There are two key principles to consider when designing the window installation detailing for your energy efficient retrofit.
1. Continuity of insulation (minimising thermal bridging)
Windows and doors are usually the weakest point in the insulation envelope, as well as potentially one of the worst areas for thermal bridging, so a lot of care is needed when installing windows into an existing home. We need to aim for a continuous line of insulation around the house with a minimum of gaps in or penetrations through the insulation layer. This will minimise thermal bridging and optimise the value of insulation measures.
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 wastes energy but also leads to the risk of condensation and mould growth on these cold areas. No one wants to install new high performance windows only to have black mould developing on the reveals!
If you pay careful attention to the window installation details then you should avoid any mould growth and also get the most from the investment in high performance windows and doors. If you are installing a good triple glazed window (eg Uw value around 0.8 W/m2K), with bad installation details then the overall heat loss of the window can be doubled to an effective installed Uw value of 1.6 W/m2K or more. By contrast, a really well-detailed triple glazed window can actually improve on the Uw value of the window itself.
Sometimes we have no choice about the positioning of the window. However, there are still things we can do to improve the installation. This might mean using extra insulation on the window reveals. If space is tight we may need to use specialist insulations with particularly good insulation values, such as Aerogel or Vacuum Insulated Panels. To be sure of your details they can be modelled in thermal modelling software such as Flixo or Therm to establish the thermal bridge Psi values, something that will certainly need be done if you are working to the Passivhaus retrofit standard.
Wrapping insulation around the frame
As far as is possible, to minimise any thermal bridge, we should think about bringing the insulation over the face of the window frame, effectively ‘wrapping’ part of it in insulation.
- If you are using external wall insulation, then you want to bring the external insulation over the face of the frame. This will be most effective with inward opening (tilt and turn) type windows as the external face of the frame will be larger than the inside.
- If you are using internal wall insulation, then you will want to bring the insulation over the inside face of the frame. This will be most effective with outward opening windows where the internal face of the frame is larger.
Unfortunately, in many retrofit situations, it is not always possible to install windows at the same time as the EWI (external wall insulation) or IWI (internal wall insulation) strategies. So, we need to think ahead and plan carefully the sequencing of any radical retrofit.
If using EWI we will ideally want to position windows towards the exterior of envelope in line with the insulation and, if possible, bring the insulation over the face of the frame. However, sometimes it is not possible to complete the EWI at the same time, in which case, if possible, position the windows standing out from the wall (so that they will eventually be in line with the EWI) and provide temporary protection until the EWI can be installed. This can be weatherproofed with timber reveals. Remember, if you are installing the windows and EWI in different phases, the external sill depth change needs to be factored in too.
If windows are installed before IWI: specify outward opening windows and possibly thicken the timber frame (depending on the size of window and amount of glazing) to allow the insulation layer to wrap internally around a good part of the frame.
If IWI is installed before windows: if windows can be externally removed, you will need to ensure IWI detailing will fit and be continuous with any future replacements. If windows can only be internally removed, this will change the existing IWI and new IWI reveal detailing will have to be created when the windows are replaced.
In small window openings, or where space is tight, we can use VIPS Panels or Aerogel which have much higher insulation values than even the most advanced phenolic rigid insulation boards. These are expensive, but if just used for window reveals they can be very cost effective and are certainly worth considering.
Forming a continuous airtight seal around your house is critical to the concept of retaining and reusing heat energy in the air to reduce energy demands, as well of course as making it more comfortable by eliminating drafts. As with cold bridging, one of the areas of highest potential leakage can be at junctions between walls and windows.
Choose windows and doors with class 4 airtightness
Class 4 (BS EN 12207) is the best grade of airtightness for windows and doors and is available on our ULTRA/ PERFORMANCE/ PROGRESSION windows as standard. Our products also have the advantage of always having 2 compression seals which result in the window having greater longevity of the airtight seal. Providing space for two seals means that the frame has a slightly deeper profile but our designers believe that the advantages of longer life airtightness easily outweigh the disadvantage of the slightly bulkier window frame.
Use of airtightness tapes around windows
To achieve good airtight seals around window frames we will need to use high quality proprietary airtightness tapes. It is particularly important to understand that silicone and expanding foam will not provide the permanent airtightness our buildings need, nor will cheap tapes provide sufficient longevity. The best airtightness tapes have adhesives developed for the aerospace industry which are designed to continue to give good adhesion and airtightness for 100 years plus.
Below are some of the key airtightness tapes that we use in retrofit projects:
Pro Clima Contega Solido SL
Solido SL is ideal for connecting window frames to plaster surfaces.
- Airtight window and door sealing tape with vapour control properties.
- Full surface waterproof solid adhesive backing offering excellent bonding to mineral surfaces.
- Can be plastered over the fleece face
- Vapour retarding and airtight sealing of indoor window and door joints.
- 2 strip adhesive backing available in widths 80, 100, 150, 200mm
Pro Clima Contega Solido SL-D
Solido SL-D is similar to Solido SL but with the added advantage of an integrated adhesive zone on the fleece side for pre-taping windows before installation in the opening.
- Pre-taping windows before installation offers the quickest way to get an airtight window.
- Careful attention is needed at the corners as a “Rabbits ear” needs to be created (pinching tape as you go around to make up the slack between frame and opening).
Pro Clima Tescon Profil
Tescon Profil is suitable for sealing reveals at windows, roof windows and doors to the reveal such as intelligent airtight vapour, masonry, render, timber frame, etc. before finishing with plasterboard or timber, etc.
- Flexible multi-purpose corner tape for windows, doors and corner joints.
- Simplifies the sealing of angled joints thanks to two release paper strips, which allow the activation of a single part of the adhesive surface and to seal corners junction by junction.
- Used for taping window/door after installation.
- Ensures high protection against piercing in corners, due to its high elasticity.
- Ideal where pre-taping integrity cannot be guaranteed during installation particularly with large heavy windows and doors.
Bill Butcher, Director, Green Building Store
Thank you to Architecture student Isaac Shepherd who contributed to this blog during a recent ‘virtual’ work experience placement at Green Building Store.
Bill has given a Carbon Coop webinar on Window installations for energy efficient retrofits and you can view the recording and slides here.
Green Building Store offers FREE product technical support for its window customers, which includes installations PSI values, technical advice on detailing and PHPP-ready data.