Retrofitting under floor heating on suspended timber floor - insulation options

[hanvyj]

Just had a burst pipe under the suspended timber kitchen floor and need to rip a load of stuff up to fix it. I'd like to properly insulate, and also fit underfloor heating while I'm doing the work. We might have to get a heat pump at some point in the future, and having no radiator in the kitchen would mean we could have a much better layout etc - so while I'm ripping flooring up and fixing plumbing, I'd like to sort out the underfloor for the long term.

 

The joists are 5 inches deep, so would maybe fit 100mm of PIR insulation + a space for screed. However, I've seen people recommend rockwool on top of a dpm e.g. here 

 

Would it be a bad idea to have insulation under the joists - this would reduce them acting as a cold-bridge, but they would no longer be exposed to the air to keep dry? 100mm is less than I'd like.

 

Options I'm considering 

 

Curious what people's thoughts are.

 

Edit: I also have a bunch of spare 200mm EPS. I'd love to be able to use that!

Edited June 20, 2023 by hanvyj

[JohnMo]

Question is do you need screed at all in any option - no. You could do a pug mix, basically a weak sand cement mix. Or moulded EPS for the pipes, with additional insulation below. Flooring OSB or ply above then your floor finish.

 

Don't think 100mm PIR is deep enough, not to stop downwards heating.

[hanvyj]

Quote

Question is do you need screed at all in any option - no. You could do a pug mix, basically a weak sand cement mix. Or moulded EPS for the pipes, with additional insulation below. Flooring OSB or ply above then your floor finish.

 

Don't think 100mm PIR is deep enough, not to stop downwards heating.

 

 

Yes by screed I was thinking weak sand and cement mix like you describe, sorry if that was the wrong word. I've seen an 8-1 mix recommended e.g: https://www.centralunderfloorheating.com/underfloor-heating-installation/underfloor-heating-screed.html 

 

Yes, 100 doesn't feel like enough to me... that leaves me with option 2 - or I could use thicker insulation, like the 200mm eps I have and leave the joist bottoms open to the air?

Edited June 20, 2023 by hanvyj

[Nickfromwales]

Option 3;

Rip the floor joists out and convert away from a ventilated floor in that room. The current direction for cross-flow of air would need to be considered, eg would filling in this area stop airflow to other spaces? If not, it's bye-bye joists and hello fully filled floors with insulation and screed.

[Sparrowhawk]

@Nickfromwales when I go the "unimaginative" builder round to quote I asked about doing this and he laughed at the idea. Said it was uneconomical as you have to dig down a long way, then put hardcore etc and build up the floor. If the depth from floor level to ground is currently 0.37m you'd have to at least double that. Brought in the word "foundations" along the way.

 

What's the minimum build-up BCO will let you get away with for a solid floor inside an existing property?

[Nickfromwales]

1 hour ago, Sparrowhawk said:

@Nickfromwales when I go the "unimaginative" builder round to quote I asked about doing this and he laughed at the idea. Said it was uneconomical as you have to dig down a long way, then put hardcore etc and build up the floor. If the depth from floor level to ground is currently 0.37m you'd have to at least double that. Brought in the word "foundations" along the way.

 

What's the minimum build-up BCO will let you get away with for a solid floor inside an existing property?

It does depend on where the footings start / finish (depths) of course, but as any general builder will just regurgitate the same shart they used on the last 40 jobs, I'd take everything they say with a pinch or 3 of salt.

Firstly, they should ONLY comment after conducting a survey, and that involves pulling some flooring up and getting a tape and possibly a shovel out. The depth would be made up with layers of insulation, so no need to import huge amounts of logistically difficult hardcore etc, just a base layer of type1, a sand blinding to get you snooker table flat and level, and then use cheaper EPS for bulk build up and finish with PIR (minimum 140mm). Set the top of that to be FFL -75-80mm for dry screed (relatively slow response from UFH), or -50mm for liquid screed (quicker response for 'timed on - off' of heating).

Unless he has used a calculator to work out differing costs for different heating methodology, has asked you how long you'll be in the property for, has worked out heating requirements per annum, and then added the value to you as the owner re floorplans and benefits thereof, then how the feck would he know what is economical or not?!

 

BCO will only be interested in the damp proof course, and the amount of insulation, but you should exceed what they ask for as UK building regs is dire on a good day. They'll 'maybe' be interested in you preserving airflow to the other ventilated floor spaces, so maybe it would help if you uploaded some floorplans showing these areas vs others etc.

[hanvyj]

Hmm, interesting idea. I measure 460mm to the top of the joists. So the amount of insulation needed might be costly. The floor looks to be sandy ground, with a few bits of broken brick here and there, haven't dug down to see what it's like. Floor is already pulled up to fix a leak.

 

I'd be doing the work myself - would I need a structural engineer or building regs involved? Presumably wouldn't with just joist-insulation.

 

I'll consider insulating the rest of the property in the same way at some point.

 

Floor plan: Green area is the area in question, we're planning on knocking down the wall and extending the kitchen into the hall:

The ground:

Edited June 21, 2023 by hanvyj

[hanvyj]

Converting it to a solid floor - would 100mm or so of concrete not be needed? Hardcore -> eps insulation -> Concrete

I just DIY poured a concrete slab for a garage I'm building with the same set-up. The concrete was costly!

 

I suppose it just replaces the screed in the original plan.

Edited June 21, 2023 by hanvyj