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Q:

How does R-21 at 5.5 inches have better insulating value than R-19 at 6.5?

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Dave writes from Waterbury, Vermont: "I plan to finish an attic space with 2 x 6 ceiling joists. I planned to install your r21 kraft faced pink insulation, before wall board. On your website it appears the best application is the r21 at 5 1/2 deep and 16 inches wide. However, the building supply stores I have been to say compressing r-19 down to 5 1/2 isn't a big deal (contrary to your web site) AND that it will take months to order r21, so two questions: 1) How is r21 at 5.5 have better insulating value than r19 at 6.5? 2) Assuming r21 is the best, how do I get it in Vermont?" Answer: How do we increase the resistance to heat flow in the same or less thickness? One way is to increase the number of fibers to reduce the size of the pockets of trapped air that resist convection. This is what happens with the insulation for a 3-1/2 inch wall going from R-13 to R-15. The amount of glass in the wall is increased in the same overall volume, which increases the density and hence the R-value. ... more
saveenergy.owenscorningblog.com
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