To insulate or not to insulate a basement bathroom??

Discussion in 'Remodel Forum & Blog' started by mrmichaeljmoore, Mar 29, 2010.

  1. mrmichaeljmoore

    mrmichaeljmoore New Member

    Messages:
    128
    Location:
    Connecticut
    I am in the process of doing a basement bathroom.
    It will be connected to an already existing finished basement.

    I have a question regarding what to do for insulation for the bathroom wall on the poured foundation wall. The wall is completely below grade. The sink and toilet will be on the foundation wall. Shower is completely interior. And the pex warer supply pipes that are on that wall are near the bottom third of the concrete wall.

    Originally, I was going to do fiberglass batts in the stud wall cavities.
    Then I did a little research on this topic and found a better way to go is the rigid foam board insulation.....

    Unfortunately, though, I have no way of doing the rigid foam insulation. The stud walls are already up, and are completely fire blocked. So, I can’t even slide the 1/2" Dow Styrofoam behind the studs....

    For what it is worth, the bathroom will have radiant floor heat (Warm Up brand) and a Panasonic Whisper Warm 110 CFM vent fan....so the bathroom will be heated and vented sufficiently, I would assume to remove any dampness....

    Plus, I always run dehumidifiers on both the unfinished and finished parts of the basement in the summer. And my pellet stove is always running in the finished basement in the winter. So the basement is usually around 40-55% humidity.

    Also, I have never got any water through the poured foundation wall that will now be part of the bathroom (knocking on wood right now). Also, one of the previous homeowners put a coat of what appears to be Dry Lock on the wall as well.

    So, this is my question....
    Should I just put some batts in the stud cavities? Or should I just leave it uninsulated?
    If I do fiberglass batts, should I do faced batts toward the warm in winter side? Or unfaced batts?


    thanks.
    mm
  2. mrmichaeljmoore

    mrmichaeljmoore New Member

    Messages:
    128
    Location:
    Connecticut
  3. cacher_chick

    cacher_chick Test, Don't Guess!

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    Do not use a vapor barrier below grade as it will trap the water vapor coming from the ground and cause future problems.

    This product would be fine on the wall for the part that is above grade, as the water vapor will transfer outward. Once below grade, the water vapor can only transfer inward.
  4. mrmichaeljmoore

    mrmichaeljmoore New Member

    Messages:
    128
    Location:
    Connecticut
    Thanks for replying, cacher_chick....

    But what’s amazing to me is that there are so many differing opinions (on this forum and other forums that I have asked the same exact question)....frustrating for a homeowner.
    Yes vapor barrier, no vapor barrier….
    Faced insulation, unfaced insulation…..


    I am wondering though…...... would the safest option to be leave it uninsulated??
    That way if I do get any moisture, it can dry out on its own and I wont have to worry about any insulation retaining moisture…
    Pipes will not freeze….so that’s not an issue….

    Like I said, the bathroom will have radiant floor heating and a Panasonic 110CFM vent fan (with the heater)....
    Plus, I always have two dehumidifiers running in the basement….along with my pellet stove in the winter.
    So the bathroom shouldn’t be damp…

    Unless someone can give me an overwhelming reason why I shouldn’t leave it uninsulated, I may go that route…

    Thanks again.
    mm
  5. Dana

    Dana In the trades

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    Location:
    01609
    Vapor retarders have no place in a basement- foundation walls need to be able to dry toward the interior. NEVER use just a studwall & batts against a foundation wall or ground moisture will rot the cold side of the studs. It might be 45-50% relative humidity in the heated interior of the basement, but the dew point of 45%RH 70F air is above 50F, and the cold side of the studs will likely get well below that. But if you put in a vapor retarder on the interior side, ground moisture will be trapped in the studwall to raise the relative humidity well into the mold-growth zone. If you put a vapor retarder between the foundation and studs, the foundation saturates, water is driven higher to dry toward the exterior on the above grade portion to effloresce & spall, or even into the foundation sill to create rot conditions therer.

    This isn't just an opinion- it's the physics of water, and the biology of mold.

    It's best to use semi-vapor-permeable foam insulation against the foundation, and finish out the R with UNfaced batts in a studwall, as long as you can keep the wintertime average temp of cold edge of the stud well above 50F. It's often less work to just use thicker foam (up to 2" /R10 for XPS, or 4"/R16 for EPS beadboard.) Be sure to avoid product with foil or poly facers, since they are both highly vapor-retardent. See: http://www.buildingscience.com/documents/information-sheets/5-thermal-control/basement-insulation/ As long as the foam insulation can pass water vapor at some minimal level the assembly can dry. Pink/blue XPS becomes semi-IMpermeable at thicknesses over 2", making it harder for the foundation to dry toward the warm interior. With beadboard or fiber-faced polyisocyanurate (sold as roofing insulation in commercial space) you can go much higher in total R-value.

