Structural waterproofing

Hi all

As I’ve posted elsewhere, we’ve had a difficult start to our retrofitting journey. Following a retrofit survey, it was decided that the extent of damp problems in the walls of the property needed to be addressed before any serious additional retrofit efforts could get underway, so that’s what we’ve been focusing on.

A damp survey by a reputable damp specialist identified a number of remedies, including the removal of masonry paint and repointing, which seems to have helped. He also recommended the installation of a the installation of polypropylene dimpled membrane, or Stormdry’s Flex Membrane, to be installed between the masonry and plaster work on walls affected by penetrating damp.

However, we are struggling to find contractors who are prepared to do this work. Those that specialise in damp proofing generally seem to wish to undertake their own surveys, to be able to provide guarantees that their work solves the damp issue. But we do not wish for additional surveys, and are wary of the reputation of damp proofers for attempting to inappropriately sell rising damp solutions such as DPC injections.

So we’re wondering where to turn next. Perhaps we need a plasterer? Does anyone on this forum know of any companies in the Manchester area that could possibly help, ideally those working on restoration and retrofit who may understand better where we’re coming from?

Than you

Putting a membrane between structural wall and plaster helps maintain reasonable indoor conditions and reduces mould risk but it is a symptomatic treatment. Ideally you should be stopping the damp getting into the wall in the first place. Penetrating damp doesn’t carry hygroscopic salts, unlike rising damp or damp induced by combustion in chimneys, unless it is sea spray that is penetrating the wall. It follows that once it dries out the wall can be plastered as normal, without special additives or membranes.

You should avoid impermeable wall coverings like plastic paints and vinyl wall papers. The wall needs to “breathe” out the trapped moisture.

Thanks Tim. What you say is correct, but we are following the advice of a highly experienced damp expert which was recommended by People Powered Retrofit. So we are most certainly looking for recommendations for carrying out the work described, rather than seeking to re-assess the need for such work. Thanks.

So long as the assessor was aware of the rest of your plans that is fine.

But I would still select a breathable internal wall finish.

That’s good advice, thank you

I’ve recently watched the AECB members webinar series on:

Challenges of retrofitting damp masonry walls with Dr Eric Rirsch & Clementine Walker - Safeguard

There are plenty of suggestions there - including plastering.

I personally would primarily treat water exposure.

Rain being the main source. Addressed by greater overhangs (reduce exposure), or by repointing (reduce absorption) - which you’ve done already.

On the ground floor, there are other approaches. Prevent splash back from the rain, when striking the ground, by having the ground slope away from the wall (reducing exposure), a perimeter skirt render/waterproofing system by the base of the wall (reducing absorption). These splash back renders may reduce drying, but reduce wetting more, therefore, on aggregate, they reduce water accumulation.

Drainage or dimple mats “tanking” is best done below grade on exterior wall where possible. They can be integrated into a French perimeter drain (they reduce absorption from the surrounding wet ground, and eliminate hydrostatic pressure from your foundations). Insulation can be added to reduce thermal bridging to the frosty ground during winter reducing internal moisture sources from condensing on the wall, and overall improving diffusive drying in both directions.

In some cases with high exposure and groundwater level, the only solution is to retrofit a DPC.

Beyond that, you’d use more moisture tolerant materials. The video referenced above mentions using internal plasters specially designed for this.

Or, making sure that humidity is controlled from within and you keep the wall warmer.

Hope that helps.

The paint that was holding in moisture has gone and the pointing has been replaced. If the brick or stone is not too absorbent the wall should start to dry without further action. :crossed_fingers:t2:

With a vapour open wall finish the wall can start to dry out in both directions (except where flex membrane is fitted, as that will hinder drying). As the wall is drying into the house you will need full time ventilation such as MVHR to keep down the humidity. If you use impermeable wall finish the moisture will collect behind it. Hence my advice above.

I have recently been surprised at how high splashback can go. I have very deep cills in anticipation of EWI (long story) and on the wall below a first floor window there is a line of earth splashed up from the vegetable bed below to a height of 50cm. I will be using Stormdry to treat the bottom meter of the wall. In that area.

Thanks, but as I mentioned before I’m after reccomendations for contractors for the work described, rather than a reappraisal of the work, having already conducted retrofit and damp surveys by qualified experts to identify the problems and solutions.

I appreciate that.

The plasterer is a good idea - I offered a direct reference to a reputable source, and the type of product you should ask to be used is suggested there as well.

You’re essentially limited to using moisture tolerant materials. I don’t think tanking is worthwhile, as it pushes up the evaporation band higher on the wall - again, this is covered in the video. Tanking is often limited to below grade for this reason.

I mention the rest of the works if the opportunity presents itself to do something to the exterior of the building. It’s part of recognizing that old buildings have some inherent durability issues that require substantial intervention.

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