Efficiency & Bathroom Humidity

Reader Frank recently commented:

“Another strategy that I’ve found useful is the proper use of the bathroom vent fan. I know the cfm rating of it and after figuring the size of the room determined that the fan is capable of completely exhanging ALL the air in the room in under two minutes! Excess humidity is no good either; the windows throughout the house frost up noticeably more when the fan doesn’t get used and interior humidity is too high. Anyway~ fan stays off during shower operations and gets flipped on for 1-2 minutes right after, and that’s it. Most of the humidity goes out, most of the heat stays in.”

This reminded me that I’ve been meaning to to discuss bathroom vents.  To start with my conclusion: unless you use them as Frank does (i.e., most judiciously) I think they are a horrible idea in winter.  They are the equivalent of cutting a hole in the wall and blowing cold air in through it at somewhere between 50 and 200 cubic feet per minute.  Oh, I know– they don’t blow air in, they blow air out.   But that air is replaced with air from some other part of your house… which is replaced with air straight from outdoors.  There’s no way around this fact.  You don’t really notice it, probably, because it’s sneaking in through a thousand cracks, and you’re likely in the shower while it’s happening.  But it’s a travesty.

In fact, it’s a double travesty, because besides replacing warm air with cold air, you are losing the (much larger) amount of heat contained in the water vapor.  One of my Cold House mottos is that water should leave the house in the same phase of matter that it came in.  This is because of the enormous heat that it takes to melt or vaporize water.  If water comes into the house as liquid, but leaves as vapor, you’ve put in a lot of heat that you aren’t getting back.

But of course you can’t just let the bathroom get and remain sopping wet all winter.   My solution at the Cold House is controversial, but I’m prepared to defend it vigorously:  we use a very small dehumidifier.  It sits in the corner of our very small bathroom.Here is the routine:  In the morning, I switch it on just before showering, set at 50% humidity.  It stays on steadily through a shower, and for perhaps 15 minutes after, as it dries out the air.  Then over the course of the day it will briefly re-activate now and then, as the air becomes a bit humid again from wet towels, etc.  By bedtime things are quite dry in there, and we turn it off altogether overnight.  Next day, repeat.  With two of us showering, I estimate the thing actually runs a total of 1.5 hours a day. It condenses about a quart of water a day into its bucket.

The dehumidifier (running) uses about 400 watts to power its fan and compressor.  All of that winds up as heat in the bathroom.  Moreover, it claims to condense 1.6 liters of water for each 1 kWh of electricity it uses– which about agrees with what I’ve observed.  Now, each liter of steam condensed back to liquid releases a further 3,600 BTU of heat, which equals roughly 1 kWh also.  So, for each kWh of electricity used to run the dehumidifier, we get back 2 kWh of heat in the bathroom.  It is, effectively, like a 200% efficient space heater.  Plus, we get a quart of distilled water every day  (it’s enough to “small flush” the toilet, saving yet more energy!)

Let’s compare to a vent fan.  That might use 50W to run its motor.  All that turns to heat as well, but it goes out the vent– no use.  All the steam goes out the vent– no use.  Furthermore, in running 1kWh of electricty through a 50W, 50 cu. ft. per minute fan, you will vent out 60,000 cu. ft. of air.  If the air in your house is 70º, and outside is 20º, you’ve blown out 54,000 BTU of heat (= 15.8 kWh).

So, assuming they serve the same function, about equally well, and you’re happy to have extra warmth in your house, which appliance would you rather use:  the one that gives back 2 kWh of heat for each 1 kWh you put in, or the one that removes 15.8 kWh of heat for each 1 kWh you put in?

One final item: the vent-user will get out of the shower into bathroom air that is either warm but 100% humid (if the fan is used the way Frank does) or relatively dry but no warmer than it was to begin with (if a high-power vent is running while showering).  The dehumidifier user, however, steps out into a bathroom environment that is warmer than it started, yet not saturated with humidity.  And when the rest of the house is 45º, this is quite a pleasant bonus.

I conclude that the only rational reason for using a vent fan instead of a dehumidifier is if your bathroom smells so bad that you actually need to replace all the air in it eight times an hour.  But if this is so, you may have other problems.

[Addendum: some will question whether the capital cost of buying the dehumidifier is worth going this this route.  I agree, this might be a rational counter-argument.  However, the cost of many high-quality vent fans is actually higher than our miniature dehumidifier.  If  you add in the cost of vent hosing, an exhaust-side fixture, installation labor, etc., it will surely come out to more.  Plus, you avoid altogether having this unnecessary 4-inch hole in the side of the house that you’re trying so hard to keep warm.  You might not have a choice in the matter, of course– your fan may already be in place, and I understand these silly vents are often required by code for new construction– but if starting from scratch, I maintain: no contest.]


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17 Responses to “Efficiency & Bathroom Humidity”

  1. Dirk Faegre Says:

    As to your bathroom humidity issue: Of course there are Heat/Energy Recovery Units – bathroom fans with a heat exchanger to bring back 70% of the heat that was on it’s way out. They are expensive but they work well.

    It would be interesting to see what the Air Quality of your home is. Very old homes leaked like sieves and so had air exchange by default. I’d hazard a guess that your home is fairly tight. Limiting air exchange in the ways you are could be a problem for your health. We have here in Maine, an organization, Maine Indoor Air Quality (MEAIQ), that might offer some insight.

    • coldhousejournal Says:

      Thanks for the thoughts. I’m not too worried about our (1950’s) house being too airtight for our health. We’ve got plenty of cracks and gaps, in spite of my efforts to close up the worst of them. In any case, there is the empirical evidence: if the house was really tight, our wood stove wouldn’t draft as nicely as it does!

      The idea of a heat exchanger in a bathroom vent seems like a move in the right direction.

