New UFH planning

[riddellns]

Hi
And immediately sorry, I know this topic has probably been covered to death thread.

We're about to extend our old chalet bungalow, rear extension and first floor extension.

The existing floors are suspended timber on most of the ground floor and concrete slab in the porch and front room (as it's an old garage), then the rear extension is going to be block and beam.
Upstairs is just timber joisted.

We definitely want UFH in the new kitchen/lounge open plan area, and it boundaries the old and new. And also in the bathroom downstairs, and possibly upstairs. 
We need to insulate between the joists on the ground floor anyway, and I'm just wondering if anyone has experience doing this, and what the most sensible/easiest/cheapest options would be. 
I appreciate that all 3 of those could well be different, but we're on a fairly tight budget, but don't want to regret not putting it in other areas that are worth it. Hallway for example. 

I know a lot of companies offer the metal plates, above insulation, but I've seen a few folks on the forum slate them for one reason or another. 
And I've seen some of the low build up (and we definitely don't want much build up) but some of them are seriously expensive given the area we want to add UFH

Lastly, first floor, is there even any point? It'll all be new walls and roof insulation, and I'm hoping the heat from downstairs will mostly heat the upstairs. But if not, if UFH won't add much cost upstairs, then I'd probably rather than than add more radiators into rooms which aren't the largest as is!

Thanks

[LiamJones]

My last last was a Reno, we had an open plan kitchen dinner that bridged the new and the old. We had the new slab made up to existing height. The old part was also concrete so that helped, then chucked SLC over the lot, then used Wunda over floor boards throughout, tiled straight on top. No issues. I was worried about the joint between new and old, but no problems. Probably waited about 6 months for it to settle before tiling. I think the wunda boards added a layer of decoupling to for some differential movement. 

[riddellns]

Cheers. Appreciate the info.
Sounds like a great shout. Not sure on the impact of using SLC over timber suspended floors though. Guess as long as there's enough ventilation under the b+b, through to the old suspended floors then there shouldn't be an issue with moisture buildup. 
Mind me asking what sort of price the wunda over boards were? Some brands a seriously pricey, but they make it look super easy. And was there much build up over the old floor?

 

[LiamJones]

I honestly can’t remember. Less than a tenner a board I think? Buildup was the

16mm board, then primer, tile adhesive and tile. So basically 16mm more than a standard tile job. If you went 10mm porcelain, 5mm adhesive you’re 31mm all in. Plus any levelling you need. I had plus 10mm in the lowest, but next to nothing at the high spots. 

Edited October 13 by LiamJones
Wrong numbers

[riddellns]

Sorry, does anyone else have any experience doing this with suspended floors, rather than concrete pads?

 

[Super_Paulie]

ive got underfloor in my 1930s house, all biscuit mix on suspended floors, works a treat so far. 170mm of PIR between the joists all foamed and taped, then the pipes within a dry mix and then chipboard floor.

[riddellns]

16 minutes ago, Super_Paulie said:

ive got underfloor in my 1930s house, all biscuit mix on suspended floors, works a treat so far. 170mm of PIR between the joists all foamed and taped, then the pipes within a dry mix and then chipboard floor.

This was my original plan thb, but don't think we've got particularly deep joists. Did you also just encase your cables and other pipework so that the biscuit mix surrounded them? Conduit for example
Thanks for the reply

[Super_Paulie]

Nah, I ran an my services in the void underneath the insulation. Put a hatch at each end of the room incase I need to pull something through in the future, but I'm not planning on that.

[riddellns]

I've just lifted my floor boards and noticed that there's a shallow (120mm) concrete slab under the void already, and then a 225mm gab between that and the floor boards.

I'm now thinking that filling in the void with a DPM, concrete, insulation and then screed, might not be a bad idea.
Or is this a ridiculous idea?

[Super_Paulie]

if you are going to remove your joists then go for it. If i could go back id remove all the joists and slab the entire thing, but its too late for me now.

[TommoUK]

..this can be done. You need to consider the outside ground level, the dpc level and how far to bring your dpm up your inside walls prior to finishing to prevent damp ingress. 

[Iceverge]

For the amount of work that renovation will take your best option is a demolition and rebuild of a simple compact house.

 

If you must stick with the current design central heating is a good chunk lower down the list than other issues. 

 

1. What is you ventilation plan? 

2. What is your airtighess strategy? 

3. How much insulation can you fit? 

 

If you do a top job on all three of these you can really really cheap out on the central heating. Maybe an A2A  split unit with 2 fan coils, electric UFH in the bathrooms and an ESHP for DHW.

 

Assuming you just want advice on the heating. 

 

1. Dig out all floors, install a good chunk of insulation 150mm PIR or 200mm EPS minimum and do UFH with closely spaced pipes in a shallow screed for high power output to suit a high heat demand house. 

 

2. Insulated the suspended floor with this method. If the joists are shallow you can allow the blue membrane to droop below them for more insulation. 

 

 

Then install high output radiator's and large diameter piping throughout. 

 

Beware UFH over poorly insulated floors will be very expensive to run. 

[scottishjohn]

or do as suggested above 

BUT

fit a low height ufh system on top of exsisiting floor 

will be about 40mm in height  to where you fir flooring covering of choice .my choice would be tiles - on top the cement board

consists pg polystyrene foil covered boards with slots for std UFH piping  and is then covered by cement board as the heatsink and ro spread out the heat ,but yes underfloor imsulation should  be done  as well and not rely on the 25mm of insulation in the piping boards  .

that would be ok for upstairs UFH as any heat loss would be to the lower floor 

 what you will notice  as idid this on previous hopuse is the speed with which floor will change in  temp ,both ways ,so with good thermostat you will never voerheat  if sun comes inthe windows

 

,but as heat sink is thinner will not keep iit as long when heating is off  as a 60mm screed ,as it will be thinner 

always a compromise 

 insulation under flooor will be key to making it work well

 

Edited October 18 by scottishjohn

[Nickfromwales]

On 16/10/2024 at 12:31, riddellns said:

Sorry, does anyone else have any experience doing this with suspended floors, rather than concrete pads?

 

It’s down to knowing how much heat the UFH is going to actually deliver. Kitchens are the worst as there’s a lot of redundant floor area covered by units etc.

Suspended floors just need as much of, the best insulation you can afford, but also for you to foam everywhere that allows draughty airflow eg around the skirting board etc.

2’ of insulation in a room with an open window means the room will never get to temp, as cold air infiltration (and the resultant convection loss of any heated air) will render the UFH useless (at full power) in a second.

 

Wunda will cost the whole thing for you. Simples!