Phreakish said

That's not a bad investment for the benefits. I did some quick math you might find useful.

Around me, 2" r7.7 white foam with facing is about $1/sq, which is about the same cost as r19 24" batt insulation. 1/2" OSB is about $0.50 a square foot. Metal roof panels are about $1.33 a square foot.

To do just walls with 2" of foam and osb capping it should be about $1.50 per square. I'd be tempted to go 4" of foam with osb in and out, which would be $3 per square or if the inside was done in metal $3.88 per square. Or, fiberglass batts with osb outside and metal inside would be $2.88 and R19+, or $2 with osb inside and out.

But what really matters is how much does heating cost and can spending more now cost less in the long run.

The Diminishing Returns of Adding More Insulation - Energy Vanguard

R10 would cost 22% more to heat than R15, and R20 would cost half as much. If using propane, for me at $4/gallon R10 and heating to 65 Deg would cost almost $1300 per season versus $650 at R20. Actual numbers will vary though depending on how well the ceiling is actually insulated since ceilings tend to be more insulated than walls and also represent about half the surface area.

Still, it shows just how valuable insulation can be. I'd try to get all you can out of it since redoing or adding to it later could be difficult.

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