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Comment by upofadown | Hacker News Reader

People tend to confuse the passivhaus idea with other energy efficient methods. The passivhaus idea is that if the building has sufficient insulation and is air tight, you can use the air from the heat recovery ventilation system to do any required heating cooling. This simplifies the mechanical part of the system.

As a result, such houses are very well ventilated when it is cold outside. Mould is not a problem. The potential problem is instead excess dryness in places where it gets very cold.

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The dryness problem normally only occurs if the air change rate is higher than the recommended 0.3. In large buildings this requires rather sophisticated ventilation systems. There are also heat exchangers that recover humidity.

In fact I found that it seems ERV (recovers heat & humidity) is more common than HRV (recovers only heat) these days. Possibly for the simple reason that ERV suffers less from the exhaust icing up.

I can confirm the excessive dryness problem. My gf moves to a passivhaus a few years ago, and I had real trouble with cry eyes during the night. After a year, I sort of adjusted, but it definitely doesnt feel very healthy.

> I can confirm the excessive dryness problem.

You just need to cook more pasta: large pots of boiling water. :)

More seriously: a very airtight house, which is generally desirable, does need a properly balance HVAC system. In some climate zones this means perhaps having a humidifier and in other zones a dehumidifier.

Most people only think about furnaces and air conditioners when considering HVAC, but those mostly deal with temperature. Humidity has to be considered separately, often by different equipment.

I am guessing in many cases, there is no money left for humidifiers. You start out with the idea that you want to build a passivhaus. But you still have a limit on how much you can spend. So all the ventilation stuff is being installed, and the humidity control is being skipped. Which leads to each and every individual apartment considering their own humidifier, which probably pretty much kills the energy-efficientcy-idea that started the whole thing. At least that is what I observed in my limited small world. Humidity below 30% is cruelty.

it becomes dry inside no matter how you ventilate or heat, if it happens through a vent system with heat exchanger, there are built-in humidifiers for these

There is a second problem in cold places that comes from the humidity you add on the air intake. The heat exchanger has to deal with that moisture freezing on the exhaust side. There are methods for doing this but none of them are ideal.

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