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Thread: subfloor in basement which is correct

  1. #46
    DIY Junior Member JVIPER's Avatar
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    I like the idea of keeping the floor as warm a possible cause this will probably be the place I spend the most time when home. Should I skip the insulation completely and go radiant? Also can radiant floor heating be laid over bare concrete and tiled over directly? This would barely cost me an inch of headroom. Will typical moisture from the concrete ruin radiant?

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    In the Trades mtcummins's Avatar
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    are you talking about electric radiant like you see in a bathroom remodel or hydronic radiant (using water lines), which is what i have in my house. electric is a lot more expensive to run, but hydronic requires more equipment to install.

    unless you know that there is sufficient insulation under your slab already, you def want insulation between your heated floor and the slab, or most of your heat will go into the ground. I would do an inch of radiant, plywood, electric heat, and tile if that's the way you want to go. on a large scale like that, you'll want to consult an electrician for load requirements, etc.

    if you want hydronic heat and the initial expense of the pumps and such required for that, you would do an inch of xps, then they make plywood with knobs on it made to click in the pex water lines, and then you can put down whatever floor you want (that is approved for radiant heat - some laminates/engineered woods are, some aren't) over that knobbed plywood. this will be about 5/8" thicker overall than the electric option.

    either way, if you're heating the floor, you definitely want insulation. if the house was built to have a radiant slab, they should have put at least 2" foam under the slab to keep the slab's heat inside, rather than heatsinking into the dirt.

  3. #48
    Test, Don't Guess! cacher_chick's Avatar
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    Quote Originally Posted by mtcummins View Post
    I wouldn't worry too much about the thermal expansion/contraction here, but might consider doing 1" or 1.5" xps under your floor, and putting a 1/2" xps panel loosely between your duct and the wood panel if you're worried about it.

    XPS is flammable. Most building codes require that when it is used inside a structure that is must be covered with a fire block. (drywall is common on walls and ceilings)

  4. #49
    In the Trades mtcummins's Avatar
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    this is a good code point. its one i would probably ignore in this case, as while i understand the basic intent, I don't think this would really apply to the intent of the code. thats not to say that an inspector wouldn't potentially flag it though...

    that said, the OP said the duct is insulated to some degree already, so you could just leave it alone and put the panel near the duct, but not touching it, and it will probably be fine. or put drywall over it... i have in past lives attached drywall directly to heat ducts rather than creating a bulkhead for it to run inside. saved a couple inches. i don't advocate that practice anymore, but i can say that nothing happened to the drywall from doing this. i don't think the level of heat through the duct is enough to really worry about it that much

  5. #50
    In the trades Dana's Avatar
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    Quote Originally Posted by JVIPER View Post
    I used to have a humidity issue with a small rain issue until I did the foundation grading, gutters and de humidifier inline with my hvac system. 2 years have past and no moisture issues but I like to be over prepared so...

    Would 6mm poly and 2" xps be way over kill for the floor?
    I you're putting down 2" of XPS, there's no point to the poly (assuming you meant 6 mils or 0.006" not 6mm.) The foam is fairly vapor-retardent, provides a superb capillary break, and it's keeping the subfloor at room temp, not subsoil temp- the subfloor won't be wick up groundwater OR taking on room air moisture via condensation. In the event of a bulk water intrusion the poly isn't going to save you.

    2" of XPS is over code-min for new construction in MA, and may not have a reasonable 25 year present-value on the future energy savings at buck-a-therm natural gas in an 90% burner, but 1.5" (R7.5) would. (See: http://www.buildingscience.com/docum...-climate-zones you're in zone 5) If you're heating with oil, propane, or electricity by all means take it up to 2". Alternatively going with reclaimed 2-2.5" Type-I EPS (R8-10) would be dead-cheap, and payback is fast. In MA there are several vendors handling reclaimed rigid foam- The Insulation Depot in Framingham is the biggest, but if you searched craigslist materials for "rigid foam insulation" you'll find others. There's a guy in Winchendon advertising there that always seems neck-deep in EPS for cheap. Use only EPS or XPS, not iso.

    If you go with radiant heat (any type) you want MORE insulation, not less, otherwise you'll spend quite a bit of money heating up the 48-50 suboil in Tyngsboro. R15 (3" XPS) would even be cost-effective if electric radiant is used, but that's quite a hit in headroom.

