Duct and HVAC insulation · Glass wool
Duct Excel ISODEC
- Diameter
- 76–508 mm
- Density
- 16–48 kg/m³
- Service temp.
- -30 – 140 °C
What is insulated flexible duct and where is it used?
In short
Insulated flexible duct (flex duct) is a bendable round air duct with insulation, such as glass wool, and an outer jacket applied at the factory. It is typically used for short connections from the main duct to diffusers or grilles, because it is easy to fit in tight ceiling spaces. Flex duct has higher airflow resistance than a straight smooth duct, and bends, sag and compression increase that resistance further. Rated air velocity and pressure must be read from the product's datasheet.
How insulated flex duct is built
It usually has three layers:
- Inner core — the wall the air flows through, usually with a wire helix to hold its shape
- Insulation — such as glass wool, with thickness and density by product
- Outer jacket — such as reinforced aluminium foil, protecting the insulation and acting as the vapour retarder
Example from our range: Duct Excel ISODEC is described by the manufacturer as a glass-wool insulated flexible duct with a seamless aluminium-foil jacket reinforced with glass yarn, 25 or 50 mm insulation; per the manufacturer, model A2Q is rated to 20 m/s maximum air velocity and A2X to 30 m/s.
Where it is used
- The final connection from the main duct (or plenum box) to a ceiling diffuser
- Places where a small offset is needed to line up with a diffuser or avoid structure
- Cool-air systems that need continuous insulation right up to the diffuser
Flex duct is not designed to replace long runs of main duct. Suitable lengths should follow the system designer's drawings and the manufacturer's recommendations.
Why bends and sag are a problem
The inner wall of flex duct is less smooth than a metal duct, so it already has more airflow resistance (pressure loss), and that resistance rises further when:
- the duct is not fully stretched — the core bunches into ridges
- the duct sags between hangers
- bends are tight or the duct kinks at a connection
- the duct is squeezed by straps, ceiling framing or other services
The result is less air at the diffuser than designed, rooms that do not cool evenly and a fan that works harder. That is why the Air Diffusion Council (ADC) installation standard sets requirements for stretching, support spacing and limiting sag.
General installation principles
- Cut the duct to length and stretch it fully; do not leave excess coiled up.
- Hang and support it at the intervals the manufacturer recommends, using straps or saddles wide enough not to crush the insulation.
- Make bends wide and gradual; avoid kinks at the collar.
- Fasten the inner core firmly to the collar, then pull the insulation and jacket over the joint and seal with the tape and straps the manufacturer specifies, so there is no gap that could cause condensation (sealing insulation joints).
- Take care not to tear the jacket; repair any tear with tape that matches the jacket.
Take air velocity, pressure and temperature from the datasheet
Maximum air velocity, working pressure, temperature range and insulation thickness differ by product. Match them to the system design, and let the HVAC designer set the duct sizes and lengths.
Read more: why ducts need insulation and how to choose duct insulation
Sources
- Air Diffusion Council (ADC) — Flexible Duct Performance & Installation Standards
- NAIMA / Insulation Institute — A Guide to Insulated HVAC Duct Systems
- Duct Excel — ISODEC product page (construction, air velocity, pressure, temperature)
Related products
Categories and example products for this topic
- Duct Excel ISODEC — Glass wool insulated flexible duct with a reinforced foil jacket, in many diameters
- INSU-TAPE Aluminium Tape — Aluminium foil tape for sealing joints in foil-faced insulation on ducts
Installation accessories · Tape
3T INSU-TAPE Aluminium Tape
- Width
- 50 / 63 mm
- Length
- 45 m
- Service temp.
- -20 – 120 °C