Controlling Humidity

Factual Questions
1

I live in a guest house which is essentially a converted garage. I am having a very hard time controlling the humidity here. Water condenses on the inside of my windows at night and in the mornings when it gets cool enough outside, which it has, and there are issues with mold starting to grow in various places. My landlady insists that none of the previous tenants had problems with mold or humidity, so I’m wondering what I’m doing wrong, and how I can control the humidity problem.

As far as I can tell, these are the main sources of humidity in the house:
– The outside air humidity, which is usually around 70% at room temperature or somewhat below, but it also tends to be foggy in the mornings and evenings.
– I shower once a day.
– I cook, anywhere from once a day to a couple times a week.
– Water can evaporate from the toilet and any dishes I have soaking in the sink.
– When I breathe, I put out water vapor.

Here is what I have in the way of environmental controls:
– I can open the windows and door to ventilate the place to the outside. I do this after showering and cooking.
– I have a gas space heater.
– I have a ceiling fan.
– I got a dehumidifier, which I used to run after showers and whenever I wasn’t in the house, but the landlady complained about how much it increased her electric bill and I had to stop. It’s the smallest one I could use for the space, and it has an energy star rating, so I don’t think I can do much better on the electric usage.

Is there any way I can control the humidity well enough to eliminate the condensation and mold problem using the tools at hand, minus the dehumidifier? My landlady firmly believes that I can solve the problem completely just be ventilating the place enough, but given the outside air humidity, I’m not sure that’s possible.

The other weird thing is that back when I was using the dehumidifier, I could get the relative humidity down from 80 or 90% to 50 to 60%, but within a couple of hours of turning the dehumidifier off the relative humidity would be back up to 80 or 90%. Can this just be accounted for from evaporation from the toilet, or is something else at work here, like maybe the concrete slab this sits on was not sealed properly and wicks moisture up from the ground? Even not using the dehumidifier, I can have the windows open and the fan running for hours and have the place feel fine, then go to run an errand and find that the place feels extremely muggy.

Any insights or advice would be appreciated!

2

There are a couple of issues here.

  1. What is the relative humidity outside? Air leakage plus your doors and windows are open will equalize your humidity with outside.
  2. Do you have a bathroom exhaust fan that is vented directly outside? Do you close the door and run the fan during and after your shower?
  3. Is your hood fan over your stove vented directly outside?
  4. Are your windows energy efficient? Modern windows should not condensate with heat or cold tranfer due to double, triple glazing, etc. Their framing is usually vinyl or another mold or rot resistant material.

I would start with these.

3

Thanks! Here are answers to the points you raised:

There are a couple of issues here.

  1. What is the relative humidity outside? Air leakage plus your doors and windows are open will equalize your humidity with outside.

Relative humidity outside is typically on the order of 70 to 80%.

  1. Do you have a bathroom exhaust fan that is vented directly outside? Do you close the door and run the fan during and after your shower?

There is no bathroom exhaust fan at all. The best I can do is close the door and leave the window open.

  1. Is your hood fan over your stove vented directly outside?

The hood fan over my stove is not vented to the outside at all. It just recirculates the air in the kitchen area.

  1. Are your windows energy efficient? Modern windows should not condensate with heat or cold tranfer due to double, triple glazing, etc. Their framing is usually vinyl or another mold or rot resistant material.

My windows are not energy efficient. One of them consists of individual panes of glass held in by glazing compound. The others are single panes of glass in wooden frames.

I would start with these.
Unfortunately I’m not in a position to make major changes to this place! Bummer…

4

A quick easy solution may be that during the colder months you install a plastic window film. Basically, you heat-shrunk as heet of plastic over your windows which will act as a second glaze/vapour barrier and should eliminate condensation.

5

Great! Do you have any suggestions on controlling the humidity level and mold in general?

6

To prevent mold you need to eliminate conditions that promote its formation and growth. Mold requires moisture, heat and food.
Reducing your indoor humidity to 45%, stopping the condensation and cleaning the affected area with a proper mold killing agent or detergent should help.
What type of heating system is being used?
The easiest way to control humidity is to simply purge it outside using exhaust fans. You could install a small exhaust fan in your bathroom window.
You can also ventilate and purge all the air from your home during periods of low outside relative humidity and temperature.
Secondly, you need to run your dehumidifier 24/7, as soon as you turn it off you start defeating its purpose.

