How to correctly install bay windows

Bay windows are attractive features and remain popular options for new home developers and homebuyers. Yet bay windows present additional demands on detailing and workmanship which, if you get wrong the first time, can prove difficult to properly put right afterwards.

This blog explains our detailing requirements for bay windows and describes common construction errors we see where bay windows are specified, that you should strive to avoid.

How to position the vertical damp proof course and cold bridging insulation in bay windows

In Chapter 7.4 of the Technical Manual, we state that bay windows require particular care in detailing and fitting so that they are stable, weather tight and reasonably air tight.

The manual also provides a diagram, displayed below, showing how the vertical damp proof course (DPC) and cavity closer should be installed, in addition to highlighting the position of cold bridging insulation:

Bay window detailing

This detailing does require a high level of workmanship and so it is unsurprising that we sometimes find defects. Some typical issues are as follows:

Window cill level defects to bay windows

Image 1 below shows the outer leaf masonry protruding into the cavity where the splay commences, increasing the potential for damp and cold bridging. This error is due to a feature plinth course not visible in the image.

Bay Window Sill Level Defect

Image 1: Cill level defect

Below, the stone cill in Image 2 abuts the return blockwork (inner leaf) which forms the reveal above the cill. While an isolating DPC is present, once again a cold bridge is apparent in this arrangement.

Another Bay Window Sill Level Defect

Image 2: Apparent cold bridge

Reveal defects to bay windows

Image 3 shows the head of a full height PVCu bay window. The issue here is that the window frame is not overlapping the vertical DPC present in the reveal. Instead, the window frame is installed against the outer feature stone leaf, creating both a cold bridge and a path for moisture to reach the internal finishes.

In an attempt to overcome this problem, a vertical DPC wrap has been incorporated over the exposed section of the outer leaf. Even so, some of the outer leaf is still visible, shown by the arrow. In this arrangement, the plasterboard’s adhesive dab will be placed onto the DPC, which is considered to be an inappropriate substrate to adhere to, and one which offers insufficient support to plasterboard.

Reveal defect to bay window

Image 3: Cold bridge and moisture path

At the base of this full height PVCu bay window (Image 4, below), similar concerns are apparent, including the visible outer leaf and DPC positioning.

Another Bay Window Reveal Defect

Image 4: Outer leaf visible and DPC concerns

In this final image (Image 5, below), the reveal does not sit behind the window frame, as required. To mitigate this, a wide DPC has been placed over the outer leaf (orange arrow), trapped by the frame and extending into the property. This is not correct to protect against lateral damp penetration or cold bridging. You may also just notice that a vertical DPC has been incorrectly built into the reveal, tucked in too far to offer full protection against damp or cold bridging (black arrow). Once again, this arrangement would also require that plasterboard be adhered to a DPC substrate.
Bay window DPC defect

Image 5: Reveal not sitting behind frame


Watch out for bay window detailing

Please remember that the construction of bay windows requires high levels of workmanship, which our surveyors will be looking out for. As you have seen from these images, possible “fixes” typically fall short of providing a satisfactory solution. The best course of action, as always, is to get the detailing right, first time.

You can download a free digital copy of our technical manual here.

Installing glazing in areas at higher risk of causing injury? Download our technical checklist.


Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.


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