Underfloor Heating  


Adam Pearce
New Member Registered
Joined:2 weeks  ago
Posts: 1
10/09/2018 6:48 pm  

Hello all

We moved into our fantastic BISF two years ago when it was, as the estate agent put it, “in need of modernisation”. We’ve been working through the house room by room to update it and are about to embark on the kitchen.

We’d like to fit a wet (i.e. gas-heated water as  opposed to electric elements) underfloor heating system. The kitchen consists of the original BISF kitchen space and a large area within a house extension.

We’ve had conflicting opinions from engineers, one of whom suggested that we would need to dig down 300mm (1 foot) into the floor to install such a system whilst the other suggested they could simply install the system on top of our existing floor which would only mean a step of 25mm.


In trying to pick between these two options I’d like to know the answer to a key question which is,


– is it feasible / safe to dig down 300mm into the original BISF concrete standing, which would be required to fit a conventional system? I would have thought this could compromise the structure of the house as the steel columns are embedded into this concrete standing, but I’m not sure how deep the concrete goes and thus how much we can afford to lose without damaging it.


Does anyone have experience of installing underfloor heating in BISFS and of digging down into the concrete in this way? Is this safe to do or should it be avoided at all costs?

Also, does anyone have any experience of the “Wunderfloor” underfloor heating system that is placed on top of the exsiting floor, and would you recommend it?


Thanks in advance!


Honorable Member Admin
Joined:8 years  ago
Posts: 685
13/09/2018 1:23 am  

Hello Adam,

Welcome to the forums, it’s good to have you with us.

I hope the renovation is going well and I can fully appreciate the hard work involved. I am sure it will be well worth the effort when complete.

I’ve only ever fitted electric underfloor heating myself, which I chose solely for ease of installation and due to it being on the first floor, on top of floor boards.

I was aware of the Wunderfloor system which could be used with a preformed insulate pad, specially designed to hold the narrow pipework. It looked perfect for a surface mount system but I did suspect the cost to be a fair bit higher than my budget.

In relation to your question regarding the concrete footings.

The Building Research Establishment state the following:

BISF House Foundations and substructure composition.
Nine-inch brick walls are built on concrete strip foundations. These are overlaid with in-situ concrete 4in thick, thickened to 9in where it is carried over the walls.

Now this of course can vary from site to site simply because so many different housing companies were employed to build these houses on a site by site/ area by area basis. I have seen some without any brickwork at all to the edge of the foundation pad, whilst others have several courses, (Particularly on sloping ground).

I have also come across some houses with solid concrete ground floors throughout and others with part concrete and part floorboards over a shallow gap. Each house can vary and this doesn’t help you’re predicament at all.

Also, you should be aware that in many properties, there may be a gas pipe running under your kitchen floor. This pipe may lead from your gas meter, particularly if your meter is located under the stairs (Depending upon your original floor layout). This can easily be be disconnected but it’s worth bearing in mind that one may be present in your property.

In general though, the pad itself is quite shallow at around 4 inches or a little more in some cases. The deeper strips run around the perimeter as you rightly state but there is also a strip running the the middle of the house from the stair side to the party wall on which your centre stanchions are mounted.

If you plan to go open plan and remove middle walls between the front and back of the property, you will still need to leave these two central stanchions in situ. Many people disguise these stanchions using various methods or simply incorporate then into the design.

I mention this inner strip because it will mean that if you did choose to dig down, you wouldn’t be able to remove this inner middle strip which may be a problem for you.

The surface mounted systems are likely to be cheaper too but you will still need to take into account the the overall finished height increase of the floor. ( I’m sure you already know this, but I’m mentioning it for the benefit of others). The key with any form of underfloor heating is the insulation layer, otherwise you could basically be heating the ground below the foundation pad which would not be very efficient.

Have you found out how much the insulation or performance values differ between a deep highly insulated install and a surface mount?

It would be interesting to know the potential running costs of each vs loss through the different insulation performance properties.

I would also try and undertake a full comparison of installation and projected running costs between the two systems. If the installation cost say £6k vs £2k for electric ( just for argument sake), I would be thinking that the £4k difference could pay for a lot of electric over an ‘x’ amount period, if you understand my meaning.

Have you had any firm quotes yet on either of these systems?

If you take a look at the post I have linked to below, you will see the foundation strips I mentioned previously. 







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