Small joist, Wet underfloor heating, what to do??

[JayP1]

Retrofitting wet underfloor heating to my 1950s build, using the dry screed method. Took the floorboards out to find 4x2” joists. But I want to use 75mm PIR insulation boards which doesn’t leave me any space for timber battens to support the insulation/screed.

What are the options here without raising the floor? Saw on another post someone used angle brackets? Could I add battens to the bottom of the joists?

 

[Notch7]

I cant see 4 x 2 joists taking weight of dry screed

what is the span of the joists?

 

[Mr Chibs]

Why not use the overlay boards instead?

How far are the joists from the ground?

 

[JayP1]

I cant see 4 x 2 joists taking weight of dry screed

what is the span of the joists?

It's 25mm of dry sand/cement (8:1), the screed will go in-between the joists and then I plan to put plywood on top before flooring. They are 400mm apart and vary in length, held up by brick in sections. Longest is 2.4m

 

[JayP1]

Why not use the overlay boards instead?

How far are the joists from the ground?

Not sure but I think about 60cm. I wanted screed for a better heat transfer, heat distribution and cost

 

[Mr Chibs]

I think this is problem on all levels, but I'm willing to be proved wrong.

75mm PIR is not the recommended thickness, think it may be 150 now, there are lots with 100mm though.
25mm screed is not really thick enough to embed a 15mm pipe and have enough mass to provide a good thermal store. You should be aiming for 50-75mm.

Joists seem a bit on the light side.

If you are going to the expense of the doing the job, you want it to perform well when you've completed it.

I was asking about the depth of void, perhaps making it a solid floor would be better, but 600mm is a lot to fill, its a lot of work as well.

You might want to have butchers through this, p54 onwards may be interest.

Polypipe install guide

 

[JayP1]

I think this is problem on all levels, but I'm willing to be proved wrong.

75mm PIR is not the recommended thickness, think it may be 150 now, there are lots with 100mm though.
25mm screed is not really thick enough to embed a 15mm pipe and have enough mass to provide a good thermal store. You should be aiming for 50-75mm.

Joists seem a bit on the light side.

If you are going to the expense of the doing the job, you want it to perform well when you've completed it.

I was asking about the depth of void, perhaps making it a solid floor would be better, but 600mm is a lot to fill, its a lot of work as well.

You might want to have butchers through this, p54 onwards may be interest.

Polypipe install guide

Thanks mate, p71 in the guide you sent talks about the method I want to do with 25mm dry screed mix but only briefly. Probably because It's not a profitable system for them as no extra aluminium plates!

 

[Mr Chibs]

Thanks mate, p71 in the guide you sent talks about the method I want to do with 25mm dry screed mix but only briefly. Probably because It's not a profitable system for them as no extra aluminium plates!

Cheers, They don't screed it though, they slap chipboard down.
Not saying it can't be done.

How large is the floor space... thinking about weight of screed etc.
Also depending on floor covering this may reduce the performance of the UFH.