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Thread: Floor insulation question from a newbie

  1. #1
    DIY Member richrc1131's Avatar
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    Default Floor insulation question from a newbie

    I'm planning on insulating the bottom floor of my house soon due to very cold floors and while I've checked out the steps necessary to do this, I still have a question:

    (Preface) - My basement isn't finished but we do use it for storage, laundry, and my tools. Occasionally, a little bit of water will seep in along the walls during heavy rain.

    I'm assuming some kind of barrier will be needed between the insulation and the used space.

    Sorry if this question seems very basic, but this is new to me.

    Also, if possible, I'd like to comply with Connecticut code.

  2. #2
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    You want to insulate the basement walls?

  3. #3
    DIY Member richrc1131's Avatar
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    I'm looking to insulate the bottom floor of my house which is the ceiling in the basement. I'm tired of cold floors.

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    DIYer, not in the trades LLigetfa's Avatar
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    I don't know what code calls for in your area but here we need to have contiguous vapor barrier on the warm side of the insulation which is nearly impossible to do on the underside of a floor. You could spray foam it but then the foam needs to be covered with a fire rated product.

    Why not insulate the walls and heat the basement?

  5. #5
    In the trades Dana's Avatar
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    Whether you actively heat the basement or not, it's often easier and cheaper (and always better) to insulate the basement walls than to attempt a retrofit-insulation of a basement ceiling/first-level floor. But you'll have to deal with the water seepage problem first.

    Or...

    You could insulate down to the level where that seepage starts (with a little bit of clearance just to be sure.) More than half the heat loss out of the basement is from the freeze line up to the floor, and half or more of that is likely to be air-infiltration. If you do NOTHING else, seal & insulate the band joist and foundation sill to the concrete with 2" of closed cell foam. Insulate the top half of the foundation (down to your seep line with up to 2" of unfaced rigid XPS foam board (pink blue green whatever, it's about R10) or up to 4" of unfaced EPS (R16). Don't use any thing with foil or vinyl facers or you'll risk rot in the foundation sill. The rigid foam can either be trapped in place by a studwall snugged up against it, or held in place with furring through-screwed to the foundation. To meet code the foam would need an interior ignition barrier (half-inch wallboard mounted to the studs or furring counts.) If the seepage is only at the very bottom of the walls only, seal the concrete with silane based concrete sealer (specialty masonry shops would have it) before insulating, and take the rigid foam down to within a foot of the bottom only. If you go the studwall route, put a sill gasket under the bottom plate of the studs, and insulate the bays with R11-R13 UNFACED batts, which adds another ~ R10 (after factoring in the thermal bridging of the studs). If you stopped the foam half way, you can still do a full-length batt, but if there's any chance of flooding it's better to stop it a foot or so above the floor (whereever you think the high-water mark would be +2".)

    Key to making this work is ensuring that the ratio of the foam-R to the fiber-R keeps excessive condensation from occuring in winter on the face of the foam and running down to rot the sill. But to keep mold from happening inside the studwall in summer from even modest groundwater drives it has to be fairly vapor permeable (but air-tight) to the interior, ergo the batts must have no facer, and no poly/foil/vinyl can go on the interior wall- only standard latex paint on gypsum. If the foam is too vapor tight the concrete can't dry toward the interior, and you can get efflorescence on the above grade portion of the exterior, and/or high moisture in the foundation sills creating rot conditions, which is why you shouldn't use foam that's too thick or foam with facers. In CT you can get away with as little as R5 (1" of XPS or 1.25" of EPS) with a 2x4 interior studwall, and the wintertime condensation risk falls to near-zero. There are also local dealers in used foam (check Craigslist) scavenged from commercial flat-roof re-roofing projects, which can make it QUITE cheap as a DIY project.

    Read up on the various options here: http://www.buildingscience.com/docum...study-analysis The hygric analysis is for Mineapolis, which is quite a bit colder and tougher to design for than CT, but R15-R20 foundations are still cost effective in a CT climate.

    But air-tightness is the first & most critical aspect. The stack-effect of the house causes the greatest infiltration pressure to be into the basement. If you can slow down the air leaving the attic through the upper floor ceilings and slow down the air entering the basement the floor will be warmer, and you'll spend less on heating. If you seal AND insulate the temp at the basement (and the) floor will likely be in the mid to high 60s all winter, assuming a 68F first floor, and boiler or furnace in the basement. (That's the way it worked for me in central MA, anyway.)

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    DIY Member richrc1131's Avatar
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    Thanks for the great advice. I will jump on sealing off the little leaks at the very bottom of the walls first.

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