    Half-inch OSB or plywood would be sufficient for meeting code on ignition barriers for foam, as would half-inch or thicker gypsum board.
    Last edited by Dana; 11-21-2011 at 01:30 PM.

  6. #51
    DIY Senior Member diyfun's Avatar
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    Quote Originally Posted by Dana View Post
    Poly sheeting is a cheap & effective Class-I vapor retarder ( ~0.05 perms at 6 mils), which takes the guesswork out of it. While vapor-sealing foundation walls could have negative consequences (rotted foundation sills, efflorescence & spalling on the exterior above grade), there's no such issue with vapor sealing below-grade slabs- indeed, it can even reduce radon levels.
    Hi Dana, why vapor-sealing wall has such negative consequences? for a full basement, if we don't lay the vapor barrier up to the top and end the barrier at least 1 foot away from the top, does that still has negative affect on the wood? Any also how it will cause efflorescence and spalling on the exterior above grade? I just want to fully understand what you have said. Like many others, I really appreciate you answer my questions and help us choose a right way to make our home better.
    Last edited by diyfun; 12-18-2011 at 08:26 PM.

  7. #52
    In the trades Dana's Avatar
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    Quote Originally Posted by diyfun View Post
    Hi Dana, why vapor-sealing wall has such negative consequences? for a full basement, if we don't lay the vapor barrier up to the top and end the barrier at least 1 foot away from the top, does that still has negative affect on the wood? Any also how it will cause efflorescence and spalling on the exterior above grade? I just want to fully understand what you have said. Like many others, I really appreciate you answer my questions and help us choose a right way to make our home better.
    Fully vapor sealing the wall keeps the moisture in the concrete, which raises the moisture content to where it meets the wooden foundation sill, and the sill can rot. (In some cases it can cause the exterior concrete to spall/crumble from freeze-thaw cycles too, but that's more common in places colder than MA.) Removing the vapor barrier at the top doesn't reduce the moisture in the concrete very much, and allows wintertime condensation of moisture from the interior air occur, since at the top the foundation is colder.

    As long as surface drainage has been dealt with and the exterior of the foundation has been waterproofed, much of the moisture in the foundation is water wicking up the wall from the footing. If you leave a 1-foot gap in the insulation & vapor retarder at the BOTTOM of the wall closer to the source of the moisture it will be more protective than leaving a gap at the top. At the bottom the foundation will be much warmer in winter, so it won't condense water out of the room air there the way it will at the top.

  8. #53
    DIY Junior Member Pikazza's Avatar
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    I have a question for anyone that is able to help. My wife and I just bought a 1953 rambler. The basement had old, shag carpet that we removed. Underneath that was old tile, which we also removed. This left the black mastic glue, which I had hoped to remove so we could seal the concrete and keep out efflorescence (which we found a lot of when we removed the tile). The problem is that removing the glue has turned into an unsuccessful tar pit mess! The product I used, Bean-e-doo, has not lived up to it's claim of being easy. Since I plan to put a subfloor over the concrete, I'm wondering if I need to worry about getting the rest of the mastic glue up, or if I could just lay my subfloor over it. Obviously, laying over it would save a whole lot of elbow grease and added cost. I would appreciate any help that can be offered. Thanks!

  9. #54
    Retired Defense Industry Engineer jadnashua's Avatar
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    That black goo may contain asbestos, so be careful of it! If you have moisture, the bigger issue is what can you apply that will not rot out or create lots of mold. I'm not sure...wait for others.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #55
    DIY Junior Member Fanfold's Avatar
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    Quote Originally Posted by Dana View Post
    Fully vapor sealing the wall keeps the moisture in the concrete, which raises the moisture content to where it meets the wooden foundation sill, and the sill can rot. (In some cases it can cause the exterior concrete to spall/crumble from freeze-thaw cycles too, but that's more common in places colder than MA.) Removing the vapor barrier at the top doesn't reduce the moisture in the concrete very much, and allows wintertime condensation of moisture from the interior air occur, since at the top the foundation is colder.

