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Thread: flooring options for basement

  1. #1
    DIY Senior Member pmayer's Avatar
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    Default flooring options for basement

    I am starting a basement remodel project this fall and want to understand my options and the best approaches to dealing with moisture in a damp basement. I have regraded around trouble spots in my foundation, and my basement can now survive a heavy rain without incurring any standing water. The foundation does not seem to wick much moisture, as I have tested by taping foil to the block wall and removed it after a couple days and found no condensation.

    My bigger concern is the floor. The previous home owner painted the floor, and when it rains heavily outside, the paint bubbles in some spots. Again, no standing water, but obvious moisture penetration through the floor. I don't think this is extreme, but I want to understand my options for flooring. I don't think it would even be noticeable if it wasn't for the paint on the floor.

    I am consider a couple of options:


    Choice #1. Pergo. My thought here would be to put 6 mil poly down, topped with the foam underlayment for the Pergo, then the Pergo itself. I am assuming that mold will form under the plastic. Is that a problem for anyone other than the guy who has to replace that floor someday?

    Choice #2. Carpeting. On one hand, I am squeamish about this because of the moisture, but on the other hand, if I use mold resistant pad, will this allow the floor to breath better and actually be the best option for the moisture coming up through the concrete? Am I hosed if I do get a small amount of standing water, or can I just wet vac and dehumidify the heck out of the room and not have a mold problem ultimately?

    Choice #3. ceramic tile, in case there ever is a small amount of standing water, I am assuming that this is a relatively painless cleanup experience with no damage to the structural integrity of the tile.


    Which one of these would you do? Is there a better option? Is there a better approach to one of these options?

    Thanks for your help.

    Cheers,

    Paul

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    Retired Defense Industry Engineer jadnashua's Avatar
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    How much height do you have in the basement? There are engineered panels that can be put down that allow the floor to breath, and you can then put any surface on them you wish, but they take up maybe 2" or so (depends on brand). If you decide on tile, you'd have to scarcify the floor to remove the paint. Messy, but quick with the right rental tool ( a big surface grinder). To give the floor a chance to breath, Ditra from www.schluter.com would help, as it has channels that would allow water vapor a place to go. If you want to investigate tile more, check out www.johnbridge.com. If the floor gets damp, carpeting directly on it is not a good idea.

    The engineered panels would give you the warmest floor unless you used heating mats under the tile.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member pmayer's Avatar
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    I have the ceiling height, and will look into these panels. I had thought they were more deal with standing water, but I guess if the allow the vapor to escape for collection by a dehumidifier, then that should work. The only height related issue would be the stairs, and I imagine I could just jack up the bottom of the stairs by a couple inches to accommodate, so that the inspector doesn't flag me for inconsistent stair heights?

    Also, thanks for the additional info. Will investigate.

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    In the trades Dana's Avatar
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    If you're in a heating dominated climate, laying down 1" of XPS rigid foam sheathing under an OSB or plywood sub-floor deck is usually cost-effective from a heating point of view, and is an effective capillary break & vapor retarder between the concrete and any susceptible mateirals. By putting insulation betweeen the sub-soil and the flooring materials they stay above the dew point of the room air as well, minimizing the mold hazard, even for carpeting. (Carpets have R-value and in a basement app without foam underneath the underlayment of the carpet can be cool enough to condense & create a mold problem even in an otherwise dry basement.)

    The water vapor coming through the slab doesn't "need a place to go", and slabs don't "need to breathe", since the concrete itself is moisture-tolerant. If the subfloor is above the room-air's dew point (which it will be, if insulated) there's no point to ventilating beneath it.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Condensation is one thing, ground water penetration is another. If the moisture is ground water penetration, then you need to address that first.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member pmayer's Avatar
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    i think it is more of a seepage than a condensation issue based upon the way that the paint is bubbling when their is heavy rain outside.

    I checked out DriCore panels at ******* tonight. I think they look promising. My main thought, however, is that they are overkill for my application (I'm not necessarily opposed to that). I believe that my problem is not severe, and that normal vapor barrier material would probably sufficiently solve the problem. But, I am not sure how much I am willing to bet on that. Ripping out a floor in a couple years is too big a bet. Not sure what to do.