7

I would probably disagree with the exhaust fan idea. Nature doesn’t like a vacuum, so if you exhaust 200 CFM (as an example) to the outside, 200 CFM is making it’s way back inside through every crack in windows and doors, and every time the door is opened. It’s "make up air’; air that’s made up from exhaust. (and it is unavoidable)

The OP will be replacing 80% RH indoor air for 80% RH outdoor air.

Also, there will be no need for an exhaust when the outdoor ambient RH % is low, because the outdoor humidity is the source of the humidity indoors in the first place. When outdoor humidity falls, indoor humidity will fall in lockstep. Showers, cooking etc have a very small, and temporary effect on indoor humidity.

Similarly, improving the envelope; windows, walls doors, attic etc will slow down the transfer of latent heat (humidity), but not arrest it.

In the end, a dehumidifier is what is needed.

8

Not sure what your outdoor temperatures are like, but if I assume they are cool (30-50°F) and at the stated 70-80% RH, by the time you heat that air up to 72°F or so, your RH should be down to around 30%.

If your measured indoor RH is 70-90%, there is A LOT of moisture coming from somewhere. With indoor RH that high you are definately going to have the mold/mildew problems you’re reporting.

Although showering and cooking are periodic contributers, I suspect your gas space heater may be the culprit… If it is not vented to the outside (one of the ventless models) it could be the source of your moisture. Water vapor is one of the components left for the burning of natural gas. And it’s probably safe to assume it is running for far more of the day than the shower or stove.

9

Err… Nope.
I think you have misunderstood what we are discussing.We’re talking about purging the added humidity by running a shower, cooking, etc. which can be up to 100% RH, wherever the make-up air comes from, it’s humidity will be significantly less. Indoor humidity is always a direct function of water use, cooking, occupancy, etc.
Finally, the OP has already stated that improving the building envelope is not an option.

10

The heating system I have is, hrm, I don’t know what it’s called. Basically it burns gas to heat a metal radiator thing, which then heats the air around it. It’s not central air or anything.

I wish I could install an exhaust fan, but I have casement windows (I think), which open with a hinge along one edge, and so I don’t think I could install anything and still have the window be able to close at all.

Unfortunately, running the dehumidifier 24/7 isn’t an option for me, as my landlady told me to stop when I was running the dehumidifier even half that amount. She claims that none of the previous tenants had problems with the humidity or mold, so I should be able to manage it by just ventilating the place well-- do you think this is even a reasonable claim?

11

The outdoor air temperatures are usually in the range of 60-80°F, so, basically room temperature. I haven’t run the gas heater yet this season, because it hasn’t gotten cold enough, so it’s not going to be the problem, except for whatever amount the pilot light contributes.

12

If that’s the case, just remove the glass windows completely! :smiley:

13

Err…yep.

You’ve got that wrong I’m afraid.

14

Nope. What part do you not understand?

15

What the OP is describing is almost certainly humidity from the outside. It is an incorrect statement that “Indoor humidity is always a direct function of water use, cooking, occupancy, etc.”

Water use, showers and cooking are a small and incidental part of indoor humidity. The overwhelming amount of indoor humidity is from infiltration from the outdoor environment. An A/C system does a significant amount of it’s job as a dehumidifier. It removes sensible heat (temperature) and discharges it outside via the condenser, and removes latent heat (humidity) through condensation. In both instances it is removing sensible/latent heat which has entered the structure from outside.

At any rate, the OP opens by saying “Water condenses on the inside of my windows at night and in the mornings when it gets cool enough outside, which it has, and there are issues with mold starting to grow in various places” and ends with “The other weird thing is that back when I was using the dehumidifier, I could get the relative humidity down from 80 or 90% to 50 to 60%, but within a couple of hours of turning the dehumidifier off the relative humidity would be back up to 80 or 90%”.

