Floor Construction questions / debate

sydthebeat

1. in a situation where UFH is installed and a subfloor is poured, i see lots of contractors persist with a DPM or radon membrane BELOW the subfloor? Isnt it counter productive to allow the subfloor drying out into the dwelling.

Plus, with increased depths of insulation being laid under UFH (say 120mm for this example, with 75mm screed) , and aiming to have a 150mm step outside, if you start the radon membrane or DPM below the 150mm subfloor, you have to carry the RM or DPM 350mm up the enveloping walls.
Laying the RM or DPM ABOVE the floor slab negates this problem. Discuss.

2. Would most contributors to this forum agree that a DPM slipping sheet is now almost compulsory between foilbacked insulation and a concrete floor/screed?

Martron

well once the subfloor is dried the dpm will protect it from further exposure to moisture. so it does not matter is its inside or not.

as for the stepped dpm i dont think it should matter how high it steps allong as it is sealed thus creating a mini tank if you will.

as for the slipping sheet i have never come across it. and would gladly like to hear an explaination

RKQ

This post throws up interesting details!

If we assume there is a need for a slipping sheet, to prevent the conc. screed reacting with the foil insulation then maybe the slipping sheet needs to be upgraded to a dpm.

The radon barrier could then be laid flat over the blinding and blockwork, in one horizontal layer - no corners to fold! (blockwork laid flat to underside of radon level)

The blockwork could then be built to finished floor level and the dpm could be laid as it used to be - up vertical at inner leaf only. The blockwork dpc would join into the dpm to prevent damp. (The radon barrier below covers the cavity) This detail may be easier to install.

Any water in the cavity would be prevented from entering the side of the floor by the vertical part of the dpm.

The need for a slipping layer is something I've just heard about on this site. It hasn't been advertised greatly.

Maybe its need, is just another example of why Homebond is slow to advocate new details / materials without full IAG Certification.

The slipping layer makes sense, as conc. will burn through skin if not washed off.

Time will tell no doubt, but we do need to be careful. For example I learned solar panel tubes have a long life, turns out to be 5 - 8 years! Thats a long life if you are a hamster! At 4 - 7K you need a lot of hot water in 5 years to break even!

As regards ground floors, I really think each should be considered individually and assessed regarding radon area, insulation type and floor depth - conc subfloor or single pour / 150mm slab etc

Martron

RKQ wrote: »

The radon barrier could then be laid flat over the blinding

not sure where you are coming from. will this not then to exposed to unnecessary possibilty of damage from the mesh in the floor .

mind you i have seen some dodgy installations before where i would imagine they were just wasting there time with it. i feel that if seversl methods of installation were introduced this would just cause confusion.

keep it simple possibly one method.

but i would agree that if you just had one membrane / slip sheet would save time and money and combine them all methane/radon/damp proof. so you have just a flat sheet with no upstands

sydthebeat

Well, you cant have 1 membrane because you need to protect the insulation from below, wet insulation is useless.

The below pdf is my query.

RKQ

2 no. membranes - radon and dpm! Never suggested one membrane????

I'm assuming that you are using a radon barrier, in a standard floor slab - no steel mesh! Ground supported sub-floor.

Radon is laid flat / horizonal / over entire structure.
Assuming we need a 500 guage slipping sheet, why not upgrade it to 1000 guage and use it as a dpm?

Foil insulation on conc. sub-floor on radon barrier.
1000 guage dpm on foil insulation.
75mm - 100mm conc screed on dpm!

It makes radon easy to install as theres no upstands!
It converts slipping sheet to dpm - minimal cost!
Stronger blockwork detail.

So it you need to ensure radon seal, use foil insulation and wish to prevent conc reacting with foil, then use above.

sydthebeat

RKQ wrote: »

2 no. membranes - radon and dpm! Never suggested one membrane????

I'm assuming that you are using a radon barrier, in a standard floor slab - no steel mesh! Ground supported sub-floor.

Radon is laid flat / horizonal / over entire structure.
Assuming we need a 500 guage slipping sheet, why not upgrade it to 1000 guage and use it as a dpm?

Foil insulation on conc. sub-floor on radon barrier.
1000 guage dpm on foil insulation.
75mm - 100mm conc screed on dpm!

It makes radon easy to install as theres no upstands!
It converts slipping sheet to dpm - minimal cost!
Stronger blockwork detail.

