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Bulging interior window sill

Wooden interior window sill beneath bay window in upstairs bedroom has a bulge in the inside corner. The window is double glazed and secondary glazing has also been installed. The area isn't damp and the bulge is firm to the touch and doesn't move, even when I press it. Any advice on how to get this investigated and fixed would be welcomed. Thanks.

3 Answers from MyBuilder Window & Door Fitters

Best Answer

Hi Triena,

It sounds like you may have damage from a previous water ingress issue. This may have happened during the winter and the window sill has dried in the warmer weather and bowed.

The most common cause I have found for this is if there are gaps to the sealant on the inside of the property or the mastic around the window on the outside. I would advise checking both of these areas.

Another cause can be condensation settlement on the window ledge consisting of temperature differences inside to outside with this then drying into the woodwork.

it should be relatively easy to remove and replace the existing window sill and reseal.


Answered 2nd Aug 2019

Hi Triena,

It will probably be where foam was used during the installation and has seeped from a small gap.
As it it not showing any signs of damp then you can sand make good and paint and it will be fine.
I hope this answers your question you.

All the best



Answered 16th Aug 2019

Use a damp meter and a thermometer or infra red camera to see if you are losing heat or gaining water. Both of which would account for the bulge past or present and it could easily be both. Another factor to consider would be the material used for the internal sill. Hardwood or Upvc are both extremely stable but compressed waste wood aka Mdf/ chipboard/ flaxboard/ osb could all pose problems when absorbing moisture. Always the possibility of rusting present or previous fixings causing expansion leading to a bulge appearing. Start with a damp meter and check more than one location to get accuracy, then use temp gauge or better still thermal imaging to find any cold spots. Hope this will help you.


Answered 21st Aug 2019

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