Cavity wall design — maximum recommended width of the air cavity In a standard cavity wall (two leaves tied across an air gap), the width of the hollow space between the leaves should not exceed which value for typical building practice?

Introduction / Context:
Cavity walls improve resistance to rain penetration and enhance thermal performance. The cavity width, however, must be controlled so that wall ties function properly, the cavity can be kept clear of mortar droppings, and thermal and moisture objectives are met without excessive overall thickness.

Given Data / Assumptions:

• Residential or commercial buildings in typical exposure conditions.
• Standard cavity ties used at normal spacings.
• No special insulation boards counted inside the cavity in this question.

Concept / Approach:
Common practice adopts cavity widths in the range of 5–10 cm. Narrow cavities risk bridging by mortar droppings; overly wide cavities complicate tying and stability. Hence, many specifications cap the plain air cavity at or below 10 cm unless special ties and design details are used. This balances moisture control with structural integrity and economy.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer guidance for cavity wall ties usually specifies maximum tie spans that align with a 50–100 mm cavity range for standard applications.

Why Other Options Are Wrong:

• 5 cm, 7.5 cm: permissible but not the maximum; question asks for the upper limit.
• 15 cm: generally beyond standard practice without special design.
• None of these: incorrect because 10 cm is the commonly used limit.

Common Pitfalls:
Allowing mortar droppings to bridge the cavity; ignoring weep holes and cavity trays above openings; not adjusting tie type/spacing for nonstandard widths.

Final Answer:
10 cm