Module 2 · Buildings and How They Handle Moisture
2.5 Timber Frame — Where the Moisture Control Actually Is
9 min read
A timber-framed house with a brick outer leaf looks, from outside, exactly like a masonry cavity house. It is not one, and treating it as one produces both wrong diagnoses and genuinely dangerous recommendations — filling the cavity of a timber-framed wall being the clearest example. The structure that matters is behind the brick, cannot tolerate sustained wetting, and is protected by two membranes whose continuity is the whole game.
Learning objectives
By the end of this lesson you will be able to:
- Identify timber frame from external and internal evidence.
- Describe the roles of the breather membrane and the vapour control layer and which side each belongs on.
- Predict where timber-frame moisture failures occur and why they are point failures.
- Explain why cavity fill in a timber-framed wall is a category error.
Key terms
- Rain screen
- The outer leaf — brick, render or cladding. Sheds most water, is expected to leak some, and is not the weathering line.
- Breather membrane
- Sheathing membrane on the cold side of the frame: resists liquid water inward, passes vapour outward. Its lapping direction is what makes it work.
- Vapour control layer (VCL)
- Sheet on the warm side of the insulation, limiting vapour entering the construction from inside. Continuity matters more than the material.
- Drained and ventilated cavity
- The void between rain screen and frame, which must stay clear and open at top and bottom so water drains and the sheathing dries.
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