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Community and Q&A

Fiber cement siding over existing plywood siding

DB_Seattle | Posted in General Questions on

Hi all,

Our 1970s house is constructed using plywood T1-11 siding that we think also functions as sheathing of the house. The siding is in need of repair and repainting, so we thought this is a good opportunity to just replace the existing siding with new fiber cement siding. In talking to several people, we were told that we can install fiber cement siding over the existing plywood. The installer we spoke with suggests they’d wrap the existing plywood siding with WRB, and install the new Hardie siding on top of it. I was wondering, is this standard practice, and if so, are there any disadvantages or concerns that we should be aware of taking this approach?

We are also interested in upgrading insulation of our walls at the same time (possibly adding rigid foam insulation below the new siding). Is this an advisable approach, or would we need to install new sheathing if we want to add insulation?

Thanks in advance for your help. As you may have surmised from above, I’m not well versed with construction techniques and I’m looking at this from a homeowner’s perspective.

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  1. Expert Member
    Dana Dorsett | | #1

    T1-11 is structural sheathing, and it's fine to install other siding over it.

    It can might be tough to make T1-11 fully air tight working from the exterior side, but a liquid-applied WRB will help out in that regard. There is no way to detail a housewrap as an effective air barrier over T1-11.

    It's fine to install rigid insulation over the exterior of T1-11, as long as it has a sufficient R-value for dew point control for your climate. Where is this house located?

    Before re-siding or adding a layer of foam is a good time to assess the air tightness and existing insulation. If need be it's possible to dense-pack fiberglass or cellulose over existing batts drilling from the exterior, which clogs up all of the infiltration paths with fiber insulation, which tightens up the assembly by quite a bit.

    1. DB_Seattle | | #2

      Dana -- thanks! Forgot to add that detail: our house is in Seattle, WA. Interesting point on the liquid WRB - I'll definitely bring this up with the installer. It also just occurred to me that if we add more materials on top of the existing plywood siding, our windows would now be recessed. Do those need to be moved so they align with the new siding?


  2. Jon_R | | #3

    Also consider fully adhered WRB with some assist from a little canned spray foam at the top and bottom (because of T1-11 grooves). Then some unfaced EPS (any amount), furring and new siding. Verify air tightness with a blower door.

    1. DB_Seattle | | #4

      Thanks Jon!

  3. Expert Member
    BILL WICHERS | | #5

    Check around the perimeter of the T1-11 panels too, and look for any holes made by critters in the faces of the panels. That material tends to start delaminating at the edges and that’s something you’d want to repair (or replace any bad panels). You don’t want to seal in any degraded panels so fix them now before your new siding goes up.

    On my own house, I’m replacing all the T1-11 with new 3/4” plywood prior to putting up fiber cement siding and a lot of exterior rigid foam. I had a lot of delamination of the lower edges and woodpecker/insect damage to the faces of the panels. If your T1-11 is in good shape then you don’t need to worry about replacing it, I’m only recommend you do a through inspection of the panels now while you have the chance.


    1. DB_Seattle | | #6

      Thanks Bill. Good point, and one installer that spoke with did say that they'll inspect the existing T1-11 and replace as necessary. Do you you think it's a good idea to bring in an independent inspector to make sure the installer isn't overlooking anything?

      Speaking of the installer: the person I spoke with suggests that they can achieve R-15 with blown in insulation, which is the minimum requirement under local city code (for a 2x4 wall -- our current walls have R-7 batt insulation within the wall cavities). If I were to add additional rigid foam insulation, I would need to move the windows and doors (unclear what the cost would be at this point). Is this something worth taking on? The installer also seems to suggest that unless I put in rigid foam, I should not install rain screen between the fiber cement and the WRB, since it could cause moisture to build up over time and cause bubbles to form in the paint of the siding. This seems confusing to me since I thought that the purpose of the rain screen is to move condensation off the plywood sheathing. Is this person correct here? thanks!

      1. Expert Member
        AKOS TOTH | | #7

        Dense packing over the exisitng batts does a great job of sealing up air leaks and it does buy some R value. Unless your studs are true 2x4, I doubt you'll get R15, but you can get R13 center of cavity (R12 whole assembly including your siding/drywall), which is much higher than the R9 your existing batts give.

        You can get the same performance with 1" rigid exterior insulation (you have to check if this is sufficient in your climate zone) over the existing batts, so if the dense packing is less than the cost of the rigid plus dealing with flashing then go for it.

        Rains screens ALWAYS help with siding durability, there is no such thing as causing bubbling, with or without rigid foam, complete BS.

      2. Expert Member
        BILL WICHERS | | #8

        >”Do you you think it's a good idea to bring in an independent inspector to make sure the installer isn't overlooking anything?”

        Doesn’t hurt, but probably not necessary if you have a competent contractor. You could always pay time and materials for T1-11 inspection and replacement since that way there is no incentive for the contractor to cheap out on checking for and replacing any bad panels — the more they find to replace, the more money they make. You can always ask them to point out the damaged panels to you prior to replacement to make sure they aren’t going overboard.

        I would seriously look at putting up exterior rigid foam now while you’re doing your siding project as this is the only chance you get to do it. You will get better whole-wall energy performance that way, and quite possibly enough to make a noticeable different with your heating and cooling costs.


  4. DB_Seattle | | #9

    Thanks all -- if going the exterior rigid foam route -- what R value should be the goal? We're in Zone 4 Marine. This has been incredibly helpful and I'm so grateful for all of your input!

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