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Can I Install New Windows After EWI?

Last modified: 14.12.2023

We receive many questions from customers about whether it is possible to install new windows at a later date than their EWI installation.

It’s a tricky question. Ideally, it’s best to leave the external wall insulation undisturbed by installing new windows first. However, for some homeowners, this just isn’t possible for reasons pertaining to added costs. Alternatively, their current windows at the time of installation are in good condition. It is possible to install new windows after your EWI installation. However, we would advise that extra care is taken.

When installing EWI, new windows are often installed before the insulation to ensure that the EWI fits snugly against the window panes. The best kind of windows to install ahead of your EWI installation are windows with trickle vents. This is because, with the added insulation, gaps in the windows can be blocked up. This can increase the chances of condensation build-up, so trickle vents are necessary to alleviate this.

It does make sense to install new windows before you go ahead with an EWI installation. It saves you carefully removing the windows at a later date - at the risk of damaging the insulation. However, we are aware that often the windows may be perfectly okay at the time of installation. For varying reasons, they may not outlive the 25-year life of the external wall insulation.

Unfortunately, replacing your windows after external wall insulation is not the easiest job. Therefore, you will need to make sure that you choose a professional to carry out the task. The ultimate aim is to prevent any damage to the insulation or rendering.

If you are very lucky and your windows installer does a perfect job and measures the windows exactly right, then they can be easily fitted into the wall and then sealed up against the insulation. Unfortunately, it is really common for windows to be slightly under or oversized. As a result, cutting back some of the insulation or adding insulation to fill out any gaps is the solution. In these situations, it’s unlikely that you will avoid any marking or scarring of the external wall insulation. If any marking or scarring occurs, extensive repairs will be required. This can be difficult, time-consuming, and an extra expense.

As you can imagine, this is all going to add an extra cost to the job. So it’s worth thinking about how old your windows are and whether it’s viable to get them replaced before your external wall insulation installation.

The key thing is to ensure your installer is as careful as possible to not damage the rendering when taking the old windows out. We recommend removing them from the inside of the house to avoid knocking the render on the reveals. If any damage is done to the reveals, this is usually fairly easy and cheap to repair. Unfortunately, if any damage is done to the main elevation, this can prove difficult to patch without leaving a scar on the wall. And it may be necessary to re-render the whole elevation.

Windows form a crucial part of the overall thermal envelope. Any gaps in the envelope act as a thermal bridge, as heat always looks for the path of least resistance. Therefore, any part of the assembly that has a lower R-value will inevitably act as a cold bridge. These cold bridges are heat highways and cause havoc when it comes to keeping your home warm. New double-glazed windows offer the benefit of minimising thermal bridging and increased airtightness.

Just installing a new window will not offer both of those benefits. Great care must be taken to ensure that the windows are flush with the insulation and any reveals are sealed by membranes and verge trims. The creation of a thermal bridge not only wastes precious and expensive heat energy but also allows for condensation to build up, leading to mould growth.

New verge trim regulations concern themselves mainly with the introduction of a secondary membrane. The membrane serves to prevent any water ingress behind the insulation. The following is the guidance summarised by EWI Pro.

  • Redundancy of seals: At least two lines of weathering protection. Sealants shall not be employed to provide the primary barrier to water penetration. The additional redundancy is achieved with an additional trim (cover trim) or suitable membrane or flashing. All joints and render abutments must have a double seal to comply with PAS requirements.
  • At eaves with insufficient roof overhang: Protection afforded to the top of the EWI system must include a secondary waterproof membrane and/or flashing that tucks under the existing sarking felt where present.
  • The overhang must be appropriate for the exposure zone and profile type: Fore ‘verge trim’-type profiles: min. 40 mm for moderate exposure, minimum 50 mm for severe or very severe exposure based on BRE wind-driven rain map: Ref – BR 262 Thermal insulation: avoiding risks (Appendix A: WP2 (publishing.service.gov.uk). For ‘integrated gutter type’ profiles, a minimum of 10 mm and as recommended by the profile supplier.
  • Gable-to-eaves junctions must be achieved with overlapping, pre-fabricated units/connectors: Site fabrication is not permissible although minor site trimming/bending is permissible if per the manufacturer’s recommendations. Joints between primary and secondary seals are offset/staggered by at least 100 mm.
  • To reduce thermal bridging effects, thermal insulation at gable-to-eaves junctions must be maintained up to the level of the top of the loft insulation or greater, per the Retrofit Designer’s requirements.
  • Gable apexes must be formed using pre-fabricated elements: Site fabrication is not permissible although minor site trimming/bending of pre-fabricated apex profiles is permissible if per the manufacturer’s recommendations. Where site bent flashings/trims are employed a lead/lead replacement flashing that covers the full depth of the apex profile is necessary. Site-mitred apex joints using two separate profiles are not permissible.

Source: EWI Pro