GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulating rafter bays with sistered roof rafters

David Lewinnek | Posted in Energy Efficiency and Durability on

I have a newly constructed home in Massachusetts, built to minimum energy standards (or maybe a little below).

The attic is unfinished, but the builder planned to finish it one day so he insulated the 2×10 rafters with a “flash & batt” approach. There are 4 inches of a sprayed on foam in the rafter bays (I would guess R-24 to R-28) and 3.5 inches of R-15 fiberglass. The fiberglass is held in place with vinyl straps and a layer of Membrain vapor barrier. There are soffit vents and a ridge vent, but these appear to be unused and blocked by the foam, at least in the rafter bays.

The problem is that part of the roof has a span too long for a 2×10, so there are sistered 2×10 rafters (basically a 4×10 with a crack down the middle, still on 16″ centers). I can look between the sistered rafters, and see daylight through the clear membrain and the soffit vents. I’m worried about thermal bridging from the roof assembly having twice as much wood per 16 inches, but also about condensation from warm humid outside air hitting coming through the soffit vents and hitting the vapor barrier.

What are my options for sealing between the sistered beams? Should I add a piece of trim below the soffit to seal it for good, and caulk/foam that trim in place? Should I cut away some of the vapor barrier and add caulk from the inside? Should I assemble 2x4s perpendicular to the rafters and add thin fiberglas insulation (I can probably add R-10 before I get too concerned about condensation) to combat thermal bridging?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    The standard air sealing method for doubled framing members (studs or rafters) is to caulk the crack on the interior.

    In your case, your poorly-sealed soffit sounds sloppier than usual, so you might want to take off the soffit and see what's going on in there. A better air sealing job in the soffit would certainly make sense.

    Flash and batt between the rafters has several disadvantages. After you have addressed the first disadvantage -- air leaks -- you might consider addressing the other disadvantage -- thermal bridging through the rafters. A continuous layer of 2-inch-thick EPS foam on the underside of your rafters wouldn't be a bad idea.

  2. David Lewinnek | | #2

    Martin, thank you for the answer. Should I be concerned about condensation and mold growth if I add insulation on the underside of the rafters? I'm concerned that adding insulation will decrease the temperature of the foam surface to below the dew point during winter.

  3. GBA Editor
    Martin Holladay | | #3

    Since you have an R-24 to R-28 layer of closed-cell spray foam on the exterior side of your assembly, and Membrain on the underside of your rafters, I wouldn't worry.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |