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rotten siding and window flashing tape

hhjason | Posted in Energy Efficiency and Durability on

We’re a building company.    We have a problem with a few client homes.   We’re stumped the reason.    And hope ya’ll have some thoughts / advice.   

Please don’t crucify us.   We’re just a bunch of folks trying to figure it out like the rest of you.    And not backing away from the problem.    

The homes are in Climate Zone 6.   

These two homes tested under 1 ACH50.    

The entire wall assembly from inside to out.   

1/2 GWB with vapor barrier primer and paint
“Bonfiglioli” wall assembly.   R21+5
1/2″ CDX with seams taped with Tescon Vana
Hydrogap WRB
Marvin Triple Pane Windows flashed with Protecto Wrap Super Stick and Zip stretch tape.
Boral TruExterior Trim
CVG Hemlock Clapboard (purchased primed front and back)
Caulked and Painted.   I’m trying to find the spec on the paint.   But do believe it was Ben Moore Acrylic.    Maybe a now-discontinued “Green Spec” line.   

Mitsubishi heat pumps.    Owners use some cooling.    HRVs (not ERVs) installed.  

We’re having issues with rotten clapboard.    These homes are only three and four years old.   The rot is at the ends of clapboards at the window casing only.   And it’s one or two clap ends in the middle of the side casing (not top, bottom, or entire length).     No rot issues at corner boards or butt ends of clapboards.    And the rotten claps are only on the southern elevation of both homes.     

One of the houses the carpenters did NOT prime the end cuts of the clapboards.    The other home they did prime the end cuts.   

I just can’t believe that in three and four years enough bulk water would enter a vertical surface (no reason for water to hang out here) to rot the clapboards.   I think something more “constant” might be going on.

I took moisture readings yesterday and almost all other surfaces (butt joints of claps, middle of claps, claps against corner boards, etc) are 10 – 14%.    But the ends of the claps against the window trim it’s 16 – 20%.    With a few fully rotten clapboard ends.   

My theory is that the sun is trying to push water through the clap (remember, it’s south side only) and the foil faced protecto wrap under the end of the clapboard is reflecting it back (the protecto wrap is under the Boral and extends a few inches under the ends of the claps).   The clapboard fields are drying adequately by the HydroGap behind . . . . . what do you all think?

The no primed cut ends on one of the homes isn’t going to gain us any sympathy (I know, I know).   But I honestly don’t think that’s the reason these claps are failing.   


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  1. Expert Member
    Michael Maines | | #1

    Jason, it sounds like solar vapor drive may be contributing to your problem:

    What type of insulation is in your wall cavities? How is the window cap flashing detailed?

    1. hhjason | | #2

      insulation is Dense Pack Cellulose (sorry, should have mentioned that). Window itself is flashed to the plywood with protecto wrap tape and WRB over it (seams in WRB taped). Boral head casing is hard flashed under the cvg hemlock clapboard.

  2. hhjason | | #3


    Thanks for the response. We do have RH Sensors installed in another house in this neighborhood. Unfortunately only the sensor on the north wall is working. It's installed on the inside edge of the exterior sheathing. I'll take a look and see what that house is doing. Andy maybe it's worth a RH probe into these troubled walls. . . . . . . Clients say they don't run the AC full time. And our assembly doesn't really include anything considered "reservoir cladding". And we do have a minimal vented cavity (hydrogap) EXCEPT where we're seeing problems.

  3. Expert Member
    Akos | | #4

    A couple of pictures of the windows in question would help. It does sound like water is pooling in those areas. Sometimes siding installers caulk the wrong location preventing water from draining out/down.

    1. hhjason | | #5

      Akos. Here's a picture of the worst spot (if any rot could be worse than another). The caulking is surely failed. But is that the horse or the cart? The clapboard above registered only a 12% moisture content. So I don't think the water is coming from above.

      1. hhjason | | #6

        PS the painter did the caulking. Not the carpenters.

      2. Expert Member
        Akos | | #7

        Do you have picture of the full window? Roof over head would also be good.

  4. hhjason | | #8

    Here's the entire wall with the problem spots circled.

  5. hhjason | | #9

    14" overhangs BUT a predominately south wind during rain storms rendering any overhang useless. . . . . . . but if it were a bulk water issue I'd expect issues at the corner boards or clapboard butt joints too. And the claps above the right most window should be wet. They are not.

