Insulation Under or Over Slab?

[learner]

My builders have installed 100mm insulation under the slab.

Is this correct??

[A_L]

@learner , it can be. It will slow the response time of any UFH but will enable you to store more energy in the slab. 

[learner]

There's no UFH going in, i just wondered if it was correct procedure

[Jilly]

Yes, probably. Can you look at your Building Control drawings, if not on a Building Notice?

[learner]

Building control are signing jobs off as we go and this has happened today

[James Newport]

31 minutes ago, A_L said:

@learner , it can be. It will slow the response time of any UFH but will enable you to store more energy in the slab. 

how does insulation under the slab slow the response time of UFH?

[A_L]

19 minutes ago, James Newport said:

how does insulation under the slab slow the response time of UFH?

 

The temperature of the slab has to be raised before it starts to heat the room

[Gus Potter]

Yes putting the insulation under the slab is fine. What you are doing is bringing the slab into the inside of the insulation envelope, just like if you have some masonry walls inside.

 

In some ways you have a big storage radiator (hate to say it but often called thermal mass) which can help stabalise the temperature in the room. It's important that you carry the insulation below the slab up the sides so you "wrap" it fully in the insulation blanket.

 

There are practical benefits in that you often have something more solid to lay your floor on and build non load bearing partition walls off.

 

 

[James Newport]

9 hours ago, A_L said:

 

The temperature of the slab has to be raised before it starts to heat the room

But how does insulation under the slab slow that process?

[dpmiller]

It doesn't. But Insulation *over* the slab with UFH in a screed or board above will be more responsive.