?Hi all,
?I’m currently planning the underfloor heating (UFH) installation for a bathroom / en-suite on a suspended timber floor as part of a whole-house central heating upgrade.

?In our previous home, we fitted UFH between joists using Celotex insulation boards with a sand/cement "biscuit mix" laid on top around the pipes, covered with OSB, and then tiled directly over. It worked well in terms of heat, but I’d like to see how best practices have evolved before starting this project.

?I’d appreciate the forum's advice on a few key points:

?Biscuit Mix vs. Modern Alternatives: Is a sand/cement biscuit mix still recommended for suspended timber floors, or should I be looking at aluminum spreader plates or routed foiled boards?

?Subfloor Substrate: Would you recommend OSB3 or exterior-grade Plywood as the structural floor before tiling? Should an uncoupling membrane or cement backer board (e.g., HardieBacker) be added on top to prevent cracked tile joints?

?Pipe Routing Layout: Do you run UFH pipework under baths, shower trays, vanity units, or fitted cupboards? The goal is max thermal mass, but I want to avoid issues with trap evaporation or overheating fixtures.

?Controls Strategy: The rest of the house will be running as an open zone governed by weather compensation. Should the bathroom UFH be part of this open loop, or is it better to isolate it with its own thermostat, manifold actuator, and floor probe to achieve the higher target temperature (21°C–22°C)?

?Thanks in advance for your insights and recommendations!

When I built my t/f house, I really wanted Lewis Plates for the first floor for a solid feel and thermal mass for the UFH. But as above, weight is a major factor and it seems my guidance to the t/f company went awry as the extra weight was not taken into account at the design stage.

Consequently, I had to use ali spreader plates.

Easy to install and lay the UFH piping, but..... Warms up quite quickly; also cools down quickly as there is minimal thermal mass. The biggest downside (IMHO) is that as the pipes heat up, they must cause the ali to expand; the result is considerable clicking and popping from the floor. I reckon it is due to the kinda soft plastic outer surface of the UFH pipe in tight contact with the ali, something has to give. While not a big deal in a bathroom, it is in the bedrooms, particularly in the early morning when the heating starts.

Personally, I would not recommend the ali spreader plates but if funds allow, something like the Fermcell floor panels which have a degree of thermal mass.

Heating under the bath, etc, waste of time and as mentioned above, should it be necessary, no access.

I did fit thermostats everywhere but the boiler installer suggested weather comp, and I have found that w-c and stats do not work well together. So all stats are off, the UFH actuators are all locked in the fully open position and I have kinda balanced the system by restricting the flow to rooms that do not really need it so much. The g/f guest bathroom is around 7x7 feet with a solid concrete floor; it gets too warm (thermal mass) so the flow has been reduced considerably.

Personally, I would not recommend the ali spreader plates but if funds allow, something like the Fermcell floor panels which have a degree of thermal mass.

I got all my stuff from Jupiter (https://jupiterunderfloorheating.com/) and they certainly have considerable knowledge on UFH flooring and heating systems / installation.