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Community and Q&A

Critique My Retrofit Plans

Wunderbar | Posted in Green Building Techniques on

I’m about to embark on a massive renovation project of an old farmhouse in the Oregon countryside (Zone 4). Long time GBA reader and welcome any constructive criticism/ideas on the retrofit plans. I’m taking a year off work to do this and have construction experience.

1906 2 story farmhouse with a daylight basement. Structural adobe block foundation in good shape. Old growth fir 2×4 wall framing. No insulation with single pane windows.

I’m redoing the entire top story as it’s basically a horribly finished attic. Siding, roof, insulation and windows are all being replaced/installed.

New Roof assembly is 4×8” rafters at 48” o/c with 2×6 t&g decking (plywood sheathed for shear) 2 layers of 3.5” polyiso fiber faced, standing seam metal roof. Simple gable roof is south/north facing

New wall assembly is 2×6 with zip sheathing. 3”polyiso fiber faced, rainscreen, siding. I should mention the only reason I’m going with 3” polyiso is because I got loads of it from a commercial site for very cheap.

I’ll be installing solar on the roof, heat pump, ERV system, efficient appliances.

My main questions/concerns are.

1. I’m hoping with the fiber faced polyiso the perm rating is better than the foil faced. Is that correct?

2. There are several thermal breaks with the rafters for the overhangs. but since they are 48” o/c it’s not a ton. Do you think it’s worth the effort to chop those, frame out the overhang after foam or just take the hit on the thermal break?

3. What’s the best way to attach a ledger board for a deck with 3” of foam on the exterior.

4. There’s a large sliding glass door 8×16’ on the east facing side. What’s the best way to frame that out with exterior foam? I’d like to do an outie as the windows will be the same.

5. there’s an attached greenhouse on the south side. Since it will now be more separated by exterior insulation, I’m wondering if there’s anything wrong with having a fresh air output from the ERV into that greenhouse to still provide some heat during the winter months.

that’s it for now… whew

thanks in advance

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  1. Expert Member


    Let me take a stab at a couple of them:

    2. I don't think the thermal bridging is significant enough to go through the gymnastics of separately framing overhangs. Pay particular attention to air-sealing the t+g where it goes over exterior walls, either by caulking under it and in the grooves as it is installed, or by stopping it and providing a small gap, then continuing it on the overhangs.


    Good luck with your project!

  2. Wunderbar | | #2


    Thanks for your reply, great tip on the T&G details

    Those deck brackets seem like the ticket too.

  3. Expert Member
    Akos | | #3

    4. You can make bucks out of LVLs to cantilever out or you can rip 2x4s to 3" and nail it around the the rough opening.

    With fiber faced iso you want your WRB over the foam which also makes an outie install much simpler as it avoids all the flashing tape origami. I would skip the ZIP and use CDX with taped seams.

    1. Wunderbar | | #7


      Ive already got the Zip sheathing stacked in the barn, you think i need to install an additional WRB over the fiber iso?


      1. Expert Member
        Akos | | #12

        The facing on the fiber faced iso is a mix of fiberglass an paper, so not the most water friendly facing. Having said that, a while back I forgot some roofing polyiso out over a winter exposed to the elements and it held up pretty well.

        My thinking about the exterior WRB is that it is less work and cost than dealing with liquid flash or tape origami around the rough openings. As a bonus it does provide some additional protection to the foam as well which doesn't hurt if the walls will be left exposed for a while before the siding goes on.

  4. walta100 | | #4

    You say new roof does that mean you are rebuilding with new materials or did you make your unusual selections because you had them on hand? If this is a blank slate a set of trusses is generally the fastest lowest cost option.

    Do you think brining the ERV air thru the green house will be a net positive energy wise in your climate 24-7-365? In my climate greenhouses tend to live close to the dew point and be on the musty side of things.


    1. Wunderbar | | #8

      Yes, im rebuilding the roof with new materials. Already have the plans back from the structural engineer.

      I was thinking ERV air in the winter into the greenhouse and close it during the summer when its ventilated.

      1. walta100 | | #11

        Seems to me the roof plan uses a lot of materials 4x8 covered by 2x6 and 3/4 ply. The material and labor to build is likely 2.5 times the cost of a truss roof 24 inches on center.

        Note the truss company will do the engineering at no cost.


