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Reno: vented roof with 4 inch rafters. Spray foam?

vap0rtranz | Posted in General Questions on

What are options for renovating an old (1940s) vented roof with 4″ rafters? 

Contractor is recommending a 1st Phase of spray foam applied from underside to get R20 (and I’ll confirm open vs closed cell with him).  And recommending 2nd Phase of rigid foam on top of the roof (to get at R60 total).

Currently, the attic floor and knee walls are insulated and not the roof.  Attic floors are either 4″ blown in cellulose in attics and 8″ loose fill cellulose elsewhere.  Knee walls are 4″ fiber batts with house wrap facing exterior (that is poorly sealed).  Contractor is recommending we remove all this old insulation after the roof is insulated as it may present “problems” … though it wasn’t clear _what_ problems that would be other than “more isn’t better”.

Another contractor recommended also dropping the sloped ceilings that are above the knee walls attics and sister in deeper rafters … but this is problematic on some of the sloped ceilings because upstairs windows get in the way.

Curious to hear what others have done or recommend.  EDIT: our house is in zone 6 — sorry, I was switching between our garden plans vs house plans where plants have a hardiness zone of 5 here 🙂

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  1. Expert Member
    Michael Maines | | #1

    Vaportranz, I avoid foam whenever it's practical to do so, due to its high embodied carbon and other health and environmental factors. Can you furr the rafters down into the interior space? If you can create at least a 14" framing cavity you could use a benign insulation such as cellulose, and still provide a 1" vent space.

    If that is not an option, closed cell spray foam is your best bet. Insist on low global warming potential foam, sometimes labeled as HFO-blown or high-lift. Don't let the spray foam contractor confuse you into thinking that zero ozone depletion potential is a differentiator, or that conventional foam is better in any way.

    Using foam on the exterior, it's probably not worth getting to R-60, unless you can find inexpensive, recycled foam. 4" of polyiso with R-28 foam on the interior will meet IRC requirements for condensation control in your zone.

    1. vap0rtranz | | #5


      >Can you furr the rafters down into the interior space?

      Not down to 14". Maybe 4", though that isn't possible on at least one ceiling. It's a 1.5 story farmhouse where the 2nd story is all knee wall and sloped ceiling that abut the rafters to form the upstairs rooms.

      Agreed on avoiding foam where possible. I've got rockwool batt, tape, etc. in storage because I was originally going a "green" DIY route of stuffing the rafter bays with batts, then a rigid layer across the rafters, etc. ... but this old farmhouse presented some challenges (sloped ceilings with drywall attached to rafters, and air leaks everywhere from attic to roof). Two contractors recommended foam because it will both a) air seal and b) give more R per inch.

      >sometimes labeled as HFO-blown or high-lift

      Great, I'll ask about HFO.

      Ty, Justin.

  2. MattJF | | #2

    You really want closed cell foam here. The challenge with closed cell is that it is very difficult to fill the bays as it is challenging stuff to shave flat. How much area is it, because it can be shaved flat. HFO closed cell (the better for the environment version), will run R7/in.

    What is your timeline for the roof? I would be temped if you are considering redoing the roof to just go ahead and do it, and put compressed batts in the bays. You should be able to get around R17 with a R23 batts squished to 4 inches.

    Can you get away with any furring? Furring down 1.5" with foam strips over the rafters and doing 2" HFO closed cell with R15 Rockwool would get you R29 center cavity and address significant thermal shunting through the framing.

    1. vap0rtranz | | #4


      >What is your timeline for the roof?

      We're wanting to do "interior" insulation (walls, basement, attic) now but re-roofing later. For better or worse, our roof was re-shingled 2-3 years ago (before we bought) and no contractor nor I have found evidence of problems with the roofing, so it's a huge expense to "fix" something that is basically working. ... the one problem with the roof is ice dams.

      >Can you get away with any furring?

      1.5 - 4" is doable. So could the roof material "sandwhich" be:
      roof sheathing
      closed cell foam
      rockwool batt


  3. Expert Member
    BILL WICHERS | | #3

    If you’re going to be putting rigid foam on top of the roof, and R40 of it at that, the insulation on the inside becomes far less important. I’m assuming you’re converting this to a non-vented assembly here due to the exterior foam. I’m pretty sure you could just skip the spray foam step and just use batts between the rafters and save a bunch of money. The R40 of exterior rigid foam would be doing all the work, and should be plenty to avoid moisture problems on the underside of the roof sheathing.


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