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R is a resistance to heat flow, just like electricty, R11-13 is equal to a 3.5" fiberglass batt of insulation, R19 is what 5.5" of fiberglass (2x6 wall's instead of 2x4 walls) the greater the R value the slower the heat flows. Foam has a way higher R value per inch than fiberglass does, something like R5 per inch. Spray in foam is even beter because it stops inflitration (air flow), you know the day's with a good North wind when the air comes out of the electrical outlets.
Personally I'll sacrafice infiltration as it improves indoor air quality vs a really sealed house, so I'm OK if my door leak a little. But my t'stat is never set over 63 degrees and my family are perfectly OK with it.
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