Underfloor Heating between Joists

[studeywebs]

Hi Everyone,

Recently moved into a 1920's mid-terrace. We're planning to add underfloor heating downstairs and i've been looking at the best way, plan to do most of the work myself. There are 2 options im considering,

1) Remove floorboard, add a timber to joist, sit 100mm celotex 120mm down, run 16mm PERT pipe, notch joist for pipes and then add a 'biscuit screed mix' to make level with joist before adding 6mm plywood sheet on top before laying floor

2) Same process, but set the celotex inline with the top of the joist, add groove board, run PERT pipe and then screed on top, before laying floor.

Is there any preference from people in terms of what is best or even best for the floor joists?

 

[Madrab]

Floor covering?

Approach one would be the best as far as thermal mass is concerned out of the two.

You could also look at an overlay. Remove floorboards - fit strapping between the joists refit floor level with the top of joists then fit an overlay system then final floor covering. Faster reacting that tradition thermal mass UFH.

 

[studeywebs]

Floor covering?

Approach one would be the best as far as thermal mass is concerned out of the two.

You could also look at an overlay. Remove floorboards - fit strapping between the joists refit floor level with the top of joists then fit an overlay system then final floor covering. Faster reacting that tradition thermal mass UFH.

Any guidance on the timber size for holding the Celotex?

 

[studeywebs]

Just found out that my joists are like 4 inches, so plan 2 is the only option. overboard and screed

 

[Deluks]

Just found out that my joists are like 4 inches, so plan 2 is the only option. overboard and screed

Or you could use thinner insulation.

 

[Madrab]

Joists are 100mm deep? Are you sure?

What size of span, must be less than 2m @400mm centres then if they're 100mm? Floor must feel like a trampoline. If they are only 100mm I'd be careful about loading them up with a screed on top.

Have you considered the 3rd option? XPS overlay?

 

[studeywebs]

Joists are 100mm deep? Are you sure?

What size of span, must be less than 2m @400mm centres then if they're 100mm? Floor must feel like a trampoline. If they are only 100mm I'd be careful about loading them up with a screed on top.

Have you considered the 3rd option? XPS overlay?

Yes, I was surprised. I thought they would be at least 6" - but the house is 100 years old and its a floating floor. So i think 2"x1" batton on the joists, Add 50mm of celotex, which brings it inline with the top of the joist. Add XPS and run pipe and then i would pour liquid screed, but i had considered 12mm plywood as well.

What do you think?

Might consider adding some support underneath the joists

 

[Madrab]

then i would pour liquid screed

Why pour screed, what flooring is going on top? You can floor board/laminate and tile directly on top of XPS400 board. You can even carpet but ideally use a 6mm cementitious board on top of the XPS - say like no more ply. No need to screed, in fact it will slow down the reaction times of the floor as it's adding a thermal mass and especially given the weight it could add to what are smaller joists. No need for celotex either IMO especially if the joists are the way they are, an 18-22mm wood flooring insert will provide plenty of insulation especially if using XPS boards on top for the UFH loops, will help to lock it all together and stabilise and also provide a solid base for the XPS.

 

[studeywebs]

Why pour screed, what flooring is going on top? You can floor board/laminate and tile directly on top of XPS400 board. You can even carpet but ideally use a 6mm cementitious board on top of the XPS - say like no more ply. No need to screed, in fact it will slow down the reaction times of the floor as it's adding a thermal mass and especially given the weight it could add to what are smaller joists. No need for celotex either IMO especially if the joists are the way they are, an 18-22mm wood flooring insert will provide plenty of insulation especially if using XPS boards on top for the UFH loops, will help to lock it all together and stabilise and also provide a solid base for the XPS.

Thanks for the advice,

Right now there are 18mm solid wooden planks on top of the joists (assume the original floor). My plan was to remove these, add the celotex and work on top of that, not to add to much floor hight. (360mm between centers)

I am laying a 20mm Herringbone engineered oak floor.

So do you suggest laying the XPS400 board on top of the original wooden floor and then lay like 6mm Hardiebacker and then just lay the 20mm floor on top adding about 50mm to the height of the current floor.

 

[Madrab]

Ok - follow what you were going to do with the celotex.

Add in strapping along the lengths of the joists, say with 2x1CLS, down 18mm below the joist level, make sure it's well screwed in nice and tight - Then cut and lay your original flooring into that space to bring it all nice and level with the joist. That will drop the original floor level down by 18mm. You can then lay a 6mm no more ply or a 9mm ply top onto that to bring it all level for your engineered oak - (though not really needed IMO) then the XPS400 will lie on top of that and your flooring on top of that.

I have done several retrofitted floors with XPS overlay just like that, though they were onto a concrete slab but the final floor laying process etc would be same.

 

[Madrab]

Just to add, the key is the strapping - it needs to be super secure and well supported onto the joists - as that will be carrying a lot of the main flooring load.

 

[studeywebs]

Just to add, the key is the strapping - it needs to be super secure and well supported onto the joists - as that will be carrying a lot of the main flooring load.

Yes, I was looking at a 19mm x 38mm batten, and probably using OSB board between the joist rather than the original timbers. The original timbers would definitely be stronger, lot of cutting though

 

[Madrab]

If they are aged pine timbers and are nice and flat then yes, they would be better than the OSB3 but OSB3 sould be fine too. If it's a reasonably sized job then get a Jiggy or circular and it'll be cut in no time, just do it in batches.

If you are going CLS then I'd recommend 38x63 (2x1 equiv) as it will be load bearing with nice long m5 x 70mm screws every 100mm. Better to be over than under IMO.

 

[studeywebs]

Thank you for the detail, really appreciate it - about 15m2 is the room, i'll have to send some pictures when it done but seems like I have a plan!

 

[Madrab]

Good stuff - when it comes to these things always best to over engineer - it's better in the long run to build resilience into anything like that.