    For much of the US an inch of XPS and a 2x3" studwall with R8 unfaced batts is the cheapest way to go, for a nominal R13 total. For extra-cold areas, 2" of XPS and a 2x4" studwall with unfaced R13-R15 batts gives you a result over R20.

    [edited to add...] ... so bottom line, from a mold-potential point of view you're kind of stuck if the studs are already in. If there's enough space to slip in some 6-mil polyethylene between just the stud-edges and the foundation with an inch or so exposed on either side of the stud, leaving the rest of the stud-bay without a vapor retarder, you could spray ~2" of closed cell foam in there for ~R10-R12. If it's a small amount of area the DIY kits from Fomo-Foam, TigerFoam, Dow Frothpak etc. are about perfect. IF there's at least 1/4" of gap between the stud & foundation everywhere you could use the foam itself to be the vapor-retarder to limit ground moisture from wicking/condensing into the cold edge studs. But if the studs are tight up against the concrete, forget about it. Be sure to use vapor-permeable interior finishes (no vinyl or foil wall papers, just latex) and keep the RH in the bathroom under 50% with it's own dehumidifier.
    Last edited: Mar 31, 2010
  6. mrmichaeljmoore

    mrmichaeljmoore New Member

    Messages:
    128
    Location:
    Connecticut
    Here are a few pics of what my wall looks like now.

    The gap between the back of the studs and the concrete wall is no more than 3/4â€.
    Plus, it is all blocked off and fire foamed, so I can’t slide some 1/2†rigid foam behind it.
    Plus, as you can see, I have all the plumbing and wiring running through the studs…so not much room in the stud cavity.

    Like I said, I think I may go the no insulation route…...

    thanks for the help….
    mm

    Attached Files:

  7. Dana

    Dana In the trades

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    It looks like you have access to at least part of the band joist & sill in there- that should be spray-foam sealed/insulated before you close it all up. If you feel like it, a cobble-job of 1-2" of foam board glued to the concrete with a 1" clearance from the studs on either side will give yo more than 50% coverage- a big improvement from a thermal point of view, yet keep the studs from chilling below the dew point of the interior air. Spray or foam-board, either will do. Spray foam will be less labor, foam board less cost. With foam board good performance will depend on keeping it tight to the concrete (sealing the edges with 1-part foam or caulk wherever the gap is more than 1/8".)

    The most-critical/best-payback would be from the frost-line on up. But if your deep subsoil temps are below 60F there's payback on having at least 1" of foam on the lower portion too.
  8. mrmichaeljmoore

    mrmichaeljmoore New Member

    Messages:
    128
    Location:
    Connecticut
    well....the whole wall is below grade, so the Refelctinx stuff wont work.
  9. mrmichaeljmoore

    mrmichaeljmoore New Member

    Messages:
    128
    Location:
    Connecticut
    Well, let me ask the question this way.......
    What potential problems can i possibly run into if I DO NOT insulate?
    Is it merely that it will cost more to heat the bathroom?
    With no insulation in there (fiberglass or foam or any other kind), there is no potential that moisture or vapors will build up in the stud wall causing mold, right?


    I did call a local company to inquire about spray foam insulation......the guy said it was too small of a job for him....at least $1000. he said it was not worth it to me or him.
    And, to be completely frank, I do not see me being able to do the spray foam. This project has run long already...that would only delay it that much more.

    And unfortunately, I cant get the rigid foam panels behind the studs.
    Would un-faced fibergalss batts work? that way, without the kraft paper, moisture can pass through?
  10. Dana

    Dana In the trades

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    2,841
    Location:
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    Unfaced batts would allow the foundation to dry toward the interior, but would lower the temp of the foundation and that end of the studs, potentially to below the dew point of the interior air of a humid bathroom, increasing the mold risk.

    If you don't insulate it'll cost more to heat, and there's still some mold potential from interior air getting to the end of the studs nearest the foundation, depending on just how cold that foundation wall is, and how humid the bathroom is allowed to get. (Do you have an infra-red thermometer?) If you foam both the wall to insulate it and the sides of studs to at least 1/2", the studs may still pick up ground moisture from the foundation, but any interior air couldn't get to the cold end of the stud- condensation would then be on foam, not wood, and the stud would still be able to dry toward the interior through the thinner layer of foam when temperature/humidity conditions were favorable.

    Spray foam with a DIY kit would be well UNDER $1000, if you're talking 125-150 square feet of foundation wall at 1.5-2" thickness, that's only 200-300 board feet. A 200 bd-ft kit runs ~$350 from online vendors, 600bd. ft kits are about $700. If your band joists & foundtion sills aren't sealed & insulated, it's worth it to put at least an inch (2" is better) on them on the entire perimeter of the foundation. If that + the bathroom job is big enough to interest the pros, go for it, otherwise a 600b.f. kit can do quite a bit of band-joist & sill. At ~800-1000 board feet it's usually the same or cheaper to let the pros do it.