  5. Expert Member
    Michael Maines | | #5

    1. Fiber-faced is more vapor permeable than foil-faced. Whether that's a pro or a con depends on the situation. How do you plan to attach your metal roofing? Your wall detail sounds good.
    2. In your climate I wouldn't worry about the overhangs. In a colder climate I would worry.
    3. Maine Deck Brackets:
    4. Heavy doors should be located over structure. Long-term durability trumps aesthetics.
    5. I don't see a problem with that, other than you'll be introducing moisture. If your greenhouse is leaky, as most are, it shouldn't matter.

    1. Wunderbar | | #9


      Great advice, thanks!

      I was going to install 1/2" zip sheathing over the iso with long structural screws. Then screw down the metal roof to the zip.

      The heavy doors will go out onto a deck so will have the deck framing as support.

      Greenhouse is pressure treated lumber and double pane glass windows. Not that leaky but has ventilation windows that can open. I could just include the greenhouse into the overall CFM calcs and have an in/out as part of the system?

      1. Expert Member
        Michael Maines | | #13

        Wunderbar, have you considered using sleepers over the roof foam? I think you'll find that it leaves you with a flatter surface than screwing through the sheathing alone. But my question was mostly about whether you would have an air space directly under the metal roofing, which produces condensation that needs to be managed. Installing metal roofing directly over sheathing (or an underlayment, then sheathing) eliminates the condensation risk.

        I have always avoided using an exterior deck as support for something that is part of the house, because decks are changed/rebuilt more often than the house and I've found that it's best for many reasons to have a clear line between what is "indoor" and what is "outdoor." Even "structural" foam will deform over time so you need something else to support the door. Perhaps a horizontal LVL that cantilevers over the foam could work but it's not something I would attempt, and I do plenty of non-standard details.

        Including the greenhouse on the home's ventilation system just seems strange to me because its temperature and humidity levels will be different from the rest of the house, but it might work.

        1. Wunderbar | | #15


          Thanks for the advice.

          I was thinking the zip sheathing on top of the foam would offer another protective layer under the roofing. Was also thinking having a ventilation gap under the roofing increases fire risk as im in a wildfire zone. Do you think that's a valid concern?

          BTW, just finished the PGH book. Really great job, its on my gift short list to all my construction buddies.

  6. Expert Member
    BILL WICHERS | | #6

    1. Fiber faced is more vapor open, since the foil facing is a vapor barrier, but the polyiso itself is not. When you start talking about 7" thick layers of polyiso though, I don't think the facer really matters much -- at that thickness, polyiso itself isn't going to be letting much moisture through. Even a 3" layer of polyiso isn't going to allow for much moisture migration. I don't think there is any downside to using the fiber faced polyiso you got cheaply though, aside from possibly a bit more difficulty taping the seams (which can usually be solved by putting some primer on prior to taping if the tape alone won't stay stuck).

    2. I agree with the others here that these aren't worth worring about. You may have a lot of exposed area with that overhang, but the heat has to "conduct" through the cross sectional area of whatever framing members extend through your building envelope, which is probably a pretty small amount of material overall. If these were steel framed overhangs I might be concerned, but with wood you should be fine.

    3. Use brackets or build in "hard points" with wood blocking and bolt all the way through the blocking into the interior structure. Don't rely on bolts only going through that thick layer of foam to hold something critical like a ledger.

    4. I agree with Michael -- prioritize the structure here. If you don't, the sliding of that door will act like a hammer to work things loose. Put the door where it needs to go in relation to the structure of the wall, then fix the appearance with trip details.

    5. I see no problem here, but BE SURE to air seal the dividing wall between the greenhouse and the home, including any doors/windows between the two. Greenhouses tend to be *very* humid, and that humidity can cause problems for the structure of the home.


    1. Wunderbar | | #10


      Ok great, good to know. a lot of this build is with recycled materials such as the 3" iso so i cant be picky but still want to build right without creating future problems.
      Good tip on the primer to tape up the fiber iso, i hadn't thought of that.

      There will be an airseal and 3" of iso between the greenhouse and home. There are 2 windows and a door but Ill be replacing those as well.

      1. Expert Member
        BILL WICHERS | | #14

        Be especially careful with the soffit area if it overhangs a lean-to style greenhouse. I was at a house recently that has such a greenhouse, and over decades of moisture from the greenhouse sneaking up into the soffit, they now have a bunch of rot in the roof in that area, and in some of the walls of the house there. I've recommended replacing the soffit with PVC trim board -- including using PVC board sheets instead of plywood on the underside of the soffit -- as part of the repair. I can't stress enough how important it is to seal up the side of the house that is shared with the greenhouse and make sure you have an absolutely moisture proof assembly here. You'll get a LOT of moisture drive from that greenhouse, so you need to design for that!


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