    Spinning in Minnesota.

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    In the trades Dana's Avatar
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    Bulk water conditions always have to be addressed first, and channeled floor panels won't deal with that either. A layer of XPS blocks capillary draw between even saturated concrete and the flooring materials AND eliminates condensation issues in the flooring related to the slab/subsoil DriCore et al also blocks capillary draw, but the cold-side of the finish flooring can still be below the dew-point of the room air in regions with colder subsoil temps, and can take on room moisture & mold despite being protected from slab moisture. A layer of foam insulation is a superior approach if you have the headroom- even half-inch (R2.5) XPS under OSB a huge improvement over DriCore or similar systems.

    If your subsoil temps are over 60F, don't sweat it, but if they're under 50F, don't put down anything that mold can live on without at least a hint of insulation. And places with 50F subsoil have cold enough climates for 1" of XPS to be cost-effective- NPV positive in under 10 years on heating fuel savings alone. In MN your subsoil temps are in the 40s:


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    DIY Senior Member pmayer's Avatar
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    Dana,

    Thanks for the detailed response. This is extremely interesting, and makes sense, but I wonder why it is not more the norm? It is way cheaper than Dri-Core, albeit more work.

    Would you glue down the XPS, and tapcon down the OSB? I like everything about the approach except the part about drilling a few hundred holes into concrete. Is that as slow as it sounds?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A good hammerdrill can make quick work of holes in 'normal' concrete. Without a hammerdrill, it can be excruciatingly slow.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    Quote Originally Posted by pmayer View Post
    Dana,

    Thanks for the detailed response. This is extremely interesting, and makes sense, but I wonder why it is not more the norm? It is way cheaper than Dri-Core, albeit more work.

    Would you glue down the XPS, and tapcon down the OSB? I like everything about the approach except the part about drilling a few hundred holes into concrete. Is that as slow as it sounds?
    Skip the glue- it's not necessary, but you may want to tape the seams to keep it from crawling around on you while installing it. What jadnashua said about simply hammerdrilling (aka "roto-hammer" drilling, in some parts of the US) for long-screwing the OSB or ply to the slab. Even the low density 1.5lb/ft^3 XPS used as insulating wall sheathing (comes in 4x8 or 4x9, sometimes 4x10 sheets) can handle the dynamic loading of humans & clotheswashers on "spin" with 7/16" OSB or ply. If you use carpet rather than hardwood/tile you'd have to use 1/2" goods or thicker to meet fire code (the ignition barrier minimum for plastic insulation). With 3/4" subfloor it could handle even much higher loads, but I'm gonna assume you'll try to avoid parking you D9 Caterpillar in there, right? ;-)

    Underslab insulation IS becoming the standard for any higher-performance house, but when codes only called for R13 walls and R38 roofs the heat loss through the slab would be only a tiny fraction of the total. SFAIK it's not required by code for new construction anywhere yet, but that may change by 2020. The actual heat loss through the slab will vary dramatically by soil type and proximity to the local water table, but in MN it would take a rare set of local conditions and highest efficiency heating systems & low fuel/electricity prices to render R5 XPS not cost-effective. (Under the slab going R12 or higher with much cheaper EPS is usually a better way to go, but we want to limit the loss of headroom here, eh?) But even from a mold-hazard abatement & flooring longevity & barefoot comfort point of view it's worth it even without the energy savings.

  11. #11
    In the trades Dana's Avatar
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    You didn't mention whether or how the foundation walls are insulated, but if they're not, that's also pretty cost-effective MN (but easier/cheaper to do if you don't need to rip out finished walls.) Some primers on the subject:

    http://www.buildingscience.com/docum...g-your-basment


    http://www.buildingscience.com/documents/information-sheets/5-thermal-control/basement-insulation/files/bscinfo_511_basement_insulation.pdf


    Walls are trickier than floors because you need a higher-R (especially above the frost line), but you also have to allow the foundation to dry toward the interior without ending up with wintertime condensation on any studwall you set up. The solutions tend to be balancing the Rs of slightly permeable foam-board against the foundation with that of VERY permeable fiber insulation in a studwall such that the average January temp of the cold edge of the studs stay above the dew point of the wintertime interior air. (About 37F, would be the dew point of 68F, 30% relative humidity conditioned space air.)

