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Community and Q&A

Dehumidifying a Large, Cold Basement

Sal_123 | Posted in Energy Efficiency and Durability on

I have Poured foundation walls, I applied Grace self adhesive membrane on all exterior foundation walls, I did it myself, there are no areas sans the membrane below grade, and I applied a capillary break (Tremco paint on membrane) between the base of foundation wall and footing (as learned here on this site!) Under the gravel layer is a thick 15-Mil Vapor Barrier, taped and overlapped seams.  Walls are not insulated yet, I figured I’d eventually insulate the interior side at some point (metal framing, rigid XPS caulked and taped panels).  The basement is about 2,000 sq ft, temps run in the mid 50s just about all year, humidity is usually >60% on a regular basis during winter months, summer into the 80s. I am in Zone 5, Northeast, and made the mistake of installing a 120 pint a day Ultra-Aire dehumidifier, ducted to draw about 30% of air from living space and 70% from the basement with a return to basement alone. Since the dehumidifier is a typical compressor unit, the low temps do not allow it to function as it should.
The way I see it, I have a few options:
1. Heat the basement (insulate the exterior walls and maybe tap the overhead main trunks?, set up a different zone, maybe motorized dampers?, or install baseboards running them off the tankless water heater? The question then becomes best way to heat a large basement)
2.  Use a desiccant dehumidifier, (I think it would have to be a large a pricey unit for the sq footage)
3. Mini-split/s (?, not too versed in this option)
4. An air-exchange system of sorts, maybe have the dehumidier draw 100% from the living space above where temps are higher and return 100% to the basement, thus causing a cycling of air – with basement doors always kept open?
Constructive input would be appreciated.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    First, recognize that 55F air at 80% RH is not particularly wet. It is approximately equivalent to 70F air at 50% RH, which is right in the sweet spot for living. If your upstairs air is about those conditions, then the basement is acting as expected. And, if your walls are right at the air temperature (probably close), there is still no real risk of condensation, even with the high RH. The surface water activity of 80% RH air is still not quite high enough to support mold growth, so this may or may not be a problem. 80% RH will certainly cause wood to swell, paper to sag, and other cellulose-based things to get generally soggy.

    If the basement is not terribly moldy and smelly, there is no big downside to your idea of bringing 100% of upstairs air into the dehumidifier, drying it out, and dumping it in the basement. That will bring additional heat into the basement as well as drying the air. There will of course be an energy penalty.

    Insulating the basement is the other option. if you insulate the walls and floor, the heat generated by the dehumidifier will also warm the air enough that the basement will be both warmer and dryer than it currently is. The energy penalty will be less, but the ROI on the insulation is undetermined until you have numbers to put in the equations. The upside of option 2 is that the basement will be warmer and dryer and generally more comfortable than option 1, at a lower ongoing energy cost.

  2. Expert Member
    NICK KEENAN | | #2

    It's a little counter-intuitive, but the biggest factor in how much energy it takes to heat and cool a building isn't the size, it's how well-insulated it is. And with your basement un-insulated you've essentially got one side of the house exposed to the elements.

    You want to seal and insulate your basement, and make it part of the conditioned space of the house. You will use less energy then you're using now. Your winter humidity problems will go away. You might have to do something about the summer. In places with humid summers that aren't that hot the need for air conditioning isn't enough to provide all of the dehumidification that is needed. But get insulated first.

    You're on the right track with foam, but I would recommend against XPS, the EPA was just about it ban it a year ago because its manufacture releases so much pollution. Instead look at EPS or polyiso.

  3. Jon_R | | #3

    That Ultra Aire 120H is supposed to work down to 49F. Is the CFM correctly on the low side (perhaps 230 CFM)?

    For minimum basement humidity, use 100% of air from basement and 100% return to basement. Warming the basement will also help.

    The best way to reduce Summer basement humidity is to improve air sealing.

  4. Sal_123 | | #4

    Thanks all for the comments.
    My next step is to insulate all the walls and reassess.
    The current conditions are awful. Humid, cold, smelly, cardboard boxes are soft. I guess the unit will work to 45 degrees but not pulling 120 pints out in a day. I hear it cycling on and off all the time. I assume to prevent freezing of the coils. The trouble shooting section of the manual shows possible reasons for inefficient function listing temp below 55 degrees.
    I need to insulate then warm the air. Once insulated, it will make it easier to cool, heat, dehumidify and will incorporate filtered outside air influx, season permitting, to increase air quality. Next question, How do I heat it once insulated???

    1. Expert Member
      NICK KEENAN | | #6

      Once insulated you may find that the heat it absorbs from the rest of the house is enough to keep it reasonably warm. And your heating bill will go down.

      1. Jon_Lawrence | | #7

        I agree. I would insulate first and then see what happens.

  5. user-2310254 | | #5


    How do you heat the rest of the house?

  6. willymo | | #8

    During the winter and shoulder seasons look into an HRV. Simple ducts to and from exterior; as long as the outside DP is less than the basement DP, you will get de-humidification for ~60 watts. Summer is when to run the dehumidifier.

  7. Sal_123 | | #9

    House has forced hot air. Then exploring diy radiant, working with an engineer that designed the system, I did entire first and second floor with radiant, in 1.5" of mud and porcelain tile everywhere to maximize thermal mass. I wish I had planned in advance and insulated the basement slab from the ground, could have done radiant as well. I have ducts overhead in the basement, could tap into those. I put in a hrv to serve second floor and an erv to serve the first floor. Looking at core drilling the wall somewhere to pass a duct to bring in fresh air into the basement. The ultra-aire 120H has an auxiliary vent to draw in air from outside, could pipe it in to that during summer months. I'm now installing 1.5" XPS against the foundation walls. Sealed and glued on all sides to each other, floor, ceiling. Will erect a metal framed wall to finish interior side. XPS will serve as a moisture barrier and thermally isolate the metal studs. Thanks for your comments.

  8. Expert Member
    NICK KEENAN | | #10

    In the summer outside air is the enemy, there's a good chance the few point outside is higher than the inside temperature. While you're insulating you should also thinking about air sealing, particularly the area from the top of the foundation to the underside of the subfloor, which tends to be leaky.

    That will make the first floor more comfortable in winter as well.

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