Lowering RH in house, mission impossible so far...

Hi guys,

First of all, merry Christmas (a bit late) and a happy new year (a bit early)!

So as the title says, I have been experiencing significant RH problems with my condo unit. Since I moved in 2 years ago, the RH has never really dropped below 60% but lately, it is staying in the 70-72% range which gets me worried about mold and other health problems.

Of course I tried everything already mentioned on the internet, that is, mainly:

-Running the range hood fan when cooking (believe me, a 800CFM fan is PLENTY of power)
-Running the bathroom fan about 10 minutes before and 1 hour after I take a shower (I take very quick showers, usually within 5 minutes)
-I do not create excessive moisture by running tea kettles, humidifiers, etc
-I have literally 0 plants/trees, etc..

Some interesting info. I did some testing, for the RH to go below 70% (around 65%), I need to crank the living room's baseboard heaters AND my natural gas fireplace until the ambient temp is around 26C... Ridiculous. Its gonna cost a fortune in energy and its really uncomfortable..

At 17C (the night/away setting), the RH is around 71-72%.. So a raise of 9C yields to a drop in RH of only 7%... According to my understanding of the psychometric chart, its too little.. This is when someone suggested to check for water leaks (in walls, under cabinets, etc..) Couldn't find any. Anyways, if there were a leak so big to create RH problems, I'd expect to see some leak spots or marks somewhere...

Finally, a bit more info on the condo setting:

• The building consists in 4 units in a 2x2 arrangement. I am one of the two top units.
• The neighbor downstairs has mentioned that his air exchanger has a RH control allowing to run the exchanger based on the desired RH in the house. He sets the control to 55%, the exchanger apparently runs 24/7. That indicated a problem to be. Other than that, if the RH outside is 75+% he will only let moisture in his condo, maybe that doesn't help me either..
• This downstairs neighbor often has condensation problems in his windows. As a matter of fact, I noticed that out of the 4 units of the building here, I was the only one without condensation problems on my windows and my RH is around 70%!!!. The neighbor downstairs also often has heavy humidity in his condo because they take long /hot showers, his wife cooks all the time, etc. Every time I go there, I feel the moisture when I walk in.. Its almost like a sauna..
• The neighbor next door (the other upper unit) is very often away and there;s nobody in his condo. The other owners, we all suspect that he drops the heating set-point in order to save money while away. Not sure yet, but when he comes back, I will ask him at which temperature he keeps his baseboards heaters while away...

On the 24-25-26 of December, I was away for Christmas eve. When I came back on the 26, the ambient temperature in the condo was around 18C while I had put the thermostats to 16C meaning my condo was heated by the neighbors (or maybe the fireplace just ran for an hour?) I dunno.. The RH was also pretty low to 52%. That confuses me. Could I be releasing so much humidity? I mean my body and sweat/breathing?

Also, the neighbor downstairs was here on the 25-26. If he would be causing this, why would the RH be so low in my unit?

So as you can see, I am lost. Too much info, no consistent trend. I am hoping someone can shed a bit of light on this before I run into mold or worst health problems...

I forgot to talk about the dehumidifier. I have a small portable dehumidifier. I tried running it last fall but it managed to lower the RH only by a few percents nothing major. I am not too excited by the idea of running a dehumidifier 365 days a year. To me its like a band-aid more than a real solution.

Some questions:
-Could the neighbors be really causing this problem?
-Could the building be so badly constructed that the moisture comes from outside? Building built in 2005, has vapor barriers on all outside surfaces (walls, ceiling, etc). And there is a floating concrete slab between me and the guy downstairs.
-Are there any inline moisture remover equipment I could install on my air exchanger ? My unit is a Venmar EA1500 and does not offer any moisture removal or heat recovery.. Very basic unit. Perhaps installing such add-on equipment would dehumidify the air going through the exchanger so putting it in recirculation mode would in fact dehumidify the whole house..

Thanks and happy holidays!

Rent several or one dehumidifier(s),
put them in several rooms or it in one room at a time,
monitor their or its runtime(s) and
therefore determine the magnitude of your humidity problem and, perhaps, its source.

At least in principle!


An added advantage is that this will also be a stress test for each your breakers and branch circuits.

Max RH for no condensation on glass
outdoor temp in F| single pane| double pane
40 39 59
30 29 50
20 21 43
10 15 36
0 10 30

so for 30F outside and double pane, condensing on the inside surface means more than 50% RH.

According to your numbers, extrapolating for in between 20F & 10F, I get around 41% for no condensing on windows (outdoor temp is 18F here). The RH in the house is around 64% right now and I see next to no condensation anywhere (maybe on the patio door, in one corner, a little fog the size of a dime)..

Since the beginning, I have the impression that there are no specific source for the humidity, but its coming from everywhere...

I will measure the RH now that the entire house is at the same temp and see which room(s) are higher or lower..

condoowner said:

I get around 41% for no condensing on windows (outdoor temp is 18F here).
The RH in the house is around 64% right now and I see next to no condensation anywhere

Click to expand...

I don't see a reason for anyone to have spun my data either way.

Get more opinions? Get ASHRAE's opinion? Maybe confounding factors involved? Wide tolerance band on my numbers due to alternative methods of making windows?



We might all learn something on this thread or this could devolve into a game of "Why don't you - Yes, but."
Stay tuned!

You also have to consider the RH outside at the same time as you are checking inside. Any air being exhausted is being replaced with outside air. You also have to consider dew point because that changes how you feel. With standard vents and fans all you can expect is to match outside humidity. If I understud I might explain it but??
The other thought I had about your neighbors humidity, if you don't have enough fresh air intake you might be sucking humidiy thru some internal leakage in the building and maybe when the other neighbor is home he is sucking some of that moisture into his condo, lowering yours.
Understanding how and why you set a humidistat may help you understand some of this https://www.google.ca/#q=setting+a+humidistat
This page has a chart of humidity and dew point in Montreal http://weatherspark.com/averages/27985/Montreal-Quebec-Canada

To close this topic...

Turns out, my Venmar air conditioner was not recirculating air from inside 24/7 but was exchanging with the outside! A defective internal louver was to blame. I discovered this when I happened to be right underneath a ceiling register on my stepladder and could feel crispy cold air coming out of the register.. Then a technician's visit confirmed the internal louver of the air exchanger was stuck on "exchange" mode..

Good to know!!

Thanks for the update and glad to here you have it figured out.

Thanks for the follow up, it's always good to know what the cure was to a problem.