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Comment by paiute | Hacker News Reader

Those dehumidifiers make these places uninhabitable for me. Constant low humidity is really unhealthy. Without them these ultra insulated homes have major vapor and mold issues. Being highly insulated is good but at a point it's just silly. I'd rather see efforts to insulate old trailers, manufactured homes, etc. Rich people experimenting with zero energy homes don't need a credit.

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I agree on low humidity being bad, but your remark on insulation does not seem correct. I'm only familiar with cold climates, so perhaps I'm overlooking something, but mold needs moisture to grow, so it will appear where water condenses out of warm air on a cold surface. You can typically see this in windows.

So you need to prevent cold surfaces. The best way to do that is improving the insulation. The reason you can see water drops in the windows is precisely because they are inadequately insulated (e.g. only double paned).

Now, it's true that if you make an airtight home, then you need some kind of air exchange. And if it's really cold outside, then even if it's wet, once that air is warmed up, it ends up at a low relative humidity. In that case you need a humidifier. I guess depending on the outside air, you'd probably need both to stay in a comfortable range.

It's not super intuitive at first, but its cold climates were this is actually a big issue. Water vapor "wants" to equalize from a more humid to less humid environment. So when it's cold out - the humidity can drop rather low. Then you take a shower and create water vapor - which flows into the walls, through the insulation, and condenses against the colder exterior. Open a wall in any stick frame house in the rocky mountain region and mildew will be present. I am building a highly insulated house in a cold climate area and have blasted all the wood with copious amount of a silver based coating to inhibit mold growth.

This installation chart is a pretty useful starting place, regrettably local codes often require you to do the wrong thing https://www.certainteed.com/insulation/resources/do-i-need-v...

Have you investigated a combination of additional vapor permeable exterior insulation and impermeable primer for the interior walls? You may be able to minimize the amount of moisture staying in the walls and still hit your thermal goals.

Ah, I see where you are coming from. The standard solution (for > 25 years) where I live is that you install a vapor barrier (plastic) to prevent the moisture from diffusing outwards. It needs to be on the interior side of the insulation in a cold climate.

Edit: Sorry, I can see that you're aware of these. Perhaps my point is just that all standard houses where I live in cold climate uses vapor barriers. Moisture control is part of the building code. Trying to solve the problem with overly dry air does not make sense to me - the whole point of a house is making a comfortable living space.

We have a highly insulated home with a heat recovery air exchanger.

Great insulation and hermetically sealed homes increases the needs for good ventilation. You can't just build a super insulated home without great ventilation. And obviously for energy efficiency you need heat exchangers between in/out air (Alternatively as a slightly cheaper and easier to retrofit option, a heat recovery in your heating system such as an exhaust air heat pump). Maintaining good humidity in any weather also isn't very difficult.

I agree subsidizing new homes doesn't seem necessary. This is simple to enforce in building codes. Defined efficiency limits for construction and building components (e.g. requiring triple glazing and 12in insulation in all walls) would be trivial to mandate and it's as reasonable in Florida as it is in Maine.

The location of the home (Fairhope, Alabama) is in a humid subtropical climate where the average humidity is above 70% year round. I couldn't find any data but seems possible that the net average interior humidity is not actually that low?

Thats what HRVs are for. You can have a tight envelope with decent humidity with no mold.

That's part of what bothers me when they combine residential and commercial use into a single figure, and then put all the onus on making homes as energy efficient as possible while entirely overlooking one of the major functional purposes of a home: to provide comfort.

It's a daffy one-sided approach that is unsustainable.

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