Overwhelmed Wet Walls?
Can masonry really be overwhelmed? This was the question posed by the owner of a recently built house that became rather damp during storms in the early part of this year. Apparently he had never considered a masonry skin might become saturated, and never considered to what extent following saturation water could free-flow within the cavity. It had never crossed his mind until the 2013/2014 storms arrived.
Consider masonry manufactured and tested to BS 4315. This involves spraying water onto a given area at a rate of 2.5 litres per minute. The test takes place over 48 hours. Upon completion of the test it is not unusual to read the ‘bricks indicated very low levels of rates of rain penetration.’ It is often not realised the spraying is not continuous. It lasts just one minute, followed by an approximately half-hour pause, after which there is another minute of activity. So over 48 hours the extent of water spraying can amount to less than 1½ hours. If the rain in your district only ever falls for one minute and is always followed by half an hours’ drying time, then perhaps the Standard is meaningful, albeit not realistic of the climate experienced in the UK? Especially when one remembers the continuously wet winter months with the accompanying high winds.
This is no criticism of the masonry, but accompanying effective DPC control measures are essential. Controlling water migration through masonry requires arrestment and evacuation. Once an average brick becomes saturated, its conductivity can double. So the thermal behaviour of the outside skin is less helpful to heat conservation during the wetter periods. Controlling penetrating water and evacuating it out of the structure as swiftly as possible makes good sense. Water retention within a wall is not helpful – it can contribute to the masonry being overwhelmed.
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