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Community and Q&A

Should I leave the door open or closed between basement and first floor (dehumidifier)?

Alan B | Posted in Energy Efficiency and Durability on

I live in a century plus old home with a 2/3 unfinished basement 1/3 crawlspace and a first floor above it. Its an energy efficiency and air tightness disaster that is very difficult to mend (though i am trying to reach parts of the crawlspace that are buried yet have gaps to outside.
My basement humidity gets to 100% very easily, as i am located near Toronto, Canada. Our outdoor humidity is rather high in the spring/summer/fall.
So i have been using an energy star rated dehumidifier since i moved here, it tends to use about 3-4 kWh/day, set at 60% to prevent mold that starts easily if i don’t use it.
My question is that since i don’t use a lot of air conditioning (except for heatwave days above 30ºC) should i leave the door between first floor basement open or close it?
I have been closing it on the theory that extra humidity from the first floor will quickly replace humidity the dehumidifier is taking out. But on the other hand the basement tends towards 15-20C with the door closed but would get closer to above grade temperature with the door open, potentially reducing the relative humidity level.
What are your thoughts on which would be more energy efficient, open or closed?

I would love to solve the fundamental problem but that is very difficult with how the house is built (not that i have given up but i don’t have easy solutions available at the moment).

TIA

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Replies

  1. Jon R | | #1

    The answer depends on how much moisture the first floor brings in that then gets unnecessarily removed. Could be anything from none (eg, you want the first floor quite dry) to lots (first floor windows are open). For what it's worth, I close the door and wish I had even better air sealing between basement and first floor.

    1. Alan B | | #2

      I mostly care about dehumidification to prevent mold, i have none on the first floor so i don't want it dehumidified at all if it will cause a net energy usage increase.
      Though i don't know how to calculate the difference, humidity and temperature varies daily so getting good test data is hard.

  2. Tom May | | #3

    I tend to leave my basement door cracked open, as well as one basement window, in the summer and leave the furthest window in my house open at the top with most all other windows closed or slightly open. The volume of air leaving the highest open window draws the same volume from the others as well as the basement combined to keep the house cool. Moving air in the basement is more efficient to remove moisture than leaving it stagnant to collect and trying to remove it mechanically. Think evaporative cooling, just like blowing on your arm.

  3. Expert Member
    Dana Dorsett | | #4

    >"I tend to leave my basement door cracked open, as well as one basement window, in the summer and leave the furthest window in my house open at the top with most all other windows closed or slightly open."
    ------------
    >"Moving air in the basement is more efficient to remove moisture than leaving it stagnant to collect and trying to remove it mechanically."

    That's not a generically adequate prescription- it works in some climate zones, but not most. It's important to understand the source of the moisture.

    In most of "-A" climate zones the summertime dew point temperature averages exceed the subsoil temperatures, often even exceeding the basement temperature. Creating an intentional leakage path with cracked windows to convect outdoor air into the basement INCREASES rather than decreases the moisture levels in basements. This is particularly true in the heating dominated 4A and higher climate zones. (This is less of an issue in the "-B" climate zones.) Air sealing the basement from the outdoors will limit the moisture source (= humid outdoor air).

    1. Alan B | | #5

      Do you have thoughts on my situation, which is better, door closed and lower basement temps or door open which will raise the temp somewhat but add the upstairs to the dehumidification load?

    2. Tom May | | #19

      Put two puddles of water on the floor, one in front of a dehumidifier, one in front of a fan. Which puddle do you think will disappear first and why?

      1. Alan B | | #20

        The fan should all else being equal (until humidity equals 100%), however this is not a water puddling issue, this is overall humidity and mold. If the humidity was 150% and no mold formed and no wood rots i would not care.

        1. Tom May | | #21

          Alan, you missed the point. Convective air movement will remove moisture better and faster.

          1. Alan B | | #22

            My point is to prevent mold and wood rot.

        2. Tom May | | #24

          ...so what causes mold and wood rot?????

          1. Alan B | | #25

            Poltergeists :D

  4. Tyler Keniston | | #6

    Can you get a cheap RH meter and try some experiments with the two scenarios? Maybe use a standard time frame (2 days?) and track the RH and the load on the dehumidifier—run it in both configurations.

    Various ways to track the load: energy metering or, could you just measure the water level and compare to basement RH? If the RH is about the same, but the water level is higher, you know the dehumidifier had to work harder to get to that RH. Or, if the water level is about the same, but the RH is higher, that will show you which is more efficient also.

