Attic Insulation – Rafters or Attic Floor?
On new construction, in Northern NJ, Spray Foam companies are huge advocates of insulating at the rafters. I am an advocate of spray foam, but my experience tells me that unless there is much ductwork in the attic, the attic plane is very complex, or there is a very low ceiling height, I would rather seal and insulate any ductwork properly, and insulate at the attic floor, to keep the overall conditioned space to a minimum.
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Since your first name is "Eco," how about moving the ducts into the conditioned envelope of the home?
Why does your experience tell you that it's better to insulate the attic floor?
It may cost a little more to insulate the roof plane, but once insulated you're essentially paying nothing extra to heat that space. It's usually much easier to get a good thermal envelope at the roof plane than at the attic floor.
I agree that simplicity is best - seal at the ceiling plane, pile insulation above that, and avoid attic ductwork if at all possible. Simpler solutions are more likely to be implemented correctly.
The big question about sealing attic ductwork is, do you have a level of quality control that ensures it is completely sealed? And can you maintain full R-value above it?
we are planning to do duct-blaster testing to confirm how tightly the ducts are sealed. And in response to Micheal Maines, by minimizing the volume of conditioned space, this would inherently keep the heating/cooling load down. Especially in a high hipped roof, which adds almost 1/3 to the overall conditioned space of the homes.
May be a quibble, but conditioned volume is not the significant metric. Heat transfer through the enclosure of a conditioned space is dependent not on the volume of the space but on the exposed surface area. A gable attic with a 10/12 pitch will have 50% more surface area than the flat ceiling below it, so in energy terms it's certainly not 'free space'. Hipped roofs improve the situation somewhat by presenting a slightly more efficient exterior form factor.
The more contentious discussion is the R-value and airtightness of the enclosure. Flat ceiling insulation values of R-60 are relatively inexpensive and easy to achieve. Roof deck insulation not so much. I must assume that Michael's comment that "it's usually much easier to get a good thermal envelope at the roof plane than at the attic floor" is directed at airtightness rather R-value.
The third major issue is the value of the attic space. If it absolutely HAS to contain both ductwork AND mechanicals, that is certainly going to weight the decision in the direction of roof plane insulation. If those concerns can be addressed in other ways, if the space is not to be inhabited, and if financial and environmental cost considerations are given any weight, ceiling level insulation seems generally the better choice.