    If you make the foam too impermeable it rots the foundation sill and causes exterior above-grade spalling on the foundation since the moisture content of the foundation rises, and can only dry via the exterior above grade portion. If you use vapor retarders/vapor barriers (even kraft facers on batts) ground moisture created mold conditoins in the fiber and studwall. It's a balancing act, and the optimal balance will vary by climate. But it's not too tough to figure out- it doesn't take high math.

    And to be clear, we're talking water-vapor permeability, not air permeability. The foam & finish wall has to be as air-tight as you can make it- taped seams, spray-foam sealed edges, etc.

    Uninsulated basements account for something like 25% of the heat load in places as cool as MN if the rest of the house is up to code and the place isn't an air-leaky sieve. (Insulating my basement walls in a ~7000 heating degree day climate took nearly 20% off heating fuel use. I still haven't insulated the slab due to already restrictive headroom, but it's on the "someday maybe" list. I'd have to demo the slab and dig to make it work in my case, so it's not exactly a high priority.)

  12. #12
    DIY Senior Member pmayer's Avatar
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    Great stuff - thanks Dana!

    The walls are not finished. On the exterior of the foundation, I have 1" XPS plus 6 mil poly. for inside walls, here is what I am thinking. Please tell me where you think I am off base:

    - 1" XPS glued to wall and sealed with tyvek tape.
    - foam top of foundation walls
    - 2.5" XPS glued against rim joist, and foamed in at seams
    - Stud walls framed with 1" gap between back of studs and XPS for air flow. I know that theoretically the XPS on wall should negate the need for this, but will I get extra credit for doing this, or a total waste?
    - unfaced bats in stud walls, no poly if my inspector will allow me to skip this (can I skip the bats if I go to 2" XPS, given the 1" on the outside of foundation?
    - mold resistant green drywall on exterior walls. I don't know anything about this stuff. Seems like cheap insurance. Is there a downside?
    - Was thinking of putting a few air vents in the walls to increase air exposure in case water gets in.

    Anything else I should be thinking of here?

  13. #13
    In the trades Dana's Avatar
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    An air gap between the studs and the XPS buys you- less than nothing it's a hit in thermal performance. XPS can't wick water, and the stud edge would be at the same temperature regardless. To keep convection loops from the gap from robbing the unfaced batts of R-value you'd have to put Tyvek or something on the exterior face of the studwall tight to the batts. Butt 'em up tight to the XPS and let it be the exterior side air-barrier for the batt. A batt left open to the room or a cavity is an air-filter, not insulation. The performance hit is large.

    Does the exterior XPS & poly extend all the way up to the rim joist & sill? If yes, you have an exterior vapor barrier, and the hazard of an interior side vapor retarder is amplified (water would have no way to escape but up through the foundation sill.) If it's only XPS on the exterior the foundation can dry toward the exterior through that just fine, but you still shouldn't put an interior vapor barrier on the interior- (print out the documents for the inspector and do the math if you have to.)

    3" total (1" exterior, 2" interior) of XPS gives you a total of R15, and 2" would be only R10. Let's assume you're going with R10 on the foam.

    R10 + R11 cheapo 2x4 batts gives you R21. The R11 batts will lose some R on the coldest days due to internal convection, but since it's only half the R value the delta-T is bounded, as is the loss of R. When it's -20F outside the above-grade portion of the batts will be experiencing a 40-45F delta, and only perform to R8-ish, but the total R would still be R18- not much of a performance hit. If you went with better that bottom-of-the linte batts, say R13 or high-density R15 batts the R-loss wouldn't be as high a fraction, and the overall R higher. At R15 only 40% of the total R is still in the foam, where condensing conditions wouldn't cause any damage. Is that a problem for the studs? Depends on where you live.

