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Foamboard and creep deformation

Mike M | Posted in General Questions on

I have been trying to find a way to insulate a brick ledge on the north side of my home. I am planning a large insulation upgrade to help get my floor heat somewhat working and seal up the house some.

The hydronic slab is not insulated as the seller claimed, and the hydronic system has too many losses to heat the home effectively or sometimes even at all. I plan to put 4″ of Roxul, 4′ down around the slab exterior, and use 4″ of XPS starting at the sill. This should raise the R value of my walls from 10 or 11 to about 30, and the slab edge from about .8 to 10. I am also planning on sealing up the ceiling and adding blown in cellulose up there for at least R60

The only reason I’m even considering touching the brick wall is it is the north facing side of the house and has at least 60′ linear exposed slab edge and 60′ of walls that are only R11ish.

If my brick wall is ~17 courses tall, and an average brick weighs 5 lbs with ~ 24 sq. in of area, the pressure exerted at the bottom of the stack would be only 3-4 psi plus mortar. Owens doesn’t recommend using the foam under a brick wall due to creep.

If this is truly a concern, why do people regularly put foamboard beneath slabs, and sometimes even footings?

The compressive weight on the foam would be way higher in the footing situation and I would think cause long term creep.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michael,
    If you want to proceed with your plan, you should talk to an engineer.

    I have an idea why creep would be more of a problem under a brick veneer wall than under a slab. Here it is: If a slab settles 1/8 or 1/4 inch, the house won't experience any problems. However, if a brick veneer wall settles 1/8 or 1/4 inch, the flashings would break and the mortar would crack above and below the window openings.

    I have no idea if my idea is the same one that concerns Owens Corning, however. One thing I'm sure of: structural engineers tend to be conservative.

  2. Mike M | | #2

    Martin, thank you for the response.
    I am an engineer (nuclear) and have spoken to structural, civil, and mechanical engineers I work with. The mortar cracking was the only thing we came up with also.

    I won't be able to attempt this until next year so I'll try to build a test stand and go on the conservative end for the variables. I'll post my findings if they end up bring of any value.

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