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Community and Q&A

How to waterproof and insulate old foundation walls from the outside?

AndySosna | Posted in GBA Pro Help on

Is 2″ XPS over dimpled membrane over blue skin a good idea? Should the foam go up all the way from footing to the rim joist which is 3′ to 4′ above grade? Or is it better to stop the foam at grade level? If going up is better choice, how to protect and finish the above grade portion of the foam? The foundation is 8″ concrete block which is flush with brick veneer of exterior walls. Location – Ontario, Canada. Your input will be very much appreciated.

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  1. Riversong | | #1

    Yes, run the foam to (and over) the rim joist, and finish with a 1/8" coat of surface-bonding cement, with additional acrylic mortar modifier. You can use a "lathe" of 1/2" galvanized hardware cloth (welded wire) for additional "tooth" and tensile strength, as long as you can get it to lie flat on the foam board.

    The most important part of the basement to insulate is above grade, and there is no sense in leaving a water intrusion path at the top edge of the foam board. The hand-troweled surface-bonding cement (fiberglass reinforced modified cement) will create a strong, durable, and attractive finish.

  2. GBA Editor
    Martin Holladay | | #2

    Assuming this is a new-construction question, here's my advice: instead of bringing the foam up from the footing to the top of the rim joist, why not extend the foam from the footing to the underside of the roof sheathing? In other words, continue the layer of rigid foam on the exterior of your above-grade walls.

    Of course, if you have a ventilated roof, you would stop the foam short of the roof sheathing, in order to leave a ventilation gap.

  3. AndySosna | | #3

    Thank you very much gentlemen. I really appreciate your help and your knowledge.
    The house is a 60 year old brick veneered bungalow. I'm planning on adding a second story next spring. The house has stucco on the northern side over the brick and foam. This existing stucco stops at the sill plate, right where foundation begins (or where it ends I should say). So, 3 to 4 feet of foundation is exposed.
    I am planning on replacing existing stucco and adding new foam and stucco all around and up to the roof after the second floor is up. For now though...while basement waterproofing is happening, trenches are dug and it's easy to add rigid foam where do I stop it?
    If I stop it right below the existing stucco and cover above grade portion of foundation, cup it for now let’s say and purge it for protection, how do I connect it with the new foam that will be added next year? Unless I go with double layer of foam on the walls next year and bring it down to overlap the long continuous joint? Is that a good idea? How much foam can I add over the brick? Instead of removing existing stucco from northern wall could I go over it with a new layer of foam? Do you see any potential problems with it?

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