Extra large windows and glazed screens – how to de-risk project specification

Tom Fowkes, VELFAC Housebuilding Business Manager

For many housebuilders, generous glazing is considered an easy route to market differentiation with extra-large windows and impressive glazed screens used to boost curb appeal and deliver light filled interiors. Improved window performance has steadily increased the ratio of glass to wall in many new build homes, but developers need to plan carefully in order to make the most of the benefits these windows can offer, and minimise the risks.

When planning large window installations, we recommend a ‘whole building’ approach. This draws on Active House principles, which aim to heat and ventilate a building using natural, rather than artificial, means. Start by assessing the impact of site orientation on potential light levels in every room, how these will vary over the day and year, and also the impact of local shade. This will determine the ideal location for extra-large double glazed window units which can maximise solar gain in the winter months, with summer heat gain regulated by solar shading. Colder, north facing facades can be optimised with smaller triple glazed windows in order to retain heat and thereby reduce overall thermal loss. When placed in the optimum locations, high performance windows will maintain a comfortable and uniform temperature across the home all year round, even in open plan designs. With no draughts or cold spots, homeowners can make maximum use of floor space right up to the glass, and the additional daylight flooding the home can further reduce energy bills by reducing the dependence on artificial light sources. To fully exploit the glazing, we encourage designers to specify slim framed window systems and to simplify window configurations as much as possible. Fewer transoms and mullions reduce costs while increasing glass area, together with light and thermal efficiency, as glass is a better insulator than frame elements.

Many of these measures will have a substantial impact on building design, so consult with your window supplier at an early stage. Their valuable insights into glazing performance, in terms of light and heat gain and associated costs, will ensure optimal thermal performance across every home you build, and help mitigate any risks of overheating. Internal blinds will not reduce excess solar gain (as the heat is already in the room) so work with your architect and window supplier to determine the physical barriers required to reduce overheating, such as extended eaves, small balconies, external shutters or brise soleil. Ventilation is also key to optimum thermal performance. Larger window openings can provide large purge ventilation options, while built-in trickle vents can ensure a good flow of air throughout a home. Also consider installing high level or clerestory windows, which can be operated by sensor or remote control, to help extract heat by convection.

All windows must also meet relevant building regulations (and compliance will also be demanded by your insurer). For example, if you are planning a glazed screen that crosses a floor slab (as often seen in entrance screens, for example) your building insurer will often require the screen to meet CWCT (Centre for Window and Cladding Technology) standards. Any window installed on the ground floor, or any location 2m above a flat or sloping surface (such as a balcony, garage or porch) must also be Part Q compliant. Non-compliance will require remedial work, with associated project delays and extra costs, but early supplier consultation can significantly de-risk this part of the building process, as can the specification of building products which already meet necessary standards.

The physical installation of extra-large window units should also be discussed at the start of the build, and not as an afterthought. Very often, the breakdown of window elements depends on site installation circumstances, so these need to be determined at an early stage – before prestart meetings – as they could also impact costs. Large windows can be exceptionally heavy so may require specialist lifting equipment for delivery and installation. Also check site access for delivery, and that on-site storage is ready. If you need help with installation then ask your window supplier if they run an ‘approved installer’ scheme, where third party installers are trained to install specific products to the right standards, with the supplier often happy to guarantee their work as a result.

Large windows can certainly maximise ROI, but as specification and installation is a specialised process, poor planning risks compromising all the financial benefits the glazing can deliver. As with so many building projects, the key advice is to consult early with your window supplier – this will ensure your specification is realistic in terms of design and performance, meets all regulatory requirements, and that the windows can be fitted on time and to your budget.

For more info https://velfac.co.uk/housebuilding/