Dampproofing using cavity drained membrane

[MattW]

I was asked to post some photos of the dampproofing of the basement in my georgian town house while in progress. The basement had clearly suffered persistent damp problems over the years with a tide mark of damp to the touch plaster up to c.4 feet. Prior attempts to solve the problem included patches of cement rendering, chemical injections in the walls and a bitumen floor coating. My neighbours have both covered the walls and floors with waterproof cement render to the ceiling, so any water in the parties walls would likely to be coming out on my side. We decided that rather than fight the damp with more of the same that we would cover the floor and walls with a cavity drain membrane (a studded plastic sheet) that covers the walls and floor, but allows any water to come through the wall and either evaporate or flow down into floor drains and then into a sump. This means that the water isn't trapped in the bricks and so the wall structure isn't damaged. It is also easily removeable so if a subsequent owner wishes to remove it they can do so.

Having decided on the approach, we needed to dig out the 1950s concrete floor which had no DPM in it or any insulation, and rebuild the floor with the following structure bottom up: hardcore, 50mm insulation, concrete slab, thin coat of leveling screed, 50mm insulation, cavity drain membrane, underfloor heading pipes, screed, stone.

The following pictures show:

1) the original basement prior to an interior wall being knocked through,

View attachment 2

2) the membrane being attached to the walls in the under pavement vault. The floor here is the screed put down to level it to ensure that any water will flow in the right direction. The membrane is held up by plastic pegs that are knocked into holes drilled into the walls, with a mastic around the head to seal the hole.

View attachment 1

3) the membrane up, the floor drains in place around the perimeter and the floor insulation being laid. Note the rodding point on the right - this enables the drains to be flushed with water from a hose to clear any sediment that builds up.

 

[MattW]

4) the floor membrane having been laid on the top of the insulation

View attachment 1

5) the whole room showing the batons that are screwed to the pegs holding up the membrane to the walls. Plasterboard will then attached to these batons to create a dry interior surface.



What is not shown yet is that the floor drains flow into a sump / pump chamber with a water activated pump that pushes any water into the drains. It has taken the fitters one week to prepare a space of 60m2 in area.

 

[Tom]

Wow!! Without any knowledge of this, it certainly looks like a very good way of dealing with damp cellars. Certainly preferable to tanking with cement products. I'm really impressed!!

 

[JoceAndChris]

Me too! What a fascinating system! Pity I can't copy it for downstairs!

Any idea how many years this will last?

 

[MattW]

Any idea how many years this will last?

Well it is guaranteed for 10 years. But who knows - the materials are all very stable / inert plastics so there is little reason to believe that it wouldn't last decades.

The major downsides to it, apart from the large number of holes in teh walls, is the loss of space as the walls are c.8cm thick including plaster board and also the loss of any internal features. In my basement neither of these was a big issue but it would be for some.

 

[Matt Green]

Any idea how many years this will last?

Well it is guaranteed for 10 years. But who knows - the materials are all very stable / inert plastics so there is little reason to believe that it wouldn't last decades.

The major downsides to it, apart from the large number of holes in teh walls, is the loss of space as the walls are c.8cm thick including plaster board and also the loss of any internal features. In my basement neither of these was a big issue but it would be for some.

The lifespan is likely to be affected more by the time it takes to block up rather than the degradation of the materials themselves. That depends on all manner of things including the wall construction and the amount of water movement through them.

Regarding the loss of space, removing the existing plaster might get you a few cm (but might increase the risk of silting too). Alternatively rather than battens and board you could use a membrane with a mesh lath on the internal surface and plaster directly onto that.