Out of curiosity, what is the effect of this thick insulation on indoor CO₂ levels? Do these houses usually have ventilation systems that can compensate?

Yes you need active ventilation systems, because the thermal efficiency is highly dependant on the house being airtight - to the point where efficiency scores are calculated by overpressuring the house (basically taping a huge fan into the front door with all other doors and windows shut) and measuring the pressure loss. When applying insulation, contractors have to meticulously use foam to seal everything up. It's a major part of the work when building a new house and just a few mistakes can seriously affect thermal performance.

Our house (with a ground based heat pump) uses active ventilation with heat exchangers (and pollen filters) and as a result we have better indoor air than we've had in rented flats in all years before.

Usually heat exchanger ventilations. Fresh air comes in and exchanges heat with used air coming out, so you get fresh air but with the indoor temperature.

HRV (Heat Recovery Ventilator) systems are really cool, too. They solve the ages old problem of bringing fresh air into a home without losing all of the temperature and humidity from the internal environment.

They are designed for a number of air changes per hour/day which would be analogous I guess. I can't remember the number but a Google will tell you, it's different depending on the kind of environment (office/home etc)

We have insulation thickness standards in the US, varying by weather "zone", but for residential construction they seem to be lightly enforced, and efficiency tests are rare to non-existant. (For an illustration of how we typically insulate, see [0].)

It's only one data point, but I'm in New England and from my home office I've watched builders replace 3 1960's-era homes with large colonials. Each is insulated with fiberglass, no visible foam, and nothing outside the sheathing (exterior plywood) except vapor paper / tyvek.

Sure, it meets code requirements for inches of insulation, but it's far from air tight or efficient.

[0] https://www.energy.gov/energysaver/where-insulate-home

The isolation is made to keep the house hot. About 10 degrees hotter then the outside (without the heater on). This is usually not a problem as temperatures are around 15 during the summer and you can open a few windows to let the hot air out. It does become a problem however when we get extreme temperatures like +30 C as you cant lower the temp by opening a window. Compared to south europe and other warm climates where the houses are built to stay cool.

Glass helps trap solar heat, but other than that any form of insulation keeps heat from entering and escaping.

This is how your thermos knows wether to keep your drink hot or cold.

So except for the sun shining through windows, insulation keeps heat out of your house.

new product idea: black plate (same as Northern roofs) thermos with glass windows. Would only work if you wanted to keep the drink hot though.

As far as I know, Insulation should keep the house cool in summer, because it keeps the heat out.

Of course, once the heat is in, you need a way to get it out (at night, through windows).

Unlike a greenhouse, insulation is not directional.

It basically becomes a greenhouse if you have windows, and of course the windows have three layers which prevents the precious warm air to leak out. Also the roof is black metal. You basically don't need to have the heater on if it's a sunny winter day - because the heating from the sun.

The biggest problem though is the dryness during the winter, cold air comes in and expands, lowering the relative humidity, warm moisturous air is pushed out by mechanical fans. You get extreme dryness, lower then in the desert, which is not good for mucus membranes. I believe that if we could revolutionise air conditioning systems we could prevent the next pandemic (and maybe help the global climate).

If you are only worried about natural cooling, insulation is not the answer. You want a poor insulated house with a lot of shade, and large open windows for the wind to blow through.

Most people these days want more than natural cooling. As soon as you decide to have an AC system installed more insulation is better for the reasons you state.

No. You never want poor insulation. Poor insulation means that the indoor temperature will always match outdoor temperature. There's not such thing as "natural cooling", only thermodynamics.

Natural cooling is about sweat (water evaporation), and staying in the shade. Lots of wind helps.

It depends on the insulation type. If you're using mass as insulation, it can keep the house very cool. This is a common effect in stone houses and barns as they stay closer to the average daily/weekly tempature because it takes a long time for the tempature of that much mass to change.

Mass ≠ insulation. You can have mass as a buffer and regulator, but normally stone or brick and mortar houses simply shift the issues to later in the hot season. It is usually a more even temperature over any given timespan though, as you say.

I'd think barns would have very low mass in comparison to their size, but would stay close to surrounding temp by way of having very little insulation and being very "drafty"

Yes, they aren't the same but can be applied to a similar end goal.

It doesn't completely shift the issue to later in the season. Yes, it will shift some of it, but you still have the night/day average effect. This can still be beneficial, especially if in a shady location.

Barns can be drafty if you are talking about wooden barns. Stone barns, or stone foundation barns built into a hillside, can actually be cooler inside than outside during the heat of the day. That's part of why horse owner sometimes leave their animals in the barn during the day and turn them out at night (typical summer schedule).