Drying out after burst pipe - condensation on DPM

15 Aug 2008
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United Kingdom
The day after I stopped to "relax" for Xmas , our converted outhouse/office water pipe burst :confused:

Thus, xmas was a chaos of ripping up the flooring and borrowing heaters and dehumidifers and running to and fro to the tip.

The chipboard floor was sodden, and the mineral wool insulation underneath was waterlogged. Got rid of both wood and wool, and stripped floor back to the wooden "joists" and the black plastic damp proof membrane underneath (see first pic).

I also slit a section of membrane to find that the concrete base underneath was also very wet. So I then slit each channel of membrane (each lane between the joists - see pic 2) in several places and directed a fan heater through, like a wee wind tunnel, until each channel was dry as a bone. That was just before New Year.

I went in today, and all above the membrane remains bone dry. BUT, I slid my hand in one of the membrane slits... only to find that it was wet again in there?!

The water supply is and has been off and thoroughly drained since the burst, so it can't be that. The wetness seems to be oddly on the underside of the membrane as opposed to on the concrete floor (even though the membrane lies on the concrete) - see pic 3. I wonder if this is just condensation collecting in there? The outhouse has a few air bricks which might be bringing moisture in from outside.

What do folk think? Should I be concerned about the new moisture, or can I - as planned - tape over the membrane slits with membrane tape, sealing it up again, and just let the membrane do its job to keep that moisture from the joists and the flooring (whenever I get around to replacing it)?

Thanks in advance - this has been utterly doing my head in!


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Is there also a DPM under the slab?

Probably condensation from the slab condensing on the underside of your membrane.

You say air vents, was the floor originally a suspended floor, replaced by concrete?
If the rest of the floors are still suspended, pipework should have been used in the concrete floor, to carry on the ventilation.
No dpm under concrete as far as I know.
Floor was concrete originally. Renovators added DPM, joists and chipboard floor.
Air bricks are at shin and head height.
Moisture will always be present on the underside of the DPM if there is no DPM under the slab.
Are your walls cavity (any insulation) or solid brick?
You might be able to fill in the air vents.
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Moisture will always be present on the underside of the DPM if there is no DPM under the slab.
Are your walls cavity (any insulation) or solid brick?
You might be able to fill in the air vents.
Ah, so moisture underneath isn't "bad" so to speak?
I didn't realise concrete was porous!

Walls are brieze block, shingle on outside, dpm, insulated (wool) and plasterboard on inside.
It wouldn't be my choice, as by rights the slab should have had DPM underneath it, rather than on top, stopping damp transmission a lot lower down. In your situation, you have what you have to work with.

You can patch the piece you have slit and tape the patch up, make the patch quite a lot larger than the slit, same goes for any tears.

Also PIR insulation would be a better choice than rockwool for the sections in between the wooden joists, will make for a sturdier feeling floor.

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