cause of condensation in ceiling?

Discussion in 'HVAC Heating & Cooling' started by ugh, Jul 15, 2013.

  1. ugh

    ugh New Member

    Jul 15, 2013
    We refinished our dry basement 9 months ago including bat insulation, vapor barrier and moisture resistant drywall on walls; bat insulation and regular drywall in ceiling. Finished area has wooden access panel into adjacent crawl space which is vented. Told contractor we wanted hvac to heat, cool & dehumidify without the need to empty the tank. HVAC sub installed a Mitsubishi mini-split which we were told to run to dehumidify the basement during the summer (we live in PA). The basement would get musty smelling but never wet before being finished. Last week we noticed black mold on the ceiling near the window access into the crawl space; water dripping out of recessed light fixtures and ceiling access points (which open to reveal pipe shutoffs etc). Restoration contractor thinks basement got too cold running dehumidifier without a low temp cutoff, the cold air traveled into the crawl space via the window and condensed and then traveled basck in to the finished basement ceiling where it dripped out the vents etc. Finishing contractor said that we did not do anything wrong, that this is unfortunate but that he won't pay for repair. Mold remediation set to start. Will definitely have to replace the ceiling and one wall of drywall as well as all the insulation that was dripping wet. Can anyone help identify the true cause of the water if there is no pipe leak found? Who is responsible for remediation & repair? Thanks in advance for your assistance!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Without proper vapor barriers and insulation, cooling a wall or ceiling when the other side has access to the outside air exchange, you'll get condensation. Simple fact...when ANY surface gets below the dew point, and on some of the hot days we've had this year, that dew point has been above 70-degrees, at least where I live - you will get condensation. If that area does not get enough air circulation once it gets above the dew point, the moisture will accumulate rather than dissipate. Mold is pretty much everywhere, and only needs moisture and food to grow.

    If the contractor failed to provide proper insulation, vapor barrier, and manage air leakage in the areas he installed, IMHO, he's responsible. Now, I'm not a pro or a lawyer, just providing my opinion.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    In almost all cases the source of the humidity is outdoor air infiltration. All realistic solutions start with air sealing both the basement (including the crawlspace venting) and attic to kill of stack effect 24/365 infiltration drives, then mechanically dehumidify. The copious access to outdoor air of the vented crawlspace is the root of the problem and there is no rationale for crawlspace venting in a PA climate- venting always ADDS more moisture to the building than it purges.

    To close up a crawlspace requires putting a 10mil poly (or EPDM) vapor barrier on the crawlspace floor extending at least a foot up the foundation (all the way to the foundation sill if it's a stone or CMU foundation), sealed with duct-mastic or 1-part can foam at the walls. Seams can be overlapped by a foot and mastic sealed or taped with housewrap tape. While it's better from an energy use point of view to also insulate the stubby exterior walls of the crawlspace, that's not absolutely essential for humidity & mold control. The key is to keep ground moisture and outdoor air from entering the crawlspace, and making the crawlspace into semi-conditioned space, with reasonable air communication with the rest of the house (even if it's just replacing the hatch to with grilled hatch.) In winter there may be condensation events on the vapor retarder when it's cold outside if you don't also insulate the walls- a couple of inches of rigid foam held in place with 1x furring through-screwed to the foundation, with half-inch wallboard hanging on the furring as a thermal barrier meets fire code.

    A comprehensive primer on the topic of conditioned crawlspaces can be found here.

    Vapor barriers are two-edges swords, and create as many problems as the cure in a PA climate, and need to be used very selectively. Air-barriers, not vapor barriers are key. The stackup of your finished basement walls & insulation may contributed somewhat to the problem, but we don't know what that stackup is. While I'd be curious to know what that is, just in case it can be improved upon or needs remediation, it's the least of the issue- your main problem is the vented crawlspace that puts 70F+ dew point air in direct contact with the subfloor of air-conditioned spaces. In almost all cases in a PA climate you would need a bit of air-impermeable foam, not just a vapor barrier, between batt insulation and an exterior foundation wall, but will sometimes be OK on a foundation wall with a crawlspace on the other side.

    The sensible cooling loads of basements are very low and Mitsubishi mini-splits do not have the capability of dehumidifying without actively cooling. In "dehumidify" mode they optimize flow & temp for maximal latent cooling, but there is no eliminating the sensible cooling, and Mitsubishis don't operate to a setpoint (either temperature or relative humidity) while running in dehumidify-mode. The only mini-splits out there that currently control for temperature and humidity setpoints independently are the Daikin Quaternity series, which can also dehumidify in heating mode. To keep from over-cooling the basement, setting up a room dehumidifier with a hose to drain into a sump works. You can still use the mini-split for cooling & heating the zone though.
    Last edited: Jul 16, 2013
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