    I believe best case scenario is to isolate the moisture sources and remove the minimum excess moisture. In other words, close the system off.
    Do you have any option to lay a vapor barrier on the floor?

    >"But on the other hand the basement tends towards 15-20C with the door closed but would get closer to above grade temperature with the door open"

    Have you measured the temperature increase with the door open? Does it actually raise the temp of the mold growing surfaces adequately, are does ground coupling ruin chances of that?

    1. Alan B | | #8

      Our weather and humidity is not very constant day to day so i can't get any good test data.

      That said i have not tested the temp increase with the door open, i can at least use temperature on the first floor on some consecutive days if they are close enough.
      I will try this and post my results, though the dehumidification adds heat, a great deal of it if i recall some GBA posts correctly.

  5. Expert Member
    Akos | | #7

    If your place is hitting 100%rh without dehumidification you have a major moisture surce in your basement (leaky foundation, high water table). I would try to fix that first. It will make a much bigger difference on indoor humidity and energy costs.

    As for leaving the door open. Most older houses have a lot of air leaks near the top of the foundation. Opening the basement door allows for more airflow because of stack effect. This means drawing in extra outdoor air and humidity into your basement, which is what you want to avoid.

    I would leave the door closed and get something to condition the main floor. Toronto area can get muggy enough that you can have mold in north facing closets on outside walls.

    Even a cheap window shaker or two pipe ducted floor console will make a big difference, not only for comfort but indoor RH and mold risk.

    1. Alan B | | #9

      I have gaps to outside that i can't easily reach. This house is an efficiency disaster.
      My preference would be to fix the issues but i can't easily reach the problem areas or even identify how bad the issue is :(

      The main floor has not had many problems, i have central air but its an old inefficient 15 year old unit. I only use it when it gets above 30C. Plus the load of the house is quite high, insulation is mediocre and hard to retrofit (knob and tube i can't bury in insulation, balloon framing that has gaps from basement to attic, low ceiling above part of the house that would not hold much insulation without filing it to the low ceiling meaning no ventilation...

      Finding someone who could solve the messed up situation here is not easy, most energy auditors and companies have given me ridiculous "solutions". If Dana ever comes this way i would be very happy to pay him for a consult. Hint hint.

  6. Expert Member
    Dana Dorsett | | #10

    It's unlikely that the basement temperature will rise by opening the door, since warm air is lighter than cooler air. If the upstairs is air conditioned leaving the door open will lower the basement's humidity level somewhat, due to the high vapor permeance of air.

    The only way the basement can approach the reported 100% RH (do you have fog or visible wetness on the basement floor?) is if there is copious outdoor air leakage along with other moisture sources, such as a leaking clothes dryer vent.

    What is the temperature in the basement in summer? What is the temperature of the basement floor? Subsoil temps in your area are around 50F/10C :

    https://greenhome.osu.edu/sites/greenhomes/files/imce/US-ground-temps.gif

    ...mid-summer outdoor dew point averages are under 65F/18C more than half the time:

    https://weatherspark.com/m/19863/7/Average-Weather-in-July-in-Toronto-Canada#Sections-Humidity

    I suspect your basement floor temp is in the 55F/13C range in mid-summer, and the basement's air temp is about 60F/16C, so if the ventilation rate is low (a very tight foundation wall, windows, and exterior doors) the mechanical dehumidifcation load to keep it under 65% @ 60F16C (= dew point of 48F/9C) will be pretty low. The air film at the floor would still have a high RH- a cardboard box on the floor might develop mold on the bottom, but the rest of the basement would not.

    1. Alan B | | #11

      "It's unlikely that the basement temperature will rise by opening the door, since warm air is lighter than cooler air. If the upstairs is air conditioned leaving the door open will lower the basement's humidity level somewhat, due to the high vapor permeance of air."
      Interesting. I am going to try testing this tonight, its closed now but i will check the temps a couple hours after sunset, kill the dehumidifier, open the door and leave it open overnight then test again before the sun comes up.
      I use the AC only on >30C days, which is less then 1/4 of an average summer (the last few we have had heat waves so i would use it a couple hours a day usually in the evening, which didn't seem to help the basement that much.
      So most of the time the upstairs humidity tracks the outdoor humidity in theory. Currently its 24C outside, 29% humidity, 23C inside and 53% humidity on the first floor according to my ecobee.

