Screed and UFH on a timber first floor

[andyrob]

Anyone done this?

In my stripped out house (!) I'm thinking of installing UFH upstairs on top of the existing timber floors using Myson insulated egg boxes on top of the existing, old but hefty timber joists/floorboards, and a thin screed on that. Some people call this a pug - not sure why? The idea is to have a heat emitter that is better than plates or whatever and to use the egg-boxes which I decided against using downstairs.

Anyway, if you've done it (or indeed decided against it) I'd love to know what you took into account. Weight, obviously, is significant and so keeping the depth of the screed thin is important. Is their a lightweight material, similar to lightweight blocks, that could be used? Other considerations?

I used 70mm over concrete/Kingspan downstairs but upstairs I would like to go barely across the top of the (17mm) pipework and egg boxes and then board over that. As there is likely to be a bit of bounce in the floors I've got to consider cracking, but if I use a strong, fibre reinforced screed (and how about a light mesh) that may be avoided?

I understand a wet, flowed-in mix is ultimately stronger than a dry mix but what would you suggest?

Many thanks for any contribution.

Andy

(To come: pairing 2 small ASHPs and a vented woodburning boiler with the unvented ASHP DHW system (Chelmer tank?); and then external wall insulation. Nothing complicated!)

[rondurrans]

Martin TWO ASHPs.........................

[A.L.]

hello and welcome,

Is their a lightweight material, similar to lightweight blocks, that could be used?

lighter materials are almost inevitably more insulating which would mean a higher temperature for the water in the circulating pipes for a given output, perhaps not good if you intend to use ASHP for space heating

why not put a dry screed between the floor joists and relay the timber floor saving on loss of room height, door realignment etc?

[Ted]

I've done this in a new-build extension.

We just put plywood down on the joists, used clips to fix the pipes to the ply, screwed in battens between the pipes, levelled off with a dry screed (this wasn't totally dry - just no extra water added to building sand that was kept outside and so was damp anyway), then covered over with engineered oak floor boards (once the screed has dried completely).  Screed 25mm thick, same as batten depth, but thinner than that over the actual pipes.  Some flexing resulting in cracking of the screed is going to be inevitable but, without taking floor boards up I don't know. In our case all the strength (and loads) is in the plywood, battens and oak floor boards, so the screed is just there for thermal mass and helping with heat spread.

Been working fine for 5 years now.

Oh, all the space in between joists, below the ply, was filled with insulation.

[andyrob]

Thanks for the advice so far - what a forum.

And if this reply becomes a duplicate it's because somehow my first went astray and I've no idea where!.

Might I be interested in Martin and 2xASHPs? Like minded?

The floor height/door frames bit (A.L.) doesn't matter as the house is stripped out - everything - no walls and even some floors are gone! But very interested in Ted's work - 5 years and going strong - must be fine then.

So, just a question of dry or wet screed (use with fibres and/or mesh = stronger?) and/or anything else that others can think of.

Andy

Billi font size reduced
 

[Countrypaul]

Might I be interested in Martin and 2xASHPs? Like minded?
Andy

I think if you review a few of the older posts, you might get the feeling Martin is less than enthusiastic about the use of ASHPs.

Paul

[Richard Owen]

I'd do what Ted suggests. You can add fibrin to the screed to increase the strength and lower the possibility of cracking. You might also like to consider adding some edge insulation to absorb what expansion and contraction there might be in the screed.

[Ted]

Yes, 10mm polystyrene around the edge as an expansion joint/insulation as well.  As far as the strength of the screed is concerned then it shouldn't be a major issue as you should not be relying on it to carry any load. But if you have springy joists to start with then it would be a good idea to strengthen them first.

[titan]

I used Lewis plates a Dutch product that allows a thin screed and becomes a reinforced floor  www.cdinortherndistribution.co.uk/product/lewis-dovetailed-sheeting

[andyrob]

Thankyou

Well, twelve hours on and I think I have had several very helpful and workable suggestions.

Many thanks to all and having found a couple of Martin's self-confessed negative vibes on other topics let me move on to a new thread which might interest him


Andy