Building pathology: Water ingress
- Date: 30/10/2009
Water penetration can be a problem for brick walls – even if a cavity is included to prevent moisture reaching the building interior.
Some brick walls function perfectly well after hundreds of years of use. But they can be susceptible to problems, not least water ingress. Today’s modern cavity walls are just as susceptible.
Until the 20th century, solid brick walls were normal. But the trouble with them is moisture penetration. Water is absorbed by bricks and mortar, and it travels between them by capillary action or wind pressure. The BS 5628 code of practice for masonry only recommends solid clay brick walls for low or moderate wind-driven rain exposure zones. For moderate zones you need a 440mm-thick wall to have a sporting chance of keeping out moisture. Solid brick walls are deemed unsuitable for severe or very severe exposure locations.
As such, single-skin cavity walls (brick cladding) has became the solution of choice in the UK. In this system, moisture that penetrates the brick skin drains down the inner face of the external wall and back out through holes lower down, while the cavity prevents moisture reaching the interior.
That’s the theory, but moisture penetration still prevails. A major drawback of nearly all brick clad construction is that the cavity can be compromised. Wall ties are needed to hold the cladding in place and cavity trays bridge the cavity to protect openings such as windows. If not installed correctly, these components provide a route for water to reach the interior.
The risk is exacerbated by poor workmanship: inevitable mortar “snots” on wall ties absorb water to the inner skin, wall ties angled towards the inner leaf direct water inwards, and perpends not completely filled with mortar make it easier for water to get through. Cavity trays need to be carefully designed to ensure they lap with other layers and direct water outwards.
Installing cavity insulation can improve the thermal efficiency of the wall. It can be argued that full fill insulation or injected insulation completely negates the weathering function of the cavity. Partial fill insulation maintains the cavity. Although insulants are usually water repellent, water can find a route through the joins between batts. In the case of injected insulation it is difficult to achieve complete integrity in the cavity.
Water penetration is related to local rain conditions, the more rain and the more wind driving the rain onto the brick cladding the greater likelihood of water penetration. The high-risk zones are exposed edges and corners where wind driven rain concentrates.
The most likely route for water entry is through the bond between the brick and mortar or cracks in the brickwork. Hard, well burnt bricks have lower water absorption than softer bricks, but the interface between hard bricks and mortar often results in thin gaps owing to shrinkage. There is evidence that lime-cement mortars are more effective than cement-sand mortars at reducing and delaying penetration. Recessed mortar joint finishes are more vulnerable than flush or bucket handle joints. Brick work cracking may result from a range of causes: lack of provision for thermal or moisture expansion and contraction of the masonry; movements in the supporting structure or foundation; short returns; cracks owing to corrosion of embedded metalwork including wall ties.
Water penetration because of wall tie bridging may be recognised by the pattern of damp on the inner leaf replicating the position of the wall ties: a typical pattern might be ties at 450mm vertical centres and staggered horizontally at 900mm. Water penetration owing to detailing around openings invariably reveals itself at or close to the interface between the opening and the wall. Water penetration associated with defects in the insulation might be more difficult to determine.
Assessment of moisture penetration may be by visual inspection, or use of detection equipment such as metal detectors for wall ties. Analysis of the moisture content may also help determine if the damp is owing to condensation. A controlled hose test may be useful to determine where water is entering the cladding.
Remedial options depend on the cause and scale of the problem. Options include repointing, applying an additional protective cladding over the brickwork, removal or correct installation of insulation, and a final option of rebuilding the defective areas.
BLP provides building latent defect insurance for dwellings and commercial buildings www.blpinsurance.com