oysters said:
Hmmm... I'm not really sure what you've calcuated in column I is correct, or meaningful.It should probably just be the difference between cells in column G.
yes column I (lets call it the 'incremental method')is odd - but the logic seems sound to me - If i have R6 and add R1 at $1.52 for a m2 it saves me a couple of cents a year = really long payback - This logic seems just as valid to me as column g ('overall method') when making a decision on what R value to install. please pick it to bits show me where its wrong.
my spreadsheet is getting a bit unwieldy now, it needs a good reformat... ive added a modified internal rate of return calculation as well. Using the overall method at R12 im getting 10.5% which is around what solar pv returns. so.... If your cold with r12 (your doing something wrong) turn on your heat pump and install some polar pv rather then add more insulation
If you setting up your own spread sheet google docs lets you make a copy of mine as a starting point if you want.