Whether you're installing a kitchen, bathroom, or commercial ventilation system, choosing the wrong ducting can lead to higher energy bills, condensation problems, and poor air quality. Understanding the difference between insulated and non-insulated flexible ducting can help you make the right choice.

What Is Flexible Ducting and Why Does It Matter?

Flexible ducting, also known as flex duct, is one of the key components of a ventilation system. Unlike rigid sheet-metal ductwork, flexible ducting can bend around joists, fit into awkward spaces, and connect fans and grilles where a straight-line route is not possible. This flexibility makes installation faster and more convenient. However, flexibility is not the only factor to consider. The key question is whether your application requires insulated or non-insulated ducting. The right choice depends on where the duct is installed, the surrounding temperature, and the requirements of the ventilation system.

Insulated vs Non-Insulated Flexible Ducting

Both types have a similar inner structure—a wire-supported aluminium or foil liner that carries air from one point to another. The main difference is whether an additional insulation layer surrounds the inner liner Insulation is not required for every ventilation system. However, in certain applications, leaving the duct uninsulated can lead to significant problems.

Runs Through Cold or Unheated Spaces

If the duct passes through a loft, garage, crawl space, or another unheated area, the air inside can lose heat in winter or gain unwanted heat in summer. Insulated ducting helps maintain the desired air temperature from the fan to the grille.

Condensation-Prone Environments

When warm, humid air passes through a cold duct, condensation can form on the duct surface. Over time, this can contribute to mould growth and moisture damage.

The vapour barrier of an insulated flexible duct helps reduce the risk of moisture and condensation.

HVAC and Heat Recovery Systems

In systems where maintaining a specific air temperature is important—including MVHR systems, air conditioning systems, and commercial HVAC systems—duct insulation is often required by the system design.

Acoustic Performance

The insulation layer can also help reduce sound transmission. This can be particularly beneficial when ductwork passes near bedrooms, meeting rooms, or other noise-sensitive areas.

Frequently Asked Questions

1. What is the difference between insulated and non-insulated flexible ducting?

Standard flexible ducting consists of an inner liner that carries air from one point to another. Insulated flexible ducting adds a layer of thermal insulation—typically glass wool or mineral wool—around the inner liner, followed by a vapour barrier jacket. The insulation helps reduce heat loss, minimise condensation, and provide some acoustic dampening.

2. Can I use standard flexible ducting for bathroom ventilation?

It depends on the installation route.

If the duct runs a short distance through a heated space and exits directly through an external wall, standard flexible ducting may be suitable. However, if any part of the duct passes through an unheated ceiling void, technical space, or loft, insulated flexible ducting is generally a better choice to reduce the risk of condensation and mould.

3. What sizes are available for insulated flexible ducting?

Common internal diameters for domestic and light commercial applications include 100 mm, 125 mm, 150 mm, 200 mm, and 250 mm. Because of the insulation layer, the outer diameter will be larger. Therefore, installation space should be planned based on the outer diameter, not just the internal duct diameter.

4. Can insulation be added after installation?

Technically, yes. Additional insulation can be installed around an existing duct. However, this can be considerably more difficult in confined or inaccessible spaces. If insulation is likely to be required, installing insulated flexible ducting from the beginning is usually the more practical solution.