Membrane finishing

[Spinny]

Hoping to have the flooring peeps in soon. Wondering how I should prepare the various membranes poking out at the windows and bifolds. (See pics)

1. The window people put a piece of DPM under the bifolds and floor length window. They used double sided tape to stick it upwards against the inside edge of the frame/threshold on the inside. Naturally the DPM has pulled away from the tape and the tape has become gummed up with dust and crap and is no longer sticky. Should I scrape and cut the tape off the frame/threshold, and then what should be used to restick it ?

2. The floors radon/dpm membrane is directly alongside the window DPM. Thinking of sticking this to the window DPM, and then the floor levelling compound should run up against it when they pour it.

3. The black vapour barrier is then on the inside of the 25mm PIR edge insulation. Either I cut this off at the current floor level and it gets covered along with the edge insulation by the floor leveller. Or I try and cut it down to fold over the top of the edge insulation and then get covered, possibly sticking it too to the radon/dpm.

4. The window DPM has got  a short 20mm cut in it just outside the window - how do I repair this ?

 

Many thanks.

[Spinny]

Any thoughts on this one folks ?

[jack]

@craig, is this something you could help with? Many thanks.

[craig]

On 06/04/2026 at 21:22, Spinny said:

1. The window people put a piece of DPM under the bifolds and floor length window. They used double sided tape to stick it upwards against the inside edge of the frame/threshold on the inside. Naturally the DPM has pulled away from the tape and the tape has become gummed up with dust and crap and is no longer sticky. Should I scrape and cut the tape off the frame/threshold, and then what should be used to restick it ?

 

Sticky tape? They should have used something like CT1 or stickslike***t would have been good enough and then trim if required.

 

On 06/04/2026 at 21:22, Spinny said:

2. The floors radon/dpm membrane is directly alongside the window DPM. Thinking of sticking this to the window DPM, and then the floor levelling compound should run up against it when they pour it.

 

Should be fine, not much else you can do from what I can see.

 

On 06/04/2026 at 21:22, Spinny said:

4. The window DPM has got  a short 20mm cut in it just outside the window - how do I repair this ?

Any left over to patch? If yes, stick over the top with the adhesive mentioned previously.

Edited yesterday at 11:05 by craig

[Spinny]

Thankyou.

Any input on Q3 ?

 

I am relieved to see stickslike***t doesn't come in brown.

[craig]

13 minutes ago, Spinny said:

Any input on Q3 ?

 

Fold it over the top of the PIR and seal it to the DPM/radon barrier I think would be the best approach.

[Spinny]

OK.

Around half of the room/extension perimeter one of the builders cut off the vapour barrier to slab level as soon as the concrete slab had set. I caught him before he could do the other half. The Architects drawing was for it to run to the wall behind the skirting and get plastered onto the wall, although the plasterers thought that daft. The 25mm edge PIR is inevitably tightly pushed against the inner leaf blockwork by the weight of the concrete pour, so I guess not much chance of any colder air from below the concrete making it through to the room anyway. Certainly over 2 years of of it being in place we have had no sign of any such issue or condensation or anything.

 

Fortunately the radon barrier was run into the cavity wall and up to join the wall DPM so no-one could cut that off.

 

Just googled whether PIR is vapour permeable and got this AI response...

 

Quote

PIR insulation, even without foil, is not considered vapor permeable; it is generally classified as a vapor retarder or a low-permeance material. While removing the foil (e.g., using tissue-faced PIR) increases its permeability slightly compared to foil-faced versions, the closed-cell foam structure still significantly resists moisture movement, preventing it from being "breathable".

 

So I guess slightly better to do as you say, but if the flooring people want to cut it off it shouldn't be a problem ?

 

[Spinny]

oh sorry Craig or others, just thought of another question(s)...

 

I have a set of twinwall flexible conduits and a 110 drain pipe coming up under the kitchen island position carrying insulated water pipes and cables etc. Therefore these come up through the concrete. Is there something I should do to try and seal up the top of the surrounding hole around the conduits. I can't say how well membranes were cut and sealed to the conduits - knowing our builder probably not perfectly.

 

Q5 I guess definitely not a good idea to use floor leveller there for fear it all runs down the hole into the insulation layer ?! Maybe try and practice my insulation carving again and try and cut some 30mm XPS slab to fit around the pipes ? What do people usually do

for this ?

 

Q6 Re conduits themselves with pipes/wire coming out under kitchen units etc - any need to seal the ends around the pipes/wires to stop air moving along the pipe from colder levels ?

See pics

 

[craig]

It’s not really my area of expertise tbh but @Nickfromwales might be able answer these questions better than I can?

[Russell griffiths]

Trim off all the foam layer down to screed level

get a can of illbruck fm330 foam, good squirt around all ducts and down the ducts, as it dries and expands a bit you can pat it into shape or just trim it back afterwards. 
or don’t bother trimming and just leave a big blob under the cupboard if your worried about catching the pipes or cables. 

[saveasteading]

11 hours ago, Spinny said:

another question(s)...

Having just had this done I can advise one concern.

The guys putting the insulation in were very cavalier at such intrusions, and were keen to hurry the job and get away. Anticipating such issues I put my heel on the membrane at all such, and one gave way drastically. I settled on the fact that screed would fill the dent and make it solid. That's provided that the membrane isn't punctured and tape still in place.

 

In your case thus concern applies outside the green foam. Inside the green circle perhaps stuff with insulation offcuts and put a sandcand cement screed on top.

 

For anyone approaching the insulation stage I'd say either watch these guys all the time or do any fiddly cutting yourself and leave them with the easy cuts. Eg we had drain pipes with mortar bedding, some near stud walls,  leaving slopes into small voids. I spent a day cutting triangular pieces to leave a flat and solid surface, and very glad I did.

 

My mental test was to imagine a heavy concentrated load, eg bed leg or grand piano.

Under an island it is more a matter of tidiness