Installing Windows: How to Get it Right

Green Building Store's Mike Shufflebotham gives his expert advice on how to ensure your windows deliver the performance you expect
Green Building Store
by Green Building Store
17th February 2021

It is not enough to have chosen high-performance triple glazed windows and doors for your project. To ensure they perform as designed, you also need to make sure they are installed well.

Window U-values measure the thermal transmission of the window, with lower ratings indicating better performance. A good triple glazed window might have a whole window (Uw) value of around 0.8 W/m2K. But with bad installation details, the overall heat loss of that window can be doubled to an effective U-value of 1.6 W/m2K or more once fitted.

By contrast, a really well-detailed installation of a triple glazed window can actually improve the unit’s baseline Uw-value. So, you can see how important this is to getting the right result for your project.

Want face-to-face windows & glazing advice for your project? Green Building Store’s Mike Shufflebotham will be available to speak to LIVE at Build It’s Expert Advice Weekend on 20th & 21st February. Book your free place here >

Good window design

Windows and doors are usually the weakest point in the insulation envelope. They’re also potentially one of the worst areas for airtightness and thermal bridging. So, a lot of care is needed when thinking about how they will be installed.

Your architect and window supplier need to be planning good installation detailing at a very early stage. So, always look at the whole package of what’s on offer, including the design and installation guidance you’ll receive.

Green Building Store passivhaus certified windows

This project features Green Building Store’s Passivhaus-certified Progression windows

For instance, Green Building Store offers product technical support for its window customers. This includes technical advice on detailing, indicative or modelled installation Psi values, and Passivhaus Planning Package-ready data. This level of detail will be invaluable in getting the design detail right.

Thermal bridging

Windows generally perform best when located within the insulative wall layer. If you position windows and doors within non-insulative construction layers, such as stone or brick walls, you will create a thermal bridge where heat can escape. This not only wastes energy but also leads to the risk of condensation and mould growth on these cold areas.

To minimise any thermal bridge, it is often possible to bring the insulation over the face of the window frame, effectively wrapping part of it in insulation. The best approach is to model window installation details in thermal modelling software, to establish the thermal bridge Psi values.


As with thermal bridging, one of the areas of highest potential air leakage can be at junctions between walls and windows. To achieve good airtight seals around window frames, it is important to use high-quality, long-lasting proprietary airtightness tapes.

Green Building Store air tightness tape window

Applying air tightness tape as part of a Green Building Store window installation

Depending on the construction type, at Green Building Store we use the following Pro Clima tapes:

  • Contega Solido SL/ SL-D is ideal for connecting window frames to plaster surfaces.
  • Tescon Profil tape is suitable for sealing windows to other surfaces, such as intelligent airtight vapour membranes, masonry, render or timber frame.

Good window installation

Unfortunately, things can and do go badly wrong at the installation stage. For example, doors might not be installed fully square, which can create a lot of problems and is not easy to rectify. So, appointing an experienced installation team is essential.

Where to find installers

Often you will have a specialist fitting team connected to the window supplier. Alternatively, the builders who are working on other aspects of your self build or retrofit project might undertake the window install. When looking for installers, try to view case studies and get testimonials from other customers.

Green Building Store’s window installation top tips

  • Good installation detailing is essential for a well-performing windows and doors.
  • Early design and consideration will help ensure that there are fewer problems on site.
  • Think about thermal bridging when designing window installation.
  • Positioning the windows and doors within the insulation zone for optimum performance.
  • Don’t forget airtightness detailing around windows and doors.
  • Choose an experienced and trusted fitting team.

Again, speak to your supplier to understand your options. Green Building Store offers several fitting options, including our own FENSA-accredited supply-and-fit service for mainland UK, or recommended regional installers. We can also provide a site visit service with training and support for your building team.

Measuring up

We always recommend that the team who will be fitting the windows is the same team that measures up the openings on site. This gives you a single point of responsibility for the install.

Quality assurance

How can you ensure that your windows and doors are being installed well on site? Windows and doors should be fitted plumb and square, and be either mechanically fixed through their frames or with substantial straps or plates. And importantly, they should be fitted at the agreed depth and position within the wall envelope, with tapes and seals properly applied. Always check they operate freely and smoothly after fitting.

Mike Shufflebotham has a wide technical knowledge and understanding of low-energy and Passivhaus building products. He is sales manager at Green Building Store and has been with the company since 2008.

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