"The roof or attic is the most obvious place to up the R Value by adding insulation." Why do people think this all the time? Yea I get it, because the ceiling temperature is higher so there is a higher temperature differential and more heat loss, but not with radiant floor heating. Anyway it doesn't matter, ceiling, walls or floor it's all about temp differential. Do you know why they put R19 in a 2 x 6 walls? Because that's all they can get in it! The only reason.
Because the ceiling is warmer the differential will be higher. With R40 and a 60 degree temperature differential you would lose 1.5 btu's per square foot per hour, but if you increased insulation 50% to R60 you will lose 1 btu /sq.ft..
A wall with R20 and a 50 degree differential as an average would lose 2.5 btu's/sq. ft. / hour but if increased it 50% to R30 in the walls the loss would be 1.67 btu's.
You would save a little more energy adding an additional R10 to the walls as you did adding R20 to the ceiling for 1/2 the cost of the insulation.
Now if we could only get windows that were R30, life would be better. The loss would jump from 16 btu's/sq. ft. to only 1.67 btu/sq.ft. and you could heat a home with a hair dryer. (almost).
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