Is insulation needed on 1st floor concrete slab?

galwaytt wrote: »

...going back to OP's original query though, it most certainly would. The screed needs to be insulated from the slab & walls.

Even my own house, an engineered timber joist and deck to FF, I insulated the screed and in a timber build it's in a 'better' environment to begin with.

Is it really necessary especially if you are going to insulate between the joists for sound proofing? I am using posi joists with 18mm WPB and 40-50mm easy screed.

Just my own opinion, but I would be with GalwayTT on this....

If you rested the first floor hollow-core precast concrete slabs onto your ground floor block walls and the floor slabs did in fact extend out to the cavity along the edge of the external block walls, which does happen on a lot of builds up until recently, then common sense would tell you that if you did use UFH on the first floor, with or without insulation, surely heat will leave the house through the precast concrete slabs into the cold bridge at the cavity. What did happen up until recently in a lot of new builds, was that the outer edge of the precast slabs would align up and rest on the outer edge of the inner leaf blockwork of the external walls, and then the first floor blockwork would start on top of the precast slabs. Hopefully this would not be the case for the Original Poster??? :rolleyes:

Even with using airtight membranes on the first floor the cold air in the cavity is still getting to the precast slab and hence keeping it cool/cold. Even with full fill cavity insulation, the cavity is still a cool sometimes moist space between the two blockwork leafs. You would in the long run spend a lot of money on oil/gas or whatever type fuel been used on wasting heat out through the slab at these locations.

Regards the BER, yes the BER does not really take into account the first floor heating in precast slabs etc. as the BER views this as been within the thermal envelope of the building. However if the slabs extended out to the cavity resting on the ground floor block walls, then there is a thermal bridge which would have to be calculated on the buildings envelope walls. Which will bring the U-Value in the envelope walls down. Which is not good for over BER.

Back to the original posters query.....YES DEFINITELY, you must use floor insulation on top of the concrete precast slabs. The point of the insulation is to minimise heat loss downwards into the precast slab, so yes use floor insulation on top of the precast concrete slabs to keep heat movement upwards so that you are only heating the sand/cement or liquid floor screed above the insulation to try keep as much heat into the rooms upstairs as you can. As for depth of insulation, Technically you could use 100mm insulation if you wanted, or had the height for it, but that would cost more money, with no real need for it, but in most cases 50mm is used.

In most cases it is the ground floor where you sink the money into using between 150-200mm as this is the baby where you really need to prevent cold coming up from.

For me, I am like GalwayTT on the first floor. I used 50mm liquid screed, on 50mm floor insulation, on 22mm OSB floor decking, on top of Posi open web joists designed to take the loadings. All external walls have 63mm insulated plasterboard which extend from the ceilings down to the floor OSB decking. The edge insulation to allow for expansion of the floor screed, runs around the bottom of all external and internal walls. The joists used on our house, were installed by our timber frame company and internally hung from the ground floor external walls using joist hangers, so this eliminates cold bridging to the cavity. I then insulated the open web joists on the ground floor ceilings with 150mm of wool insulation.

I've seen them cross the entire cavity and form a balcony at the other side in the good ole' days. Thankfully thing have moved on since then a bit.

Hi guys,
1. is the ground floor ceiling void better to be warm or cold?
2. On the AT, Is it better tape the ceiling plasterboard or the hollowcore, to the walls?
3. Solid ends on the external ends of the HC are an optional extra, would this be better than wrapping the ends with AT membrane?

Calahonda52 wrote: »

Nice choice of FF flooring G-tt

Can u share the following details please:
Did u use wall hangers, or as they say in Dublin Wha!
How did u deal with A/T issues at the ends and along the walls where joists are // to walls?
Thanks

All walls full height, joists on hangers between walls is std detail, no membranes.

I'll dig up a sketch.

[edit] sketch attached [/edit]

I haven't read all of this thread, but Knauf do an insulated cement particle board. Their regular cement board (12.5mm thick) is called Aquapanel and I think that the insulated version is called Aquapanel Thermal. To the best of my knowledge it is for exactly the type of application that you are looking at.