seismic retrofitting & basement wall insulation questions

iron Posted By iron, Dec 17, 2015 at 8:28 PM

  1. iron

    iron
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    next summer, i'm planning to seismically retrofit part of our house (the basement part for now). we have lots of cripple walls coming off our basement and those, i've learned, are usually the key failure point in an earthquake (assuming the house is bolted down to the foundation - which i question in ours...). we're just north of seattle and awaiting the big one....

    rather than removing siding and putting plywood up on the outside, i think i'm going to tackle it from the inside since it offers a good number of benefits, not to mention that i wanted to redo the insulation anyway.

    my question is pertaining to the use of plywood or OSB to act as my new shear walls for the cripple walls. if i put it on the inside wall, my understanding is that they (especially the OSB) will act as a vapor barrier. one can read tons of articles about vapor barriers and get 10 different opinions, so i'm not sure if this thread will solve that issue. but, after installing the seismic anchors/brackets/etc, here's what i plan to do:

    1. spray closed cell foam against concrete wall
    2. insulate remaining wall cavity with roxul or fiberglass insulation
    3. cover with plywood/OSB for shear walls
    4. drywall
    5. paint with latex paint

    i also plan to install 1" foam on the floor and 1/2" ply over the top of that before carpeting. from the few exposed areas of concrete floor i've seen in the basement, things seem very dry and well constructed. similarly, on the one wall where i used to have a downstairs fireplace, it appears the studs are dry and the current batt insulation looks okay (r-11, though installed poorly). this all makes me think the basement is performing reasonably well in its current setup. my concern isn't that i will not gain increased warmth/insulation after my project, but that i will inadvertently create a moisture problem since things will not get to breath in the same manner. that said, the basement is drywalled and painted now, so that's effectively a vapor barrier anyway.

    thoughts?

    thanks.
     
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  2. semipro

    semipro
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    Its a vapor retarder not barrier so it shouldn't trap moisture in the wall unless the paint is oil based.
    The experts at Construction Science and elsewhere that used to recommend that basement walls be allowed to "breathe" have changed their stance in the last few years. I believe they now recommend pretty much what you plan to do: establish a moisture barrier immediately inside the basement walls and then build typical walls inside of that. Since very little water should make its way through the closed cell foam the plywood you install inside of that should not trap any water. The plywood is still somewhat permeable though. I would recommend that you install enough foam against the walls so that condensation can't form on that surface, probably 2" min.

    If you haven't already done this I'd tape some plastic down on your slab to see if condensation forms under it. This, as a precaution since you plan to install carpet over it. I'd consider installing some poly under the foam on the floor to prevent any moisture from making its way up into your decking and carpet.
     
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  3. Highbeam

    Highbeam
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    Wow, retrofit for earthquake eh? I live in this alleged earthquake zone and have for 40 years and now in a 1963 house that is also not bolted to the foundation and has these posts and beams under the house that are not built to withstand lateral movement at all. Our house is not worth the cost of retrofit at least on a cost to risk analysis done by the insurance company. You must have a more valuable home and/or one more easily retrofitted.
     
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  4. iron

    iron
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    well, there are several factors here:
    1. i'm a structural engineer, so now that i live in a 1979 house instead of a 2006 house, stuff like this concerns me.
    2. i plan to redo the insulation in the basement walls anyway, so anchor bolting, installing clips, etc is not that hard at that stage
    3. the cost to do the retrofit will be comparatively cheap from a materials standpoint, somewhat labor intensive
    4. cripple walls are the big culprit in seismic failure in residential buildings. we have cripple walls on 3 sides of our house, primarily under living room (where the fireplace is!) and kitchen. if those wall fail, you're house is toast.

    so, to me, spending $1-2k and a few weekends and weeknights isn't the end of the world to get a little piece of mind knowing that $/value of a house rebuild would be on the order of $200k, not to mention not having a place to live during that time period and all the other chaos that would be occurring in the region at the same time.
     
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  5. jdavis_206

    jdavis_206
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    hello,

    Informative stuff what everyone has posted. If I insulate behind the sheer wall, put 2in foam against the pony wall and frame out wall, do I need to put insulation between the studs? Since sheer paneling doesn't cover entire wall, I am thinking no fiberglass insulation behind sheer wall and put insulation between the studs on new framed out walls.

    Thoughts?
     
  6. iron

    iron
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    what i did is:
    - get down to studs
    - do retrofit (anchors, extra studs, holddowns, etc)
    - put R11 insulation back in cavities
    - sheathing over studs (3/4" ply with nails at 2" OC)
    - 1.5" XPS over floor slab
    - 3/4" T&G subfloor over 1.5" XPS
    - 3" XPS foam board over plywood and over concrete foundation wall, air sealing gaps with great stuff equivalent (get the dispenser gun)
    - build 2x4 wall with a small gap between 3" XPS
    - electrical, drywall, etc

    none of the interior finished walls touch any part of the insulation. i wanted to do this to prevent contact of hot and cold air and the possibility for moisture/mold.
     
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  7. gzecc

    gzecc
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    Getting all the structural stuff done is great, except for when the tsunami comes after the quake!
     
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  8. Ashful

    Ashful
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    Interesting. I’ve seen houses with cinder block walls crumbling to mush, and owners claiming that’s because they were painted with DryLox paint inside, to remedy a moisture problem that should have been fixed on the outside. Was their conclusion, as to the cause of their wall failure wrong, or are there some pretty important caveats to your statement above?
     
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  9. semipro

    semipro
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    I believe that this is the article that I was referencing - though I wrongly attributed it to "Construction Science" rather than Building Science Corp.
    https://www.greenbuildingadvisor.com/article/joe-lstiburek-discusses-basement-insulation-and-vapor-retarders

    CMUs, or cinder blocks are typically made of concrete and water exposure usually only makes concrete stronger as long as corrosion of any embedded reinforcement metal (e.g. rebar) doesn't cause spalling. I'm guessing that those crumbling block walls were constructed of poor materials and that waterlogging only made things worse.

    That said, obviously its better to block water entry at the soil/wall interface if possible so you don't need to deal with it on the inside of the structure.
     
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  10. Highbeam

    Highbeam
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    If that water is acidic....
     
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  11. semipro

    semipro
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    or contains any salt...
     
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  12. Highbeam

    Highbeam
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    How does salt effect a cmu block? I see lots of concrete boat ramps in salt water.

    Maybe you’re trying to say that the steel reinforcement would corrode?.
     
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  13. semipro

    semipro
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    Yes.
    My point was that water shouldn't normally cause concrete degradation unless steel reinforcement inside corroded, expanded, and damaged the concrete.
     
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  14. blades

    blades
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    salt will attack concrete although it is a very slow process- more likely the mortar was damaged by salt exposure, over here where temps drop to the minus negative numbers at times and can vary from 30+ to -10 in an 24 period salt on old concrete can aggravate spalling issues- cracks allow more intrusion so the effects become worse with freeze thaw cycles. frost depth here can be on the order of 4 ft.
     
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