    As long as surface drainage has been dealt with and the exterior of the foundation has been waterproofed, much of the moisture in the foundation is water wicking up the wall from the footing. If you leave a 1-foot gap in the insulation & vapor retarder at the BOTTOM of the wall closer to the source of the moisture it will be more protective than leaving a gap at the top. At the bottom the foundation will be much warmer in winter, so it won't condense water out of the room air there the way it will at the top.
    Dana,
    I know this is an old thread but it is the exact issue I am having with my basement as well. I just wanted to know is it necessary to leave the 1 foot gap at the bottom. Would 2 inch XPS be ok on the walls and poly with fanfold on the floor with laminate? The foundation is not sealed on the exterior. House was built in the early 40's. I would greatly apprecate any help. Thanks

  11. #56
    In the trades Dana's Avatar
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    If you have decent roof overhangs (one foot of overhang per story) good surface drainage sloping away from the foundation, and a foot or more of above-grade exposure on the exterior it hardly matters what you do to the interior face of a poured concrete foundation. Any efflorescence on the concrete is a point of concern though.

    Most of the moisture in the foundation wall would be wicking up from the footing. A foot of naked concrete just above the slab provides quite a bit of drying path to limit the amount that wicks up to the top. Jacking up the house 1/4" a section at a time and slipping in EPDM sill gasket or metal flashing would save the foundation sill from even saturated concrete. It's not as hairy as it sounds, but most people don't want to go there.

    If you use 2" of unfaced Type-I EPS (1.lbs per cubic foot nominal density, and about R7.8 @ 2") on the interior of the concrete it would provide quite a bit of drying capacity toward the interior, as long as you don't screw it up by low-permeance finishes or install vapor barriers. Leaving the foot of free air just above the slab would also provide a lot of drying capacity. EPS is a heluva lot less environmentally damaging than XPS too, since it's blown with pentane instead of HFC134a, the latter of which packs a powerful greenhouse punch, and most of which leaks out over the first 25 years, lowering the performance of the XPS in the process. At age 50 XPS is barely higher R than EPS of equal density, despite starting out about 20% higher.

    Fanfold XPS on the floor is not sufficient R to protect the subfloor from summertime moisture drives at RI type deep subsoil temps. Ideally you would have at least R3 between the nearest wood and the slab so that the wood tracks the room temp fairly closely. Average summmertime outdoor air dew points in RI are in the mid-60sF, and your subsoil temps are in the low 50s. Without some R-value between the wood and the slab, the cold side of the wood will linger near or below the room-air's dew point, and it'll be susceptible to mold/rot, and any rugs on that laminate floor would become a dust-mite ranch. With 3/4" Type-II EPS (1.5lbs per cubic foot nominal density, R3.1 @ 3/4") and 7/16" OSB for a subfloor you'd have enough stiffness & fastener retention for laying down laminate flooring. Type-II EPS is more vapor-tight than Type-I, and has higher compressive strength, but you'd still need the poly sheeting.

  12. #57
    DIY Junior Member Fanfold's Avatar
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    When you say don't screw it up by low-permeance finishes or install vapor barriers-would that be something like Dry-Lok or Zinnser Watertite oil-based paint?

  13. #58
    In the trades Dana's Avatar
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    I couldn't find an ASTM E96 vapor permenance number in the spec. Many things that are waterproof to liquid water are still somewhat permeable to water vapor (take rigid foam insulation, for instance). Most concrete sealers that block moisture wicking are semi-permeable, maybe 2-5 perms, But without a spec it's hard to say for sure where a particular product ends up.

    But I was referring to the interior finish wall, and not trapping moisture between the foam and the finish material to grow mold on the studs, not the treatment you give the concrete.

  14. #59
    DIY Junior Member Fanfold's Avatar
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    Quote Originally Posted by Dana View Post
    I couldn't find an ASTM E96 vapor permenance number in the spec. Many things that are waterproof to liquid water are still somewhat permeable to water vapor (take rigid foam insulation, for instance). Most concrete sealers that block moisture wicking are semi-permeable, maybe 2-5 perms, But without a spec it's hard to say for sure where a particular product ends up.

    But I was referring to the interior finish wall, and not trapping moisture between the foam and the finish material to grow mold on the studs, not the treatment you give the concrete.

    Got it. Thanks for your help

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