Can you explain how the different R values are achieved? Is it just sprayed on thicker? If so how is this done on the underside of the attic if the roof is constructed of 2x4 lumber? Does it expand beyond the beam thickness of 3.5 inches?
Now that I have some quiet time. The primary reason the Icynene is a game changer is it utilizes the synergy of R-Value with Air Barrier. 1+1=4.
R-Value is a mathematical denotation for the resistance to heat transfer by conduction and conduction only. Higher the R the slower the warm molecule are apple to transfer form warm side to cold side.
Air Barrier is either air barrier or it is not. That said once an air barrier achieved added product does not improve the air barrier. Icynene achieve air barrier at 3.5” (aka R13). Short story at an R20 the foam prevent 95+% of heat transfer and 97% at an R30 (8”).
Icynene is designed to be used to encapsulate a conditioned space, when utilized in a non-conditioned area it performs only as a thermal product providing R-Value.
Does it expand beyond the beam thickness of 3.5 inches?
Yes, an R20 thickness will encapsulate a 2x4 truss system.
Is it just sprayed on thicker?
Yes adding thickness increases R-Value to spray foam. There is a diminishing value return which occurs beyond a R13. Typical residential code has always required a R19 minimum so generally we offer R20 as the optimum thickness choice.
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