Nailbase insulation vs. 2 layers of tuff-r with staggered seams

We are going to be rehabbing a home in CT in which the third floor is part of the living space. We want to get the insulation level up by using either nailbase or 2 layers of 2" tuff-r with staggered seams on top of the deck. We are going to use close cell spray foam with polyisocyanurate insulation to increase insulation and stop bridging on the inside. Has anyone used nailbase over spaced wood slats that use to have a cedar shake roof over it? We believe that the closed cell spray foam underneath would act as a vapor barrier, the slats would provide more that enough support and the hot roof is o.k.  but real-world experience would be much appreciated.

Tags: hot, insulation, nailbase, roof

Views: 446


Reply to This

Replies to This Discussion

I would consider using cellulose insulation blown in between the rafters, rather than spray foam against the skip sheathing. Foam on both sides is risky, in my opinion, and will make any future repairs to the roof structure a real challenge. Since you are addressing thermal bridging above the sheathing I don't see the real advantage to more foam below. Proper air-sealing of the drywall lid will probably be adequate, and I'm assuming you are making plans to correctly control interior moisture load anyway.


An alternative would be to install all of the insulation below the roof deck. Install baffles made of rigid foam board between the rafters, with a 1"+ vent space, then install cellulose or possibly foam between the rafters, and follow up with foam board across the bottom. That way you preserve the vented roof. Obviously this approach uses up some of your headroom.

Building assesmblies need to be able to dry in at least 1 direction.  Rigid insualtion on top will limit the drying to the outside so you should allow it to dry inward. I would agree with David and use cellulose. I would not use poly on the inside. Vapor diffuison through the drywall is not a big worry. What is a worry is air leaks carrying moisture in to the ceiling. You should consider and air tight drywall system along with a vapor retarding paint. You will also want to use more than 1 layer of rigid insulation with staggered and taped seams.


Do not use batt insualtion. Only use a dense pack material as that performs better than batts and cuts down on air movement close to the point of being an air barrier.  I said close but it is not an air barrier.



  • Add Videos
  • View All


Latest Activity

Sam Goode is now a member of Home Energy Pros
1 hour ago
Tina Gleisner commented on Tom White's video

How to Insulate a Tiny House (or a Big Fat House)

"Really like how Corbett tells his story in a simple manner that homeowners should be able to…"
3 hours ago
Tom Mallard commented on Tom White's video

How to Insulate a Tiny House (or a Big Fat House)

"So consider the conduction paths of studs-rafters show quite clearly, to counter this requires…"
4 hours ago
Davide Lanzoni commented on Tom White's video

How to Insulate a Tiny House (or a Big Fat House)

"Very cool the thermal tuning with body in white and glasses in black !"
5 hours ago
Quinn Korzeniecki posted a discussion

Applications for Jon Siemen Scholarship Due by 11/31

Thinking of obtaining a BPI Quality Control Inspector (QCI), Energy Auditor (EA), Crew Leader, or…See More
5 hours ago
Benny hani replied to Benny hani's discussion Manual J online
"Thanks Bob"
Benny hani replied to Benny hani's discussion Manual J online
"Thank you Isaac"
Bob Blanchette commented on Amber Vignieri's blog post Even with Polar Vortex, Hourly Pricing Participants Saved
"Looks like the days of paying a fixed amount per KWh are rapidly coming to an end. Many utilities…"

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service