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Community and Q&A

Siding update with insulation install – but no sheathing

ScipioUSA | Posted in GBA Pro Help on

Hello Everyone.

New here, but have been trying to suck up the knowledge as best as possible. Im sure this is the first of several questions, so forgive me if Im not providing enough/the right info to get things started.

I own an old farmhouse, right on the 5A/6A climate border. Its a foursquare type, with a few additions tacked on the back of it. My questions focus on the main part of the house, since its mostly original everywhere. Build date is circa 1880, built with a mix of new/reclaimed timbers (based on beams in the basement).

There is currently wood clap siding that is in pretty poor shape (needs paint, many are cupped/warped/split, brittle when trying to nail them). So, I dont like the siding currently, but the rest of the original details are there and in great shape. I want to keep all those. First choice is to use fiber cement as a replacement, and there is the distant thought of vinyl also depending on costs.

Part of the reason that we are also considering siding replacement is that there is not any insulation in the walls, and in one part there are some bricks. (this weekend I just discovered there were not bricks in all the walls, which is a good thing!) I had a contractor out this week to give an estimate, and we also discovered that there is not any sheathing either, the claps are direct nailed to the studs.

So now for the actual questions.

I am looking for a siding option that doesnt require me to insulate externally, since I really dont want to mess with the reveal on the windows or play with any of that trim work. Can anyone advise on what would be the best bet for insulating? My first thought was to use some rigid foam board, cut it to fit in the stud bays, and then use expanding foam to air-seal it to the structure. Then I at least have an air barrier. I know fiberglass is really not an option because it will just filter the air as it whistles through the wall. Would it make sense to blow cellulose into the wall cavity as new siding goes up?

Im not sure if fiber cement can be hung without sheathing, and if not, any other thoughts on what to install?

Sorry for the dis-jointed nature of the question, but there are a lot of moving parts and pieces, and I guess many of them are still fluid.

Ultimately, I dont want to have to re-trim the windows, but want to do what I can to help the energy efficiency of my house. (More questions to come about the awesomeness in my attic).

Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    If you are replacing the siding, it makes sense to strip the old siding and install plywood sheathing first. The best type of insulation to use between the studs is dense-packed cellulose. The best time to install it would be after you install plywood sheathing; then you can install insulation through holes drilled in the sheathing.

    Then you can install a WRB (for example, asphalt felt) and new siding (with or without a rainscreen).

  2. ScipioUSA | | #2


    Thanks for your quick reply. I was hoping to avoid sheathing the house to try and avoid the expense and also what it would do to the reveal at the window trim, but I suppose that its really going to be a necessity if I have any hope of trying to improve the energy efficiency and comfort of the house.

    But I suppose it makes sense to do sheathing and dense pack (Im pretty sure the knob and tube is gone, will be easy enough to tell when the siding is gone) when the walls are open.

    In this circumstance, there is probably not much benefit to going with another insulation method, correct?

    Thanks again for the quick assist!

    I will create some additional questions RE: siding selection and attic insulation.

  3. Expert Member
    Dana Dorsett | | #3

    If the lath & plaster are in rough shape, with nails that might blow under the pressure of dense packing, open cell spray foam might be in order. But dense packed cellulose has more to offer for moisture protection, assuming you flash the windows properly to the felt or housewrap. Dense packed (as opposed to damp-sprayed or low density dry blown) cellulose can be almost as tight as open cell foam if you also attend to air sealing details on the sheathing, since the fibers find their way to and clog up exfiltration points from the wall cavities. Unlike open cell foam, it can buffer significant moisture loads from incidental air leaks, as long as it has reasonable paths to drying.

    If there's not sufficient space to provide a rainscreen gap, the crinkle-type housewraps offer a better capillary break than either felt or flat housewraps.

    Many homes of that era are balloon framed from the foundation sill to the attic without benefit of fire blocking or top plates. You may have to add at least the top plates (or maybe some blowing-mesh) to successfully dense pack this house.

    I don't know of any fiber cement siding that would provide the necessary structural stabilization against racking forces, but it may already have corner bracing or something handling those loads. Either way, you need some sort of drain-plane layer sufficiently well broken against capillary draw between the fiber cement and wood/insulation or it will take on substantial moisture loads under wind-driven rain, loads that would take forever to dry when you have insulation is limiting air flow in the cavity, and isolating it from the drying heat of the interior space.

  4. ScipioUSA | | #4

    Thank you for your input Dana.

    The walls are in pretty good shape for the most part, with some exceptions on the second floor near the ceiling plane where there was once upon a time some water damage.

    I am going to have to find the proper details on flashing the windows to the new sheathing, since Im honestly not sure how that will be done with the existing trim staying in place. I suppose that is because I never really thought about it before...

    Also, I was always under the impression that balloon framing used the sheathing as a structural member, and as such the studs were not the only load bearing portion of the wall. Since I dont have any sheathing, I figured this could not be the case. That said, there is probably a wall cavity or two that is open all the way to the attic, but I suppose we will know for sure once the siding is all the way off. I would certainly be OK with having a top plate, it would help keep the errant critter from keeping us awake at night after crawling into the ceiling...

    My thoughts are leading me towards the use of some exterior sheathing, and then I just need to determine the actual siding to be utilized. Based on the way things are adding up (as they always do), I may end up going the vinyl route for a siding selection. I just really dont like the way that J-channel destroys the details at the window and corner trim....

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