Jonathan Ducker, Head of Regulatory Affairs at Kingspan Insulation, discusses the growing trend of rooms in the roof in new builds and how the Future Homes Standard is set to affect it.

When designing rooms in the roof as part of new builds, the first consideration tends to be around headroom. Many developers typically aim for a headroom of at least 2.3 – 2.4 metres below the ridge and integrate dormers to maximise overall headroom within the space. The thermal performance of the roof also needs to be carefully planned. Under Approved Document L 2021 (ADL 2021) in England, the Notional Dwelling specification sets a roof U-value of 0.11 W/m2K for new domestic roofs. This specification 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.

Developers are free to vary from this specification providing they meet the performance targets, limiting values (0.16 W/m2K for domestic roofs) and any other mandatory requirements. It’s worth noting that we’re expecting the Future Homes Standard (FHS) in England to be published before the end of this year – the Government has stated that this will be published ‘in the Autumn’. Whilst we are still awaiting the final text, the Government has recently announced that new homes will have low-carbon heating, such as heat pumps, together with high levels of energy efficiency, in order to cut people’s energy bills and to boost the nation’s energy security. They have also noted that the Future Homes

Standard will explicitly promote solar, subject to practical limits, with flexibility for new homes surrounded by trees or with lots of shade overhead. 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 enabling them to be heated affordably. It is therefore worth improving the target roof U-value to a similar level as that used in the current Notional Building in Scotland — 0.09 W/m2K.

Warm roofs

When looking to reach these low Uvalues, 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, which can allow the Uvalue 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).

In addition to making use of thermally efficient insulation, some thought also needs to be given to the construction being used. Historically, many developers have tended to install insulation between the rafters, with a further layer of insulation or insulated plasterboard below. However, even when designing the construction to allow phenolic insulation to fully fill the gap between 150 mm rafters, this can still require a substantial thickness of insulated plasterboard beneath just to reach a Uvalue 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.

As a result, it is often more straightforward to reverse this approach, fitting insulation above and between the rafters. In addition to helping to maintain internal headroom, this “warm roof” approach can also simplify detailing as a continuous layer of insulation can be maintained across the roof surface, limiting repeating thermal bridges through the rafters. Minimising losses from these bridges is a key step to ensuring compliance both with ADL 2021, and the expected FHS, and should help to reduce any gap between the designed and actual energy performance of the property.

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.

Room to grow

Constructing properties with rooms in the roof can be an effective means to maximise property value on new builds. By installing insulation with a low thermal conductivity, such as phenolic boards, as part of a warm roof approach, it should be possible to ensure a good level of headroom within the space whilst also helping to meet key carbon emission and energy performance targets.

For further information and technical support, please visit www.rdr.link/dbe025

The post Kingspan Insulation discusses the growing trend of rooms in the roof appeared first on PHPD Online.

Adrian Pargeter, Head of Technical and Product Development at Kingspan Insulation, reveals how tapered roof insulation can ensure effective drainage while keeping properties warm.

Keeping out the elements is the most fundamental purpose of any dwelling. Here in the UK that often means dealing with a good deal of rain and cold temperatures, particularly at this time of year. For buildings with flat roofs, it is crucial to ensure that the structure is well insulated and has suitable drainage provision through a designed fall. Tapered roof insulation can meet these requirements in a single solution, and recently introduced systems now incorporate Phenolic packer boards, allowing the overall build-up depth to be kept to a minimum without compromising performance.

The name ‘flat roof’ is something of a misnomer as these structures should always have some level of pitch to allow effective rainwater runoff. BS 6229: 2003 defines flat roofs as “having a pitch less than 10^o to the horizontal”, although some may extend this to pitches of up to 15^o.

The standard recommends that flat roofs should have a design fall of 1:40 and a minimum “as-built” fall of 1:80. Failure to provide a suitable fall will often result in water ponding. If untreated, this can lead to a number of further issues including alkaline formation and mould growth. The additional roof loading can cause further deflection, aggravating the issue and potentially reducing the design life of the roof.

Roof insulation

Properly insulating the roof is also an important consideration, both to achieve compliance with the relevant Building Regulations or Standards and to help to maintain a comfortable internal environment for homeowners. Kingspan Insulation recommends specifiers aim to achieve a U-value of 0.11 W/m^2.K (in England & Wales) and 0.10 W/m^2.K (In Scotland) for new domestic properties and 0.14 W/m^2.K for new non-domestic buildings.

Typically, this can either be achieved with a warm roof construction, where the insulation layer is installed above the deck, or a cold roof construction, where insulation is installed between and below the roof rafters with a minimum air gap of 50mm between the insulation and the deck.

Recently, a trend has emerged where installers fit a hybrid solution with a thin layer of insulation above the deck and the rest below. Whilst in principle this may seem like a sensible way to reduce the above-deck construction depth whilst eliminating the need for a below deck ventilation gap, it can lead to significant long-term issues.

In a warm roof, condensation is prevented by keeping the deck at a warm temperature, therefore there is no cold surface for the condensation to form on. In a cold roof, condensation is prevented as the ventilation current takes away the moisture vapour before it can form as condensation.

With the hybrid solution, however, the above-deck insulation layer often offers insufficient performance to keep the deck warm and, as the ventilation void is absent, there is no way to remove the moisture before it forms condensation. This construction should only be considered if a full condensation risk analysis is carried out and even then, it will rely on an almost perfect level of workmanship.

Tapered insulation systems can provide a better alternative, delivering excellent drainage and thermal performance in a single solution.

All-in-One

Tapered systems typically comprise sloped tapered boards, hip and valley boards and flat packer boards. They can be installed above concrete and metal decks either with standard waterproofing or as part of a green roof system.

The systems offer a number of practical benefits for contractors. Tapered insulation systems can be fitted with a dry installation process, meaning no time is lost waiting for the surface to dry. In many cases, these systems can be fitted above the existing roof surface, further streamlining project time scales. They can also weigh as little as 1.5% that of a screed to fall system – removing structural loading concerns.

Tapered systems are typically manufactured from rigid insulation such as EPS (expanded polystyrene), XPS (extruded polystyrene) or Polyisocyanurate (PIR). For enhanced thermal performance, systems are now available which combine PIR sloped, hip and valley boards with Phenolic insulation packer boards with thermal conductivities as low as 0.018 W/m·K. The packer board’s enhanced level of thermal performance can allow desired U-values to be met with thickness savings of 20 – 40 mm, depending on the individual scheme and build-up, when compared with a full PIR system.

Some suppliers also provide a tapered roofing design service to support installers. The system boards are provided with a clear layout tailored to the specific design of the roof. This allows a simple installation and ensures the desired level of rainwater run-off and insulation performance to be achieved with a minimal construction depth. Some also include a full condensation risk analysis as part of this process.

Flat roofs can provide distinctive style to conventional detached and semi-detached properties as well as a more easily maintainable roof space for multi-unit housing. Tapered roof insulation systems incorporating phenolic insulation can provide a quick, thin and lightweight solution for these roofs.

The post Roofing | A guide to Tapered Roof Insulation appeared first on PHPD Online.