There is no need what so ever to waste alot of money insulating beneath a hollow core ceiling, this expanse is not part of your building envelope, this being the four walls, roof and floor.
Indeed you WANT to be heating the hollow core as this is a great source of thermal mass and will act as a giant radiator when the heating is off and help to keep a comfortable temperature during non heated periods.
I too have hollowcore and believe me it is exceptional, even during the recent very cold spell because of the thermal mass of the walls and hollow core, it was like a summers evening in July throughout the nights.
If you want to spend money, spend it in the right places and ensure you get the best cavity/ floor/ rafter/ attic insulation possible. Slabbing the hollowcore in anything other than regular half inch plasterboard is actually counter productive.
Do a search on Thermal mass to fully appreciate its benefits in construction and you will see what I mean.
.....Hollowcore rests on the walls of the building, out past your internal insulation(if you use it), and past the inner leaf, into the cavity. As such hollowcore is one of the finest examples of cold bridges you can buy....and can deliver it all nicely right into the deepest recess' of your build. And remember, as thermal store, you've pumped huge energy into it. Why do this, when you only need to heat the space for the people who live in it - why heat the fabric of the building at all ? As much as it jars, the Laws of Conservation of Energy still apply, and with every step in the heat chain, there are losses. Imho, you'll find a more moderate system is more likely to respond effectively. And the single biggest reason people use hollowcore is the (perceived) benefit of sound. But that is only because, to quote the joke, 'thats the way we always do things around here...'. There are ways to get the benefits, using all kinds of materials.
+1 on Legs akimbo post..from my research and understanding thats the way to go....
Plus if you are putting in a metalled ceiling, the void between the plasterboard and the slab acts as an insulator anyways I believe (air being a great insulator, so I'm told....)
Actually it's not. Hollowcore is not air tight, and I can guarantee you the connection of it to the wall isn't, either. So the heat you will put up there (and you will...) will leak from the building. And plasterboard is not a (measurably) effective insulator. If it was, people who dry-line wouldn't need to put insulation behind it.......
Make sure you have the loading screed properly detailed at the edges.
Enjoy the utterly wonderful sound insulation and sense of enclosure you get when its finished
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you don't have to have hollowcore to have effective sound isolation. You can have both concrete screed over non-concrete decks, and non-screed decks that are equally effective, without the expense of hollowcore, and the added expense of the support it requires from walls and foundations as well.
Thermally isolating the upper floors fom the lower seems to follow on the "efficient heating controls" manta promoted by DOE/ governmen.
"New thinking" in Part L started this isolationist relationship with the upper floors/bedroom areas which has led to problems in some private houses and apartments.
I agree that weat should stay within the outer envelope, but cutting off heating in some rooms within that envelope can lead to problems.
Only if each room sealed from the others in terms of both heat and moisture does this make sense but then how can you move around the house?
When warm moist air circulates from hotter areas like bathrooms and kitchens, it finds colder surfaces in these rooms and condenses to for damp spots on any cold bridges..
I don't think anyone advocates cutting off heat in some rooms. Different temperatures, yes, but that's not a bad idea, especially for bedrooms vs living rooms. Issues from bathrooms, utilities and kitchens should be addressed through proper effective ventilation.
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Why is it not possible with excellent design and workmanship to contemplate a traditional concrete cavity wall house with absolutely no cold bridges ?
Depends on the building design you have to work with. If structure requires it, structure will trump it every time - ask your engineer.........