Solid Walls and Damp

Over here in the Channel Islands a lot of the old housing stock was built of granite from local quarries. The mortar appears to be mud or a poor clay mixed with a little lime.

Internal wall plaster was beach sand mixed with lime slaked on site. Once dry a fine sand lime was applied. White washed and freshened up as necessary. These walls kept the rain out because of their thickness. Usually at least 400mm thick. Rain water would penetrate then evaporate due to the open matrix of the mortar beds. I would expect window reveals and door reveals to be damp. This was exacerbated by the door and window openings being formed in poor quality bricks. Again, a local clay and locally fired.

No form of horizontal DPC was installed although the very old properties were built on very large granite stones. Rainwater goods would have been minimal.

I can well imagine that the ground floor of these early buildings were very damp. This is probably why this level was used to keep cattle during the winter months.

Move forward several hundreds of years and in untouched houses of this type the walls are very wet at all levels. There are may contributing factors but the main reason is the sea salt that has migrated to the surface of the plaster. This is of course hydroscopic. I have seen walls running with water when the humidity is high.

The British Standard for managing moisture in habitable buildings in the UK is BS 5250:2021. This is a very useful standard although it may not have been adopted in the islands.

So what has happened to upgrade these buildings?

Outside

For many years the external elevations were repointed with strong sand/cement pointing. Sometimes this was very thin and being cement based cracked and even de-bonded allowing storm water ingress. Frost damage can occur because of the build-up of moisture.

Water Repellents

Following the strong pointing quite often water repellents were applied. Acrylics were used along with Aluminium Stearates then Silicones.

The problem with these spray on coatings was the limited penetration. They not only reduced rainwater penetration they also reduced evaporation so further frost damage became a problem.

Today following lime pointing we apply emollient preparations containing Silicones and Silexines.

External Renders

Renders were also used in an attempt to reduce rain water penetrations. Sadly, these were also very strong sand/cement with a waterproof additive. These renders were not only very strong say 3 to 1 but also used a fine sand so shrinkage cracks were inevitable, of course rain water was drawn in but could not evaporate back out.

External Paints.

The next attempt to reduce storm water was the application of modern paints.

A lot of modern masonry paints contain significant amounts of acrylic.

Apply several coats and quite a good membrane is achieved. The problem with this is water gets in and cannot get out also moisture builds up in the wall causing damp problems on the internal faces. Sometimes its quite easy to recognise this problem as bubbles full of water can be seen on the outside of the walls.

Inside

The old lime plasters were removed to be replaced with very strong sand/cement plaster, quite often containing water proofers.

What is Best Practice today?

Check the external paint and the render.

1.    If impervious remove.

2.    If the paint only is impervious remove the paint. Repair cracks and repaint with a silicone or mineral paint.

Internal

Providing the external elevation has been made vapour permeable it is ok to have an impermeable face to the finishes on the inside.

We would use an SBR modified sand/cement render or a pre bagged Salt Resistant Plaster. Because today we have to have insulation the best approach is a frame system, however, this must incorporate a vapour check to avoid the risk of interstitial condensation.

Often, we fit Delta Air Gap Membranes to overcome all damp and salt problems but a vapour check in the insulation lining is essential.

Rising Damp

Easily dealt with today by the introduction of a new resin damp proof course.

Watch out for our next article on Solid Block Walls.