Thinnest possible screed options for underfloor heating over concrete floor

[bmj1]

Hi all,

 

Our build-up is:

 

1. Concrete floor (beam/block)

2. PIR

3. Screed / underfloor heating

4. Engineered wood 15mm

 

Please can I ask this forum: what is the very thinnest screed option available? That will be effective and durable.

Edited June 4, 2023 by bmj1

[Dave Jones]

how much PIR ? 

75mm is average for screed.

[Conor]

Liquid screed, 40-50mm.

[JohnMo]

Thinnest screed is no screed, just do a floating floor on pre grooved insulation panels.

 

Not suggesting this company but the first one that came up on a search

https://ambienteufh.co.uk/product-category/floating-floor/

[Temp]

+1

 

We have 21mm Engineered Oak over UFH and B&B without screed. We actually fitted battens the same thickness as the insulation and secret nailed the Engineered oak to that.

 

We did find we needed high flow temperatures to push enough heat through the floor when it's very cold but that's possibly because we built to 2005 Building Regs and have a full height window and no curtains.

 

Put as much insulation under the UFH as you can. We only have 80mm and if building again might double that. I know better now.

[Alan Ambrose]

>>> We have 21mm Engineered Oak over UFH and B&B without screed

 

That works OK heating wise? Gut feel suggests that (a) wood isn't the best heat transfer medium and (b) that the wood might get upset (bows etc) with the heat below. Good to know if that works well?

[bmj1]

How about this ?

 

https://www.ready2pour.co.uk/liquid-floor-screed/gypsol-ts-15/

[Nickfromwales]

On 05/06/2023 at 11:23, JohnMo said:

Thinnest screed is no screed, just do a floating floor on pre grooved insulation panels.

 

Problem here is that the block & beam floor will need to be SLC'd before the floor is anywhere near uniform enough to do away with a screed.

[Temp]

2 hours ago, Alan Ambrose said:

>>> We have 21mm Engineered Oak over UFH and B&B without screed

 

That works OK heating wise? Gut feel suggests that (a) wood isn't the best heat transfer medium and (b) that the wood might get upset (bows etc) with the heat below. Good to know if that works well?

 

Engineered wood is better than solid over UFH. Much less likely to warp and cup so you can risk wider boards. Ours are 210mm wide and you can't tell they aren't solid wood. 

 

We don't have it but many others have Carpet over OSB and ufh in dry mix on first floor.

 

We have a mixture, just about everything somewhere in the house. From best to worst I rank them...

 

Tiles over ufh in screed.

Stone over ufh in screed.

Wood over ufh (foiled insulation/heat spreader plates)

Carpet over ufh in screed (but it does depend on the underlay and carpet).

 

As I said above we do need to run high flow temps  but see reason above. We have an oil boiler so that's no issue. 

 

Note that to meet building regs wood floors spanning joists are meant to be 18mm thick. So be careful when shopping for engineered wood as some is 14mm and may require chipboard under it. You don't want 18+14 = 32mm of wood.

[Gus Potter]

On 04/06/2023 at 18:34, bmj1 said:

Hi all,

 

Our build-up is:

 

1. Concrete floor (beam/block)

2. PIR

3. Screed / underfloor heating

4. Engineered wood 15mm

 

Please can I ask this forum: what is the very thinnest screed option available? That will be effective and durable.

I'll come at this from the practical installation / designer side.

 

Start with the beam and block flooring. See manufacture's literature.. the beams tend to be bowed upwards as they are pre stressed concrete. That is your starting datum. The thin point is in the middle. beam and block floors are not flat!

 

Next is the PIR. Often you find the PIR sheets have a bit of a bow.. they should be flatish but you get sheets that are definitly not when they turn up on site, but you are buying a small quantity as a one off.. so you lack clout to some extent..  you have two possible obvious choices.. have a barny with the PIR supplier or live with what you have... a barny will cost time on site, and you have to know your stuff about tolerances to be in a position to reject material. You need a bit of weight to bend the sheets back flat.

 

Pipe congestion. There is software that you can use to lay out your UF pipes.. looks great.. but all the pipes coalesce as you near the boiler / manifold.. that is the bit where the softare falls down. Also you may have to get around structural components, things the SE / Architect / you are not keen on you altering.

 

For me I would go for a 90mm to 100 mm screed. The cost lies not so much in the thickness but good preparation. Setting plenty level datums and so on and making sure you can get back in to polish the odd rough spot.

