Colin Wells, Head of Technical at Keylite Roof Windows (Keylite), discusses how closer collaboration between manufacturers and SAP assessors can bridge the gap between product design and compliance.
Keylite understand that SAP assessors face challenges in balancing increasing energy performance requirements with complex design and technical variables, especially when products are not designed with energy assessments in mind.
One of the biggest challenges is the heavy reliance on standardised, verified data to calculate a home’s energy performance. When manufacturers fail to design and test products with SAP frameworks in mind, gaps in the required technical data can occur.
Assessors can spend considerable time gathering the correct data, which can significantly delay the overall housebuilding process, particularly if the same problem affects multiple plots on a development.
Traditional apex dormers present significant challenges for SAP assessments due to the need for detailed geometric, thermal and regulatory calculations.
Where a standard roof window has four linear junctions to consider, a dormer window has multiple, more complex junctions, each with its own linear thermal transmittance (Psi) value. These must be accurately assessed to account for potential thermal bridging.
If dormer footprints exceed 20 per cent of the roof room’s total floor area, assessors must model the dormer separately.
Should the assessor not have the precise certified technical data from the manufacturer, they may need to use government default values, making compliance much more challenging.
Keylite’s ModuLite system is designed to operate as a standardised, single-source component, incorporating a highly insulated timber upstand rather than a complex multi-element build.
ModuLite is treated as a single proprietary component and allows Keylite to provide pre-calculated, certified U-values for complete configurations.
Along with Keylite’s patented Integrated Expanding Thermal Collar as standard on every roof window, and the insulated upstand, ModuLite is designed to minimise cold bridging between the frame and roof structure, providing a single Psi value for the installation junction to help account for heat loss at this point.
The system offers SAP assessors four Psi junctions totalling 6.5 linear metres
(based on Keylite 06 sized roof windows). This is a 74 per cent reduction in linear junction length compared with traditional dormers.
With one overall U-value of 1.27 W/m2K (for a 04 sized roof window) and 1.26 W/m2K (for a 06 sized roof window) along with an 8 per cent reduction in thermal bridging compared to standard GRP dormers, ModuLite can deliver a 2-2.5 per cent improvement in assessments.
ModuLite’s benefits can give housebuilders greater flexibility when optimising the wider building specification. SAP assessors can increase design air pressure from 4 to 4.5 and relax floor U-values from 0.11 to 0.14, potentially saving developers £5 per m2.
Ultimately, by designing products with the SAP assessment process in mind, manufacturers can streamline assessments and help accelerate the overall housebuilding process.
To learn more about Keylite’s ModuLite system visit www.keyliteroofwindows.com/modulite.
