laying flagstone floor with electric underfloor heating...

[Andre Standing]

Hi,

First post...

My wife and I have recently bought an old stone cottage in Dorset. It has been subject to the usual treatment of heavy cement renders and a horrid cement floor--laid directly on earth--which was cracking badly. I am removing the cement renders and have already removed the cement floor in the living room. I wanted some advice on how we are going to redo the floor in that room.

I want to lay the floor with reclaimed flagstones. The floor at the moment is bare earth, which is essentially thick clay. It is a small room - about 10 square meters (with a wood-burning stove).

I want to avoid a limecrete subfloor. I have previously built earthern floors (having built a few cob houses in Kenya), and have laid stone floors directly in earth, which works well. But here I want to improve the insulation and I would love to install underfloor heating. Unfortunately this will have to be electric.

I am thinking of digging down and putting in a layer of compacted geocell aggregate, then a geotextile fabric over that, then covered in sand, upon which I will lay the stones. The stones I have are quite uneven in depth, so I want to lay into a dry mix so I can adjust the heights of each stone. I know this is not a conventional way of doing it, but I see no need for a limecrete screed beneath the stones. What I am wondering is whether it is possible to put electric heat mats under the floor, perhaps embedded in the sand?

Is this a bad idea and do you have suggestions for alternatives?

Many thanks in advance!

 

[a twig]

Putting electric heat mats under the stone will not do much for the floor temperature and will do terrible things to your utility bills.

There have been previous threads on here - I think the conclusion was UFH (wet or electric) only really works with a modern floor construction.

 

[Cubist]

I'm with Twig on this one and agree that economically the payback argument would prove very poor and more than just a little difficult to prove. From a technical perspective at first your proposed approach seems to make sense insofar that laying a bed of insulating material would reduce the loss of heat through the floor. However, when one considers that the rate of heat loss through an un-insulated solid floor is very slow and is dependent on the general ambient land temperature in the vicinity, which is always higher than local air temperature, then you would be, in effect, using the stones as a large heat sink. At best this may contribute a few degrees of warmth in the rooms concerned but would not significantly reduce the operational cost of the direct air heating systems you would also need. In fact, I would be prepared to gamble that the direct gain from any reduction in the latter would be very small indeed.

Frankly, I'd bin the idea of putting heat mats under the stones but there is some merit in laying a bed of insulating material and perhaps laying a heat reflective membrane over that before laying the stones - and after that - investing in some woolly socks and and scatter rugs.

 

[Andre Standing]

Thanks. That all makes sense. I also have wondered whether going the extra mile in putting down the geocell would make a big difference. I can't get under the foundation, so the effect of the insulation might be quite limited?

 

[Cubist]

so the effect of the insulation might be quite limited?

Ordinarily I might say 'every little helps' but in this instance, again, I think the financial payback argument would be very poor. Considering what you plan to do with the floor it may be worth giving some thought to using a blend of sand and something like vermiculite as the bedding layer for the slabs.

 

[Rippa700]

Putting electric heat mats under the stone will not do much for the floor temperature and will do terrible things to your utility bills.

There have been previous threads on here - I think the conclusion was UFH (wet or electric) only really works with a modern floor construction.

I'm about to do similar job. We have a large kitchen/living area with wood floor on insulation (celotex). About 15mSq is laid with 30mm thick large slate slabs on bare earth. I was going to lift, dig, insulate and screed (not sure which way round yet) and then put underfloor electric under the slate slabs. I can't see why this would be inefficient. They can heat on low night tariff and be like a big storage heater, warm in the morning. Do you have a good reason for your comment about the flags? Stone is a good thermal mass and conductor. You've made me pause...

 

[a twig]

No good reason, I just make things up on here

What you’re proposing and what the OP was proposing are quite different, hence you’d get different results.

 

[Cubist]

I'm about to do similar job. We have a large kitchen/living area with wood floor on insulation (celotex). About 15mSq is laid with 30mm thick large slate slabs on bare earth. I was going to lift, dig, insulate and screed (not sure which way round yet) and then put underfloor electric under the slate slabs. I can't see why this would be inefficient. They can heat on low night tariff and be like a big storage heater, warm in the morning. Do you have a good reason for your comment about the flags? Stone is a good thermal mass and conductor. You've made me pause...

Your plans seem to make a lot of sense.... technically speaking,..... but I'm not sure they stand the acid test of 'do they make common sense'. All that work, mess and expense for a few extra degrees and the ability to go barefoot versus the simplicity and versatility of socks, slippers and jumpers? Nah, I don't thinks so.