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Case Study: Solving Damp Issues

We recently completed a project on a small home situated in a dank valley near Betws Y Coed, North Wales. This historic building had been under new ownership for a few years after having been left uninhabited for a long period. The new owners were keen to do things properly, which would require undoing some previous work, but had struggled to find local contractors with a good understanding of moisture movement through building fabric, and especially experience with the traditional building materials they hoped to employ, and the care to use them properly.

They were experiencing lots of damp internally, with yellow staining on the bedroom walls and water running down the fireplace. The chimney had been capped and they hoped to have this opened up again to allow for the installation of a log burner, and they hoped to have an old Victorian fireplace removed and the new opening plastered in lime. They also required the upper half of a gable wall repointing in lime mortar, and a site visit suggested that the stone string course on this same gable would also need some work to ensure it was dripping properly. After a chat we agreed to open up their chimney which would include rebuilding the upper portion of the chimney, plus installation of three new chimney pots and a lead drip detail. Opening up the chimney to the other fireplaces would allow ventilation, in turn aiding damp to dry - an often forgotten function of old buildings. Finally, we decided that a hot lime shelter coat and lime wash to the gable would increase the drying face of the new lime, protect and consolidate the old lime on the lower half which wasn't getting repointed, and of course give the wall a fresh and appealing look.

After some deliberation we decided that a hot lime mortar would be appropriate for the intended function, and would work given the time of year we were conducting the work and the environmental conditions that the work would encounter. Ultimately, maximum breathability was required, and this was soon seen to be a good choice. When we started raking out the mortar in the gable it became apparent that the wall was fairly voided, and we would need to spend some time conducting deep pack repointing to ensure micro porous continuity. For the deep pack mix we added some coal ash to aid in the carbonation of this deep mortar. Once this was in we could get to the final pointing, opting for a flush finish to give the maximum drying face.

The pointing all went well, and by the time we were applying the first licks of lime wash we were already seeing a great improvement internally. The permanently damp stains in the bedroom had dried up, and we were seeing the same coloured staining tracking through to the exterior, suggesting that any salts in the wall were now moving in the right direction.

Case Study: Grouting Voided Walls

People living in old stone houses such as are common in Wales tend to face two main problems: damp and insulation. The modern materials and approaches typically taken to solve these issues in such buildings can often lead to more issues down the line, and people do not realise that a lot can be done to tackle both issues at once, using exclusively traditional materials, in a manner which is sympathetic to the original design and intended function of the structure. 

In such buildings, the outer wall is very thick, generally consisting of two stone faces with a rubble filled void between. The top of the wall is rarely sealed very well, and this means that when the wind blows, it blows into the roof space and down into the cavity, then out of any flaws in the masonry, and into the house. This is one of the main sources of the drafts that such buildings are notorious for, and though it may be possible to seal many of the flaws which are letting the drafts in, this approach is very much a sticking plaster on a bigger issue.

A further problem with the voided wall in this circumstance is the tracking of water from wind driven rain to the inside of the building. Historically, these buildings were built with lime, so this wasn't such a problem since wind could easily wick away any moisture from the porous lime, because the moisture would present itself via the entire surface area of the lime mortar. These days however, many of the same walls have been pointed or rendered with cement, which invariably develops cracks that allow water to penetrate. This water can't easily be wicked away, as it is running through small cracks with a much smaller surface area, and therefore the water either becomes trapped in the wall, or continues to run down through cracks and along stones in the cavity until it reaches the inner skin. Thus the benefit of the rubble filled void is lost, and instead the trapped water is expressed as damp internally. Alongside this significant concern, the wind blowing into the void effectively reduces the insulating thickness of the wall by two thirds, meaning that a 600mm wall only has a 200mm thermal mass. But all of these problems can be solved by one simple and cost effective solution.

Grouting of the wall with a quicklime based grout not only fills the void and triples the effective thermal mass of the wall, it creates continuity between the porous lime in both skins of the wall. Combined with proper lime pointing and insulated lime plasters, the reputation that these sorts of buildings are damp, drafty, and cold need no longer apply.

In a world where the cost of energy is increasingly at an alarming rate, and the need for effective insulation and energy saving solutions such as heat pumps is becoming ever more necessary, the grouting of old voided stone walls may yet prove to be an essential part of the puzzle. 

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