Insulating a solid floor

[modellingman]

I am refurb'ing a small ground floor bathroom that was built probably in the 60's as an addition to a much older (end of 19th century) terraced building.

The bathroom is small (about 1.7 meters squared) and has a solid concrete floor. I am thinking about installing insulation in the floor to make the bathroom feel a little less cold and to alleviate some of the condensation problems.

I'm not looking for advice about condensation, just about my thoughts on insulating the floor.

I'm not going to go through the hassle of digging up the existing floor to lay insulation with concrete on top. My thinking is to lay something like 100 or 150mm of celotex or similar on top of the existing floor (which is reasonably flat and sound) and to then lay something like P5 chipboard on top of that. I have not decided on what will be used as the decorative layer but ceramic floor tiles over an electric heating mat is an attractive option.

There is considerable ceiling height, so the reduction in height caused by raising the floor level is not a concern, nor is the need to deal with the doorway.

This being a bathroom, it will have a new bath installed and I am a little concerned about whether my proposed way of insulating the floor will provide a sufficiently stable platform for supporting the bath, which will weigh well over 250 kg when filled with water and occupant(s). The bath will also have a shower over and any instability is likely to lead to failure of the seal between the top edge of the bath and the bathroom walls which will form part of the shower enclosure.

Any Foolish advice, particularly about the stability concern, will be much appreciated.

TIA

[staffordian]

I'm not sure how practical it might be, but could you consider a raised platform set on a substantial wooden frame on which to mount the bath, and insulate the floor up to this frame?

[redsturgeon]

I wouldn't have thought you would have a problem the 250 kg willI assume be spread by the layer of chipboard across a large area but if you are worried you could forgo the celotex under the bath and have a solid floor there.

John

[Gaggsy]

I wouldn't have thought you would have a problem the 250 kg willI assume be spread by the layer of chipboard across a large area but if you are worried you could forgo the celotex under the bath and have a solid floor there.

John

Or build some dwarf walls in the position where the bath's support legs will be. Two courses of bricks would do it.

[ajjones100]

We did something similar. Early 80's build with solid, uninsulated floor, which was always cold. As part of the complete refurb, I laid a plastic DPM membrane, then screwed down 3x2 battens at 400/600mm centres using frame fixings into the concrete slab. 50mm Celotex cut between the battens, with expanding foam around the edges to prevent cold bridges. On this I clipped down 16mm PEX-AL-PEX pipe and then infilled level with the top of the battens a biscuit mix (10:1 damp sand:cement). Then plywood screwed down to form a solid base for tiles in the kitchen/diner and engineered oak in the lounge and hall.
It's a lot of work for a DIYer, but the result is very satisfying.

[sg31]

This being a bathroom, it will have a new bath installed and I am a little concerned about whether my proposed way of insulating the floor will provide a sufficiently stable platform for supporting the bath, which will weigh well over 250 kg when filled with water and occupant(s). The bath will also have a shower over and any instability is likely to lead to failure of the seal between the top edge of the bath and the bathroom walls which will form part of the shower enclosure.
TIA

The load will be spread over the celotex under the bath by the chipboard. celotex and polystyrene insulation are quite good at bearing weight in this type of situation. I would suggest filling the bath before sealing the edge or tiling. If the bath moves at all it will then be upwards when empty rather than downwards breaking the seal when full.

[modellingman]

Thanks for the suggestions. I am going for a hybrid of them.