Insulation on new floor - above or below concrete slab?

[Grandad Gaz]

Just started building an extension out back and I can't decide whether to put the celotex under the concrete slab or above. Both methods are acceptable by building control.

If I put it below the slab, that will make life easier running the pipe work around, and I just lay a 75mm screed on top. Downside doing it this way, it will mean I shall be heating up the slab as well as the rest of the house.

Help, what's the best way?

[TA14]

Insulation under the slab. No that much heat will go down, you can put a break layer in or just adjust heating times to come on earlier and finish earlier if you think that it will have much effect.

[Spudler]

Concrete, insulation, screed.

[Grandad Gaz]

Thanks. Two condradicting views already. No wonder I can't decide myself!

[Drumroll]

Insulation, then concrete. but make sure you get the right density of insulation or it won't support the concrete. No need for screed if you do it that way.

[Andehh]

Makes sense sence to have the insulation as close to the floor covering as you can. No point keeping a massive slab of concrete warm is there!

I'm also for the concrete, insulation then screed. I dont see the logic of doing it any other way.

[C Lee Farquar]

We have insulation under the slab as we wanted to use the thermal mass of the concrete. IIRC a cubic metre of concrete is equivalent to 200 litres in a thermal store.

We also didn't want a layer of screed.

[Busa mav]

My stock detail has insulation above the slab.

[Grandad Gaz]

Due to massive difference in levels between inside and outside, we have decided to go with beam and block!

Either that or we will need about 30 tons of hardcore to bring it up to the required floor level. Not to mention the amount work that goes with it.

[ATTAK Z]

Don't forget to ventilate beneath the beam and block

[Grandad Gaz]

ATTAK Z said:

Don't forget to ventilate beneath the beam and block

No, have not forgotten I would have had to extend the vents from the existing suspended wooden floor anyway.

How many will I need, roughly? Every couple of metres?

Thanks

[ATTAK Z]

Grandad Gaz said:

ATTAK Z said:

Don't forget to ventilate beneath the beam and block

No, have not forgotten I would have had to extend the vents from the existing suspended wooden floor anyway.

How many will I need, roughly? Every couple of metres?

Thanks

1800 ctrs should be OK

[astroarcadia]

Spudler said:

Concrete, insulation, screed.

This.

[astroarcadia]

Spudler said:

Concrete, insulation, screed.

This.

[Grandad Gaz]

ATTAK Z said:

Grandad Gaz said:

ATTAK Z said:

Don't forget to ventilate beneath the beam and block

No, have not forgotten I would have had to extend the vents from the existing suspended wooden floor anyway.

How many will I need, roughly? Every couple of metres?

Thanks

1800 ctrs should be OK

Thanks!