  6. CMObuilds | | #10

    With that tight of an enclosure, you might be seeing warm moisture laden air finding a weakness in the window installation.
    Have you put up a blower door to monitor house pressures under different conditions?

    Unbalanced HRV, supply only fresh air into air handler, or anything that might be putting the house into positive pressure?

    I would put it at 50 Pa and see if you get leakage in the areas of damage, even a good north wind will force air out on the south side, could do some zonal pressure testing on the rooms as well under different conditions to see cause and effect.

    Based on myself building tight houses, they are sensitive to high indoor humidity which even with the best intentions need to be managed with dehumidification. Im in zone 6 as well.

  7. AlexPoi | | #11

    Does your windows drip cap have end dams like the one in the picture? Water could be coming from there.

    See Martin's comment on a previous question :

  8. Expert Member
    Akos | | #12

    For the head flashing above the window, do you have end dams like this:

    Without end dams, instead of channeling the water out, the head flashing is channeling the water down the side behind the siding. With walls that can see a lot of water and rain, this is definitely an issue.

    1. hhjason | | #13

      Akos we don't have end dams. But the head casing has a Boral Drip Edge that overhangs the head casing. YES water could roll around the head flashing, under the top clapboard, then migrate down the wall. But I just don't believe it's that much water leaking into that condition. Most of the water will drip over the front edge of the head flashing a drop off a saw kerf in the Boral Drip Cap. AND the rotten clapboards at the center and right windows are down the side of the window (not at the top) suggestion the water / moisture isn't coming from above. The claps above these rotten sports only measure 12% moisture content. . . . . . . I still think it has something to do with the foil faced flashing tape. We have another house in the neighborhood with an 8' tall slider door with a rooflet over it. The door is protected from water, but the bottom of it is exposed to the sun. I'll measure the moisture content of the claps in this location. See what I see.

      Thanks to all for your comments.

      1. Expert Member
        Akos | | #14

        I would bet my cordless circular saw (which is my favorite tool, not something I do lightly) that the problem is with the head flashing/trim detail.

        You can do a quick check by removing the rotten siding, than with a garden hose spray upwards into the gap between the siding and head trim above the window. If you see water running down the side of the window, there is your problem.

        In your case, the lack of drainage around the flange is what probably what is making your problem worst. Fixing the drainage is not easy, so I would focus on fixing the head flashing.

        I highly doubt the problem is with the non-permeable tape. After all, the window flange itself is non-permeable and it never causes any problems.

        1. AlexPoi | | #15

          I would add that given the amount of rot you have on a house that is barely 4 years old, this is definitely a bulk water problem. You wouldn't have this kind of damage just with a condensation problem.

          1. CMObuilds | | #16

            You can on a really tight house.

    2. CMObuilds | | #17

      Is it counterflashed? If you fold the ends up water can get under the head flashing from the side, you’ve got the potential for water intrusion with either method. Plus your gonna kerf cut siding as if that wouldn't wick water.

  9. hhjason | | #18

    Thanks for the responses and ideas.

    On the house shown above the trouble areas are all on the right sides of the south windows and on the first floor. This might support a bulk water / lack of turned up head flashing / predominately south west wind when it rains argument. BUT if that's the case I'd expect to see the left sides of the west windows to also show problems. And they don't. PLUS the worst rotten clapboards on this house (and the neighbor house not shown) are 3/4 the way down the window casing with clapboards above testing at +/- 12% moisture content. Suggesting bulk water is not running under the clapboards.

    I don't think it's bulk water. . . . . I guess we'll pull the clapboards and see.

    1. ohioandy | | #19

      Here's a totally alternate explanation. I've had a similar problem occur on a few wood porch floors I've rebuilt with tongue and groove douglas fir boards. Couple years later I'll get a callback from the client complaining that a board is rotten. Sure enough, everything is perfect except a single board has dramatic rot from the outside end in, appearing very much like your first photo. Obviously whatever mechanism is at work is NOTHING like your situation, unless the explanation is simply that once in a while a board just wants to rot right now, thank you very much. Maybe an internal genetic flaw in the cell structure, or the boral equivalent of an allergy. In both cases I dealt with, I pulled out the single defective board and slotted in a new one, and all's been fine. Total PIA, but wood is a living gift from God, not a manufactured commodity with a warranty.

    2. creativedestruction | | #20

      Hold a level to the head flashings. I'd bet they're 1/8 bubble low or more on the side you're seeing the siding deterioration.

      Akos's theory seems likely.

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