DPC, DPM, VCL Detailing

9 Jan 2011
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United Kingdom
I know this has probably been done to death, but the advice and recommendations also seem to change over the years with introduction of new products, etc. Anyway, my DPC/DPM seems to have been done wrong, but I just wanted to ask what I should do next to make it work.

It's a small garden room with a 2 course brink plinth, DPC, and then timber sole plate. The floor is concrete slab, 100mm Kingspan PIR, DPM and then 70mm screed. I thought common practice was to tuck the DPM under the DPC, but mine is the other way around:





I'm about to start taping the insulation around the studwork and then I'm going fully envelope the room with a VCL stapled in place. The question is, how do I treat the transition between the DPC, DPM and VCL at the floor? The DPC edge plunges under the screed, so the DPM comes up over it. Should I bother using a double sided butyl tape to seal these membranes together where exposed? Or should I drop the VCL down the walls and sandwich it between those layers at the floor? Or should the VCL instead be taped over the top of the DPM, so that it is the top layer?
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OK, just to reply to my own post, I'm thinking perhaps a bead of some sort of silicon sealant all the way around the floor between the DPC and the DPM that's on top. Then trim away whatever surplus of DPM there is, but when I come to do the VCL around the walls use double-sided butyl tape to join it to the DPM. If this causes the bottom edge to flare out a few mm's with all the tape and sealant there it shouldn't matter because the plasterboard would probably be terminated just above this point. Does that sound about right?

I've order a couple of rolls of Visqueen double sided DPM jointing tape. But my question is, what would be a suitable sealant to use between that DPC/DPM junction? And on a related note, Is there a preferred expanding foam I should use to close up the gaps between the studs and the Kingspan PIR?

Thanks all.

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