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Questions on a “not-so-deep” retrofit…

Justin Merkovich | Posted in Energy Efficiency and Durability on

Minneapolis
1948 mid century/ranch, stucco exterior
Wood shaving insulation with paper face stapled to framing
2 layers of (I believe) “plasterboard” 3/8″ over 5/8″ = 1″ total
Kitchen renovation found no evidence of moisture in the ceiling or walls, removed wood shaving insulation and replaced with R-19 batt (regrettably but can’t undo now)
Polyethylene vapor barrier on interior side of renovated walls (insulation code official laughed when I said that I expected him to closely scrutinize the vapor barrier and he said, “There are so many holes in this old house [windows and doors], this renovation could have a perfect VB and it wouldn’t matter”)
Attic space had a layer of unfaced batt insulation over the wood (where I didn’t renovate. I sealed every penetration I could find with spray foam and I put vent chutes in almost all of the cavities and blew in 12″-15″ of loose cellulose so I feel like the attic is as good as I can do for the budget that I had. We removed an exterior door in the renovation and framed it in/insulated with the batt, re sheathed with plywood and threw some building paper on – 2 years have elapsed.

It’s time to fix the “hole.”

I want to do whatever I can within my meager budget to add insulation to the exterior and break the thermal bridge to the exterior and this hole will set the precedent for the way I’d like to do the rest of the house. I don’t know what to make of the 1″ of plasterboard and the wood shaving insulation in terms of moisture permeability from inside to outside but I do know that the walls are freezing. ASSUMING vapor is traveling to the exterior (a big if and I’d love to hear opinions)…
My first shot at the wall assembly is this (exterior to interior):

Siding (horizontal, likely thermally modified ash)
1x treated furring strips (attached with a Heco Topix screw [or sim] to 1 1/2″ sheathing/framing penetration)
Roxie CavityRock (layered up similar to a deep retro fit found on this site) Mono and Double Density as suggested for furring compression
Vaproshield (Wrapshield SA) self-adhered seems like it’d be easier to install but I’m listening to opinions.
Sheathing (existing if good)
Existing cavity insulation
Existing finish
Windows will probably have to be replacement windows and I’d be spray foaming the opening prior to install
It should be noted that this “hole” is in a 3′ -6″ porch and there is a 3′ eave/overhang on the house. The “hole” is on a wall perpendicular to the main facade. Almost no weather ever gets to this particular wall.

I’d greatly appreciate any comments/ideas.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Justin,
    I think you are on the right track. Your best option is probably to improve the airtightness at the sheathing level -- and VaproShield is one way to do that -- and to install a fairly thick layer of rigid or semi-rigid insulation on the exterior side of the wall sheathing.

    Personally, I would probably choose to install EPS rather than mineral wool, because it's tricky to get the furring strips co-planar with mineral wool (because mineral wool is squishy). But mineral wool can work well in this location.

  2. Justin Merkovich | | #2

    Martin,
    Thanks for the reply.
    The reason that I'm considering mineral wool over EPS is because of the vapor permeability. Are you suggesting EPS because it is vapor permeable and inexpensive? I've read a fair amount about EPS v XPS and it appears that EPS definitely absorbs LIQUID water, it is unclear to me if it is vapor permeable. I understand that the Roxul Cavityrock will be more expensive than EPS but if it provides similar R values and DEFINITELY breathes, then it seems safer plus I'm a tree-hugger.

    For the specific wall that I'm describing I'm limited in the thickness of the Roxul Cavityboard because the wall is adjacent to a large window opening (I've probably got about 3" to get insulation, furring and cladding before I start to get too close to the adjacent window jamb trim). For the rest of the house, I'd like to go up to 4" of insulation - I'm really holding my breath on how to detail this in terms of how the Vaproshield works with existing windows, how to trim out the much deeper jambs, heads, sills in a way that will look great and let water drain. This is a 1400 sf one story house on a flat site so if I have to treat/seal material at the window openings every few years it wouldn't be so bad...

    The biggest questions that I have are:
    What is the effect of 1" of "plasterboard" (3/8" + 5/8" layer) in terms of vapor permeability?
    What is the effect of the "faced wood shaving insulation" on vapor permeability?

    It seems to me that the plasterboard and the wood shaving insulation installed in 1948 are doing little to nothing to stop the travel of warm/moist air from moving to the exterior. I'm not going to tear apart the inside of the house - it's just not feasible with two small children so I think I have to accept the poor existing insulation and consider the walls as NOT having an interior vapor barrier (hence the Vaproshield) and load up on the exterior insulation in a way that breathes to the exterior and hope that the sheathing stays warm.

    Thank you for your time.
    JM

  3. GBA Editor
    Martin Holladay | | #3

    Justin,
    I think you are way too worried about vapor permeance.

    When you install insulation on the exterior side of wall sheathing, the sheathing stays warm in the winter. There is no need for the wall sheathing to be able to dry outward. This type of wall assembly is designed to dry to the interior.

    By the way, plasterboard is vapor-permeable.

    For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  4. Nate G | | #4

    Under 3" of EPS is still reasonably vapor-permeable if there are no foil facers. But mineral wool has additional advantages; it's not flammable and is far more resistant to insect infestation than foams.

  5. Bob Irving | | #5

    Justin: have you had a blower door test so that you know where the air leaks are?

  6. Justin Merkovich | | #6

    Thanks for all of the input. Funny you should mention a blower door test. There is a company in town that does this work and then works on identifying incentives for energy improvements and financing. I just got in touch with them on Friday. I'm bracing for the bad news - the original 1948 single pane windows are super leaky.

  7. Justin Merkovich | | #7

    Any input on what I can expect at the windows?
    Since I plan on doing replacement windows, I'm going to be relying on the original flashing/detailing and I have no idea how windows were installed in 1948. I'll also have to contend with creating a frame to account for the additional depth of the insulation. Thanks again.

  8. GBA Editor
    Martin Holladay | | #8

    Justin,
    If you are buying new windows, you can buy any type of window you want: new construction windows with flanges, new construction windows without flanges, or replacement windows designed to slide into your existing jambs.

    If you are buying new windows, I would suggest that you flash the rough openings properly rather than just sliding the windows in place and relying on caulk.

  9. Justin Merkovich | | #9

    Martin,
    I have a good understanding of how to detail a new window in a rough opening/sheathing, etc. Some great ideas on this site as well. I am planning new REPLACEMENT windows (removing the inner stops, jamb liner, adding spray foam, setting the replacement window in a bed of sealant over flashing to the exterior). I'd *like* to do new windows but I just think they'll be too expensive. If they are close to the cost of the replacement windows, I'd rather do that because I think the detailing will be easier to do in terms of flashing it correctly and taping it but I just don't think it'll be feasible monetarily...

    What is unclear to me is where old windows are in relation to the sheathing. In a new installation, "outie" windows are obviously on the sheathing (or a "buck" of some kind). I just have no idea how the old windows were installed (innie?) what (if any) flashing was done at the sill, what the head flashing is like etc.

    Maybe it would be useful to make a guess and put a detail sketch on the site here for input.

    Thanks again.

  10. GBA Editor
    Martin Holladay | | #10

    Justin,
    If you are adding mineral wool insulation on the exterior side of your sheathing, and you are going to leave your windows in the same position in your wall, you will end up with innie windows. If you go that route, you will need to install exterior jamb extensions (usually made of metal flashing or plastic trim like Azek) and a new waterproof (and well-flashed) exterior window sill.

    The alternative would be to build plywood boxes (window bucks) that extend to the exterior of your new insulation, and to install your windows as outies.

    In either case, you need to carefully flash the rough opening before installing your windows.

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