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

Insulate Attic Rafters AND Joists

KKlouzal | Posted in Energy Efficiency and Durability on

I have done the research. Don’t insulate both, pick one or else you’ll have moisture problems. What about a way around that?

I live in climate zone 2B. Hot and dry.

I want to insulate my rafters with 1″ rigid closed cell foam board and cover that with 3″ of spray foam. I’m putting the foam board down first so spray foam won’t get in the way when it comes time to replace the roof deck.

Then I want to put down mineral wool insulation between all my joists, mainly for sound dampening. I’m looking at Thermafiber Fire & Sound Guard which is 3″ thick and their product page makes no claims of R-Value but it’s probably in the neighborhood of R-15. (HomeDepot says it’s R-24 but that can’t be right?).

So that’s R-21 in the rafters and R-15 in the joists. Lets talk moisture. I’m in zone 2b, hot and dry, but there is still moisture. Currently my attic has only two large 3 foot square vents on the north and south wall. No ridge or soffit venting (although the soffits are a serious air infiltration location, you can see light between the cracks, I don’t think this was intended to be a soffit vent but the degree of air leakage is probably acting like one).

The soffits will be completely sealed with the spray foam, leaving the two large vents on the north and south wall. Leave them open? That defeats the purpose of insulating the rafters. Well maybe it doesn’t, the roof decking in mid summer when it’s 115 degrees outside probably gets MUCH HOTTER. So insulating the rafters would help minimize the heat coming in this way and only leaving the 115 degree heat to blow in from the large vents where my joist insulation will insulate from the conditioned space.

Okay not a good idea? Seal up all the ventilation. The joist insulation is unfaced so moisture can get through and dry towards the downstairs living area, but this is mineral wool which is naturally hydrophobic so just how permeable is it really?

Maybe this is a horrible idea and I need to pick either the rafters or joists and insulate one and not both.

Oh I almost forgot to mention, each room has their own ducted minisplit in the attic, 6 in total, two branch boxes. So 8 condensation creating devices but they are all hooked up to a condensate drain. And I have a fresh air exchanger ducted with a single supply and return into each room.

I suppose another option could be to completely seal the attic and place a couple supply and return vents in the attic which hook up to the fresh air exchanger.

I really would like to insulate both.. OKAY go! What say the experts? There must be some science here..

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. Expert Member
    BILL WICHERS | | #1

    Mineral wool is very moisture open. The 'hydrophobic' part has to do with how it doesn't want to hold bulk water, it has nothing to do with vapor migration through the material.

    Per code, your insulation has to be in one layer. You can't add together insulation in two seperate places to arrive at your target R value. You'll also find that moisture migrates upwards, so while you can dry through the mineral wool on the floor, you're still likely to see higher average humidity levels in the attic space compared with the living spaces.

    If you want to insulate the roof deck, you are correct that you have to seal off the vents and create what is known as a "conditioned attic". If you leave the vents open, the insulation on the deck might help a bit with solar gain, but the interior attic space will still get close to the outdoor ambient air temperature due to the venting.

    I would reconsider your plans here and either put ALL of the insulation under the roof deck, then close off the vents, OR put ALL of the insulation on the attic floor. If you don't have mechanicals in the attic, I would absolutely keep your attic vented and use all loose fill insulation on the attic floor. Loose fill insulation is cheapest, and is also one of the best ways to insulate. If you want to create a conditioned attic, I would put all spray foam under the deck and encapsulate the rafters. Don't use rigid foam here, that's likely to cause problems.

    If you want to put something up to let the spray foam release, I would staple housewrap up tight to the underside of the sheathing. Anything you use like this will prevent the sprayfoam from adhering though, which will compromise it's structure and potentially allow voids to form between the spray foam and roof structure which would be a moisture issue. If you were to do this, I would staple housewrap under the sheathing only, and allow the spray foam to adhere to the rafters. This would give you your release layer, but also allow for the spray foam to have something to anchor to. I would use all closed cell spray foam here if you go this route.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |