If only I could turn back time....

[Carol W]

I have absolutely no building knowledge !

We have a new extension which will be open plan with two old rooms which formed part of our 1994 build home.

My original plan was to have radiators in the old section and UFH in the new.

The plumber suggested we use Wunda rapid response overfloor heating so we could have the same throughout the big room but I really didn't want the floor height to change. So, the builder decided it wouldn't be too much work to dig up the screed in the two old rooms.

It turned out there was only 40mm of screed before block and beam. No insulation. I've looked at replacing the blocks but I think you then need a structural screed on top and I'm not sure we have the depth for that. 

Their plan was to put slurry on the block and beam (apparently the original builders should have had this) and then a layer of ply, then the Wunda trays, then self levelling, then the porcelain tiles.

The screed in the new extension was applied (over deep insulation) to the same level as the block and beam in the old so that both parts of the room could have these same layers. 

Then I started investigating and realised that in the old section of the room there would be nothing much stopping the heat travelling down to the block and beam and I'd be wasting money heating the ground!

I really want to work out how much that would cost me in money terms so I can show the builder. How can i do that ?

So I'm going to tell the builder I want to go back to the original idea of radiators in the old part and ufh in the new.

How should he build up the layers to do the best job in each section (one with and one without ufh) given that we only have 40mm (including a 10mm tile) to play with and the Wunda trays are 20mm. 

I have been told that ply is a bad idea but the plumber says the trays must have a really solid base to sit on or be screwed onto. 

Help! My husband really isn't into any of this and although I'm learning a lot, I know my knowledge is limited. 

[SteamyTea]

With UFH it is really down to how much insulation you can get under it.

I suspect that radiators are going to be your best option.

But still add floor insulation if you can.

 

If you can put up with a bit of noise, and the new room/s are not constantly used, then plinth/fan assisted radiators may be an option.  They take up less space.

[joth]

1 hour ago, Carol W said:

I really want to work out how much that would cost me in money terms so I can show the builder. How can i do that ?

You need a room by room heat loss calculation for the proposed build.

Various ones exist online, a lot of folks here report good results using the spreadsheet from here:

 

 

[ToughButterCup]

3 hours ago, Carol W said:

...

I really want to work out how much that would cost me in money terms so I can show the builder.

...

 

What a lovely person you must be  .... Could I suggest a different approach? 

Work the problem out for yourself (the heat calc suggested above)  put your calculation in a post on this board, we'll check it (well the nerds will) and then

 

Tell The Builder What You Want Done.

 and when you've done that

Ask whether he still wants the job or not. And if he does, tell him to get on with it.

 

Deference, partnership, customer engagement, intelligent discussion, shared interest in heat calculations  - and builders - almost never mix.

Harsh? Yes. 

[JohnMo]

I would not bother with the UFH, not had a good experience with the UFH trays similar to those suggested. Seem to need very flow temps to get any heat output.

[SteamyTea]

22 minutes ago, JohnMo said:

Seem to need very flow temps to get any heat output

?

Slow flow rate or high flow temperature.

[Iceverge]

Needs a high temp I think from reading. 

[SteamyTea]

2 minutes ago, Iceverge said:

Needs a high temp I think from reading. 

High temperature UFH is not the best, higher losses to the ground, not so nice to walk on in bare feet.

[JohnMo]

1 hour ago, SteamyTea said:

?

Slow flow rate or high flow temperature.

Mistyped, should read high flow temperature.

 

20 minutes ago, SteamyTea said:

not so nice to walk on in bare feet.

Unfortunately even with 40 deg flow temperature the floor still feels cold to touch.

[SteamyTea]

16 minutes ago, JohnMo said:

Unfortunately even with 40 deg flow temperature the floor still feels cold to touch.

Where is it all going, downwards?

[Iceverge]

Here's a look at the Wunda system with 12mm ply. 46mm thick, That U value isn't the whole story because where the pipe tracks are there's almost zero insulation under them and they're the greatest point of heat loss. 

 

 

 

Here's my proposal.

 

Sand cement slurry to fill any large gaps in block and beam. Self levelling compound to level out any big dips. 

 

25mm PIR, all joints tightly fitted, foamed and taped with aluminium tape. Foamed with IL330 or taped to walls for airtightness. 

 

Float 2 x 11mm OSB layers , joints crossed, screwed and glued.

 

Then finish with lino or LVT.  Total 50mm thick. Much cheaper than the Wunda system and 40% less heat loss. No specialist materials and more solid to boot. Alternatively replace one layer of OSB with T&G engineered flooring glued down. You may need to lay the OSB on a diagonal to avoid any joints lining up. 

 

 

 

 

For the heating I would opt for radiators throughout. Easier to balance for this application and with suitable floor coverings you won't suffer from cold feet. Make them as large as you can to future proof for an ASHP.  

 

 

 

Edited December 5, 2023 by Iceverge

[JohnMo]

8 minutes ago, SteamyTea said:

Where is it all going, downwards?

Not very sure, have 200mm of polystyrene below. And the same at the sides. So only really one way to go and it's up.  Have flow through the pipes so that's not the issue have dT across supply and return. Pipe spacing is 125mm. Think the thermal resistance upwards is just too high. At 40 degs flow temp I should be putting out twice the required W/m2, that is required to heat the building. So will resort to electric heater and maintain 15 degs and then heat as required after that.

 

I should have just installed a couple of 1000mm x 400mm rads and run at a max temp of 35 to 40, job done. That's my plan for next year. Done enough messing this year.

 

Agree with @Iceverge above for this thread. UFH over rated. Size the rads for DT 20 as per the link below.

 

https://www.heatgeek.com/do-i-need-to-upgrade-my-radiators-for-a-heat-pump/

 

Heat pump ready good for gas boiler efficiency.

[Carol W]

6 hours ago, Iceverge said:

 

Float 2 x 11mm OSB layers , joints crossed, screwed and glued.

 

Thanks. Do you not think there would be any issue with ply/OSB ? It's a kitchen/lounge/diner and I've had some people tell me to avoid wood under the tiles. I suspect they are thinking of moisture getting through 

[Carol W]

11 hours ago, joth said:

 

Various ones exist online, a lot of folks here report good results using the spreadsheet from here:

 

Thanks for the link. I'll give it a go at the weekend. Never enough hours in the day !

[Carol W]

9 hours ago, ToughButterCup said:

 

What a lovely person you must be  .... Could I suggest a different approach? 

Work the problem out for yourself (the heat calc suggested above)  put your calculation in a post on this board, we'll check it (well the nerds will) and then

 

Tell The Builder What You Want Done.

 and when you've done that

Ask whether he still wants the job or not. And if he does, tell him to get on with it.

 

Deference, partnership, customer engagement, intelligent discussion, shared interest in heat calculations  - and builders - almost never mix.

Harsh? Yes. 

Yep...trying to sort the problem out myself because no builder can know it all. He's a nice guy and all the people who work for him on our job are also lovely. He'll do whatever I ask him to do but I'd rather bring him along on the journey of discovery. Not sure he'll get deep into the heat calculations but he'll listen to what I want. 

 

[Iceverge]

6 minutes ago, Carol W said:

Thanks. Do you not think there would be any issue with ply/OSB ? It's a kitchen/lounge/diner and I've had some people tell me to avoid wood under the tiles. I suspect they are thinking of moisture getting through 

 

If the floor is well tiled and sealed it'll be fine. I would be slow to put any wood on the cold side of the insulation as water vapour might condensate on it and rot. You'll need to if you opt for Wunda however unless you want to use a cement board instead or a thin screed beforehand.  If it's lightly to be in an area regularly wetted like near a kitchen sink or a bathroom then you could tank the OSB before tiling. 

 

I would consider a floor level drain like a wetroom in the case of a flood caused by a burst pipe etc. I wish we had done it in all the rooms with water in our house. Imagine coming back from holidays and finding a burst pipe has harmlessly drained for a week into a drain. Mop the puddle, fix the pipe and you're done. If it happens in our house it'd flood the entire house until it reaches the level to get into the shower tray downstairs ☹️

 

 

[ToughButterCup]

On 05/12/2023 at 21:51, Carol W said:

Yep... He'll do whatever I ask him to do but I'd rather bring him along on the journey of discovery... but he'll listen to what I want. 

 

Send him to Lancashire please . He'll never ever want for work. 

 

Lucky you are indeed