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I've put in 1" Type 1 polystyrene (estimated at R3.75) around the exterior walls and I'm deciding which insulation to use in the framing.

The R12 works out to $.38 p/sqft The R14 works out to $.54 p/sqft (+40% over R12) The R20 works out to $.63 p/sqft (+65% over R12, + 17% over R14)

I need about 500 sqft of this stuff. The different R values come in different size packages, so that could come into play in terms of the cost difference.

Is it worth to spend 40%, or 65% more on the insulation batts? With R12 and the polystyrene, I've got over R15. But more insulation means more $aving on heating.

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    You need a lot more information to calculate this - climate, square footage, energy cost, etc. Luckily you can use the Insulation Investment Calculator ;-) – Comintern Aug 28 '15 at 1:31
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    It depends on your climate and how much you spend on heating/cooling. But since you're only talking about $125 difference between the R12 and R20, it wouldn't take long to make up the difference in savings on your utility bill -- if you lower your heating/cooling bill by $1/month you'd make up the savings after 10 years. Lower it by $10/month and you'd make up the savings in one year. – Johnny Aug 28 '15 at 1:32
  • Thanks for the comments. I just realized that R20 comes in 6", and my framing is using 2x4s. So really it's just between R12 and R14 now – Daniel Aug 28 '15 at 3:04
  • You might want to also talk to your municipality's code enforcement division. I know where I live there are minimum R-value requirements for insulation on exterior walls, there may be similar requirements where you live. – John Aug 28 '15 at 12:55

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