Where to put vertical DPM?

23 Feb 2010
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United Kingdom
I'm building a new build single storey kitchen extension for ourselves. The wall opposite the house will be a retaining wall holding up 1.5 mtrs of next doors garden. The engineer has designed the retaining wall with a torched-on DPM on the INNER face of the outer skin of the cavity wall. I understand there is a risk of hydrostatic pressure forcing the membrane off the wall, although he doesn't think it's a problem.

Technical advice at Icopal recommend filling the cavity with a green mix to support the membrane against the wall up to the raised ground level. The cavity is 100mm and will have 50mm insulation down to the base, but I am worried that if I use a cavity fill I will create a cold bridge and have condensation problems in the future.

The ground is quite light and there is a plastic tanking membrane on the outside of the concrete mass fill outside of the cavity wall. Should I just go with the engineers option?
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I can't see why you would get condensation issues. If the room is tanked, heated and ventilated then no probs.
Cold bridges occur where there is no insulation or less in one area than another. If you have uniformity then the whole wall will perform the same. so no 'bridges'

If there is a dimpled membrane on the outside then that will allow drainage thus reducing hydro pressure, assuming you have shingle/pipe somewhere for the water to drain to?

Would full fill insulation right up against the membrane be an option? Might even work out cheaper, for example if foil backed boards were specced originally then 100mm polystyrene would have a similar inso value at about half the cost.

Personally I'd be tanking inside too, to be sure to be sure!

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