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Heh, I wish it were that easy so many conflicting opinions around... manufacturers recommend adding battens in places where a high load is expected. I don't want to risk the floor slightly sagging over time because I add a heavy load in a place that was previously unsupported, hence why we're going with battens. Regarding the OSB, by "glued and screwed" you mean between them? (or where is it screwed to, if it's floating on top?) Regarding VCL, please see the following for an example, they all specify a VCL above insulation, below OSB/plywood/floor https://www.kingspan.com/gb/en/knowledge-articles/a-guide-to-constructing-a-floating-floor/ https://assets.publishing.service.gov.uk/media/62a0683ad3bf7f036fc493ba/solid-floor-insulation-best-practice.pdf https://polyfoamxps.co.uk/floor-insulation/concreate-slab-insulation/ https://www.permagard.co.uk/advice/installing-vcl-vapour-control-layer

All the documents I could find recommend installing a DPM below and a VCL above the insulation (especially for vapour closed setups) to prevent moisture from reaching the insulation and getting trapped between insulation and slab. I could be missing or misunderstanding something though

so apply butyl tapes all along the battens wherever we intend to screw the boards down? (30cm spacing on long edges, 15cm on short edges, SpruceFloor recommendations) Do you mean the double-sided one, with the idea that it will kind-of seal the hole once the screw is in? For a similar reason, installing the VCL below the plywood means I wouldn't be able to glue the boards down to the battens to increase stability we have done that on first floor (screwed and glued, no VCL) with excellent results, but it seems like that won't be an option, if the VCL has to be below.

thanks for sharing that 👍 I'll be honest, I did think about the silicone

Hi everyone, we're about to install some insulation on top of a solid concrete floor. Context: Ceiling height is already very low, so we've got very little space to play with. We've decided to go with the following build-up: - existing concrete - DONE: liquid DPM (mapeproof onecoat) - 25x38mm battens, 40cm spacing between centers. This is to increase stability of the insulated floor (alternatives like PIR and wedi boards were discussed at the linked thread) - 25mm Kingspan K103 phenolic board - VCL? - 18mm SpruceFloor plywood - VCL? (alternative position) - underlay - 12mm laminate (most likely QuickStep Impressive Ultra) to gain additional stability There are a few challenges which I would like to get your opinion on: any advice on how to merge the liquid DPM with the existing DPC? The DPC is very short (60s maisonette in an ex-council block) and neither Mapei nor Ardex mention this challenge in their technical documents. Haven't contacted their technical teams about this matter, yet. given we're going with battens, the plan is to screw and glue the 18mm plywood to the battens. Installing the VCL below the plywood would mean screwing through it to fix the plywood in place, which doesn't seem like a great idea. Is that how it's usually done? I read a number of documents on insulated suspended timber floors but they never mention how the floorboards are then fixed to the joists without perforating the VCL underneath. Would it make sense to install the VCL above the plywood? The kitchen is already in place and we will not be able to Installation guides for the VCL recommend lapping 10cm on the walls, which makes sense but... Where is that taped if the wall is already built? On the finished wall? If so, how would you then install the skirting board on top of it, assuming the VCL will already cover 10cm height from the floor? Nailing the skirting board in place, perforating the VCL, doesn't sound like a good idea...but at the same time you cannot glue the skirting board to the VCL, as that's just taped to the wall... The only option I can think of getting a 15cm tall skirting board and then glueing it in place using the top 5cm or less which are above the VCL... How do people usually workaround that limitation? Does anyone have experience with similar projects?

Thank you, and did you screw through the VCL when installing the OSB? It seems like a big no-no, but alternatives are either putting the VCL on top of the plywood (though every best practice document says it should be right on top of the insulation) or laying the plywood without screwing it to the battens, which I fear will decrease its stability. We went with the battens approach as that will allow us to use phenolic foam instead of PIR, and even though there will cold bridges due to the battens, the overall gain should be positive, or at least that's the bet. Laying a floating floor directly on top of PIR (140kPa compressive strength) didn't really convince me, after seeing first hand the 120kPa strength phenolic foam K103 behaves under pressure. (we spoke about the other alternative, wedi boards, a few posts up)

@Ferdinand I think I am going to go with your plan! Which VCL did you install?

good call out...I'm a bit scared of opening another can of worms so late into the works 😅 I'll have a look, thanks

Thanks @Ferdinand I had a good read at the linked thread good tricks! In our case, the rewiring has already been started and it's going on the concrete ceiling (second ceiling to be built on top of that). Heating pipes will run below the internal walls insulated plasterboard boards, as those are 62.5mm thick We're having an ASHP installed, too, but unfortunately we couldn't go for UFH because of the big downward heat lost that would create (the concrete floor is not currently insulated). Any reason why you used laths throughout the floor, rather than just at the edges to create the service channels? In our plan there would be no laths Which @JohnMo's idea are you referring to, Wedi boards? It's a very valid alternative, it would give us comparable insulation saving perhaps 5mm of floor height (or less, depending on adhesive thickness) but it comes with a few disadvantages which I highlighted in my previous post. Still a very good option though

cool, that was all clear, just wanted to confirm The idea would be to prevent any moisture from above from getting trapped between insulation and DPM, as the DPM is on the cold side of the floor. Does that make sense? However, once the VCL is in place the new risk is moisture getting trapped in the DPM-insulation-VCL sandwich (the assumption is DPM and VCL are not fully sealed so moisture can still get in between those layers.

The DPM is a liquid one we installed on top of the existing screed as there did not seem to be any pre-existing DPM. That is under the insulation and stops the moisture from below. The VCL would be on top of the insulation and would the moisture from above. Is VCL the wrong term for it? What do you mean?

After going back and forth on what to do, I think I have settled on DONE: Epoxy DPM (already installed, Mapeproof OneCoat) 20mm PIR with fire rating E 18mm t&g plywood or 2 x 9mm plywood. The 9mm do not seem to be available with t&g, I guess because they're too thin. VCL? There are lots of people pro and against it, as it often happens it's not really clear what path to pursue underlay 12mm laminate Regarding the VCL, there are 2 schools of thought: "VCL is a must have": these people argue it is required to stop moisture from above condensing under the insulation layer, which makes sense... "don't do it": these people argue the epoxy DPM and VLC on top of the insulation will create a plastic bag around the insulation and if any moisture were to get in between those layers (sealing the liquid DPM at the edges is virtually impossible, and the VCL might not be 100% sealed either) will get stuck there. These arguments make sense too... Regarding the insulation layer: we ultimately decided against Wedi as we couldn't justify the higher price, considering their flammability and tile adhesive installation method (that's the only allowed installation method, as far as I could read on the official manual). With Wedi we wouldn't need the plywood layer but it would lead to higher installation and material costs and higher risk in case of fire. Any opinion on the proposed plan?

current options for floating floor are: - 30mm XPS wedi boards, fire rating E + underlay and laminate (no height for adding plywood unless we decrease wedi to 20mm) - 20mm PIR board, fire rating E + 18mm plywood (or 2x9mm plywood) + underlay and laminate any reason to go for one or the other? I'm tempted to go for PIR + plywood as I feel it might feel more solid despite the lower compressivity of the PIR

would the Wedi board be suitable for installing right under laminate? Or would it still require some kind of additional support layer?

thank you, that seems fairly strong. Just found XPS is quite flammable though