Spray Foam R-Value vs Blown-in Attic Insulation
We’re building a new house in Texas (Zone 3a) and trying to decide how to insulate the attic. At first, we planned for a ventilated attic with blown-in cellulose (R-35), and air handlers/ducts in unconditioned space (unavoidable given space constraints). But, it now looks like our budget might allow for spray foam. Most of the contractors are quoting prices for 6″ of open-cell (R21 or so). Is that going to perform as well for us as the blown-in? It makes sense to me that the air sealing effect of the spray foam, plus being able to get the ducts in conditioned space, might make up for the difference in R-value. Or, is that just wishful thinking?
Ryan
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You had me at “air handlers/ducts in unconditioned space” - definitely find a way to avoid that!
Someone else will have better advice for you, but I think you’d want closed cell here which is quite bad environmentally… is there any way you could bulk up your roof framing to allow for a deeper cavity for batts instead of spray foam? Check out Martin’s article, I like the look of #1: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Former spray foam contractor (New England)
I would add an 1.5" (R10) insulated nailbase to roof for a thermal break, then add 6" open-cell spray foam (R20) on the underside to get an R30...
Thanks, but that's not really an option in terms of budget. It's a large roof (7000+sf), so I estimate adding an insulated nailbase (250-260 sheets) would be about 15-25k extra. For that cost I could afford to double the thickness of the spray foam.
skip the nailbase & add closed cell first, then open cell (to address Malcom's concern below) ...
Ryan,
Two options to consider.
- Using plenum trusses, which provides a service cavity for services inside the house's conditioned space. you could then go bak to using cellulose on the attic floor.
- Building a cathedral ceiling using permeable insulation under the sheathing, with a vapour diffusion port at the peak.
Open-cell spray foam on roofs does have some risks: https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing
This is the south everyone puts their HVAC in the attic my neighbors can’t all be dumb.
Hot leaky equipment and duct work in a vented attic will not cost 40% more to operate.
Conditioning the attic with twice the surfaces area and half the R value will not use 30% more energy.
It is not like new construction and someone could make a plan and avoid doing something silly.
Since the equipment must go in the attic you might find this article interesting.
https://www.energyvanguard.com/blog/buried-ducts-allowed-2018-building-code/
Walta