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What is the maximum gap around UPVC windows?

April 12, 2024

You may well be used to seeing ‘mind the gap’ at train station platforms. 

But if you’re installing PVCU windows, they’re three words that are very much worth bearing in mind too. 

The maximum gap around UPVC windows will more often than not depend on the installation guidelines provided by the manufacturer. 

However, a good rule of thumb is, 5mm per gap if the structural opening width is up to 3m, and 7.5mm if it’s between 3 and 4.5m.


Adhering to maximum gaps – What are the requirements?  


Exceeding the recommended maximum gaps around UPVC windows can lead to a number of issues. 

If the gaps are too large, it may compromise the effectiveness of the window’s insulation, allowing draughts, and potentially reducing energy efficiency. 

Improper gaps can also impact a window’s ability to resist water infiltration, which could lead to leaks and water damage. 

Furthermore, exceeding recommended clearances can affect the overall structural integrity of the window, possibly resulting in warping or misalignment over time. 

Exceeding the maximum gap between the window frame and wall isn’t the only potential issue when measuring up either. If a gap is too small, then the installer may risk damaging the window during the installation process.

Gap rules for windows versus doors – what you need to be aware of  


On the surface, gap rules for windows and doors appear identical. 

However, while the principles of managing gaps around windows and doors do share similarities, there are some differences based on their functions and construction.

Gaps around windows, for instance, are primarily managed to accommodate the natural expansion and contraction of materials when temperature changes occur. Proper sealing is also crucial for energy efficiency and weather resistance. 

While the gaps around doors do serve similar purposes, there are also additional factors to consider such as security, sound insulation, and ease of operation. Doors often have more stringent requirements for security, so gaps may be smaller to prevent unauthorised access.

Similarly, in the case of sound insulation, windows and doors may require different approaches depending on the customer requirements. 

For example, a sliding door may have different sound insulation properties when compared to a swinging door; likewise, fixed windows when compared to ones with multiple sashes. Whether it’s windows or doors you are dealing with, in order for effective soundproofing, security, energy efficiency, and weather resistance, it is vital gaps are properly sealed, and appropriate materials are used.

What’s the best approach for filling larger gaps around windows?  


For larger gaps around windows, a combination of methods such as expanding foam tape can be used to ensure the best possible insulation, sound/weatherproofing, and aesthetic appeal. 

Regardless of size, cleaning the gap around the window to remove dirt, debris or old sealant should always be the first step. Measuring the width and depth of the gap comes next. If working with a larger than average gap, inserting a backer rod will help provide support for the sealant, acting as a foundation, and preventing it from sinking into wide and deep crevices. Be sure to choose a backer rod size that fits the gap snugly. 

A high-quality sealant, weather-resistant and designed for exterior use, will offer the greatest protection. Once applied, use a putty knife or a caulking tool to smooth and shape it, completely sealing the gap, leaving a clean finish. For added draught protection, especially if the gap is near moving parts, additional measures such as weather stripping may be considered.


How often should window gaps be checked?


We would recommend checking window gaps periodically to ensure they remain properly sealed and insulated. Regular checks will mean any issues can be identified and dealt with promptly. The frequency of these checks will often depend on a number of factors including age of window, local climate conditions, and the type of sealant used. 

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