Underfloor heating mat - isulation

31 Dec 2003
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United Kingdom
I have a new conservatory nearing completion and I am going to have an underfloor electric heat mat installed.

The floor of the conservatory currently consists of a concrete raft, then Kingspan insulation (not sure of its thickness but I would say about 75mm), then a 75mm cement screed. Is it necessary to also install a 6mm insulation board on top of the screed before laying the heat mat? I want to have the job done properly but obviously dont want to waste money on additional insulation if it's not necessary.

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There are two very different types of electric underfloor heating, simple resistive, and chemical, the latter the resistance of the element changes as it heats up, so there are no hot spots, insulation can be electrical or heat, some need an earthed mat and some have the earthed screen built into the heating wires, so there is no simple answer.

The main problem is speed, what we often want is for no heating while we are away from the home, so the slower the heating system can heat the room, the longer it needs to run before the room is hot enough to use, I had this problem with mothers wet room, from switch on to room being warm enough around 4 to 5 hours, it took an hour to get floor warm, then limited to 27ºC so very slow heating the room.

Best central heating for me is likely the fan assisted heater, inferred is faster but hard to control, the worst is likely the storage radiator, and underfloor heating is very poor with response times.

I can see the point of liquid distributed under floor heating, as the heat pump works more efficiently the cooler the liquid can be. But I simply can't see the point of underfloor heating when not using a heat pump. I am sure there are exceptions like old peoples homes where speed is not an issue as same temperature is maintained 24/7, I fitted it mainly to dry the wet room floor, it was a failure, but that's easy with after sight, but can't think of many cases where one would want such an unresponsive central heating system.
Ummmmh. A very informative response but it doesn't seem to answer my question...
if you expect the underfloor heating to make the room habitable in the winter I do hope you have deep pockets
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My pockets are deep but have some holes! Does anyone actually have the answer to my question?
If you put the heating elements directly on the screed, the heat will only appear once the 75mm screed has been heated.
That will take forever with electric.

If you put decent insulation below the heating elements, the floor will heat up quickly, but 6mm insulation is so thin as to be next to useless.

The UFH should have been pipes on top of the insulation before the screed was poured.
Any electric effort will be an expensive disappointment.
Underfloor pipes were not an option.

Thanks for the advice re. the insulation board.
There was between 9" and 12" of insulation below the element with mothers, and plywood between the insulation and the element, then the tiles put direct on the element, however the pocket for the floor temperature sensor had too steep a bend, so sensor could not be renewed without lifting tiles.

The floor did hit the maximum temperature of 27ºC but that was not enough to maintain the room temperature with the required ventilation LABC insisted on, and we relied on the towel rail to keep wet room warm.

However with that much insulation the floor always felt warm, even with the UFH off, the floor had to be dug up to arrange the drains, so all earth dug out replaced with insulation. As to if it would have worked will less can't say, but does not really matter as the UFH was useless.

In a wet room cooling would always be reasonably even, as no furniture, any other room really need reychem or similar so no hot spots, or use liquid which auto ensures all floor at same temperature.

As said main idea was to ensure dry floor so my mother an amputee would not slip and fall, that was also a failure, it took nearly ½ hour to re-warm the floor, and another ½ hour to dry, so had to be mopped after the shower to reduce chance of slipping. Clearly carpet would further reduce heat transfer, and in any room other than wet room furniture needs to be off the floor or floor will over heat.

The Romans did try with the hypocausts I now know why the idea was dropped.

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