If you’re designing or inspecting HVAC systems in British Columbia, understanding how duct insulation requirements differ between Part 9 and Part 3 buildings is critical for compliance with BCBC 2024 and ASHRAE 90.1. This blog post breaks down the requirements for ducts located in unconditioned spaces such as attics, garages, and crawlspaces — and includes a cheat sheet with drawing note examples. (Discliamer: This is my interpretation of the code. )

---

What Is an Unconditioned Space?

An unconditioned space is any area that is not directly heated or cooled, such as:

• Attics

• Mechanical rooms outside the envelope

• Garages

• Crawlspaces

These spaces are prone to energy loss if duct insulation is inadequate, which is why codes impose strict R-value requirements.

---

Duct Insulation Requirements – Cheat Sheet

Here’s a side-by-side comparison of what’s required under Part 9 vs Part 3:

Category	Part 9 Buildings (≤ 3 storeys & ≤ 600 m²)	Part 3 Buildings (> 3 storeys or > 600 m²)
Applicable Code Section	BCBC 2024 – Section 9.36	BCBC 2024 – Section 10.2.2.1 + ASHRAE 90.1
Energy Code Path	Prescriptive or Performance (9.36)	ASHRAE 90.1-2019 (or NECB 2020)
Duct Insulation in Unconditioned Spaces	Match exterior wall R-value: ~R‑17 (RSI 2.97)	Supply ducts: R‑8 Return ducts: R‑5
Vapour Barrier Requirement	Yes, if conveying cooled air	Yes, if conveying cooled air
Common Mistake	Using R‑4.25 from 9.32 instead of 9.36	Applying 9.36 instead of ASHRAE 90.1

---

Duct Insulation for Part 9 Buildings

Clause 9.36.3.2.(3) of BCBC 2024 states:

“Where heating or cooling ducts pass through an unconditioned space, the thermal resistance of the duct insulation shall be not less than that required for above-ground opaque building envelope assemblies.”

That means, in Climate Zone 4 (e.g. Vancouver), your ducts must be insulated to ~R‑17, just like the exterior walls.

---

Duct Insulation for Part 3 Buildings (ASHRAE 90.1)

Part 3 buildings follow ASHRAE 90.1, which sets duct insulation requirements based on the air temperature and duct location:

• Supply ducts in unconditioned spaces: Minimum R‑8

• Return ducts in unconditioned spaces: Minimum R‑5

Also, vapour barriers are required if the duct conveys cooled air to prevent condensation.

---

Sample Drawing Notes

For Part 3 Building:

“All supply and return ducts located in unconditioned spaces shall be insulated per ASHRAE 90.1-2019 Table 6.8.2A: Supply ducts: Minimum R‑8 (RSI 1.41); Return ducts: Minimum R‑5 (RSI 0.88). Include vapour barrier and seal ducts per Section 6.4.4.2.”

For Part 9 Building:

“All heating/cooling ducts outside the conditioned envelope shall be insulated to RSI 2.97 (R‑17) minimum per BCBC 2024, Article 9.36.3.2.(3). Vapour barrier required if conveying cooled air.”

---

Final Thoughts

Using the correct R-value for duct insulation based on building classification is essential for energy code compliance and long-term system performance. If you’re unsure whether your project falls under Part 9 or Part 3, consult with a mechanical engineer or code consultant early in the design process.

Disclaimer

The information provided in this article reflects my professional interpretation of the applicable requirements in BCBC 2024 and ASHRAE 90.1 as of the date of writing. While every effort has been made to ensure accuracy, building code requirements can vary by jurisdiction and are subject to change.

Readers are encouraged to consult with their local building authority or a registered professional engineer for project-specific guidance and official rulings.

Disclaimer

The information provided in this article reflects my professional interpretation of the applicable requirements in **BCBC 2024** and **ASHRAE 90.1** as of the date of writing. While every effort has been made to ensure accuracy, building code requirements can vary by jurisdiction and are subject to change. Readers are encouraged to consult with their local building authority or a registered professional engineer for project-specific guidance and official rulings.