[torginus]

Isn't it the other way around? 50% humidity means that air contains 50% as much moisture as it, at a given temperature. Raising the temperature means that the air can now hold much more moisture.

Bringing in cold air at 50% humidity, then warming it up to room temp makes the humidity fall, leading to dryer air indoors than comfortable.

[Notatheist]

Yep. Humidity will tank to 25% here in winter. I have two humidifiers fighting the HRV continuously when it gets cold. As I understand it an ERV controls moisture as well, but such a module for my system costs over $4000.

[littlestymaar]

But household activities like cooking, showering, drying laundry, or even just washing the dishes etc. generate tons of moisture, and this moisture is the reason why your home has ventilation in the first place: to get it out and avoid mold!

There's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to such way too much air out of your house than needed during the cold days (and it also tend to be designed to suck cold air into dry room first, and get out from wet rooms, when you want it the other way round when it's very cold).

[crazygringo]

Not in the winter though!

In the hot and humid summer you're definitely trying to reduce indoor humidity.

But in the winter when it's bone-dry? A hot shower barely makes a difference.

I keep two humidifiers running all winter long just to bring indoor humidity up to 35% or 40% where it's healthy.

Otherwise it often goes down to 15% or even 10% on cold winter days, which is terribly unhealthy.

[littlestymaar]

> Not in the winter though!

> In the hot and humid summer you're definitely trying to reduce indoor humidity.

No, you can't do that with ventilation when it's hotter outside than inside actually, that's not how thermodynamics works! But we don't care about that, because in the summer you don't have cold walls or window where water vapor can condense and let mold grow.

> But in the winter when it's bone-dry? A hot shower barely makes a difference.

The reason why it doesn't make a difference is because all the moisture is vented away by your ventilation system! And that's because that's what it's designed to do! Stop it and see how it goes! For the record a single wet sponge drying up in your kitchen is enough to raise humidity by 10%! You barely need 2cL of water per cubic meter to have 50% humidity at 20°C.

As I said the problem is that in winter, ventilation system often ventilate way too much.

Also, they are often designed so the cold and dry air enters in the bedrooms/living room and the warm/moist air is extracted in the kitchen and the bathroom, and because of that the rest of the house doesn't get any of the excess moisture of these places. This is done because the designers wanted to make sure that the humidity level never raise too much in the room, because again humidity will ruin your house and health pretty quick (having air that's too dry isn't very good for your lungs, but having fungi spores in the air is much worse!)

[quietbritishjim]

I think you are agreeing with the parent comment with a tone of disagreement.

They say they let cold air in during the winter because they want to lower the humidity.

Then you say that, if you let cold air in (and then let it heat back up again) then you end up with lower humidity.

The only thing you might disagree on is exactly what humidity you would like inside your house. But that's subjective. (As it happens, I agree with them: I often find it too humid inside during the winter, because I've restricted airflow to keep the heat in.)