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I'm about to buy and electric underfloor heating system for my new loft extension. The supplier is trying to sell me insulation boards, two types of underlay etc etc but i'm not sure if these are required.

I've just built the new floor, it has acoustic insulation between the joists and then 18mm osb over. I can tape the gaps too if needed as these were not tongue and groove. The loft itself is then timber frame and insulated with PIR insulation or rockwool and then further insulated plaster board over that.

My question is do you think the insulation boards over the osb and under the heating matt is required? It's a big expense for an area I imagine will be pretty comfortable all year round anyway but then don't want to install the underfloor heating if they will not work at all. I'm getting an underlay and carpet over around 2.3-2.5 tog

Thanks for your advice

On the ground floor the insulation is very important, however on upper floors it is down to if you want the heat in the floor below.

Underfloor heating has a limit or 27 degs C, this means it is very slow to warm up, where a room is used 24/7 and there is no alternative heating this is OK.

However most rooms are not used 24/7, and also most rooms have alternative heating, be it sun through the window, or a cooker, or the roof getting hot, so we want most rooms to have heating with a fast response time.

Be it a the suns come out, or a door opened, or a cooker used, we in general want the heating to respond fast, and also we tend not to use rooms 24/7, so we want a room to heat up fast, my mothers wet room with electric underfloor heating from switch on to floor feeling warm 1 hour, and to heat the room looking at 3 to 4 hours.

So how often will you use the room, and for how long.

It's two bedrooms that we are installing it in and will be the only active heating source for the rooms.
It is, i hope, a well insulated space, and so should remains relatively comfortable throughout the year without much additional heat. However at the same time, I don't want to pay for the cost of the electric heating if they are not going to have much of an affect because I didn't pay to install the insulation board.

It may work with your case, the main problem is UFH is slow to respond, so things like sun through a window heat the room faster than the heat can stop entering the room from the UFH so the heat over shoots.

The main reason for the use of UFH is the heat pumps work better at lower temperatures, getting a heat pump to 70 degs is wasteful, so there is a very good reason to work at lower temperatures.

However the main advantage of heat pumps is they can work in both directions, so fan assisted radiators are likely the best idea as even mounted at floor level they can cool the room.

The Raychem underfloor heating can only heat the room, but unlike the cheaper resistive wire, the chemical impregnated cable is self regulating so less likely to over heat when furniture is placed on top of it. The resistive cable type needs a pocket in the floor to monitor floor temperature as well as room temperature, and you can't put any furniture on the floor, so really only good for wet rooms and the like.

I fitted a resistive type in mothers wet room, it was a failure, idea was it would dry floor, but once cooled with the shower water, it took 2 hours to fully heat up again.

And this is the problem, around a 3 hour delay between switching on, and starting to heat room, and same delay switching off and allowing room to cool, and the modern idea of only heating when required can't be done, forget geofencing.