wet underfloor heating system insulation question

[Newby14]

Hi all, I am looking for some advice re my wet underfloor heating system. I am installing celotex 50mm between the joists and then pinning the ufh pipes to it and covering with dry mix of sand and cement. followed by 18mm ply and then the final board.
I was wondering if this sounds fine or I need to do anything more to protect the joists from vapour and to Imorose insulation. I'm not sure what to do with the edges and have currently stuffed rock wool there. Any advice would be much appreciated As I'm already half way through the installation!! Thanks!!

 

[muggles]

Sounds like a terrible plan, why aren't you using a system that's designed to be run between joists?

 

[Newby14]

Hi Andrew, this system is designed to be run between joists. I'm installing per the manufacturers instructions. My question is about the vapour barrier I require.

 

[Newby14]

And any additional edge insulation I require.

 

[ServiceNumpty]

I'm confused, there are systems out there that have preformed insulation panels, and spreader plates. Is yours for upstairs or downstairs.

 

[muggles]

Sounds very odd to me too, I've never heard of putting dry sand and cement over UFH pipes installed between joists before. Did you buy it from cheapestunderfloorsystemicouldfind.com?

 

[Newby14]

As far as I know this is the recommended way of installation. The alu plates are a less effective heating option. I'm looking to hear from someone who has installed this in their property already.

 

[Agile]

Perhaps you can explain what the purpose of dry mix with cement is? Why not just use dry sand instead?

Also I don't see what the purpose of the 18 mm ply apart from increasing the thermal resistance!

At the Ecohome exhibition there was a system which I thought was very good.

It consists of 0.5 mm square corrugated sheet on which you clip the tube and then fill with wet cement mix. Its only about 47 mm thick and the thinness and steel spreads the heat very effectively and will span a joist gap of 600 mm. Its about £20 per square m although a cheap skate might find their own cheaper material to use!

I am even considering using it in my extension.

Tony

 

[Newby14]

The sand cement mix is there to retain heat. It is standard practice for underfloor heating on joists so there are no air gaps. The alu plates, etc are alternatives to this method and considered less effective for conducting heat. Anyway does anyone here have an answer to my question On vapour barrier?

 

[Richardthe3rd]

The thermal mass of a 'Pug' system like that is always a good idea, lower mean water temperatures & lots better than alu plates.

You do need to consider the structural aspects of this though, the additional concrete can add lots of weight on the joists.

Also I'd be using Plywood or similar to support the system between the joists
HTH.

 

[Newby14]

The insulation is fully supported by boards under the joists. I was wondering if I need to polythene sheet installed as a vapour barrier and if so where? Ie just below engineered wood boards? Below the subfloor? Below celotex? Any suggestions?

 

[Richardthe3rd]

The insulation is fully supported by boards under the joists. I was wondering if I need to polythene sheet installed as a vapour barrier and if so where? Ie just below engineered wood boards? Below the subfloor? Below celotex? Any suggestions?

Never found the need for Visqueen or similar on a system of this nature. Just ensure the concrete finished flush with the top of the joists & makes contact with the underside of the flooring.

 

[muggles]

What do the installation instructions for your UFH system say?

 

[Newby14]

Yes we have ensured there is no gap between the sand cement and ply on top. But I'm wondering if there might be a possibility of some vapour going down to the joists and rotting it? Installation instructions don't ask for any vapour barrier but I feel it is a real risk. So trying to find what others have done in similar circumstances.

 

[Newby14]

Fyi.This is a basement flat in a Victorian property.