# Insulated slab ceiling on hollowcore



## thepool100 (17 Jan 2010)

am stuck in two minds would appreciate users feedback/opinions on what options people took or recommend?

two storey house with hollowcore going with rads up and down

option 1 - single foil

option 2 -insulated plasterboard 37 mill

by going with option 1 am i just heating the hollowcore will this be a waste of money on fuel heating this area huge slab? or will there be any difference noticed by going withe cheaper option of single foiled between upstairs and downstairs?


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## galwaytt (20 Jan 2010)

..insulated slab is better, as yes, otherwise you're just heating the hollowcore.    For the same reason, you'll have insulate the floors OVER the hollowcore, for the same reason.


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## Lak (20 Jan 2010)

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.


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## TripMeUp (21 Jan 2010)

+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....)


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## onq (21 Jan 2010)

thepool100 said:


> am stuck in two minds would appreciate users feedback/opinions on what options people took or recommend?
> 
> two storey house with hollowcore going with rads up and down
> 
> ...



Hi  *thepool100*

+1 what others have said re the slab as heat store.

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 

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.

You can enter a circular argument about how there should be no cold bridges, but its only recently with the advent of later generation timber frames that a no-cold-bridge construction can even be comtemplated.  Your measure would tend to make the position more severe. When the whole room is cold, moisture migration and condensation becomes an issue regardless. In the meantime the recieved wisdom is to allow a low level of heat persist even in unused rooms in older houses to help deal wirh damp.

[broken link removed]

Not connected - Eamon Markey has advised me on several projects.

HTH

ONQ.


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## LouisCribben (22 Jan 2010)

onq said:


> You can enter a circular argument about how there should be no cold bridges, but its only recently with the advent of later generation timber frames that a no-cold-bridge construction can even be comtemplated.


 

I find this comment very interesting and I'm sure its true, even if I dont understand why.

Why is it not possible with excellent design and workmanship to contemplate a traditional concrete cavity wall house with absolutely no cold bridges ?


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## onq (26 Jan 2010)

LouisCribben said:


> I find this comment very interesting and I'm sure its true, even if I dont understand why.
> 
> Why is it not possible with excellent design and workmanship to contemplate a traditional concrete cavity wall house with absolutely no cold bridges ?



Do you know, I'm not sure - it just popped into my head and I posted it. Weird.

I suppose its the coldness around the opes I grew up with in traditional cavity walling.

The details/seals/fireseals around the opes/frames junctions tended to cause problems then.

I suppose if you could engineer a window with the same u-value as the wall you'd be on the way to achieving it.

It IS an odd thing to say, especially as the last large house I did achieved an A3 rating and has no evident cold bridges... 

Mind you its a dormer bungalow, which means its three fifths roof and although the ground floor walls are cavities, they're 415mm with 100mm full fill cavity insulation.

ONQ.

[broken link removed]


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## galwaytt (28 Jan 2010)

legs-akimbo said:


> 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.



TripMeUp said:


> +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.......



onq said:


> 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 .


 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.



onq said:


> 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.
> ...


 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.

[/quote]



LouisCribben said:


> 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.........


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## onq (28 Jan 2010)

galwaytt said:


> .....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.



I think we may be at cross purposes here GalwayTT.

You may be correct in terms of some newer forms of construction, but not in terms of traditional build with insulated cavity and 150-200mmPCRC Hollow Core units according to details I have seen installed.

The edge of the hollow core rests circa 100mm in on the faces of 215mm solids on the flat, sometimes - depending on engineer's spec., load, and span with a cage laid along the outer 100mm of the block with rebar projecting into the loading screed all bounded in temporary timber formwork placed along the outer edge of the block facing the cavity.

The slab + edge beam detail lines up flush with the vertical outer face of the inner leaf - it doesn't project into the cavity nor does it bridge it.

The loading screed seals the edge and top of the floor in monolithic concrete construction, which may drip a little between unsealed  edges of the PCRCHC units when wet. This screed seals the top and the edge beam detail seals the hollow cores from the cavity. A relatively dry mix fills the cores to its angle if repose - I've never seen them sealed with formwork although some products used have insulation in them.

In traditional houses the cavity wall will have insulation placed against the outer face of the inner leaf - this will proceed past the face of the edge beam as described above as it lines up with the inner leaf - there is no cold bridge per se.

In more modern solutions options including having the entire cavity filled with insulation and the outer face of the outer wall can be externally insulated. In neither case does the PCRCHC unit cause a cold bridge AFAICS.

I'm happy to stand corrected if you can point to a detail. 

Finally, you state:



> 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.



Always willing to learn and I'd be interested to see some examples of the details you mentioned and you might post links here or PM me instead if you prefer.

TIA if you can deliver and not to worry if not.

ONQ.

[broken link removed]


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