Replies

1. GBA Editor
   Martin Holladay | Jun 12, 2018 11:23am | #1

   W. Ramsay,
   To answer your question, you either need energy modeling software, or you need to sharpen your pencil.

   The answer depends on your climate zone (heating degree days and cooling degree days), your heating appliance type, your fuel type, and your fuel costs, among many other factors.

   Your most important equation is this one:
   Q = HDD*A*U*24

   Explanation:
   Q = annual heat loss in BTU through a building component (for example, walls)

   HDD = heating degree days for your location (look it up online)

   A = area of the component under consideration (for example, walls)

   U = the U-factor of the component -- which can be calculated thus: U=1/R

   You multiply by 24 to account for the conversion from a hourly metric (U) to a day-based metric (HDD).

   Once you know the annual heat loss, you have to figure out what it costs you to get those BTU. That depends on your appliance type and fuel cost.

   If you are considering Option A (perhaps R-20, which is U-0.05) and Option B (perhaps R-30, which is U-0.033), you would make this calculation twice. Once these calculations are complete, you'll know how many BTU you need for each option (and therefore how many BTU per year are saved by upgrading your wall insulation).

   Needless to say, this approach doesn't consider cooling costs or cooling savings. For that, software is best.

2. 
   tundracycle | Jun 12, 2018 11:58am | #2

   Thanks Martin. This is in Minnesota. I'm just trying to get a rough idea. E.G., likely 2% less energy used so not worth doing more than standard or 34% less and likely worth it.

   HVAC folks think almost no difference so not worth it. Which is possible. I did a very quick run w/ an older version of PHPP and got 34% which sounds way off but that could be operator error.