UNDERSTANDING CONDENSATION AND HYBRID ROOFS

For a conventional warm or inverted flat roof, the presence of a suspended ceiling internally makes little difference to the overall thermal performance of the roof.

The ceiling system contributes no thermal resistance of its own (unless it is well sealed). In addition, fixing the ceiling system into the underside of the roof deck does not impact the main components of the roof system. The AVCL, insulation and waterproofing layers are all above the structural roof deck, and so don’t interact with the ceiling system at all.

When specifiers and installers opt for a hybrid roof arrangement, however, then questions must be asked as to whether the U-value calculated in theory will match the performance of the roof in reality.

A hybrid flat roof features thermal insulation above the roof deck (as in a conventional or inverted warm roof), and an additional insulation layer fixed below the roof deck. It therefore has characteristics of both warm and cold roof construction, hence being described as a hybrid.

The insulation below the roof deck is typically on the inside of the air and vapour control layer (AVCL), whereas general construction practice is for AVCLs to be on the warm side of the majority, if not all, insulation.

Because they sit in this grey area, hybrid roofs are not formally recognised by standards that deal with best practice in roof construction. Both BS 6229:2018 and BS 5250:2021 acknowledge that hybrid roofs get built on site, but don’t explicitly state that they’re an acceptable form of roof construction.

You can read about hybrid roofs in more detail here, including a closer examination of what these two British Standards say about them.

A hybrid roof features insulation secured to the underside of the roof deck. Where a roof build-up also features a ceiling system fixed to the underside of the deck, both the ceiling system and the insulation will interact with one another.

This interaction takes the form of ceiling supports/hangers penetrating the insulation layer, acting as a thermal bridge through it. U-value calculations can, and do, take account of some thermal bridging effect for the supports.

However, our concern is that assumptions about installation made in U-values calculation don’t represent what will happen on site. This is potentially an issue with any U-value calculation for any form of construction, but because hybrid roofs reflect an area of building design that is not considered best practice, the issue is even more critical.

A calculation is likely to assume perfect installation, with the number of penetrations for supports kept to a minimum. But what if a ceiling system needs more supports when it is being installed? What if the penetrations made through the insulation are done untidily, and leave large holes in the insulation rather than being kept neat and tidy?

By their nature, hybrid roofs already introduce an increased risk of condensation compared to a conventional warm roof or inverted roof. That’s why rules of thumb exist saying no more than one-third of the total thermal resistance should be to the inside of the AVCL.

Any penetration of the below-deck insulation layer risks an increased transport of warm air to the cooler side of the insulation. Under the right circumstances, this could result in some condensation occurring.

A worrying development we have seen in hybrid roof calculations is a highly vapour resistant insulation layer shown to the underside of the roof deck. This layer is significantly thicker than the insulation above the roof deck, but the calculation ‘works’ because the below-deck insulation is assumed to be continuous and offering perfect vapour resistance.

However, we have seen examples of such calculations where a suspended ceiling system is present. How does the system fix through the insulation without compromising the perfect vapour resistance that is being relied upon?

It is concerning to see a hybrid solution, which already is not considered best practice, being deviated from even further to address what good design and specification should avoid in the first place.

At Polyfoam XPS, we maintain the stance that hybrid roofs should be a design choice of last resort. If they are unavoidable, then they should meet the recommendations of BS 5250:2021, which deals with best practice in managing moisture in buildings.

For advice on U-value calculations and condensation risk analysis for your project, contact us.