A builder specifies a 25 mm thick rigid insulation board to be used as part of a rainscreen system behind fibrecement siding. To maintain the minimum required airspace, what type of furring strip or strapping is most appropriate?

Question: A builder specifies a 25 mm thick rigid insulation board to be used as part of a rainscreen system behind fibrecement siding. To maintain the minimum required airspace, what type of furring strip or strapping is most appropriate?

Answer options:

• 19 mm x 38 mm wood furring
• 5 mm thick woven mesh material ✅ 25 mm x 38 mm wood furring
• 12 mm x 38 mm wood furring

Correct answer: 25 mm x 38 mm wood furring

Explanation: If the rainscreen gap is to be created by furring strips over insulation, the furring strips must be at least as thick as the required airspace. As per CSA A440.4-19, a 19 mm airspace is recommended. Therefore, 25 mm furring (1 inch nominal) would provide this required depth when installed over 25 mm insulation.