 

Now you want an engineered wood floor. Have you looked at the warranty on this? The wood floor folk make it REALLY difficult to stick within the temparature range.. and that includes areas where there are conjested pipes.  If you want to stick the rules re not eceeding surface temprature.. hard to do at times.. that little bit of congestion can drive the design if you want to comply with the flooring warranty. This leads to a thicker slab / screed to try and comply with the flooring warranty.

 

Next is do you glue the flooring to the screed or float it?

 

I've tried various options over the years tackling UF.. I'm kind of.. if you don't like my opinions.. I have others.

 

At the moment I'm experimenting on my own house. I have floated an engineered floor on a 2.0mm foam layer on a structural 100 mm thick concrete slab with A142 mesh and some movement joints on PIR. Yes I know it is an insulating layer.. but I made the insulation under the floor a bit thicker.. so far been working great for the last three years. The big thing here is that the UF tends to go off in the summer. Funnily that is when the moisture content in the floor rises so swelling occurs in the timber flooring, sounds odd that the moisture content in a timber floor rises in the summer with UF!

 

I decided to float as opposed to glue as I have one floor that is big but about half is over a timber suspended floor , the other over a slab / PIR. At the back of my mind is this. If something goes wrong if floated more easily fixed. Also one part is the kitchen area. If I get a flood maybe easier to fix than if fully glued.

 

But another big thing for me is how I want to live. My wife (a buddingdesigner also) likes rugs (our spaces for example change from season to season) and the things that make a home a home.. for both of us. We have eye to the future.. we want to be able to sell the house to someone who may want to live differently from us.

 

If you don't want a sterile environment .. be able to put down rugs, bean bags, large sofas that insulate the floor that act as an insulator then I have found that embedding the UF pipes in about 90-100 mm of concrete material provides room for pipe congestion, accounts for tolerances in the substrate below and distributes the heat more than a thinner screed / slab. We have a few rugs that insulate the floor but eventually the heat gets out..  yes I know not perfect.. but we have a house that is designed to be flexible and easily adapted... depending on our mood.

 

My advise is .. go for a thicker screed as it will mitigate hot spots and make on site construction much easier. The game changes a lot if we are doing this at first floor level for example as we are adding weight to a structural floor spanning between say walls C.f a ground bearing slab arrangement.

 

In terms of overall cost I think that a thicker screed distributes the heat more eavenly but the big thing is that it cuts all the trades folk /PIR / flooring supplier and even the SE a bit more slack and that will drive the cost down much more than say reducing the screed thickness. Ask builders to work to tighter tolerances costs a lot more than making say a screed a bit thicker for ground bearing UF applications.

 

Compare the extra cost say between making the screed 30mm thicker.. 70 - up to100 mm . Now take a tradepersons rate ( a good one) at £250 - 300 per day. Work out how much extra time it will take the trades folk to work within a 70mm screed and for the flooring to work. Then compare with the extra cost 30mm of concrete. I think once you look at this holistically and talk to your builder you may find that the simple stupid thicker screed is the way to go.

 

In summary if you are thinking about this sort of stuff and it's new to you then always think.. yes I can see that folk say on the internet that they can supply a thinner / stronger product.. but look at the knock on effect in terms of the other trades that have to work around that and how used to it they are. You local builder can be great value if you play to their strengths.

 

Lastly if this is your forever home.. always think about.. what if there is water ingress, what if say in ten years time we want to lay tiles.. in the round you may regret not just putting in a thicker screed.. the extra concrete is not that much.. the cost lies in the preparation, the laying squad coming to site, the plumber having to get all the pipes so they have cover and so on.

 

It's like baking some cakes (so I'm told).. sometimes heating the oven cost more than the ingredients.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited June 6, 2023 by Gus Potter

[JohnMo]

Good post, to add at congested area you may need to insulate the UFH supply pipes to avoid general room overheating also.

 

Another advantage of a thick screed is you use it as a thermal store, so if your boiler or heat pump is too big in the shoulder seasons, you charge the floor at a hotter temp than would be normally ideal, for a few hours and it will drip feed the house with heat for the rest of the day; like a storage heater.

[saveasteading]

19 hours ago, Gus Potter said:

the beams tend to be bowed upwards as they are pre stressed concrete.  Slab beams have very big bends in the.

With T beams.i have never venvounted a bend.

If there is crown, this determines the thinnest area of screed.

 

[Temp]

On 06/06/2023 at 23:50, Gus Potter said:

Funnily that is when the moisture content in the floor rises so swelling occurs in the timber flooring, sounds odd that the moisture content in a timber floor rises in the summer with UF!

 

Now you mention it we have noticed that as well. It's because cold air in winter can hold less moisture so humidity is lower in winter than in summer. I don't think it's anything to do with the UFH itself. 

 

Its what the gap around the edge under the skirting is for.