Water repellent treatments for masonry walls

When retrofitting older buildings, one of the big challenges is reducing heat loss through solid masonry walls without creating moisture problems. Adding internal wall insulation (IWI) helps with energy efficiency — but it can also trap moisture inside the masonry, potentially leading to damp, mould, or even decay of embedded timbers like joist ends. 

In this article we look at Masonry water repellent facade treatments, or Hydrophobic facade creams, how they work and whether or not you could or should use them.

Many older buildings are built using masonry, such as brick, limestone or sandstone, which is water permeable and which absorb significant amounts of driving rain as it hits their walls. Over time, this water can soak from the exterior towards the interior of the building, raising the moisture levels within the walls, making conditions much worse for any timber within the walls.

In times past, without any insulation, the heat from the interior of the building would have passed out through the walls easily and kept them dry. However, with the move towards internally insulating buildings, this process is drastically reduced and can allow the rain to penetrate further.

What are Water repellent treatments?

Water repellent masonry treatments, or hydrophobic creams, are water-based milky solutions which are applied by brush or roller to the external surface of porous masonry or mineral render, soaking 5-10mm into the surface. They work best on porous materials as they need to soak into the surface to be effective.

These treatments work by penetrating into the pores of the masonry: the active ingredients (typically silanes and siloxanes) bind chemically to the pore walls and leave a water-repellent layer, while preserving vapour permeability — meaning moisture inside the wall can still evaporate outwards. 

Some treatments seal the surface of the wall and prevent rain penetrating by creating a barrier but these also tend not to allow moisture to be released from the wall easily, and so are not recommended for this kind of application.

Why use Water repellent treatments with internal insulation?

The main benefit is that they significantly reduce the amount of moisture ingress into the wall from wind-driven rain. This keeps the moisture content of the wall down which ensures that embedded timber stays drier and that any internal wall insulation has less moisture to contend with.

The treatments are vapour permeable (breathable) too, so moisture vapour that travels in and out of the wall is not significantly impeded and walls can dry inwards and outwards when necessary. This in turn improves the thermal conductivity of the wall, as it is not so wet, and further improves the thermal insulation.

The products are also long-lasting, as they soak into the surface of the masonry, away from any UV light and weathering which might degrade them. They do need re-application every 10 years or so but subsequent applications tend to use much less product, as it is more of a top-up.

Are there risks to using them?

Masonry water repellent treatments are not a magic bullet so proceed with caution and ensure proper hygrothermal analysis is undertaken when installing internal insulation. Just because the amount of wind-driven rain penetration is reduced, it does not mean any amount of internal insulation can be applied.

Although on balance the treatments reduce water penetration significantly, drying speed is reduced by their application. Although the walls are still vapour permeable, they cannot dry by capillary-driven drying, which is the quickest way to dry masonry and so wet walls must not be coated with these products.

There is also a risk from bypassing the hydrophobic layer and getting water into the wall which can only slowly dry back out. This can be caused by incomplete application but also from cracks in the masonry surface which are not repaired prior to application of the treatment.

This bypass can also cause issues with freeze-thaw damage and salt damage since moisture can accumulate behind the treatment, potentially creating wetter walls than they would have been otherwise.

Summary

Masonry water repellent treatments offer a sound way to make porous solid masonry more resilient to water ingress whilst still allowing water vapour to escape. This can significantly improve the robustness of well designed internal wall insulation solutions for retrofit.

However, it is worth considering the following points:-

• Don’t rely on them alone. Ensure the surface of the masonry is very well pointed and free from cracks or other defects.
• Ensure the walls are dry before you apply the treatment as it will slow drying and could make conditions worse for any internal wall insulation.
• Use hygrothermal modelling to assess firstly whether it is required, and secondly how the wall performs with different levels of bypass to assess the sensitivity of the wall.
• Perform a small trial on a section of the wall before treating the whole building.

The product we recommend is Remmers Funcosil, although there are several equally appropriate products on the market.

If you have questions about the use of these products, please feel free to contact us.