      "The only way the basement can approach the reported 100% RH (do you have fog or visible wetness on the basement floor?) is if there is copious outdoor air leakage along with other moisture sources, such as a leaking clothes dryer vent. "
      I think it was about 100%, i have been using the dehumidifier for the past few years all spring/summer/fall, but it will hit 80% in a day without the dehumidifier.
      No leaking dryer vent as i have a heat pump dryer. No water leaks elsewhere either that i can find.
      Now that i think about it i would see dampness the basement floor in spots before i bought the dehumidifier.

      "What is the temperature in the basement in summer? What is the temperature of the basement floor? Subsoil temps in your area are around 50F/10C :"
      10C is the rule of thumb around here so thats probably correct.

      "I suspect your basement floor temp is in the 55F/13C range in mid-summer, and the basement's air temp is about 60F/16C, so if the ventilation rate is low (a very tight foundation wall, windows, and exterior doors) the mechanical dehumidifcation load to keep it under 65% @ 60F16C (= dew point of 48F/9C) will be pretty low. The air film at the floor would still have a high RH- a cardboard box on the floor might develop mold on the bottom, but the rest of the basement would not."
      The blower door test i had done had plenty of air leakage coming from the crawlspace. About 14.5ACH540 for the whole house iirc.

    2. Alan B | | #15

      Interestingly after leaving the door open overnight and turning off the dehumidifier everything dropped in temp from 0.2-0.6C. The concrete floor, the brick support beam and the laundry tub and the ceiling/first floor rafters.
      I am not sure how much heat i cut off by turning off the dehumidifier, is there a way to calculate the heat liberated by converting 1L of water vapour to liquid?
      I'm not sure the data tells me much, though you did mention stack effect and that might be the cause.
      I will repeat the experiment tomorrow with the dehumidifier on.

      For the record floor 18.7/18.5C, beam 20.1/19.8C, laundry tub 19.6/19C and ceiling 20.5/20.3C
      Before/after.

  7. Jon R | | #12

    > My basement humidity gets to 100% very easily

    Depends a bit on where you measure it, but I disagree with others that this indicates copious outdoor air leakage or a major moisture source. It only takes an outdoor dew point above the basement temperature, a LITTLE bit of air leakage and some time. Look at humidifier condensate to determine the rate that moisture enters the basement.

    1. Alan B | | #14

      I don't fully understand how dew point works, but when it hits 30C it will fill the bucket in a day which i will measure the volume of next time its full.

      1. Jon R | | #17

        You can then compare your liters/day with others to get a relative idea of how well sealed your basement is.

        1. Alan B | | #23

          Its poorly sealed. Also the bucket capacity is 5L.
          Sometimes i think its best just to take a wrecking ball to the house and start over.

  8. Tyler Keniston | | #13

    One other question is what surfaces are you finding mold on?

    I think it's important to differentiate the issue of having 100 % (or high) RH AIR with the fact that an OBJECT can be colder than ambient air (especially due to ground coupling) and create a boundary layer that will sorb moisture.

    I suspect it's more important in your case to limit moisture ingress than it is to allow warmer air to enter. (In other words, the moisture that rides the warm air into the space is a net loss compared to the increase in energy that air brings into the space if/when the surfaces that develop mold are coupled to the vast thermal sink of the ground). That's what I would guess, anyways, without having every parameter quantified.

    One exception—maybe—would be if you allowed consistent and large quantities of airflow into the basement (like in the way Tom May describes in comment 3) such that the influx of energy is able to raise object surface boundary temperatures and/or if the source of moisture is internal to the basement.

    1. Alan B | | #16

      Mold on rafters, damp spots, general odour, walls. And surface rust on unpainted metal such as screws.
      I have thought about adding HVAC that would heat the basement with upstairs air, though that would take some retrofitting and if there is radon that would suck (untested).
      Overnight the humidity when i ran my test (posted above) the humidity went from 60% to 80% when i restarted it this morning.

  9. Expert Member
    Akos | | #18

    In most older homes in Toronto area, you can't rely on the upstairs AC unit for dehumidification in the basement.

    The problem is the basement tends to be colder than the rest of the house, so for example if your uspairs is at 22C/77F @ 50%RH, the same air in a typical cooler basemet (16C/60F) would work out to 72%RH which would be well into mold growth support.

    Generally, there is no way of keeping the basement RH in check in older houses without active dehumidification. If there is mixing with the air between the basement and the house, this means the dehumidifer will be doing extra duty as it will have to handle part of the humidity from the rest of the house.

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