Jonathan Ducker of Kingspan Insulation explores how warm roof construction combined with advanced insulation materials can help meet our ambitious energy targets without compromising internal space.
In the next few months, the final text for the Future Homes Standard (FHS) will hopefully be confirmed. With all new homes expected to be required to use low carbon heating (typically either a heat pump or district heating), the performance of the building fabric will play an important role in both supporting compliance with the requirements and ensuring homes are affordable to run. For this reason, many developers are looking to maximise the thermal performance of the roof construction.
For properties where the roof space will be used as a liveable space, it is good practice to fit insulation between and above the rafters. Using insulation with a low thermal conductivity can be especially beneficial in this application, helping to ensure there is suitable headroom in the space and simplifying detailing to ensure a compliant new roof.
Future Homes Standard – What to Expect?
The Government consultation on the Future Homes Standard proposed two different options for the Notional Dwelling specification. This is applied to a theoretical building of the same dimensions as the proposed home within the energy modelling software to generate the performance targets for the building.
As with the existing standards, developers are free to depart from this specification provided they meet the performance targets, limiting values (such as worst-case U-values) and any other mandatory requirements (such as using low carbon heating).
Aside from the switch to low carbon heating, one of the proposed options is largely unchanged from the existing requirements in Approved Document L 2021 (ADL 2021).
The other includes some enhanced features, limiting air loss to 4 m3/m2.h @ 50 Pa with ventilation provided via a mechanical ventilation heat recovery system, and using wastewater heat recovery and PV.
Surprisingly, neither of the specifications requires the building fabric to be upgraded beyond the values in ADL 2021. This is despite the U-values in the English Notional Dwelling lagging significantly behind the current equivalents in Scotland and Wales in most areas.
With the switch to low carbon heating appliances, which perform most efficiently at lower flow temperatures than conventional boilers, ensuring homes retain heat well will be vital to allow them to be heated affordably. It is therefore worth looking to upgrade roof U-values to the same level as used within the current Notional Building in Scotland – 0.09 W/m2K.
Warm Roof
When looking to reach this U-value, the thermal conductivity (lambda value) of the insulation is especially important, as this can significantly impact the depth of the roof construction. The lower the thermal conductivity of the insulation, the more effective it is at slowing heat loss. This can allow the U-value to be met with a slimmer construction than might be possible with materials with higher thermal conductivities. Rigid phenolic insulation boards are well suited for applications where ambitious U-values
are needed, as they tend to have the lowest thermal conductivity of any commonly used insulation board (as low as 0.019 W/mK).
Along with the choice of insulation, it is also important for developers to consider the build-up they want to use, especially for room-in-the-roof constructions. In the past, the typical approach has been to fit insulation between the rafters with a further layer of insulated plasterboard below. However, even when designing the construction to allow phenolic insulation to fill the gap between 150 mm rafters, this can still require a substantial thickness of insulated plasterboard beneath just to reach a U-value of 0.11 W/m2K (as in the existing Notional Dwelling in England). This often means ridge heights have to be raised to ensure there is sufficient headroom in the space below.
A more practical approach is often to fit phenolic insulation between the rafters, with a further layer above. This is often referred to as a “warm roof” as the roof rafters will be kept fully within the insulation envelope. In addition to allowing projects to more easily reach low U-values, fitting a continuous layer of insulation above the rafters can also help to simplify detailing and limit repeating thermal bridges through the rafters. This can support compliance with carbon and primary energy targets within the expected FHS, and should help to ensure the finished roofs retain heat effectively.
When adopting this approach, it will typically be necessary to install a breathable membrane above the insulation, followed by the cross battens for the roof tiles. A vapour control layer (VCL) applied to the internal face of the insulation behind the plasterboard lining can help provide both vapour control and good levels of airtightness.
Levelling Up
While the final format of the Future Homes Standard is yet to be confirmed, achieving excellent fabric performance will stand you in good stead to comply with any requirements and help ensure that the low carbon heating technology functions effectively. Pitched roofs offer a good opportunity to deliver ambitious U-values, and by adopting a warm roof approach with insulation with a low thermal conductivity, developers should be able to achieve U-values as low as 0.09 W/m2K without compromising on internal headroom.
Jonathan Ducker is head of regulatory affairs at Kingspan Insulation Limited