Floor insulation above or below slab?

[PeterGan]

My situation is I am renovating a ground floor bathroom which previously had a solid uninsulated concrete floor. It will become a wet room with underfloor heating.

 

The entire subfloor inside the bathroom has already been cut up and removed, and most of the loose aggregate underneath it dug out. Its now just a 1&1/2 foot deep pit with dirt at the bottom, so a blank slate in regard to floor design.

 

The original plan was to lay down a new DPM, insulation, then new slab in that order.  But I've now had second thoughts. Since the bathroom will be used infrequently it doesn't really make sense to have it warm all the time, and therefore the thermal mass of the slab over insulation could be a hindrance rather than a benefit. The bathroom is only a few square meters and probably less than 10% of the entire house footprint, most of which will never be retrofitted with effective floor insulation. The building as a whole is around 1970s level  of insulation and a bungalow so quite leaky by modern standards. This points to a requirement for room temperature controls instead of maintaining a low level of heating continuously?

 

So if I instead use an insulation over slab approach, this should allow the desired barefoot comfort floor temperature to be reached quickly when required without an obligation to leave the UFH (bathroom only) running all the time. It would probably just be left on a bit longer to help dry the floor after use and to reduce dampness in winter. Is this the correct philosophy?

 

One potential problem is the load bearing/spreading capacity of a thin layer of screed versus thick concrete for baths, wall hung toilets etc which I will probably have to create a new post for in the appropriate forum. But I feel the thermal behaviour of the floor is the inherent factor that should drive decision making. What's the right answer to this?

[Mike]

Sounds like you'd be better off with electric floor heating directly below the floor tiles. Or a wall-mounted fan heater for a really quick warm-up. But do still pack insulation into the void you've dug.

[joe90]

I agree, electric UFH run by a controller (temp and timings) it’s what I did in my en suite, very effective.

[PeterGan]

Hi, what would be the advantage of electric versus wet UFH, given that the floor is already up? Assume with the insulation above the slab response times would be similar and the wet potentially cheaper to run? The up front cost of one system versus the other doesn't worry me so much. 

[Mike]

23 hours ago, PeterGan said:

Hi, what would be the advantage of electric versus wet UFH, given that the floor is already up?

The response time - electric is directly below the tile adhesive.

 

23 hours ago, PeterGan said:

Assume with the insulation above the slab response times would be similar

Hydronic would be slower - it would be embedded in the screed, which would be thicker so take longer to heat.

 

23 hours ago, PeterGan said:

the wet potentially cheaper to run?

If it's used infrequently that's not likely to be a significant factor.

[PeterGan]

Thanks. Even with the electric, should the main concrete slab go above or below the main floor insulation? I assume the slab should go below the insulation in this setup, to avoid unnecessarily heating up all that concrete?