In our case it's something like five tons of cellulose. It's supposedly a good buffer against summer heat getting into the house but it doesn't really store heat to release it slowly when the temperature goes down again. I can imagine it works well high up north but here I think it would make it more difficult to control the temperature. It's one of the reasons why we have chosen a wood-based construction using I-joints. We do have a concrete foundation slab, screed everywhere and ceramic tiles on the ground floor.

The big stratified water tank is a well known concept but you need to have room for such a big tank. Here in Belgium space is at a premium so houses tend to be on the small side. Solar thermal panels are often connected to a tank of 500 litres or less, simply because of space constraints...

There are several possible construction models; it's a matter of selecting one that fits your local climate and your habits. Not everybody likes a stove for example.

The last couple of years we've seen a shift happening from applying the passive house concepts on new houses (pretty much a solved problem) to renovating existing ones. Here in Belgium there's a massive amount of old houses constructed before the seventies/eighties which is when they finally started putting in insulation. Now that all new houses are energy efficient it's time to tackle the massive waste in those old houses. Most people buy such an old house and start renovating so there's a lot to be gained.