That describes infiltration from the outdoor environment. An exhaust fan helps remove humidity from showering or cooking, sure. But that’s not what the OP is describing. Not at all.

This apartment needs a dehumidifier, or an A/C which is a dehumidifier as well.

16

Would the fact that I’m living in a small space (basically a two-car garage) affect the assertion that cooking, showering, and so on would not have much of an effect compared to outdoor humidity?

17

No, not really, and here’s why…

You have a smaller place, sure, but that probably also means you’re living there alone and producing less water vapor yourself. In other words, it’s relative.

Whether it’s temperature, or humidity, nature always wants to equalize. If it’s 45° outside, indoors wants to be 45°. Insulation in your walls only slows the transfer of heat; it can’t stop it. So (via a thermostat) you need to periodically “add” Btus (heat) to the indoors to replace the heat you’re losing to the outside.

But the transfer of heat and humidity through your walls and ceilings is a two way street. In the summer the heat wants in; in the winter the heat wants out.

The same goes with humidity. Nature will want to equalize. If you temporarily “manufacturer” humidity through a shower or boiling water, your place will be more humid. If the indoor humidity is higher than the outside, it will transfer through your walls, and the outdoors will “accept” the water vapor you made.

However, in the short term it may condense on the windows etc. That’s why exhaust fans are helpful: if the indoor humidity is higher than the outdoors and exhaust fan can rapidly transfer the humidity outdoors.

But think about it: How often is a pot of water boiling on the stove? How long is your shower? Really, how many cubic feet of water vapor do you think you can “manufacturer”? Do you manufacturer it continuously? Of course not.

But right outside your front door there are millions and millions of cubic feet of air, and if it is humid out side it represents an unending supply (as long as the weather remains humid) of water vapor. And the only thing between the outdoor humidity and the indoor humidity is the quality of your walls and ceilings.

And so that humidity is continuously infiltrating your walls into your living space. And…just as a thermostat senses heat that is transferred outside and kicks on your furnace to replace the Btus lost to the environment, a humidistat in a dehumidifier senses the presence of humidity that has been transferred from the outside, in and kicks in the dehumidifier to turn the water vapor into water.

Think about this: when the dehumidifier reduced the humidity from 90% to 50%-------and after you turned it off the humidity shot back up to 90%; where did that “new” humidity come from?

Were you taking a long shower? Boiling water? That new humidity had to come from somewhere.

If you live in a humid environment you can expect the indoor humidity to be the same as the outdoor humidity as long as you do not have an A/C system (which is fundamentally a dehumidifier) or a dehumidifier.

Nature will not allow you to have a lower humidity than the outdoor humidity.

18

Heres another way to think about it…

The gist of your problem (and solution) is where is the humidity coming from?

I would expect the humidity to briefly and temporarily rise when you shower or cook. That’s why exhaust fans are installed and used. Turn on the exhaust fan for 10 minutes and voila! problem solved.

However…if you get a reliable hygrometer you can check it when you’re not cooking or showering. (an hour or so after showering or cooking, or 15 minutes after running an exhaust fan) If you check with a hygrometer make sure the exhaust fans are off, and have been off a while.

****If the humidity indoors is substantially higher than the humidity outdoors the source of the humidity is something going on indoors.

*****If the humidity indoors is the same as outdoors—or close to it-----the source of the humidity is the weather.

Clear as mud? I thought so.

19

One final thought…

Your land lady is right when she says, ‘that [you] can solve the problem completely just by ventilating the place enough’ ONLY IF the outdoor humidity is lower than the indoor humidity. If it’s the same, you’re simply moving humidity around. If it’s higher, it just gets worse.

In post #3 you said outdoor humidity is typically around 70-80%. That would encourage mold growth, and without some mechanical equipment (read: dehumidifier or A/C) to remove that humidity you can expect the indoor humidity to be 70-80%. The same as outdoors.

Humidity, like temperature, will get in. You can’t stop it. Even if you had 20" insulated walls it will still get in. The 20" wall could/would only slow it down.

You want lower humidity?:

  1. Change the weather; essentially lower the humidity outside.

if you can’t…

  1. Get a dehumidifier.