We've solid brick, external walls, about 10" thick, and have moisture seeping through. We have plaster on the walls which we'd like to take off to seal the walls inside, then replaster.

So far our best suggestion is to use tanking slurry before the replastering. But is there any, hopefully cheaper, alternatives to tanking which can be plastered over?

I'd like to avoid water soluble sealants like PVA solutions.

We'll also be repointing and rerendering the external walls eventually, but need to do something before winter kicks in.

  • Have you figured out where the water is coming from in the first place? Is it all walls? Just a few walls? – DA01 Oct 7 '15 at 20:34
  • It seems to be seeping through cracks in the external rendering, which will be being patched up either at the end of this year, or when possible in the new year. – mrtimdog Oct 10 '15 at 19:07
  • We're still undecided on the best solution, but it's looking like we'll be leaving it be until the new (hopefully drier!) year ... – mrtimdog Oct 10 '15 at 19:15

Ok, I guess you have to acknowledge that preventing water from reaching the wall is the best solution, followed by exterior wall protection, but I read the OP as acknowledging this already and asking specifically for interior solutions to water proofing. Since you mention being a software engineer, I see it analogous to a question about cryptography, where you're not necessarily looking for something that's 100% unbreakable, but simply what's appropriate for the task at hand. Really awful images can come to mind when someone says "Moisture seeping through", but it seems to me like we're not talking about standing water on the basement floor, just the presence of moisture on the wall (correct me if I'm wrong), and a partial solution able to be implemented on a short time frame (1-2 months).

I recently did some work on my basement brick wall where I have a little bit of moisture seepage and efflorescence, nothing serious, just a little. I stripped the brick clean and used Quikwall over it with an acrylic modifier, and with a sealer over that. This has worked well for me since I'm not trying to hold back floodwaters, I just want to limit efflorescence.

I never heard of tanking slurry, but of course I did a quick Google search and can now talk about it like I'm an expert :) Ok, but really, I have heard about similar products before and understand the science behind them well enough and it's theoretically solid. It sounds like it'd work well, and I know there are an number of similar products out there (not sure what's best). I may end up trying it myself. You can also coat your floors if you see a need to.

A couple additional notes. Sounds like you plan on finishing with plaster. Quikwall can be used as a plaster like finishing itself. If you plan on finishing some other way, I've used RedGuard before which helps prevent cracks and is very effective at water sealing, but is bright red so is not a good finish on its own. Also, the other (although much smaller) cause of basement moisture is condensation, which you can't seal against, so there are situations where insulation is used to protect against that.

The OP also asks about cost. I'm not aware of any cheap solutions that delivery a reasonable level of effectiveness. From what I've seen, it costs money to do anything you're likely to feel comfortable with.

  • Thanks for your info, I agree with the cryptography analogy! It's also ground floor, we've no basement, floors are wooden, on wood beams, so there's space underneath to ground which'll soak up anything going down that way. – mrtimdog Oct 10 '15 at 19:12

Sealing on the interior is doomed to failure. If there is already water inside the wall that is pushing its way into the inside, what do you suppose will happen when it hits the sealant? Continuous moisture movement will, over time, cause it to bubble and peel off, and then you'll be right back where you started, but with a horrible mess on the inside.

A better approach is to prevent water from getting inside the wall to begin with. There are many, many ways to do this, from extending the roof overhangs, building a wraparound porch, protecting the exterior of the walls with a water-resistive barrier and some kind of exterior covering--you name it. Lots of options. A common approach in your neck of the woods is to render the exterior of the walls with a water-resistant render mix incorporating waterproofing additives, and potentially painted with a silicate mineral paint afterwards.

  • Thanks ... but if the moisture's trapped in the wall after sealing the outside, how does it get out, instead of further in? As far as I understand the nature of tanking slurry is that it penetrates the brickwork and forms a permanent crystalline barrier when contacted with incoming moisture, so it'd not be the kind of stuff which would be able to peel off. P.s. I'm a software engineer, not a builder, so I'm learning as I'm going along here with this old house! – mrtimdog Oct 7 '15 at 19:37
  • Hah, I'm a software engineer too. This "tanking slurry" stuff is not something that seems to be known in the USA. If it works, it works! However, you will still have the problem of moisture in your wall, which will fall due to gravity and collect at the bottom, possibly making your floor wet. A wet wall is also very thermally conductive, which will make it uncomfortable and lead to high utility bills. – iLikeDirt Oct 7 '15 at 19:53
  • Glad I'm not alone then ;) ... Moisture succumbing to gravity's not really an issue as it'd seep around the horizontal damp course and keep dropping down onto the foundations and below (there's no concrete barrier or any other sorts between the wall base and Earth, as was built in 1890). So the floor's safe :) – mrtimdog Oct 7 '15 at 20:10

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