Attic Insulation in home in Zone 3/WH
约翰·斯诺登| Posted inEnergy Efficiency and Durabilityon
I am trying to decide on attic insulation. I have read the merits of spay open cell up against attic roof. The whole triangle space then becomes conditioned from leakage up from second floor. Open cell only gives about R16-18 anyway.
I have read the merits of 18 inches of blown fiberglass and the realization that it slowly settles and doesn’t seal air movement up from second floor either.
Does it make energy sense and economic sense to spray open cell 6 inches up into roof decking and spray 6 inches open cell between ceiling joists to give R-30 plus??
Thanks,
John
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Replies
John,
Why do you write that "Open cell only gives about R16-18 anyway"? Is that because your spray foam contractor can't spray it any thicker? And if so, is that because thicker foam hasn't passed the code-required flame-spread testing?
There are many misunderstandings around this issue, so be sure that your spray foam contractor explains them. I certainly wouldn't want less than the code minimum R-value of insulation -- and if your contractor says that he can't provide it, I'd want to know why.
ThanksMartin, Rafters are 5.5 inches so 5.5 times 3.3 is 18 for open cell. Spraying a full rafter is only 18 ,right?
John,
First of all,if you plan to install drywall, you can always fur down the rafters to create greater depth.
And if you aren't installing drywall, why can't your contractor spray a thicker layer?
You should consider blown cellulose. Your loose fill insulation is not the air seal. Air sealing the attic floor should be done prior to installing the insulation. In my opinion fiberglass is the wrong product in an attic. Loose fill fiberglass is the most air permeable and is the least efficient at blocking radiant heat flow. Cellulose will trap air better and settling is not a problem as long as it is accounted for at time of install.