    Bottom line, it's spray foam or nothing in this situation, and foam done right is lower risk than doing nothing. Fiberglass alone increases the mold potential.
  11. mrmichaeljmoore

    mrmichaeljmoore New Member

    Messages:
    128
    Location:
    Connecticut
    well, i think i may go the no insulation route....

    it may cost me a bit more to heat the bathroom, but at least the chance of mold/mildew will be diminished greatly than if i use fiberglass insulation.
    I think the combo of radiant floor heat and the heated fan, it will be plenty warm in there....

    What I may do is this.....after the room is all done and the inspector has given me the CO, I will drill some "air holes" in the stud on the outside at the end of the bathroom. (Not sure if I am describing that clearly....but I am talking about where the plumbing vent pipe comes out from behind the wall, as seen in the first picture above).
    That way, there will be some airflow in and out of the cavity behind the sheetrock. The dehumdifier on that side of the basement will then hopefully draw any dampness/moisture from that area.
    Do you think that is a good idea?

    One other question...assuming I go the no insulation route, should I at least try to insulate the rim joists with rigid foam? Right now, I think there is some fibergalss stuffed in there.

    Dana, thanks for all your help with this.....By the way, are you a builder/general contractor?

    mm
    Last edited: Apr 5, 2010
  12. Terry

    Terry Administrator Staff Member

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    Location:
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    You may want to look into insulation installers in the phone book.
    They could handle that in short order and would have everything on the truck.
    When I was building homes, I always contracted out the insulation.
  13. Dana

    Dana In the trades

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    On the rim joists again, foam is your friend, fiberglass (especially un-faced) is your enemy. If you do it in cut'n'cobble with rigid foam you need to seal the edges with foam (Great Stuff, etc), since warmer more humid interior air will convect through any gaps/cracks and condense on the now-colder wood all winter. Unfaced fiberglass is worst, since it allows the most air flow, and kraft or foil facings are nearly impossible to get air-tight. With spray foam air-tightness is guaranteed. (There are no rocket-scientists wasting their carreers as foam-sprayers- the DIY kits aren't rocket-science to use- just more expensive on larger jobs than going with the pros.)

    Whether condensation on the rim joists due becomes a problem depends a lot on your climate, and your interior temp & RH. The dew point of 68F 30-35% relative humidity air is in the high 30s F at sea level. If that's your interior temp & RH, and the average January/February temp in your area is above 40F it almost doesn't matter- it may condense during cold nights & cold snaps, but spends more far more hours at temps where it's drying, not taking on moisture. But the dew point of 70F 60% RH air is ~59F, and any surface below 50F will visibly & quickly form moisture on it's surface(!). Air-tight rim-joist insulation is an inherently safer approach in almost any heating dominated climate. (In my neighborhood the average January temps are ~18F, yet I still see homes with unfaced fiberglass jammed against mildewy rim joists & sills.)

    I'm not a building contractor but my father was, and I worked for his company while putting myself throught school. I'm currently working as an electrical engineer, but they build-ing-science stuff has been with me for awhile after watching even professional designers make what was to me even at the time dead-obvious mistakes around water vapor & humidity cotnrol. I've spent lots of time trying to explain the physics of the prolblem, the difference between a vapor retarder & an air barrier etc. (something that seems somehow elusive or mysterious to a lot of folks), or the difference between waterproof & vapor-tight, but it's not that hard to develop a good intuitive grasp of it once you understand the psychrometric chart, and the sources of water vapor & vapor pressure. Closed cell foam is waterproof, but semi-permeable to water vapor at thicknesses of 2" or less- it's wonderful stuff when properly applied, but like poly sheeting or foil facers, it can be a disaster when placed in the wrong part of the stackup of an assembly.

    Mechchanical dehumidifcation works, but not as reliably as building it inherently mold-resistant with the right construction methods. The best thing is to keep all the wood on the warmer drier side of the assembly with at most modest vapor retarders to slow down the ground moisture transport into the wall assembly without making it so tight that the concrete saturates. Warm dry wood doesn't grow mold. See: http://www.buildingscience.com/documents/information-sheets/5-thermal-control/basement-insulation/ In your situation whether you foam the cavity or not, the edge of the stud next to the concrete is going to be taking on ground moisture into the wood, but as long as you don't vapor-seal it on the interior it will likely have sufficient drying time that it won't rot. Closed cell foam is sufficienty vapor permeable to allow the stud to dry toward the interior, but is sufficiently air tight & liquid-waterproof that when the interior humidity is high (say, during showers, etc.) the interior air can never get to the cold part of the stud to condense, soaking into the wood.
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