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Thread: Finish half of the basement

  1. #1
    DIY Junior Member ariapn's Avatar
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    Default Finish half of the basement

    Hi, I'm a complete newbie planning to finish just half of my basement and leave the other half with the HVAC stuff unfinished and maybe use it for storage.
    I'm going to frame and insulate the concrete walls in the finished room.
    My question is what kind of insulation I need on the wall between finished and unfinished part? Also, do I need to do anything to the unfinished concrete walls?
    Thanks.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    If the basement is entirely below grade, you probably have minimal insulation requirements.

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    In the trades Dana's Avatar
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    Quote Originally Posted by ariapn View Post
    Hi, I'm a complete newbie planning to finish just half of my basement and leave the other half with the HVAC stuff unfinished and maybe use it for storage.
    I'm going to frame and insulate the concrete walls in the finished room.
    My question is what kind of insulation I need on the wall between finished and unfinished part? Also, do I need to do anything to the unfinished concrete walls?
    Thanks.
    Mayhaps you should be asking what to put up against the concrete wall. If you put up a studwall against the studwall and insulate it with batts you'll have a serious mold condition with 5 years ( if not 5 months!). Putting a polyethylene vapor retarder up against the concrete fixes that problem but creates another: Trapping ground moisture in the foundation wall causes it to saturate, moisture rises within the foundation to the above-grade portion where it'l effloresce & spall, and often create rot condtions at the foundation sill.

    The solution is to use only permeable or semi-permeable foam insulation (spray foam or rigid-board) in direct contact with the foundation, and use unfaced batts in the studwall. (Facers are vapor retardent- the studwall and foundation needs to be able to dry toward the interior.) With enough foam between the studwall and the foundation, the exterior-most edge of the studs will stay above the dew-point of the interior air, so interior side vapor retarders then aren't nessessary. In most of IL it's advisable to use at least R5+ on the below-grade portion, R12+ above grade. If you used 1" extruded polyethylene board (aka "XPS", major brands come in pink or blue, often sold as exterior sheathing insulation) against the foundation wall that would give you R5, then a 2x3 studwall with unfaced R8 batts would finish it off to spec. Alternatively, you could just use 2" of XPS and not sweat being slighly below ideal on the above grade portion. Or, cheaper still, you could use 3" of EPS (expanded polystyrene beadboard "styrofoam") for the whole thing, in 2 overlapping layers of 1.5" for a total of R12. With foam board only solutions, holding it in place with furring strips through-screwed into the foundation gives you something to hang the sheet-rock on. Sealing the edges/seams top & bottom with caulk, foam-board adhesive or 1-part foam improves performance, and blocks convection currents from depositing condensed moisture on your now-colder foundation sill all winter. (Insulated & seal the band joist & sill with foam board or spray foam while you're at it. It must be air-tight to avoid the problems though.)

    If you have access to it, fiber-faced polyisocynaurate board (sold as roofing insulation in commercial construction) is highly permeable and gives you ~ R6-7 per inch of depth vs. R4 inch for EPS, or R5/inch for XPS. Iso makes for the thinnest layup. (I put 3" of iso on my foundation for about R18-20- it cut my heating bill by ~15-20%. YMMV.)

    As for insulating the finished portion from the rest of the basement, unfaced batts in a studwall would work, but the "right" thing to do would be to insulate the upper half of the rest of the foundation (down to at least the mid-winter frost line) to R12, bringing the HVAC stuff all within conditioned/semi-conditioned space. Leaving the air handlers/boilers/furnaces/water heaters in unconditioned space cuts into their as-used efficiency, even if you took great pains to seal & insulate all of the ducts/pipes/jackets, etc. Defining the exterior walls of the house as both the pressure & thermal boundary makes it easier to do a good job, and in IL earth-coupling the conditioned space to the basement slab will increase both cooling & heating efficiency. You'll grow less mold on your stored goods too, eh? ;-) You don't need to fully finish the mechanical room, but if you use foam insulation you DO need to put a thermal-barrier against flames between the foam and the rest of the space. (Half-inch or thicker sheet rock works.)

    See: http://www.buildingscience.com/docum...nt-insulation/

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