Suspended floor insulation for UFH

[damessiahuk]

Hi- newbie here!

I'm looking to install wet UFH as part of a heat pump install.

Floors are suspended timber, with 100mm joists. There's a c.200mm void underneath.

I plan to PIR insulate from bottom of joist up, leaving a 25mm gap for pipe and biscuit mix screed on top of PIR. This will either be through 50mm PIR board on a 25mm batten, or potentially roof soakers at joist level with a 75mm PIR level.

My question is around the edges of the room.... Do the boards need to butt up directly against the wall? Do I need to seal/protect anywhere with a membrane of sorts? If so, where does this go? Under the PIR or on top? (Struggling to find a definitive answer so really welcome views!)

 

[noseall]

Hi- newbie here!

I'm looking to install wet UFH as part of a heat pump install.

Floors are suspended timber, with 100mm joists. There's a c.200mm void underneath.

I plan to PIR insulate from bottom of joist up, leaving a 25mm gap for pipe and biscuit mix screed on top of PIR. This will either be through 50mm PIR board on a 25mm batten, or potentially roof soakers at joist level with a 75mm PIR level.

My question is around the edges of the room.... Do the boards need to butt up directly against the wall? Do I need to seal/protect anywhere with a membrane of sorts? If so, where does this go? Under the PIR or on top? (Struggling to find a definitive answer so really welcome views!)

You cand screed onto a timber floor.

Wet UFH is doable with suspended timber floors, but the heating pipe array needs to be suspended on trays, in between the joists.

 

[robinbanks]

I'd put 75mm PIR under the joists too, in a continuous layer, and go your 75mm between route. Run all PIR to the walls and seal it using a bead of expanding foam. Do not run the biscuit mix to the walls.

How long are these joists? 100mm sounds rather small for a usable room size; do they have interim support?

needs

Sounds like you're claiming there is only one way to do it; this isn't true

 

[damessiahuk]

You cand screed onto a timber floor.

Wet UFH is doable with suspended timber floors, but the heating pipe array needs to be suspended on trays, in between the joists.

Thanks, I've looked at the tray systems but they don't offer the same thermal mass to retain heat. There's a few screed based systems I've seen which are designed for suspended floors ....https://www.thermo-floor.co.uk/underfloor-heating-suspended-floor.html

 

[robinbanks]

I've looked at the tray systems but they don't offer the same thermal mass to retain heat

Indeed; I have just such a system in my upper floors and in my humble opinion, it's cack and I'd never recommend it. I now know why the engineer that specified my insulation told me he was uprating the floor joist spec in his own new build and pouring a liquid screed on every floor..

 

[damessiahuk]

I'd put 75mm PIR under the joists too, in a continuous layer, and go your 75mm between route. Run all PIR to the walls and seal it using a bead of expanding foam. Do not run the biscuit mix to the walls.

How long are these joists? 100mm sounds rather small for a usable room size; do they have interim support?


Sounds like you're claiming there is only one way to do it; this isn't true

Thanks! So fix to the underside of the joists perpendicular? I'm open to this (more the better right?) just might be tricky with available space!

The biscuit mix presumably runs up to the foam bead?

So no dpm or anything needed?

The joist lengths are about 4.2m, there's a dwarf wall at about halfway point from memory of running wires through!

 

[noseall]

Thanks, I've looked at the tray systems but they don't offer the same thermal mass to retain heat.

Why is thermal mass heat retention important?

 

[noseall]

Sounds like you're claiming there is only one way to do it; this isn't true

No. But I'd assumed the OP was referring to a standard s/c screed. The work needed to achieve a screed on a timber floor is not worth the effort, in my opinion.

 

[damessiahuk]

Why is thermal mass heat retention important?

The ashp will run at a lower flow temperature, so retaining the heat and keeping a constant temperature more efficient than heating up and cooling down constantly.

I am aware the plates are easier to install but I don't think, in my opinion, they're the most effective system

 

[damessiahuk]

I need to flag- The biggest barrier I actually have is I don't think building regs will allow me to notch down a depth of 25mm on a 100mm joist to loop pipe around

 

[robinbanks]

Same setup for me; ASHP providing 27ish degree water into a poured liquid screed downstairs and heat spreader plates upstairs. We don't use the upstairs loops any more

Thanks! So fix to the underside of the joists perpendicular? I'm open to this (more the better right?) just might be tricky with available space!

Indeed, working out a way to get it into place perhaps one of the trickier challenges. I'd perhaps score just one side longways in a couple of places (400 and 800) and snap the board so it bends, then it can be fed down between joists, but keeping one foil side intact will help it continue to act as a single sheet. If you're supporting it with a cross network of screwed battens below, running perpendicular to the joists you can fit and secure at the edges. If you want to fix it interim (board centre) you can drive fixings down through the board into the batten and fix to the joist side with a modesty block

The biscuit mix presumably runs up to the foam bead?

Around the edge? You don't have to (shouldn't) take it all the way to the wall

So no dpm or anything needed?

Close fitted, the PIR boards will suffice

The joist lengths are about 4.2m, there's a dwarf wall at about halfway point from memory of running wires through!

Maybe knock the top rows of bricks off that aren't supporting joists so you can fit insulation around the load bearing points

The work needed to achieve a screed on a timber floor is not worth the effort, in my opinion

Hear here!

The biggest barrier I actually have is I don't think building regs will allow me to notch down a depth of 25mm on a 100mm joist to loop pipe around

Indeed, but you don't need to do it; the pipe is only 16mm so you can either take it over or under the joist. If you go over you can find somewhere to sneak it at the room edge without notching the joist. If you go under you're just passing it through the under joist insulation, easy to hollow out. You put the pipe through as a series of loops then shape them out straight to fit between the joists when you get them to the length required to complete a there-n-back run between the two adjacent joists

 

[damessiahuk]

Hadn't thought about going underneath! Interesting..... Hope not too faffy. I'm leaning towards structural overlay boards to replace my floorboards.

Goes completely against the thermal mass comment but might be the only other way without raising floor

 

[robinbanks]

What floor finish are you considering?

 

[damessiahuk]

What floor finish are you considering?

LVT probably