How to calculate R-value of attic insulation?
user-1021589| Posted inGreen Building Techniqueson
Hi there –
My wife and I are in the final stages of completing a super-insulated house near Ottawa, Canada. I have requested that the attic be insulated to R-80 with blown cellulose. While in the attic yesterday installing some cabling, I had a chance to measure the cellulose. It was installed about a month ago, it is very even, but is only 16″ in depth, more or less.
From what information I have cobbled together from the net, this corresponds to about R-60. Am I correct or I have I not accounted for something (such as settling)?
Thanks in advance.
John Scime
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Replies
John,
Once the cellulose insulation has settled -- a process that takes several weeks or months -- you can expect blown-in cellulose on an attic floor to have an R-value of R-3.6 or R-3.7 per inch. So, once it has settled, you want to have between 21 and 23 inches of cellulose.
Installers need to anticipate 13% settling, so your installer should provide between 24 1/2 inches and 25 inches of insulation before settling.