Most people assume the siding on a house is what keeps the rain out. It isn’t — not entirely, and not by design. A rainscreen wall is designed around the assumption that water will get past the cladding, and the wall’s real job is to deal with it once it does.
Peel a wall apartFour jobs, in order
A wall has four control layers, that need to work together. This lesson follows the water-first framework:
- Water — shed and drain liquid water
- Air — stop air moving through the assembly
- Vapour — manage diffusion so the wall can dry
- Thermal — resist heat flow
Follow the water
Wind-driven rain hits the cladding. Some runs off. Some finds a joint, a fastener, or the gap around a window, and gets behind.
That water lands in the drainage cavity — the air space created by the vertical strapping between the cladding and the sheathing. The cavity does two things at once. It gives the water a clear vertical path to run down and out at the base of the wall, and it lets air move through so whatever is left can dry.
Follow the waterBehind the cavity is the weather-resistive barrier — the layer the wall actually depends on. It is lapped like shingles so that anything reaching it runs down and out rather than in.
Why this is a BC story
The rainscreen requirement did not arrive by accident. It came out of the leaky-condo era, when thousands of buildings on the south coast were built with face-sealed walls that depended on sealant to keep water out. Sealant is a maintenance item with a finite life. When it failed, the water had nowhere to go and nothing to dry it.
What to look for as a buyer
- Staining or swelling at the base of a wall, where the cavity should be draining
- Persistent moisture below window corners — a potential entry point
- Whether the cavity is actually vented top and bottom, not just present
- On a strata: whether the building has been re-clad, and what the depreciation report says about the envelope
The single most useful thing to understand about your home’s exterior is this: the gap behind the siding is not a construction defect. It is the whole design.
Check yourself
Rain gets past the siding on a wall built to current BC practice. What is supposed to happen next?