Should I Install Windows Before or After External Wall Insulation?

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Windows Before or After External Wall Insulation

Windows Before or After External Wall Insulation?

Windows are often installed as an accompaniment to external insulation. It makes sense that if you are spending this kind of money on the external walls, the windows should be up to scratch as well. The obvious question then is whether this should happen before or after the insulation.

Windows after insulation, what can go wrong?

If your windows installer does a perfect job and measures the windows exactly right, then they can be fitted into the wall and then be sealed up against the insulation. Unfortunately, it is really common for windows to be slightly under or oversized and that means either cutting back some of the insulation or adding insulation to pack avoid. Either of these scenarios is not ideal and will lead to marking or scarring of the insulation or having to repair it. As you can imagine, this is going to add extra cost to the job.

Installing windows in the future

Obviously, over the 25 year lifetime of the insulation system, there will likely be a point where new windows need to be fitted. Windows don’t have as long a lifespan than insulation, so it is to be expected that new windows will be required at some point. What is the best approach here? 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.

Insulation after Windows

The best way to do this is to install the windows first and then install the insulation right up to them with reveal beads. This ensures a snug fit and a great finish every time. The key thing to remember is to get the window sills pre-extended by your installer, as this will produce a much better finish than having to add over-sills and avoids extra silicon work.

Ventilation Awareness

The one thing that you should be aware of when installing both new windows and insulation at the same time is ventilation. In an old solid wall property with draughty windows, there are plenty of gaps for the moist air to escape and avoid condensation forming. Unfortunately, when the walls are both insulated and re-glazed, all these gaps can be blocked. To ensure that this does not create a damp issue, you should ensure that the windows have ventilation strips and that there are adequate vents on the wall. This needn’t mean draughts though. Modern ventilation boxes are much more sophisticated than old air bricks and thus would be able to diffuse the moisture away from the home without creating an excess draught.



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  1. External wall insulation and windows great content

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