Encapsulated Spray Foam Attic Code Requirements – GreenBuildingAdvisor

Hi All,

New member here.  First a little background on myself.  I have been an HVAC contractor all my life so going on 35+ years now.  

I run an honest heating and cooling calculation on all my project to size my equipment.  Recently I have been running into insulation contractors spraying about 6″ of open cell ( 0.5lb/ft3) foam at the roof deck creating an encapsulated attic space.  Being in climate zone 4, I thought the minimum attic insulation is R-38.  Could even be more now.

Don’t get me wrong I love an encapsulated attic space if I have equipment and duct located there.  It can easily decrease my cooling loads by 25% or so.

What I don’t understand is how the building code enforcement people allow a R-22 insulation value in an attic.  (r-3.8 x 6″) ?  The answer the insulation people give me is that it is equivalent to a R-such and such of fiberglass.

Like I say I run lots of load calculations and to me an R value of X is = to an R value of X.  It doesn’t matter the material you use to achieve that value.  Infiltration is a different calculation completely separate from the heat conduction through the material.

That being said a foamed building envelope will have less infiltration than conventional insulation methods but again infiltration is calculated separately.  To put it in math terms the equation would be—



Thermal conductivity of foam x area + infiltration

= Thermal conductivity of fiberglass x area + infiltration.

subtract infiltration from both sides of the equation and it is obvious a R-22 foam assembly has more loss than a R-38 assembly.  Not to mention the total surface area when foaming at the roof deck is substantially more than the surface area of the ceiling of the conditioned space.

What am I overlooking here?

appreciate your comments,

Axiscat