Tanking Damp Basement Walls

[joe91]

Sorry in advance for the long post. I need some advice with a basement room. Any help is much appreciated.

I have a 3 storey building split into a house & a flat. The house is the top 2 stories, above ground. The bottom story is just one room under half of the ground floor of the house. This one room, along with a 2 story extension makes up the flat, with separate access.

This room, which is the kitchen of the flat has 2 and a half walls which are underground (cellar) walls. The "half", has a sloping path / steps running up the outside of the wall, ranging from about 2 foot to five foot up the outside of the wall. Of the other 2 underground walls, one has been rendered with some kind of cement and plaster, and seems dry. This wall is more under the house (perpendicular to the front) than the other underground wall, which is directly under the front of the house, we've hacked the old plaster off this front wall, to reveal the stone & damp mortar.

We've laid a new floor, with DPM, but left a trench filled with chippings along the front of this wall, and the half cellar wall, we also left a channel full of chippings under the floor, where there was originally an open channel running, covered with flagstones. We have built new stud partition along the full damp wall, and also along the 'half' cellar wall, as this is damp along it's lower half as well. We intended leaving the damp walls unrendered, so that they can breathe, and put vents in the stud partition to alow air to circulate, both around the walls, and under the timber floor of the room above. There are no air bricks at all, but an old chimney which has been tiled over, but is open to the attic of the house above. This will remain open, behind the stud partition

We've not boarded the stud walls or ceiling yet, as I'm not really convinced that our approach is correct. I'm wondering if we should really tank the walls to try & stop any moisture coming through at all, or allow it through in the hope that it will dry out in time. I'm thinking that maybe allowing it to breathe will create more moisture in the air, and make it harder to keep humidity down in the room / cavity / ceiling void.

The ground above & outside the damp wall is covered with flagstones. We've pointed the joints in an attempt to minimise water going into the soil, but I guess this will take a long time to have any effect, if indeed it helps at all. We're also installing an extractor hood in the kitchen to reduce moisture from cooking.

 

[johnheritage]

I think you're right to stop there before doing the boards.

If the walls behind the partition are underground and damp, it's almost certainly going to end up going into the timbers of the stud wall over time.

The product for fixing it, and it's application, is balls cheap, quick and simple to use. A good thickness, continuous coat or SBR bond (following the recommendations for dilution or admixing it with other things - the manufacturers usually have a PDF with details for specific problems, drying times and other bits and pieces need a quick check to make sure the layer is as good as you can get it).

What's hard about it, is where's the moisture going to go. That's like putting a dead cat in an oil slick on a motorway (with a banana peel on the cat), dramatic changes may occur.

From the mental picture I've got from that description, it's staying in the walls or wicking up them once the SBR is on.

Trying to seal the floor upstairs and outside was a good idea in terms of rain coming back down the side, but it could still stay damp down there if the moisture is wicking through the soil underneath as it heads for the water table.

If there's a damp course over the room (in the two stories above), the water wicking up will just look like normal ground water contact to the rest of the house, and should be rejected by the damp course up there.

But it'd be worth considering things like, does the floor above that room have any damp coarse in it? If no, tanking the room below could force the water more rapidly into that floor directly above.

If the floor above is timber, where are those sitting? If it's on the damp walls you're working with, that's bad for a start. Again, if you tank it, the water will go up even quicker.

So you'd probably want to have a poke around and find out precisely what is going on where the damp walls meet the rest of the house. Doing that from inside your unfinish / unboarded room will be easier / make people happier than going from the top down.

The main thing I'd then think about is, what are those walls made out of? Are they common blocks and mortar, or some form of cast concrete?

Common blocks / mortar aren't going to be so happy about being tanked and damp. Whereas the concrete will be less bothered about it. Another possibility to look at is injecting damp proofing over the entire surface. The stuff is basically silicone dispersed in an oil of some description, the same way they add a bit of silicone to moisture resistant plasterboard to get it to reject water better. Normally, injectable damp proofing is used for a single run of bricks. You squirt it in and you can watch it spreading out (like the grease from a bag of chips) over a few hours. But doing an entire wall with the cartridges will cost LOTS and it'll be tricky to make sure you've got it all covered (need to at least draw a drilling grid).

Also worth considering putting some ventilation grills on the boards to allow the air behind the stud wall some chance of equalizing with the room.

The best way to do this, which is likely going to be a BIG job (but maybe not that big depending on the specific site, would be to try getting a membrane or diverting layer down the outside walls around room, to get the water either blocked from the outside or to at least encourage it away from the walls.

Before going any further, you might want to take a sample of the soil outside.

I mean, you could try knocking a length of copper pipe down a metre or two. Poke the soil back out like a long turd log (not a single pile) and have a look to see if it's soaking wet down near the bottom. If it's dripping, you can't leave the walls as they are. They could end up trying to sue you a year or two later when it reappears on the studs or something rots. If it starts taking out the floors above, that could get serious.

This is also a problem where it might not be so bad now with the room as it is, but tanking can create big changes in the way the moisture is moving.

If there's a damp course running around the room you're in, where it meets with the floors above, you've struck gold on a number of those points, because you can be pretty sure tanking it won't force moisture upstairs. But still worth considering what will happen if it's an old brick wall tanked from the inside.