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 an Attic With 2×6 Rafters

kiwiscott | Posted in General Questions on

I live in NJ in an old 1930’s house. I’ve only got 2×6 rafters in the roof (which is interesting). My attic is currently uninsulated and there’s a lot of HVAC equipment and ducts up there. (Yes, I’m a poster child for disaster).

I’ve been looking a how to best insulate the roof. So far I seem the settling on add 6″ of closed spray foam to the rafters and create an unvented assembly . The rafters are 2″ x 6″ (actual not nominal) so I feel like close-cell spray is the only option.

I’ve read a lot of articles on the site ( and agree in general with the sentiment – don’t condition an attic however I have little to no choice.

So my question is – it’s 6″ of foam going to cause a problem as it will create an unvented roof assembly?

Exterior foam on the roof 4-6″ was an option but no one seems to want to entertain a job.As an aside, I read something on this site the other day where a contractor was explaining that the way he’s set up the company 1 week jobs are his target and he’s figured out ho to make money that way so that kind of makes sense in this area where I live — the volume of jobs means they need to be highly predictable.

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. nynick | | #1

    I've got the same 6" rafters in the one of the attics in my 1870 home. I've decided to put the new HVAC air handler up there. The Building Inspector is fine with me filling those rafters the best I can instead of trying to make code, so we're going with Closed Cell. Should be fine.

    1. kiwiscott | | #2

      Thanks - I think so too. I hate to use this solution but what can one do?

  2. Expert Member
    DCcontrarian | | #3

    The other alternative is to effectively make the rafters thicker.

    The extra thickness isn't needed to hold up the roof, it only has to support the weight of the drywall and insulation so it can be relatively flimsy. There's a couple of ways of doing it, the best way depends on how much thickness you want to add what the existing construction is like.

    You want to have at least a 1" air channel between the insulation and the underside of the roof. If you have the clearance I'd be looking to make the effective thickness about 13", that allows for the 1" air channel and R-49 insulation.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |