Apologies if this is in the wrong Forum section, its kind of a substrate question!

Im in the early days of planning a new kitchen with tiled floor and will have UFH over 18mm plywood and 10mm Marmox, with SLC then decent BAL adhesives. My question is regarding insulating between the floor joists before I board it over. The joists are 100mm deep. Ive read up on using garden netting or chicken meshing to staple/screw onto the underneath of the joists (theres space to crawl underneath) then fill the cavity with insulation.

Should I use 50mm Celotex/similar make or normal loft glassfibre insulation? I cant work out the U factors. I can get a 8x4 sheet of Celotex for £10 or wait for a buy 1 get 3 free loft insulation deal. Opinions welcome

Originally Posted by ollymonkeynuts

Apologies if this is in the wrong Forum section, its kind of a substrate question!

Im in the early days of planning a new kitchen with tiled floor and will have UFH over 18mm plywood and 10mm Marmox, with SLC then decent BAL adhesives. My question is regarding insulating between the floor joists before I board it over. The joists are 100mm deep. Ive read up on using garden netting or chicken meshing to staple/screw onto the underneath of the joists (theres space to crawl underneath) then fill the cavity with insulation.

Should I use 50mm Celotex/similar make or normal loft glassfibre insulation? I cant work out the U factors. I can get a 8x4 sheet of Celotex for £10 or wait for a buy 1 get 3 free loft insulation deal. Opinions welcome

100 mm joists don't sound strong enough, is there any deflection in
The floor? If you use insulation boards on top of the ply you song need to insulate underneath as well
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100mm joists are no way strong enough, whats the width of them...

I was just thinking general insulation is a good thing, not just to increase the efficiency of the UFH I realise the Marmox will do that.

The joists are supported frequently on brickwork (every 1.2 metres) so once the plywood goes down im confident theyll not be deflection. Im creating a kitchen/diner, the kitchen is about 2.4m wide which will extend to about 4m.

Its an old 1930's house, nothing ive discovered so far makes much sense. Ive just dropped a chimney so im just extending the floor joists and have the opportunity to do the extra joist insulation.

Originally Posted by ollymonkeynuts

I was just thinking general insulation is a good thing, not just to increase the efficiency of the UFH I realise the Marmox will do that.

The joists are supported frequently on brickwork (every 1.2 metres) so once the plywood goes down im confident theyll not be deflection. Im creating a kitchen/diner, the kitchen is about 2.4m wide which will extend to about 4m.

Its an old 1930's house, nothing ive discovered so far makes much sense. Ive just dropped a chimney so im just extending the floor joists and have the opportunity to do the extra joist insulation.

i would use rockwool then, i don't like the idea of kingspan (or similar) inbetween the joists as they wont be draught proof so would not be as efficient

As long as it is cut to size properly and tightly butted cellotex would be far better than rock wool. Instead of netting though batten the side of the joist to support the cellotex and before you bord overlay the whole thing with polythene membrane to draught proof. As you have a crawl space you have an air gap so I would go for 35mm tile batten then 65mm cellotex.

Thanks Ajax123 but wont the membrane encourage moisture between it and the celotex? Also so ive got it right in my head do you mean batten so that the celotex sits flush with the top of the joist?

A polythene membrane will only lead to condensation issues if it is placed on the "cold" side of the system. if it is between the insulation and and screed/heating it will be maintained at the same temperature as the screed. Or to put it more succinctly and in direct answer to your question.......no.

To answer your second question....yes. You want to avoid a large air gap between your insulation and your over boarding as this will provide a space for heat to radiate downwards into thus reducing the overall efficiency of your underfloor heating system.

Smashing, That makes a lot of sense.