Water retention within cavity wall masonry can affect the aesthetics, thermal performance and life of the building fabric. Masonry walls and features should be able to return to a dry status relatively quickly, and to achieve this masonry must be able to drain.
Water can usually be evacuated out of the structure using conventional caviweeps within various perp joints. But when constructing ashlar walling or any style where the perp and bedding courses are particularly fine, there can be insufficient space for this approach. An alternative is required.
The longer masonry is wet directly affects the thermal performance of a cavity wall. The thermal conductivity of any material depends on temperature, density and importantly its moisture content. When a material becomes wet, the air enclosures fill with water and the conductivity of the material increases because water is a better conductor than air. So when calculating heat loss of an external skin, consider the effect of it remaining wet longer than it could. A masonry skin with a specific mass of 1600-1900 kg/m³ and thermal conductivity of 0.6-0.7 W/mK can change to 0.9 1.2 W/mK when wet.
A wet external surface is also more liable to support algae growth. This will occur in urban as well as rural areas normally when the atmosphere around the wet masonry is warmer and drier. BRE (D 370) gives guidance to treat affected surfaces, but it is preferable to reduce the likelihood of the algae occurring in the first place by making provision for the masonry to drain. Inability of a wall to drain also supports localised damp staining.
Water retention within any masonry increases the demands on any damp control measures within it. Minimising the amount of water accumulating within a structure rather than increasing it should always be the objective.
Where conventional caviweeps cannot be fitted or where the masonry style or layout prohibits, Pyramid Weeps provide a solution. The Pyramid Weep is a modern version of the masonry bleed straw - a water evacuation method used over 150 years ago.
Incorporating natural straws within masonry mortar beds to bleed out water from within is an old custom. Letting the wall bleed with masonry bleed straws was the forerunner to todays cavity wall weep. Selected straw reed (commonly Devon reed) was cut and dried and enveloped within the mortar beds. Straw reed provided a reasonably proportioned conduit through which moisture and water could flow. Unfortunately, reed breaks down and degrades over a period and subsequently flow reduced. Lime mortar was also discovered to sometimes leech and sludge the reed ends. The Pyramid Weep is a modern version of the bleed straw and offers numerous advantages:
Optimum placement position: The flat bed of the Pyramid Weep provides the evacuation route at the lowest position relative to the mortar bed. It is sufficiently compact to permit incorporation within a bed course. The bed course position should coincide with a perp joint where the dimensions are particularly fine this permits the triangle to extend into the perp.
Strength: Its triangular shape will not compress or distort like a round conduit.
Flow: Water and mortar debris wash is along a flat base at the widest point.
Durability: Moulded from pvcu that will not rot and decay.
Discreet: The Pyramid is hardly detectable when built-in. The front of the weep finishes flush with the external face and is almost unnoticeable within the finished masonry.