Insulation under or over slab?

[sinbad197]

Hello all,

I am about to start the foundations for a small extension, the footings are brickwork are complete up to the DPC. Having a look on my plans they show the 150mm of hardcore, 50mm sand then 70mm kingspan insulation under 150mm slab....

But having a look round I can see some people put the insulation over the slab and then screed on top of it...

I was just wondering if there is a correct was of doing this with regards to building regulations, or if this was determined by ground conditions or something else i haven't taken into account.

Given a choice I would much rather go for the insulation over the slab...

any advice would be most welcomed !

 

[freddiemercurystwin]

Either way will comply with the regs, there are loose arguments for each method, if you insulate under the slab the floor will take longer to warm up and more energy. However it will retain heat for a little longer. Obviously with the slab over the insulation you will not need a screed if the slab is power floated so a few quid can be saved here. Can't remember the last time I did insulation under the slab tbh but that's just me. Occasionally there are issues the existing building that may make one or other solution preferable.

 

[sinbad197]

Many thanks for the reply !

Think I will go for the insulation above the slab......just hoping the building inspector agrees with me.

 

[cotswoldbuilders]

If your putting insulation over slab, make sure you have glass fibre strands in the screed mix, and min 75mm deep.

 

[noseall]

Under or over makes little difference in terms of energy use and underfoot comfort etc.

However, UFH manufacturers prefer the insulation situated above the slab.

In terms of practicality or installation preferences, we have done both and would say that there is very little difference. Installing the insulation on top of the slab (at a later date) means having a deep oversite trough for the duration of the build which can be a disadvantage.

 

[htgeng]

If it was mine I'd make it 100mm insulation over slab and reduce the sand.