0 Helpful?

How to insulate unvented attic with foil faced rigid foam in 6 inch bays?

Hello,
Im currently remodeling my house in southern california. Im interested in building an unvented attic since all my HVAC equipment and ductwork are in the attic. In addition, we've had rats and mice nesting in the fiberglass insulation before we tore it all out. This is a single story, 3200 sqft ranch house that will have quite a few vaulted ceilings. With that in mind, what is the best way to use foil faced rigid insulation to create an unvented attic?

Option 1: 5 inches of JM AP foil-faced polyiso screwed directly to the undersides of the rafters and foamed and taped to air seal

Option 2: 4 inches of Foamular in the bays glued to the plywood and 2 inches of JM AP screwed to the rafters

Option 3: 6 inches of Foamular in the bays, Radiant barrier attached to the rafters.

Thanks!
Andrew

Asked by andrew shu
Posted Nov 22, 2012 7:42 AM ET

Tags:

1 Answer

Select your preferred way to display the comments and click "Save settings" to activate your changes.
1.

Andrew,
The best way to insulate your roof with rigid foam is to install the rigid foam on top of the roof sheathing. But I'm guessing you don't want to do that, perhaps because the roofing is in good shape and you don't want to replace it.

Working from indoors, the best way to insulate your roof is with spray polyurethane foam.

A distant third in the "which is best?" contest is either Option 2 or Option 3. This approach is never used by professional insulation contractors but is often suggested by homeowners. The nickname for this approach is "cut-and-cobble." It's fussy, time-consuming work. Good luck.

Answered by Martin Holladay, GBA Advisor
Posted Nov 22, 2012 10:54 AM ET

Other Questions in Green building techniques

BuildingGreen publishes special news stories for April 1st

In General questions | Asked by Martin Holladay | Apr 1, 15

Is it wise to put a product like Drainwrap behind a layer of rigid insulation?

In GBA Pro help | Asked by mike labesky | Apr 1, 15

Best way to find the experts in my area

In GBA Pro help | Asked by Mary Dorcey-Mussog | Apr 1, 15

Foam under slab details

In GBA Pro help | Asked by David McNeely | Mar 31, 15

With taped exterior sheathing, is there any benefit to caulking each stud bay?

In Energy efficiency and durability | Asked by Rick Van Handel | Mar 31, 15
Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!