    The average January temp in the twin cities is 12F. (See: http://www.climate-zone.com/climate/...apolis-stpaul/ ) Assuming 68F 30% RH basement air, the dew-point would occur at 37F. With 12F on the exterior and 68F on the interior you have a total delta-T of 56F, and the cold edge of the stud on the above grade section is 60% of the way into the R, or 0.6 x 56F=34F below the interior room temp, or 68F-34F=34F. That's below the 37F dew point of the interior air, so on average you may have condeinsing conditions in the fiber. You can either add more foam or reduce the R in the fiber. If you go with R13 you have an R23 total, and the cold edge of the stud is 57% of the way through the R. With a 54F average delta that means it's average temp would be 0.57x54F=31F below the 68F room air, or 37F- exactly AT the dew point of 68F 30% RH room air- still a potential mold issue, if condensation forms on the XPS and dribbles down to the bottom plate of the studwall. At R11 batts you have a little bit of margin, or, you can bump up the R of the foam a bit by going to 1.25" or 1.5" goods. The limit on the interior foam would be 2" before it's too vapor retardent, putting the foundation sill at risk from ground moisture wicking.

    Other stuff: A studwall should have a good poly + foam sill gasket under the bottom plate to keep ground moisture out, and limit air-infiltration from the room into the wall cavities.

    Greenboard is always a good idea, no downside anywhere.

    Air vents in the wall are a BAD idea, since it'll maximize condensation at the temperature extremes of daily/weekly conditions. Once it's in there it can only dry very slowly- more by vapor diffusion through the wallboard than by even more air infiltration. If you're anticipating bulk water incursions, stick with a foam-only solution. 4" of EPS with interior furring through-screwed into the foundation on which to mount the wallboard would add R16 to the 1" of exterior foam for a total of R21, and would be about as thick as the XPS+studwall solution. Be sure to only use UNFACED EPS (no poly, no foil), and use 2 layers with staggered taped seams. (Glue them in place with foam-board adhesive as you go, put the furring up later.) EPS is typically much cheaper than XPS per unit R value than XPS, but only ~ R4/inch to XPS @ R5/inch. ( At R20+ on the foundation it will probably still meet code for new constructino in 2025, but the rest of the house probably won't.)

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    DIY Senior Member pmayer's Avatar
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    Man, Dana, I've read your post three times, and I think I understand almost enough to respond. Holy smokes, you've got a handle on this topic, I would say Yes, the insulation on the foundation goes up to the sill plate. I have also found that only 2 of the three exterior walls that I will be finishing have insulation. For some reason they did not insulate the front wall of the house, which seems strange since it is covered and does not look bad at all.

    So, while I don't fully follow all the math, here is what I think the net of it is. Please tell me where I am off.

    I should

    - insulate all exterior walls with 2" XPS or EPS (May be overkill where there is 1" insulation on the exterior, but not a problem I assume?)
    - build the stud wall as close to the XPS as feasible.
    - insulate in the wall with R11, Kraft faced
    - use greenboard on exterior walls (do you like this better than paperless? why isn't greenboard the standard in a basement?)
    - foam gasket under bottom plate of wall (glad you mentioned; hadn't heard that one before but makes sense)

    I am extremely grateful for all your help.

  15. #15
    In the trades Dana's Avatar
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    Almost.

    Kraft facers on batts are too vapor-retardent- they run about 0.5perms, and you're really want to be 1 perm or higher at that layer. Use only UNFACED goods. If you can't find any R-rated batting without facers, cheap un-rated stuff sold as acoustic sound-abatement batting are the right density & insulative value. (Most big orange or blue box stores carry unfaced R11 batting for use in partition walls.) Don't use anything more vapor-retardent than latex paint on the interior- no vinyl or foil wallpapers, etc.

    2" of XPS would be fine everywhere. On the side with out exterior insulation you may want to go with 3-4" of unfaced EPS instead, to keep the R-ratio with the batting about the same. (Or go with 2" of XPS and a 2x3" studwall with unfaced R8 cavity fill for R18 total.)

    Greenboard is fine in an insulated basement, if marginal at best in an uninsulated studwall against a foundation wall.

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