A hybrid flat roof is the name given to a warm flat roof construction where additional thermal insulation is installed below the roof deck.

Warm roof build-ups work by maintaining the structural deck at (or close to) the building’s internal temperature. Additional insulation below the roof deck changes the relative temperature of the different layers of the build-up, which can introduce unwanted risks.

Those risks can be heightened if the insulation is also installed on the internal side of the layer acting as the AVCL. Introducing extra insulation, especially if it is a different material type to that installed over the roof deck, can change the management of moisture (including the potential for condensation to occur) in the roof. It is therefore appropriate to look at what BS 5250:2021 Management of moisture in buildings says about hybrid roofs.

Why might a hybrid flat roof be constructed on site?

A hybrid flat roof is most likely to occur because there is some restriction on the height of warm roof insulation that can be installed on the flat roof. Theoretically, this shouldn’t occur on a new-build project, though there can be miscalculations with regard to upstand heights or threshold levels.

Whatever the case, the thickness of insulation that can be accommodated is not enough to achieve the required U-value. A solution is therefore sought to accommodate additional insulation below the roof deck.

Where there is a desire to improve the U-value of an existing roof, similar height limitations might apply. Alternatively, there could be a preference to leave the existing roof finish undisturbed, so the option is taken to add insulation below the deck while carrying out internal works. Either way, a hybrid roof is the result.

What factors affect condensation risk in hybrid flat roofs?

A typical warm roof build-up features a structural roof deck with an air and vapour control layer (AVCL), thermal insulation and external waterproofing layer all installed over it.

The AVCL acts to restrict moisture vapour entering the roof structure. Without an AVCL, moisture vapour generated inside the building could move past the insulation layer and come into contact with the cold waterproofing layer. As the air could no longer contain the same quantity of moisture vapour, excess moisture would be deposited as condensation.

The thermal insulation could then get wet, reducing its effectiveness and increasing heat loss from the roof. If the condensation is not able to dry out then the roof structure could be subject to a long-term build-up of moisture that is unseen by building occupants.

In a hybrid roof, adding insulation to the underside of the roof deck acts to decrease the temperature of the deck and the AVCL. The extent to which it does so depends on the relative thicknesses of the warm roof insulation and the ‘hybrid insulation’.

It also depends on the extent to which the additional insulation allows the passage of moisture vapour, either due to its material properties or due to the way it is installed (for example, if poor installation leaves air gaps that allow the movement of warm air past the additional insulation).

What do BS 6229:2018 and BS 5250:2021 say about hybrid roofs?

BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings – Code of practice acknowledges that hybrid roof build-ups occur, including where thermal insulation is added to an existing roof, or in a roof construction where the waterproofing sits between two insulation layers (as could be the case in an inverted roof build-up where insulation is added below the roof deck).

It says there is “an increased risk of interstitial condensation with a hybrid roof”, and recommends that a full condensation risk analysis is carried out. BS 6229 does not say outright that hybrid roofs are bad practice, but it does make clear that designers “should select the type of flat roof most suitable for the intended building”.

As part of its recent update, BS 5250:2021 Management of moisture in buildings – Code of practice included a new footnote in its table summarising the condensation risk analysis methods appropriate for warm flat roofs.

Footnote B to Table 4 says that condensation risk analysis calculations are required for warm flat roofs “if thermal insulation is split both above and below the deck or AVCL although typically no more than one-third of the thermal resistance should be on the warm side of the AVCL.” The term ‘hybrid flat roof’ is not actually used in the standard.

Polyfoam XPS and hybrid flat roofs

Like the two standards referenced above, Polyfoam XPS acknowledges that hybrid flat roofs do occur and are sometimes unavoidable for a variety of reasons. Nevertheless, we always advise customers that they are best avoided wherever possible.

We have always been aware of the principle of the “one-third rule”, but we do not like to offer it as an option because the potential for poor installation on site can result in risks that are not present in theoretical calculations. Additionally, if different types of insulation with different performance characteristics are used, the differences in thickness and thermal resistance can be confusing for people to understand.

It was interesting, therefore, to see the ‘one-third’ option effectively formalised in BS 5250:2021. In our view, however, BS 5250:2021 is only acknowledging that hybrid flat roof arrangements can occur – it is not saying that they should be considered an acceptable form of construction. To find out more about flat roof construction, condensation risk and U-values, contact us about your current project.