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stevehull · 05-05-15, 08:34 AM

Gasstingy,

Yes, you can put in too much insulation. I have seen it done. I recommend about R40-R60 for the ceiling, in all but the most extreme lower 48. Walls max out at about R20-R30. One client insisted on doing R70 walls and R120 ceiling. I tried to explain that the return on this tripling made the pay off time literally about 200 -300 years. He would not buy it and spent far too much $ on this.

The performance data discouraged him. He though by putting in these large values, he could heat his home "with a lightbulb" - this was before CFLs or LEDS.

As an engineer, the major issue is "bang for the buck" with adequacy of design. With insulation, the return is not linear, but a 1/x relationship. So, for an attic, the return on the first inch is about one week, for the next four inches is 6 months, the next four inches about a few years and so on. Much beyond R-60 and you are just insulating with dollar bills.

If you look a U values, the inverse of R, then it is a linear relationship. In Europe, the U value is more widely used.

But basically it is just Ohm's law E=i/R or E=iU.

Steve

05-05-15, 08:34 AM	#11
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Gasstingy, Yes, you can put in too much insulation. I have seen it done. I recommend about R40-R60 for the ceiling, in all but the most extreme lower 48. Walls max out at about R20-R30. One client insisted on doing R70 walls and R120 ceiling. I tried to explain that the return on this tripling made the pay off time literally about 200 -300 years. He would not buy it and spent far too much $ on this. The performance data discouraged him. He though by putting in these large values, he could heat his home "with a lightbulb" - this was before CFLs or LEDS. As an engineer, the major issue is "bang for the buck" with adequacy of design. With insulation, the return is not linear, but a 1/x relationship. So, for an attic, the return on the first inch is about one week, for the next four inches is 6 months, the next four inches about a few years and so on. Much beyond R-60 and you are just insulating with dollar bills. If you look a U values, the inverse of R, then it is a linear relationship. In Europe, the U value is more widely used. But basically it is just Ohm's law E=i/R or E=iU. Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990