2.6
Cavity Drainage and Flashings in Masonry walls
Typically, a cavity wall consists of two wythes separated by a gap of at least 2 inches, which may contain insulation while still maintaining a minimu…
In cavity walls, a minimum separation is maintained to ensure unobstructed cavity drainage.
Often, a continuous drainage mat within the cavity is used to deflect the mortar droppings and facilitate a clear drainage path.
The water entering this cavity moves out through the weep holes spaced appropriately at the cavity's bottom and placed above the flashings.
Additionally, vents near the cavity's top promote air circulation within the cavity and aid in evaporation of cavity moisture.
Flashings can be installed externally to prevent moisture from penetrating the exterior wall, or internally to divert water that enters the cavity back towards the outside.
Internal flashing is installed at the bottom of the cavity and places where the cavity is interrupted.
One end of the flashing is overlapped by a water-resistive barrier provided on the interior wythe, whereas the other end extends beyond the exterior wythe and bends down to form a drip edge to facilitate free fall of water away from the wall.
Sheet metal flashings, asphaltic membrane flashing, composite flashing, and synthetic rubber are the commonly used flashings.
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Q1: What is the purpose of weep holes in cavity walls?
Weep holes are strategically placed openings at the base of the cavity that drain accumulated water from inside the wall. Created by leaving head joints open and often protected with screens to block pests, weep holes work in conjunction with internal flashings to direct moisture toward the exterior. This drainage mechanism prevents water from pooling within the cavity and causing structural damage.
Q2: How does a drainage mat improve cavity wall performance?
A continuous drainage mat inserted within the cavity deflects mortar droppings and creates a clear drainage path for water movement. This advanced method allows the cavity space to be reduced while maintaining effective moisture expulsion. The mat facilitates water flow toward weep holes, ensuring that moisture does not accumulate and compromise wall integrity.
Q3: What is the difference between external and internal flashings?
External flashings, including metal copings and counter flashings, protect vulnerable areas like parapets and roof intersections from water ingress. Internal flashings are integrated during construction at potential water entry points like window heads and sills, catching water and directing it toward the exterior via weep holes. Both types work together to prevent moisture penetration and manage water that enters the cavity.
Q4: Why should durable flashing materials be used in cavity walls?
Flashing materials range from expensive, durable metals like copper and stainless steel to flexible, cost-effective options like asphaltic membranes and synthetic rubber. Given the complexity and expense of replacing internal flashings after wall construction, investing in high-quality, durable materials is advisable to avoid future water damage and costly repairs. Proper installation and material selection ensure long-term moisture protection.
Q5: How do vents contribute to cavity wall moisture management?
Vents positioned near the cavity's top promote air circulation within the cavity and aid in evaporation of cavity moisture. Combined with weep holes at the base and internal flashing systems, vents create a complete moisture management strategy. This ventilation approach reduces humidity levels and prevents condensation buildup that could lead to deterioration.
Q6: What minimum cavity spacing is required for effective drainage?
Cavity walls typically consist of two wythes separated by a gap of at least 2 inches, which may contain insulation while maintaining a minimum clear space of 1 inch to facilitate adequate drainage. Advanced drainage mat methods can further reduce this space while ensuring effective moisture expulsion. Proper spacing is critical for allowing water to flow freely toward weep holes and preventing moisture accumulation.
Q7: How is internal flashing positioned to direct water away from the wall?
Internal flashing is installed at the bottom of the cavity and at locations where the cavity is interrupted. One end overlaps a water-resistive barrier on the interior wythe, while the other end extends beyond the exterior wythe and bends down to form a drip edge. This design facilitates free fall of water away from the wall, preventing moisture from re-entering the structure.