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Community and Q&A

Humidity Control for Hardwood Paneling and Floors

orange_cat | Posted in General Questions on

I live in the Easy (Ontario, Canada) with high humidity during cooling and shoulder season, and low humidity during heating season (in a heating-dominated climate); zone 5A.

I would like to add a humidifier and a dehumidifier to keep hardwood dimensionally stable (we have solid hardwood paneling and flooring) .

I am looking for a solution that does not rely on whole house ducts even those I have ducted air conditioning and ERV for two reasons
(1) I may wish to remove humidity in shoulder season when cooling is not required
(2) I may wish to add humidity when heating is infloor hydronic radiant heat.

I asked a mechanical engineer two questions and was told “no” to both, with “just have a stand-alone dehumidifier in the basement” and room humidifiers.

I would much prefer to have a plumbed  automated solution mostly because I want to go away for two-three weeks anytime during the year and not worry about the humidity levels and having to pay someone to come in and fill/empty the humdifiers/dehumidifiers.

The house is 2.5 floors with a single central staircase (think of a single vertical well almost like for a spiral). All floors above grade (the half floor has a garage on the front, family room at the back fully above ground, grade on slab).

1. Humidifier: I wonder if a standalone humidifier installed at the bottom of the staircase would suffice?
or this: (not these particular products – this is just something I found on google).
My understanding (very limited) is that humid air is lighter and will rise and much of the wood paneling is around the stairwell or adjacent with doors that are likely to be open much of the time. I like that there are no ducts and all else equal steam seems least mold-prone.
Dehumidifier. a whole house dehumidifier with dedicated ducts. I am curious if some really simple ducting (one register per floor near the staircase?) would suffice? Or even a small dehumidifier unit per floor hidden  in the walls?

Help please?
(I do not have open floor plan anywhere)

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  1. Expert Member
    DCcontrarian | | #1

    The question boils down to how well humidity dissipates throughout a house. I don't know.

    But if it did dissipate evenly, I would expect the dew point, not the relative humidity to be constant, because the temperature tends to vary within a house. And there is a funny relationship among wood, temperature and humidity. If wood is left in constant conditions it will reach what is called "equilibrium moisture content" or EMC, where it stops absorbing moisture or drying. At a given temperature, with higher humidity the EMC will always be higher. At a given humidity -- absolute, not relative, and measured by dew point -- at higher temperatures the EMC will always be lower.

    So for example at 70F, with a relative humidity of 40% the EMC is 8.5%; at 50%RH it's 10.1%; and at 60% it's 12.0% (this is moisture expressed as a percentage of the dry weight of the wood). So if you keep the temperature at 70F, going from 40% to 60% increases the moisture content of the wood by almost 50%, that's why you get seasonal variation.

    OK, now let's look at a house where the air has the same moisture content throughout, but the temperature varies. Let's say the air has a dew point of 50F, which at 70F is a RH of 48%, which is really comfortable for most people. At 70F the EMC is 9.0%. At 60F the EMC is 13.1 and at 80F the EMC is 6.7. So same air, at 80F the EMC is barely half of what it is at 60F. Admittedly that's a pretty big temperature swing, but it's conceivable between basement and attic. You could also see swings like that between the sunny side and shady side of a house with a lot of windows.

    I think you should try to keep the humidity in the house within reasonable limits, but for your own comfort. It's a bit of a mug's game to try and keep wood at constant moisture. Properly installed woodwork should have allowance for seasonal movement anyway.

    1. orange_cat | | #3

      I only want to keep humidity in a 30-50 range (with temp in 60-80) - I just want the ability to bring it up or down within that range.

      1. Expert Member
        DCcontrarian | | #7

        That humidity is pretty low. I find anything below about 45% RH makes my throat uncomfortable.

  2. Expert Member
    DCcontrarian | | #2

    Also, there are alternatives to portable humidifiers and dehumidifiers that you have to fill and empty. At the simplest, most dehumidifiers allow you to have them empty into a drain so you don't have to empty them.

    1. orange_cat | | #4

      That requires placing a drain where a dehumidifier is installed and that is precisely what I was told a 'no" to.
      I am having drains laid out now - this is the time to add dehumidifier and humidifier but the moment I am not relying on forced air circulation the mechanical engineers seems to have no options. And I think that cannot be right...

      1. Expert Member
        MALCOLM TAYLOR | | #5


        "That requires placing a drain where a dehumidifier is installed and that is precisely what I was told a 'no" to."

        That seems odd. Mini splits and HRVs have condensate drains. They go somewhere.

        1. orange_cat | | #8

          Sorry it may be my description - I asked about adding a standalone whole house dehumidifier and a standalone whole house humidifier and was advised to use just those portable units. That was odd to me. I want them to connect to water supply and water drains.

      2. Expert Member
        DCcontrarian | | #6

        You have ductwork. Why not use it? You can get dehumidifiers and humidifiers that are meant to add on to ducted systems. The HVAC doesn't have to be running when they're running, just the fan. You can either run the fan all the time, or hook the fan up to a dehumidistat/humidistat.

        Here is a humidistat I've used:

        It was around $14. It has two switches. One is heat/cool/off. The other is humidify/dehumidify/off.

        1. orange_cat | | #9

          There are ducts and ERVs.

          But - would fan alone, without air conditioning (i.e. no need for cooling, only for dehumidification) dehumidify even with ERVs? Does not sound like it could - sounds as if a separate dehumidifier is needed (with own fan)? Sharing ducts with ERV?

          For humidifier, again - without the hot air circulating, using the AC ducts for moisture - relying on infloor heat - sounds like it would not even work. What would move the humidity through ducts? Again fan?
          What about mold with humidity and no heat?

          I understand mechanical engineers automatically assume air is always blowing, through the ducts so add-on humidifier to it and dehumidifier to it too. But if it does not? (during shoulder season or heating with infloor) - that is why stand alone humidifiers and dehumidifiers exist?

          I understand even having ERV share the same ducts is not an obvious solution, but I cannot reverse that one. But no humidifier or dehumidifier had been installed yet.

          1. Expert Member
            DCcontrarian | | #10

            In the simplest way you install something like this for humidification:

            and something like this for dehumidification:

            You install them near the return for the HVAC, but not connected, and you run the fan on the HVAC all the time. So the output of the device gets sucked into the return and circulated around the house. That's pretty simple.

            If you want to go up the complexity ladder, some improvements to consider:
            * Run a wire to somewhere central in the living space, and put the humidistat there.
            * Also attach the fan on the HVAC to the humidistat, so instead of it running all the time it only runs when one of the devices runs.

            Both of those devices can also be ducted into the HVAC ducting. Step 3 in increasing the complexity would be to do everything above, and also duct them in.

          2. Expert Member
            DCcontrarian | | #12

            I should add that I'm not recommending those particular units, I don't have any experience with them, they're just examples of the kind of equipment you should be looking for.

  3. Expert Member
    DCcontrarian | | #11

    The #1 thing to do to control humidity in your house is to make sure it's as air tight as possible. In the winter, outside air is much drier than inside air, in the summer it's much moister. The reason humidity swings from season to season is that outside air leaks in. The more you can seal your house the less those swings will be.

    Air sealing makes your house quieter, cleaner and more comfortable too. The fact that an ERV is planned is a good sign.

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