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I am finishing my attic and I live in Southeast Michigan (zone 5). I have installed baffles and I have narrowed down my insulation choices to a few options.

Option 1 would be very similar to what is going on in this picture. R21 batt with 2 inch rigid foam board.

Option 2 would be adding 2x6s to increase the depth of the rafters so I can use R38 insulation with foam board.

I'm leaning toward option 1. I'm wondering if the added work and expense would be worth it for option 2.

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    I believe you've made this into an opinion question. To determine whether or not it's worth it, you should do some analysis and compare the up-front cost of the added materials and labor vs. the energy costs down the road.
    – jwh20
    Commented Feb 28, 2023 at 20:16
  • Batts are usually much more flame proof than foam(toxic fumes when burning), but are behind drywall or similar fire stops.
    – crip659
    Commented Feb 28, 2023 at 20:17
  • Despite this being opinion driven, it would be beneficial to know the location and specifically the climate conditions to have an informed answer.
    – RMDman
    Commented Feb 28, 2023 at 20:34
  • Did you do the calculation with heat saving R21 vs R38
    – Traveler
    Commented Feb 28, 2023 at 20:37
  • learnmetrics.com/insulation-r-value-chart-per-inch
    – Traveler
    Commented Feb 28, 2023 at 21:04

1 Answer 1

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You didn't say what kind of foam board, but I'm going to assume something with R5 per inch, or R10 for a 2 inch foam board.

Option 1: R21+R10 = R31 => U value of 0.032

Option 2: R38+R10 = R48 => U value of 0.021

So on the surface, it looks like option 2 reduces heat loss ~30% compared to option 1. But this is optimistic because it ignores the heat loss through the wood roof rafters. So in reality the difference may only be 25%, or 20%.

But, let's look at this another way. What if there was no roof insulation at all, and so assume an R (and U) value of 1 (roof sheathing, shingles, air film). Then going from uninsulated to option 1 reduces your heat loss by 1-0.032 = 94.3%. And going from uninsulated to option 2 reduces your heat loss by 1-0.021 = 97.9%.

So is it worth all the effort to reduce your heat loss from 94 .3% to 97.9%?

You would have to convert these two heat loss values into BTUs per hour and then based on your fuel source & costs, heating system efficiency, etc, to arrive at a dollar estimate of heating cost savings between option 2 and option 1.

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