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Vapour Barrier With Underfloor Heating Pipes

Fri, 22 Nov 2024 18:04:32 GMT

Do I have to use a vapour barrier with underfloor heating pipes? This is a question I get asked at least once a week..

The quick answer is Yes , you do.
Often referred to as a vapour barrier, vapour control layer (VCL), or a separating membrane. This must be placed on top of your insulation, before clipping your UFH pipes down.
There are several reasons for this:

As it suggests, it is to control the moisture from within the subfloor, essentially stopping any moisture from being brought to the surface.

Prevent any potential reaction between the screed and the PIR foil faced insulation, which can in turn damage your screed.

Help to prevent the insulation from being degraded by the wet screed.
Prevent liquid screed from migrating under the insulation and floating it, which can be extremely costly.

Another very common question is "Should I put the VCL under or over the pipes?" I see this installed incorrectly every week on social media.

It must go below the pipes for the above mentioned reasons.

Also, if you put the VCL over the pipes, the screed will not fill all of the voids around and under the pipes. This will induce cracking in the screed and cause problems.
Having the VCL on top of the pipes will also cause the floor to sweat. Again causing problems.
Having the VCL below the pipes stops the heat from the pipes being in direct contact with the cold from the subfloor.

Here is a link for a full rundown for the correct procedure of floor preparation: www.superflowscreeds.co.uk/preparation-guide

Making the right liquid screed choice

Wed, 24 Nov 2021 12:49:54 GMT

An overview of liquid screeds

There are essentially two main types of liquid screeds. Calcium sulphate based Anhydrite/Gypsum screeds and Cementitious screeds. Both are extremely good products when used correctly.

Liquid screeds have many benefits:

Self compacting, so great when used over/around underfloor heating pipes, filling all voids.

Good heat transfer with underfloor heating.

Increased productivity- 2,000m2/day can easily be achieved.

Can be walked on in 24-48 hours.

No reinforcement required.

Low shrinkage - does not curl and reduces the risk of cracking.

Significantly reduced thickness when compared to traditional sand & cement screed.

Much larger bay sizes without the need for expansion joints, depending on application.

There is a wide range of Calcium based liquid screed products, to cover all project requirements.

Something to bare in mind when opting for a Calcium based screed, is that they will form a surface laitance (crust) when drying. This will have to be removed by means of mechanical sanding/grinding before adhering any floor finishes. If this is not done, then the floor finishes will almost definitely "pop" and lift in time.

The Cementitious liquid screeds tend to have shorter drying times, which is great where projects have time constraints. They are a little more expensive (Approx' £45-£50 per m3) but they do not form a surface laitance, so more cost effective in the long run.

Post screed guide

superflowscreeds@gmail.com (Ian Wilton) — Wed, 24 Nov 2021 12:17:11 GMT

Improving the drying time

Once your liquid screed is placed, the building should be sealed for 24-48 hours. You can then walk and work on the screed.

After the initial 24-48 hours, the building should be ventilated daily, by means of opening windows and doors to encourage an exchange of air. This will aid the drying time.

If you choose Anhydrite or Gypsum liquid screeds, they will form a surface laitance (crust). You will need to have the floor mechanically sanded within 7 days of install. If left longer, this could result in mechanical grinding with diamond segments, which will be more expensive. Good site planning can save you money here. The removal of the surface laitance will make sure that any adhered floor finishes will stay stuck and not lift. Mechanical sanding also opens and increases the surface area of the screed which again will aid the drying time.

If you choose a Cementitious liquid screed, they do not form a surface laitance.

It is important that you do not store things like sheet materials directly on to your new floor screed. They should be placed on bearers first to allow good air flow over the screed. Again, this will aid the drying time of your liquid floor screed.

If you have underfloor heating within your liquid screed, you can gently force dry the screed after 10 days by introducing a low heat. The heating should be commissioned and turned on in this way before adhering your floor finishes. Turn heating off and allow 72 hours before tiling etc.

When it comes to your final floor finishes, your floor fitter should check your floor screed for moisture content and make sure that it is dry enough before commencing with their floor fitting.

If you choose an Anhydrite or Gypsum liquid screed, it is important that the correct primers and adhesives are used. Cement based adhesives should not be used as they could react with the screed.