Over floor underfloor heating

[Ed209]

Hi just after some advise regarding the expansion gap of 10mm ply.
I live in 1930s bungalow and wish to instal underfloor heating, my existing floor is suspended floor boards, the skirting is 6" stripped and waxed (don't want to remove)
The process would involve laying 15mm insulation with groves for 10mm barrier pipe then 10mm ply, then carpets & not sure in kitchen/bathroom yet.
My question is expansion gap for the ply as in ideal world would leave 10mm and skirting over, but can't do that so what do you think the minimum I could get away with?


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[RobinBHM]

I think you are best to discuss with the manufacturers of the microbore system. It always seems counterintuitive to have underfloor heating below timber and carpet as they are both good insulators.

Are you removing the floorboards and insulating in between the joists? The feedback Ive had from almost every customer who has had retro fit underfloor heating is thats it is very expensive to run because of insufficient insulation below and insufficient heatsink in the form of screed above. If you are thinking of using the underfloor heating as a primary heat source you might be disappointed.

Wet underfloor heating is at its most efficient as a in screed system, pipes set on top of 100mm celetex and 75mm screed over the top.

There are wet systems that fit into an aluminium diffusion plate using standard 15mm pipe, height is kept low because the pipe sits down below the top of the joist.

 

[meccarroll]

Hi it is best to follow the manufacturers instructions with this type of installation otherwise you could invalidate your warranty and house insurance if things go wrong. If it means taking the skirting off to do the job properly and re-fixing after, I would do it. If you follow the manufacturers instructions regarding installation you should be ok. I'm not sure you should be cutting grooves in the insulation?

I think RobinBHM is primarily referring to a wet system of underfloor heating rather than electric, but the advice in relation to insulation is also good to note for an electric system. As a matter of interest which system are you installing and who manufactures it? What is the specification of ply you are using and manufacturer? Is the insulation provided by the underfloor heating manufacturer? The only reason I ask is because all the materials have to meet the correct specification and be installed to the manufacturers instructions for the system to work.



Mark

 

[Ed209]

Was considering the Polypipe 10mm in the insulated aluminium reflector trays that and the 10mm ply raise the floor level by 25mm excluding underlay/carpet
But may reconsider after Robin comment, it has crossed my mind to lift some boards and celotex between joists but its all getting a bit expensive and time consuming


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[Phil Pascoe]

Expensive and time consuming? Yes ... but not a waste of money - if you don't you're trying to heat the subsoil. While you're at it clad the T&G with hardboard or 6mm ply. Stop draughts and insulate underneath and it'll probably achieve as much as your heating would achieve without doing so.

 

[Random Orbital Bob]

I have polypipe 10mm over 100mm celotex, held in place by that egg box type plastic matting with little clips holding it in and then 75mm screed over the top. If you do anything that doesn't insulate effectively beneath it, you're on a hiding to nothing. We have a very considerable square footage of underfloor on 8 separate manifolds with two separate circuits heating circa 5000 sq foot and our gas bills are only marginally more than a regular house.
Installed properly it is really efficient.

Make dam sure you commission it though so there is no air in the pipes.

 

[DavidJHolmes]

10mm ply seems a bit thick for going over the pipe. We install the Wundafloor system and they only recommend a 3mm ply for carpet. You also need the specific underlay (I think 2.5tog) for carpet. Carpet doesn't transfer the heat as good as a tile or laminate. You can tile straight on top the Wundafloor system with a thin matting between to allow for expansion movement so its a bit thinner. IIRC the trays are 20mm thick.

 

[meccarroll]

Was considering the Polypipe 10mm in the insulated aluminium reflector trays that and the 10mm ply raise the floor level by 25mm excluding underlay/carpet
But may reconsider after Robin comment, it has crossed my mind to lift some boards and celotex between joists but its all getting a bit expensive and time consuming


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Just took a quick look at the Polypipe system and they state if you use their "OVERLAY" system there should be no need to use any additional insulation other than what they recommend (supply). I'm pretty sure you will need to pressure test the system also have to allow for some wiring work done for the electrics so I'd think taking the skirting off is going to be the least of your concerns. Polypipe do several systems and I'd be inclined to telephone their technical department to ask for their advice before proceeding any further. There is a fair bit more work to these systems than most people think, I've worked on a couple of large installations taking around three months. I expect yours is a lot smaller but still not a quick job if done properly.

Mark