In this ongoing series, Rob Firman, of Polyfoam XPS, looks at different aspects of flat roofing design and construction and helps to demystify them. This month Rob focuses on hybrid flat roofs, ‘the one-third principle’ and BS 5250:2021…
A warm flat roof with additional thermal insulation below the roof deck is known as a hybrid flat roof. Adding insulation below the structural deck changes the relative temperature of the different construction layers, potentially introducing unwanted risks. Those risks are heightened if that insulation is also on the warm side of the AVCL.
Extra insulation, especially if it’s a different type of insulation to that installed over the deck, can alter the movement of moisture and the possibility of condensation occurring in the roof. As a result, it’s worth looking at what the recently published BS 5250:2021 Management of moisture in buildings – Code of practice says about hybrid roofs – see our article in the last issue for an overview of the revised standard.
Why is there potential condensation risk in hybrid flat roofs?
A typical warm roof features a structural roof deck with an air and vapour control layer (AVCL), thermal insulation and external waterproofing layer installed on top of it. Installed correctly, the AVCL restricts moisture vapour entering the roof structure.
In a hybrid roof, insulation to the underside of the roof deck lowers the temperature of the deck and the AVCL. How much it lowers it by depends on the relative thicknesses of the warm roof insulation and the ‘hybrid insulation’.
It also depends on the performance properties of the additional insulation, and whether it is more vapour open or not. It can also depend on the way the insulation is installed. Poor workmanship, for example, can leave air gaps that allow the movement of warm, moist air past the additional insulation.
Why do hybrid flat roofs get built?
A hybrid flat roof is most likely to result when there’s 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.
Ultimately, the thickness of insulation that can be installed is not enough to achieve the required U-value, so attention turns to accommodating additional insulation below the deck.
Similar height restrictions are more likely when looking to improve the U-value of an existing roof. Alternatively, the client might want to leave the existing roof finish undisturbed, so insulation is added below the deck during internal works.
Hybrid flat roofs in BS 5250:2021
BS 5250:2021 Management of moisture in buildings – Code of practice features a table that summarises the condensation risk analysis methods appropriate for warm flat roofs (alongside all other roof types).
The term ‘hybrid flat roof’ is not used anywhere in the standard, but this table includes a new footnote B that says 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.”
At Polyfoam XPS, we’ve always been aware of this principle being used within our industry, and we found it interesting to see it effectively formalised in BS 5250:2021. As we said in our summary of the standard last month, this new version of BS 5250 is a highly practical document that seeks to acknowledge the realities of what is built versus the theory of what is designed.
Hybrid flat roofs do occur and are sometimes unavoidable for a variety of reasons. In our view, BS 5250:2021 is only recognising that fact, and is not saying that they should be considered as a common or acceptable form of construction.
We’ll continue to support our clients if they are looking at hybrid roof options, but we’ll always advise that they are best avoided wherever possible. The “one-third rule”, as mentioned earlier, should not be used as an excuse for poor design, because the potential for poor installation can result in risks that are not present in theoretical calculations.