So it you need to ensure radon seal, use foil insulation and wish to prevent conc reacting with foil, then use above.

the problem with that makeup, RKQ, MAY be that the radon membrane would possible be 150mm below outside path level. Therefore the subfloor can be considered wet, and any insulation laid on top can also be considered wet. Therefore my argument would be to lay the RM over the subfloor and under the insulation, in the typical up and over inner leaf manner.

I can see the advantage of laying the radon membrane without upstands, perhaps it could be done. If the subfloor is levelled off the top of the rising walls, then the radon membrane could be laid horizontally across a complete flat surface. This radon level would be outside path level. The cavity wall can then be started over this radon level.

On the slipping sheet, it would also be my opinion that it should be min 1200 gauge anyway.

RKQ

If you are using a 1200 guage slipping sheet, then I suggest loosing the radon barrier and using the slipping sheet as a 1200 g radon barrier.
Detailed appropiately.

A 1200 guage slipping sheet is good enough to be a radon barrier. Why lay 2 membranes, at radon quality? ( and cost + labour)

I have noticed many housing estates with a flat radon barrier, laid 225mm below FFL. Timber shuttering is used to cast the floor slab, no dpm used!
I assume water penetrating the cavity will penetrate the side of the floor slab.:(

sydthebeat

we need 2 layers.

One below the insulation to keep it dry.
The other above the insulation to restrict reaction with concrete.

Do you think theres any issue with moving the lower membrane from below the subfloor to above the subfloor?

Slig

While I agree with the DPM/Radon barrier above the floor slab, raises your waterproof layer higher above the ground, means that your floorslab isnt drying into the building etc. but it does pose a problem.
Typical construction means that barrier is positioned and then the slab poured over it. With this system the barrier is left exposed while the blockwork goes up for the walls, plastering, first fix etc meaning there is a much higher chance of it getting damaged. The best way I can think of to negate this is to either:
a) cover the membrane with ply throughout the build until insulation & screed are poured or
b) fit a DPC to all the walls with sufficent lap to connect to the radon barrier when the building is ready.

sydthebeat

Slig wrote: »

While I agree with the DPM/Radon barrier above the floor slab, raises your waterproof layer higher above the ground, means that your floorslab isnt drying into the building etc. but it does pose a problem.
Typical construction means that barrier is positioned and then the slab poured over it. With this system the barrier is left exposed while the blockwork goes up for the walls, plastering, first fix etc meaning there is a much higher chance of it getting damaged. The best way I can think of to negate this is to either:
a) cover the membrane with ply throughout the build until insulation & screed are poured or
b) fit a DPC to all the walls with sufficent lap to connect to the radon barrier when the building is ready.

or fit the insulation and pour screed at floor stage.....

Slig

sydthebeat wrote: »

or fit the insulation and pour screed at floor stage.....

services? plumbing, underfloor heating etc. very handy to have a screed

sydthebeat

Slig wrote: »

services? plumbing, underfloor heating etc. very handy to have a screed

yes, but this can all be done at floor stage, before blockwork..... the same way as it is done with a traditional 150 slab floor.

sinnerboy

sydthebeat wrote: »

Well, you cant have 1 membrane because you need to protect the insulation from below, wet insulation is useless.

The below pdf is my query.

I agree with this ^ ^

Advantage of radon barrier below slab is that the barrier is protect then . Sand blinding provides even surface to base of structural slab , so reduced levels are easy to check . My fear would be that less care will be taken with the floor slab which may be poured over fill containing contaminants .

Also if the radon barrier is placed over the slab - the site activity of placing insulation slab over over it is potentially hazardous ( ok not highly so ) but you can imagine dragging boards over a slippy rain wet membrane .... . even in dry conditions there is in increased risk of damage to the membrane compared to pouring concrete over .

I too only became aware of the "risk" of wet screed attacking foil facings . Mr Lion was very coy about it when I asked him . "well you don't HAVE to do it .... it is a precaution in our literature now" . B******x ..... Still, I have added layer of 500 gauge polythene below screed even though I don't "HAVE" to .

Changing tack a little ... see on you sketch syd - the screed edge insulation junction with the bottom of the CPB . Accredited Details call for foam fill here - REASON - contractors sensibly will keep bottom of board up and away about 10mm from (drying out ) screed . Otherwise skirtings warp and decorations spoil . I have taken to detailing min 60 or even 80mm vertical ins to screed so tha CPM lands tightly on insulation board , not screed . Then silicone seal continuous bead to base of skirting masks junction