Ductwork Insulation

Duct insulation is a layer of material around a ventilation duct. It limits heat exchange through the duct wall and keeps condensation from forming. In Slovak practice the term is izolácia vzduchotechnického potrubia. It matters most in a house with heat recovery ventilation, where some ducts carry air at outdoor temperature and others carry warm air through cold spaces.

What does duct insulation do?

The US Department of Energy gives two jobs on its guidance pages. High-R insulation with a vapour barrier helps prevent heat gain or loss through the duct walls. It also keeps condensation from forming on the ducts. The two jobs are separate. Insulation that saves heat but lets a duct sweat still causes moisture damage.

The ventilation company Alnor adds that condensation on duct surfaces causes corrosion. This reduces the service life of the installation. Alnor also states that excess moisture inside ducts creates conditions for bacterial growth.

Which ducts in a heat recovery house need insulation?

A heat recovery unit has four duct connections. Outdoor air comes in, supply air goes to the rooms, extract air returns, and exhaust air goes out. The UK Passivhaus Trust describes the two ducts that connect to the outside in its guide How to build a Passivhaus. The main intake and exhaust ducts carry air at outside temperature, so they form part of the external insulated envelope. The Trust states that these ducts are insulated, but they cannot be insulated to the standard of the wall. So it recommends short ducts. It places the unit next to an external wall, or within two metres of one.

The table sorts the cases that the opened sources address.

A warm duct in a cold space loses heat, and a cold duct in a warm humid space collects water. The dew point of the surrounding air decides when water forms on the duct surface. See the balanced ventilation system entry for how the four ducts fit together.

Why must the insulation resist water vapour?

Moist air that reaches a cold duct surface condenses on it. Insulation that lets water vapour through lets that moist air reach the cold surface. The US Department of Energy therefore recommends thermal insulation with a vapour barrier. A closed cell material such as rubber foam can do this without a separate foil. Alnor states that rubber foam has a high resistance to water vapour diffusion. Alnor calls this valuable in home heat recovery systems.

As one product example, the retailer Ahlsell lists the Armacell AF/Armaflex 13 mm plate. The listing names ventilation ducts in cooling and climate control installations. The page gives a water vapour resistance factor above 10,000 and a thermal conductivity below 0.033 W/m·K. These are the manufacturer figures for that one product. A different product has different values, so use the data sheet of the product that you specify.

How is duct insulation installed?

The US guidance asks for sealing before insulating. Seal the ducts with duct mastic or with metal tape that meets UL 181, UL 181A or UL 181B. These are US product standards, and they do not set a Slovak rule. The sealing stops air leaks and the heat loss that comes with them. The insulation and its vapour barrier follow over the sealed duct.

The opened sources name two ways to give a duct a vapour-tight cover.

What thickness and standards apply in Slovakia?

This page gives no Slovak minimum thickness. The Slovak text checked for this page, Decree 364/2012 Z. z. in its consolidated wording from 10 March 2020, contains no rule on duct insulation. The decree's numeric annexes are not part of the text that was read. No Passivhaus Institute source and no manufacturer sheet opened for this page gives a typical thickness for ventilation ducts. The US Department of Energy page gives R-8 from the US ENERGY STAR programme for ducts in unconditioned spaces. That value is not a Slovak requirement.

The designer therefore writes the insulation type, thickness and vapour barrier detail into the duct specification. The data sheet of the chosen product and the building services design set these values. The principal checks that detail in the permit documentation.