth is about $600. There is a $400 utility company rebate that I would qualify for if I got it sprayed. This is close enough for me (~$200 after rebate) that I'm thinking that I shouldn't bother with DIY and just have it sprayed by a pro.

Is there something that I'm missing in my math below?

137.6 lineal ft.
7.2   height ft.

986.0   sq. ft.
2       in. depth
1972.0  board ft.
$1.00   closed cell spray foam per board ft.
$1,972  closed cell spray cost (contractor -$400 utility rebate)

38      XPS (4x8) sheets
$31.22  Home Depot per sheet price
$1,186  XPS sheets cost
4       Foamular JointSealR tape rolls (90 ft. per roll)
$36.00  Foamular JointSealR tape cost/roll
$144.00 XPS tape cost
12      GreatStuff cans
$4.00   GreatStuff can cost
$1,378  XPS insulation cost (DIY)
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    137.6 * 7.2 does not equal 986.0 but other than that your math checks out. What about tax? Are there any other fees for the sprayed foam? Double check. Do you need to add supports, or wood or anything? Also spray is extraordinarily messy!
    – Ariel
    Commented Jan 8, 2015 at 7:46
  • Where are you located?
    – DMoore
    Commented Jan 8, 2015 at 14:51
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    Are you simply asking us to check your math, or are you trying to compare the two types of insulation?
    – Tester101
    Commented Jan 8, 2015 at 15:00
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    My understanding is that professionally done closed cell spray would be better than DIY XPS panels. So, I expected there to be a sizable cost differential between the two that would make it worthwhile to spend my time to DIY and get a (slightly) worse result. However, based on my rough cost estimates (I'm in Seattle, WA), it looks like cost difference is actually fairly small. In which case it seems like I should go with having someone else do the job more quickly and with a better outcome. Just checking to see if I'm missing some big cost gotcha (pro) or savings (DIY).
    – Boris
    Commented Jan 8, 2015 at 22:48
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    Somewhat off-topic, but make sure to caulk or mastic all of your exposed double-stud gaps and headers. Those tiny 1/4-1/32" gaps lead to a lot of air infiltration and should be quickly handled with caulk before installing drywall. If you don't do those but have closed-cell foam sprayed, this will inevitably be your biggest source of heat loss. If the contractor who quoted the foam didn't mention this, find another contractor. My contractor included this in the quote but was happy to take it out of their work and let me do it.
    – pbristow
    Commented Jan 6, 2016 at 13:53

2 Answers 2

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There's a sizable difference in how "tight" (well-sealed) you'll get a DIY installation, even with the tape and GreatStuff in your calculations, vs a proper closed-cell foam spray job. In most homes air movement is as big of, if not greater, culprit for heat loss than passive heat loss. So if the cost of DIY is even close to a pro job, then the pro job will pay for itself in spades.

However, 2" of foam is below what's required for code here in Illinois, if that's all the insulation the wall would have. So check on that. It's important you get the right amount of foam... if not then you can build up condensation on the inside of the foam and end up with mold. There's a lot of misinformation out there about this, so the simplest solution is to just make sure you follow code.

Also, make an effort to check up on the contractor. According to the experienced guys who did my house, if the two components of the foam aren't mixed just right, the foam can either not work correctly or spontaneously combust while curing. I'm sure the latter is rare, but the former would be undetectable to the naked, untrained eye, I would imagine.

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I used 'foam it green' and it worked just fine (closed cell) as a DIY. Just dont try and 'stretch' out the foam in the end and pay attention to the color of the foam. Green is good, yellow or blue is wrong. You also need to make sure the surface you are spraying into is the correct temperature (I think the best is 75 F). You get 27 R in a 2 by 4 wall. It can be sprayed over electrical wires. Holes are sealed and the foam is rather rigid. The biggest problem with spray in foam is it is very expensive. I used great stuff to fill in any areas I missed.

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