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Community and Q&A

Insulating at the deck ledger

Stephen Edge | Posted in General Questions on

Zone 6
New construction
Layers from the outside in are as follows:
2×10 PT deck Ledger
2×4 Spacers
20 in tall copper flashing
20 in tall I&W shield
Plywood sheathing
20 in tall 2×4 webbed floor trusses sitting on PT sill and stepped foundation wall(I can put as many inches of insulation required here because it’s above the celing of the basement)

Dry, daylight, live in foundation with lots of windows. Passive air inlet. Wood stove.

My question is how to insulate the inside of this truss (rim joist) area to keep the sheathing as cold as possible under the I&W, while at the same time allowing drying to the inside.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, when you have wall sheathing under Ice & Water Shield, your goal is to keep the sheathing as WARM as possible, not as cold as possible. Warm sheathing is dry; cold sheathing is wet and therefore risky.

    If you insist on using Ice & Water Shield and copper flashing on the exterior side of your wall sheathing, you really have to install an adequate layer of rigid foam on the exterior side of your sheathing. This article [click here] tells you that you need between R-7.5 and R-11.25 worth of exterior foam, depending on the amount of interior insulation you plan to install against your rim joist.

    One possible improvement in your details: forget the deck ledger. Instead, build your deck on independent footings.

  2. Stephen Edge | | #2

    Martin, in a perfect world, I would agree with you. But the ledger and flashing are already attached to the house and the siding is on. Exterior foam is not an option here. So now should the goal should be to keep the sheathing temp as close to the outside temp as possible to reduce condensation, using thick interior insulation. And to allow drying to the inside for this area. Correct??

  3. GBA Editor
    Martin Holladay | | #3

    If you keep the sheathing close to the outdoor temperature, you increase the chance of condensation -- you don't decrease it. In winter, the sheathing will be cold. Condensation happens on cold surfaces, not warm surfaces.

    Since you can't keep the sheathing warm, your best approach is to create a tight air barrier and vapor retarder to prevent interior moisture from migrating toward your cold sheathing. The usual way to accomplish this is with closed-cell spray polyurethane foam -- the more the better.

    (Remember, though, that the foam should be installed in lifts, with each lift no thicker than 2 inches. You don't want smelly foam.)

    This approach does not allow drying to the interior, but you closed that door when you installed the Ice & Water Shield. Once that was installed, you have no choice but to create an air barrier and a strong vapor retarder on the interior.

  4. Stephen Edge | | #4

    Thanks Martin. I thought condensation happened where warm surfaces meet cold air rather than cold surfaces meeting cold air.

    The I&W is on the outside of the sheathing. If I put a vaport retarder on the interior, I'll be creating a vapor sandwich with no chance of drying in either direction

  5. GBA Editor
    Martin Holladay | | #5

    You're right -- you are creating one of those bad sandwiches. But hey -- you designed it, not me.

    As long as the sheathing is dry on the day you choose to install your spray foam, you'll probably be OK. But it's not ideal.

    Condensation occurs when warm, humid air encounters a cold surface. That's why aluminum beer cans get condensation on the exterior during humid summer weather.

    In the case of an uninsulated rim joist or cold wall sheathing, the danger is that warm, humid indoor air will come in contact with the cold rim joist or cold wall sheathing. That leads to condensation -- or, more accurately, moisture accumulation in the cold wood.

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