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Deep energy retrofit from the inside or out?

C Chrusch | Posted in Energy Efficiency and Durability on

We have a 2 x 4 house with standard framing in Calgary (very cold and semi-arid – zone 6).

It is a full gut job, so we can insulate from inside or out, aiming for R-50 walls. We’re thinking an offset frame wall with 4.5″ spray foam;1.5″ of XPS; synthetic stucco with appropriate water control. Existing 2 x 4 stud cavities would get cellulose. The foam is the air/vapor control layer so no housewrap or poly. But building on the exterior with a sloped roof means little space to get R-50 above the top place and still provide adequate roof ventilation.

1. What do you think the trade-off on less than R-50 on the top plate vs building the wall on the inside is (aside from loss of floor space)?

2. 2″ of foam is enough for an air and vapor barrier, but if I use less foam and more cellulose how do I know where the dew point is in the wall and if I can still go without 6 mm poly from the interior?

3. If the roof has enough ventilation (based on area calculations and 1:300 rule) in general, do you need to vent every rafter bay? If you don’t, is the roof in that area as risk of moisture problems?

Thank you in advance your thoughts.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    C Chrusch,
    Q. " What do you think the trade-off [is] on less than R-50 on the top plate vs building the wall on the inside is (aside from loss of floor space)?"

    A. You never described your roof insulation plan, so I don't know why you can't achieve R-50 over the top plate.

    Q. "2 inches of foam is enough for an air and vapor barrier, but if I use less foam and more cellulose how do I know where the dew point is in the wall and if I can still go without 6 mm poly from the interior?"

    A. The answer to your question can be found here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "If the roof has enough ventilation (based on area calculations and 1:300 rule) in general, do you need to vent every rafter bay?"

    A. The answer depends entirely on your roof insulation plan. Are you insulating a flat ceiling or a sloped roof? What type of insulation are you using? What is the R-value of your insulation?

  2. Dick Russell | | #2

    Regarding roof ventilation, here is a good piece, just a few months old, from an established expert:

    https://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs

    His opinion is that you really ought to vent each rafter bay. He disagrees with code on having more venting area up top than down low, stating that the reverse makes much more sense. His reasoning is easy to understand.

  3. C Chrusch | | #3

    Thanks for your reply.

    1. It's a sloped roof with 7-8 inches over the existing wall top plate, which will be even less over the new exterior wall because of the slope. Even with spray foam at R6-7 an inch this is just enough to approach the R-50 of the wall.. That isn't counting 1-2" for roof ventilation. The attic/roof will get blown cellulose to R-80.

    2. Thanks for pointing out the foam calculator.

    3. Sloped roof, blown cellulose, R-80.

  4. C Chrusch | | #4

    Thanks Dick,

    It's not that I don't want to ventilate each bay. It's a problem of choosing between enough insulation over the top plate vs need for roof ventilation. I would hate to go to the the trouble of beefing up the walls to R-50, but leave a perimeter of less insulation. Beefing up from the interior, however would mean being able to adequately insulate over the top plate of the wall. If I can get R-30 over the top plate that's still pretty good, but does it still make sense to spend $ on a thick wall when you can't insulate a critical part of it?

  5. GBA Editor
    Martin Holladay | | #5

    C Chrusch,
    You are planning to install 21 or 22 inches of cellulose in your attic floor, and you now realize that you don't have enough room at the eaves. This is a common problem.

    If you really want to address the issue properly, you need to add insulation above your roof sheathing or raise your roof by sistering on new framing members above the existing ones.

    As you know, even if you use closed-cell spray foam, you'll need at least 12 inches to get R-80. And many spray foam contractors are unable or unwilling to install foam to that thickness.

  6. C Chrusch | | #6

    Martin,

    I was going to accept that I can't get the full R-80 for the entire ceiling. The compromise is a perimeter of foam for it's higher R value then the remainder in cellulose.

    The main thing I'm deciding on at the moment is whether to build the walls on the inside or outside. It is worth building out and getting an R-50 wall that isn't R-50 over it's top plate?

    One of the appeals of the proposed wall assembly is how well it adresses thermal bridging. It is a house from the 1950s with much framing in the walls. I suppose if we build the wall from the inside we could still put exterior XPS to help stop the bridging.

  7. GBA Editor
    Martin Holladay | | #7

    Well, you can still get R-80 if you are willing to modify your roof.

    A deep energy retrofit is expensive. Because few homeowners can afford all of the measures required to make an existing house superinsulated, most homeowners accept compromises.

    I think you understand the issues involved, and you know your own budget.

    Any goal is achievable if your checkbook can handle the cost.

  8. C Chrusch | | #8

    Still fiddling with how to do this. The current roof slope is 3.5:12 and we are aiming to maintain that. Otherwise we would have to go with less attractive roofing options (tar, membranes,etc) rather than metal. The metal roof folks here aren't keen on metal on a roof with a slope less than 3:12.

    Having said that, we were thinking of adding rigid on current sheathing with sleepers for a cold vented roof. If we were to do that, we still have lots of air in the attic. Would that need to be vented as well? If it is vented then the rigid doesn't add insulation value where it is above air. Is that just a waste of money? A sketch is attached to give an idea of what we're thinking.

    Thanks.

  9. C Chrusch | | #9

    not sure what happened to the file.

  10. GBA Editor
    Martin Holladay | | #10

    C Chrusch,
    Once you install rigid foam on top of your roof sheathing, you are committed to an unvented conditioned attic. So don't vent your attic!

    If you want, you can move some of the R-value that you plan to install on your attic floor to the underside of the sloped roof sheathing -- but you don't have to.

    Here's more information: Creating a Conditioned Attic.

  11. C Chrusch | | #11

    Thanks Martin. Do you still call it a conditioned attic with 6 mil poly in ceiling and 1" spray foam then cellulose? I was concerned about the sheathing being unable to dry from under the rigid foam.

  12. GBA Editor
    Martin Holladay | | #12

    C Chrusch,
    I don't recommend installing poly in your ceiling.

  13. C Chrusch | | #13

    Martin,
    It's a retrofit with existing poly. Sounds like leaving it in place together with rigid foam = double vapor barrier = trouble.

    Choices are 1: Remove poly and go with roof/attic insulation and ventilation as drawn (maybe go 2" spray foam over ceiling). Having siad that the huge amount of air between the cellulose and sheathing bothers me. Should it?

    2: Keep the poly and raise the entire roof enough to put all insulation under sheathing and vent from soffit with baffles.

    Did I get it right?

    Thanks.

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