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Comment by sweettea | Hacker News Reader

Not discussed: hot water in houses increases humidity. In modern well sealed houses, that can result in unfortunate mold growth.

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From the article:

> Extra moisture in the air might cause problems with things like mold. Use your judgment. It’s probably fine as long as things don’t stay damp all the time. For many, extra humidity in the air would be welcome. (Maybe you can stop using that ultrasonic humidifier.)

I've always experienced extremely low humidity in winter months, so I would think the net effect might be a reasonable positive if this saves me needing to run a humidifier. Am I missing some detail? Or is that just unusual?

You’re in the uk?

No, but I was given to understand that winter => cold => dry air was driven by the basic physical properties, so I would be very curious if it was different there. Is it different in the UK, and if so, why?

The UK climate has moderate temperatures and a lot of rain. Excess humidity is a common problem here, and excessive dryness is rare.

It tends to often be humid here, even in winter.

Right now, it’s 81% humidity. Our laundry has been taking two days to air dry.

Cold air can hold much less water (hence why it's relative humidity). This is good if you take in cold air and heat it to 21°C or so because the air you end up with will have very low humidity, but if the outside is a drizzly 10°C and you are only heating to some 18°C, it can be a problem.

someone else can explain better than me, but.. humidity is lower when it's cold but relative humidity is higher. Being outside in the uk when it's cold is often like being inside a cloud, with 100% relative humidity levels. Combine this with the old, badly insulated, difficult to heat housing stock, and you can guarantee that there is one part of the house (a north facing wall, say) that is at or dangerously close to the dew point for some part of the day/night. You wouldn't want to leave a steaming bath for longer than necessary.

It depends massively on your climate and how air in your house is managed.

If you live somewhere which is significantly colder outside than in (e.g. 0-5C out, 21C inside) heating outside air to comfortable temperatures will greatly decrease humidity. Couple this with a system which circulates air, or even exchanges inside&outside air and you will have incredibly low humidity inside.

I live somewhere which often sees 10-15C @ 80% humidity over winter, and the only air circulation in my house is from opening doors/windows. 70% internal humidity @21C is incredibly common. It takes a bit of moisture management to keep it below 60%.

Ventilation with heat recovery sounds like it could work in those conditions, but just barely. 80% @ 15C would turn into 55% @ 21C.

Unfortunately, very few homes in the UK have any air ducting. I'm looking at installing a MHRV system at the moment, but planning the ducts is a nightmare. Brick walls, lintels and structural joists everywhere with no headroom between floor and ceiling.

We absolutely should be doing it in new builds though, and we're not.

New build flats have some air ducting. At least, mine did in two rooms (it wasn't for AC). But maybe you meant new build houses.

That sounds rough. Consider putting in air to air mini-splits instead, and leaving window cracked open, etc. As a bonus, each room will get its own thermostat, and cooling / heating.

The mini-split will only require coolant lines, and will have a sufficiently high coefficient of power that this + whatever ventilation exists might use less energy than heating with natural gas (or resistive electric), even with the loss of efficiency from not using heat exchangers.

(Doing both things would be much better, of course. Retrofitting is hard.)

Define “massively.” Sounds like a Reddit-esque waffle word like “wildly” or “dramatically.”

Moisture is the #1 destroyer of buildings in its various forms.

https://www.buildingscience.com/bookstore/books/moisture-con...

Moisture control and mitigation is a very important part of building to last, and building to minimize maintenance and energy usage (simplistic "insulate everything and seal all air leaks" can result in a doomed structure).

This strongly depends on the climate. In some climates like where I live, we run a humidifier in the winter to deal with the super dry winter air. It is built into the central HVAC for our modern well sealed house.

Why do you say "not discussed"? That's point #4 in the article.

It's because the house builders don't do the job properly and install MVHR, which not only removes the excess humidity, but also recovers most of the energy of vaporisation.

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