  2. Jon Silverman Says:

    Dehumidifiers keep the bathroom from molding, but eat lots of electricity and act too slowly to assure reliable results. In winter, most houses are too dry anyway. Seems silly to run humidifiers throughout the house while running exhaust fans in bathrooms, which presents an opportunity:

    Try leaving the door to the bathroom open while you shower (yeah, I know; not an option for some folks), and run a big box fan in the doorway. The same fan you use to cool the house in summer is perfect, and you already own it. If the fan is to be set on the floor, it’s a hair better to blow air from the other room *into* the bathroom, so this cooler air will cooperate with the natural convection current created by the shower’s warm, moist air; but, really, you can blow it either way is more comfortable. Let the fan run for a few minutes after you finish showering to finish the job. This scheme dries the bathroom and humidifies the house in one shot, and saves you the time, hassle, and expense of running other humidifiers.

    If you can not have the door open while you shower, just close it and run the fan inside the bathroom while the shower is running, to prevent moisture from settling on surfaces. Then, after showering, open the door and blow the moist air into the rest of the house for pretty well the same effect.

    Critics may presume this would induce mold in the rest of the house. It does not, because the moisture is distributed into a huge volume of dry air. If you live in a tiny studio and find it’s an issue, that would be a special case.

    • coldhousejournal Says:

      I think this idea of sharing the bathroom humidity with the rest of the house is an excellent one, for most “normal” people. But we really don’t need or want it. As you can read ad nauseam in the “water vapor” related blog posts, the perceived “dryness” of winter air is really a result of heating it to indoor temps. The colder your house is, more relative humidity the indoor air will maintain. For our place, I don’t feel we need extra moisture at all. (Note: we are a bit torn on this, because J’s parents gave us a beautiful cast-iron humidifier-kettle thing for the top of the wood stove, which smells great if you put some spices in the water… I’ll discuss that at some point.)

      “Dehumidifiers… eat lots of electricity and act too slowly to assure reliable results.”
      It does use some electricity, but as I mentioned, you get more than that much heat back– due to reclaiming the latent heat of vaporization in the water you condense. So, this mitigates. In comparison, water vapor vented out into house doesn’t give back its heat. It might condense and drip down the windows, or somewhere else, but eventually it’s going to find its way back outside as vapor, not as water. Lost heat!

      Also, our dehumidifier acts plenty fast. Granted, our bathroom is not more than 10 sq. ft., so that helps. One thing I found, too, is that it works even more efficiently f you can get the dehumidifier towards the ceiling, or run a duct from ceiling-level down to the air intake. Then you’re sucking the warmest, moistest air right in to the unit.

  3. felix Says:

    bathroom fan wont clear the air in 2 min. do the math please. dehhumidifier worx great, been doing it for years & even used a lit candle on the air intake of the the dehumidifier to burn poopy stink. dehumidifier worx better than exhaust vent for showers & takes the place of a small heater, but i prefer fan for poopystink.
    BTW- 1cubic foot of air with a 50* differential results in a .6 btu heatloss. so if its 20* outside & 70*f inside & the fan exhaust @ 50cfm then 60 min. x 50cfm= 3000 x .6=1800btu or about 10 cents electricity from BANGOR HYDRO as actual heatloss.

  4. felix Says:

    it helps to place the dehumidifier so that it blows away from the person & towards the coldest spot. mine is located & blows at the window which buffers the feel of the cold. the cold coil in the dh builds up with frost as it temporarily borrows heat for the room. when the dh is turned off, the borrowed heat is reabsorbed by the melting frost.

  5. Tom Says:

    I’m reminded of the page on SunFrost’s website (yes, the super-efficient fridge makers) called Concepts for Sustainable Living. They have a totally enclosed shower design that they claim reduces the temperature you need to set your water at.

    Basically, water cools down on its way from the showerhead to your skin due to evaporative cooling. However, if the air is saturated there’s no cooling, so you can set the temp lower.


    The profile of their CEO’s energy-efficient house is pretty inspirational:


    • coldhousejournal Says:

      That’s a pretty good idea. I was inspired by that website last year, and tried the enclosed shower concep, in a crude way, at the Old Cold House. It wasn’t pretty– involved a bunch of bubble wrap and duct tape. It did more or less work. But what I found was that as the shower approached being truly air-tight, it also approached being unpleasant from a breathing perspective…

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  7. More Google Queries « Cold House Journal Says:

    […] Answer:  Uh… where where you hoping it would move to?  Akron?  Time for my annual plug for using a dehumidifier, rather than bathroom fan vent, in the winter. […]

  8. Sam Says:

    with all that being said about the pros and cons , and we with crummy ceiling vents are willing to try the dehumidifier method- should the existing vent be covered at the point of entry into the room. i.e. put bubblewrap or some other insulating material over the opening so
    no air can go out or come in?

    • coldhousejournal Says:

      Sam– I wouldn’t tend to think that’s very necessary. Usually vents have a duct going at least a few feet away from the fan, then exiting the house through another opening that has an air-pressure activated louver. So when the fan is off, it’s unlikely there’s much warm air escaping through the vent. But I guess it wouldn’t hurt to cover it up for the winter.

  9. The Craftsman BlogNo More Cold Bathrooms - The Craftsman Blog Says:

    […] pointed out a post on Cold House Journal (you can read the post here) where the author does a little science experiment to keep his bathroom warm and dry in the […]

  10. Should I run the bathroom fan while showering? | Grist Says:

    […] doorway fans are particularly nice for forcing air from one room to another. Or how about a small dehumidifier, Barb? Its sole purpose is to wring out sopping air, and a small and/or Energy Star-rated machine […]

  11. restranged Says:

    aren’t you normally not supposed to put a dehumidifier so close to a wall? or is that just my crazy dehumidifier?

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