Screed flooring - anybody got any ideas?

[Snuggy]

We bought a house last year. Despite paying for the most expensive survey and thinking no probs, there was a rotting floor in the extension area.
We replaced some to get it liveable but it is not a long term solution.

The existing floor is screed, liquid dpm, sand layer to even out, 35mm polystyrene insulation,18mm chipboard finish. It butts on to a concrete floor original building.

I have heard there are some insulating screeds.

Is it feasible to lay a screed on top?
I have heard there are insulating screeds available...anybody used these and which product? Do they really insulate?
Have I enough depth to lay wet underfloor heating into that screed?
Any other suggestions?
Thanks.

[Stinsy]

I could tell you a few "war stories" about surveys not being worth the money you pay for them! The "issues" they do find tend to be spurious and waste time/money confirming they're not a problem and they often miss really obvious genuine problems.

The first step is to find out for certain where the water is coming from.

Extensions tend to be poorly insulated and are sometimes seldom used and therefore heated less. This causes damp air from cooking, drying washing, cleaning, making and consuming hot drinks, perspiring mammals, etc., from the whole house to condense and cause damp in the one slightly colder room. The solutions for this are: better insulate the extension, heat it more, improve ventilation, use a dehumidifier.

The next thing I'd look at would be rainwater leaking in. This can be from a damaged or improperly fitted roof, a problem with flashing, or failed gutter allowing water to round down an external wall, or whatever.

Damp rising up through the floor is very rare. There was a time when chemical damp-proof courses were proscribed to remedy every instance of damp. This rarely solved the dam because (as I said) "rising damp" is very rare.

Now if you want to re-screed in order to bring the floor of the extension to the same level as the house or because you have determined that its poor insulation is the cause of the dam then go-ahead. 100mm of EPS followed by 100mm of screed is a good starting point. And laying UFH pipe on top of the insulation below the screed is pretty simple. Screeding is hard work but well within the remit of a DIYer if you have the motivation.

I wouldn't worry about "insulating screed" snake oil. Insulation and screed are two different components.

[Snuggy]

Thank you for replying. I know where the water came from and that is resolved. I can not have the uproar of 200mm as that would mean digging out.

[Countrypaul]

You wrote "The existing floor is screed, liquid dpm, sand layer to even out, 35mm polystyrene insulation,18mm chipboard finish." I assume this is from the lowest level rising to the Finished Floor level.
Are you sure what you refer to as screed in that mix is a screed layer over concrete or is it simply a concrete layer on the ground? If the latter, and you are unable/unwilling to dig out to create more depth then I think you have limited choice. An insulating screed would seem pointless to put UFH pipes in as it should be insulating not transmitting the heat. Screed normally has a minimum thickness often around 25-40mm but if over insulation then thicker.

If you have a screed layer over concrete then removing that screed layer putting PUR/PIR insulation down and then a screed layer might be an option.

If are stuck with just the 48mm abouve the existing screed/concrete I suspect the option of pre routed chipboard (for UFH pipes) over insulation might be the simplest option.

If you do go for UFH check out what flow teperatues will be needed as with chipboard these can be higher if the pipes are say 10mm. Obviously the floor finish will also affect the flow temp.

[Stinsy]

Don’t be scared to dig out. Doesn’t take long to break it up and barrow it to a skip.

For UFH to be ecconomic and effective you need decent insulation below the slab and the UFH pipe embedded in a thick slab. If you were building new you might consider 300mm of EPS below a 200mm slab. 100+100 is therefore pretty conservative.

[Countrypaul]

I agree with Stinsy, whilst digging down may seem very intimidating, don't dismiss it without serious consideration. The longer term benefit is likely to more than make up for the short term disturbance.

We are on a valley side so the front of the house is 5ft about ground level and the rear is at the excavted ground level (4m back he gound is 3m high). Breaking up the concrete floor on two rear rooms and excavating down 2 feet only took 1/2 day (admittedly this was only part of a much larger renovation project so a couple of people). We put a beam and block floor in (100mm thick blocks 1500 deep beams irc) then DPM, 120mm PUR/PIR insulation, VCL to limit contact of screed with aluminium foil on the insulation, then UFH pipes and a 70mm sand and cement screed with fibres in. One or two places we had to put some floor leveling compound down after someone put their foot on the screed brfore it had set properly but other than that the floor covering was laid directly on top of the screed.

[Snuggy]

Are you sure what you refer to as screed in that mix is a screed layer over concrete or is it simply a concrete layer on the ground?

I think I must be muddling my terms as you imply. I believe the stuff under the dpm is concrete, so I could do away with sand/polystyrene and chipboard, lay a 38mm "screed" to match the floor to the main house level,and forget about insulation. But no underfloor pipes could be suspended and do a proper job in that?

[Tinbum]

Personally, I'd dig it out for the extra effort and do a proper job.

[drjim]

Are you sure what you refer to as screed in that mix is a screed layer over concrete or is it simply a concrete layer on the ground?

I think I must be muddling my terms as you imply. I believe the stuff under the dpm is concrete, so I could do away with sand/polystyrene and chipboard, lay a 38mm "screed" to match the floor to the main house level,and forget about insulation. But no underfloor pipes could be suspended and do a proper job in that?

You can do that if you want a cold floor.

I converted the last bit of my built in garage to be a utility room last year, most of it had been done by the builders when we did our extension, 100mm kingspan wet UFH pipes and screed then ceramic tiles. For my extended bit (took down concrete block wall and created split utility/boot room) I found I could get 70mm of insulation on top of the existing garage floor then electric UFH and ceramic tiles but it only just fitted. I'd much rather have had 100mm but couldn't quite face the effort (plus my Wife was going mad about the dust and mess it was all causing!).

If 70mm wouldn't have fitted the big drill would have had to take the concrete out irrespective of mess, that's what the builders did in the rest of the space. No insulation is just putting an enormous cold radiator in your house, not what you want in winter.

Why not just put some timbers in and flooring boards over the top, just treated battens is all you need and concrete screw into the sub floor. You could even get a bit of insulation in there as well, but bear in mind air gaps.

[cojmh]

I am currently converting a small garage into a downstairs bedroom and wet room and I decided to dig out the floor and also entrance hall to put in underfloor heating (get rid of radiators as obstacles for a wheelchair).

Like the others have said .... seriously consider digging out .... it might take a bit of time but it is not as bad as I imagined to begin with.

The architect that drew my drawings specified the following layers to meet current U value building regs.

(from bottom to top)

1. Hardcore with sand (Like MOT) - in my case the builder who is helping me has suggested reusing the concrete we are breaking up to save on skips space and also moving it!
2. DPM
3. 100mm of Concrete
4. DPM (not sure why there are two layers one above and one below the concrete but there are)
5. 150mm PIR insulation
6. 75mm of Screed with UFH in it.

I have watched the builder and to be honest if I was fit enough and we didn't need to put sewer lines and other draining etc. into the concrete layer then I think I would be quite happy to do it myself.