Blown Windows
What is a blown window?
A blown window is a double-glazed window where the airtight seal has failed allowing moisture to enter between the panes. This can cause condensation, as well as make the windows look cloudy. If this happens the insulation of the window may not work as well, meaning that you may feel a draft and colder.
What are the causes of a blown window?
There are many reasons for why a window might blow. Recognising the signs early can prevent energy loss and further damage.
Age and wear – Over time the windows seal naturally degrades. Most double-glazed units last around 15-20 years, but older windows are more prone to seal failure.
Poor installation – If the windows are fitted incorrectly the seal and frame can become stressed, leading to premature failure.
Temperature changes – The glass can expand and contract with the heat and cold. Overtime this can cause the seal to become loose or crack.
Excess moisture – Accelerated seal deterioration can happen if there is high humidity or poor ventilation, which can allow condensation to form inside of the window.
Frame structure – Shifts in a buildings structure or uPVC frame can end up misaligning the glass, causing stress of the seal and causing the leaks.
Environmental factors – Exposure to harsh weather conditions can weaken the seals, this is even worse on south facing windows.
How to dry out a blown window?
You cannot permanently dry out a blown double-glazed window. This is because the failed seal allows moisture to remain or return between the panes. Wiping away surface condensation and using a dehumidifier may reduce moisture in the room, but they will not repair the sealed unit. Avoid drilling into the frame or glass, as this could damage the window and may affect any warranty. Ask a qualified window professional to inspect the unit and advise whether the sealed glass unit or the whole window needs replacing.
Do blown windows need replacing?
Majority of the time blown windows do not need a full replacement, as long as the frame is still in good condition you only need to replace the sealed glass unit. This is done by removing the trip or beading that is holding the glad into place. This means that the glass pane is now accessible and can be removed. The new pane is then put into the existing frame and the beading is refitted, securing the window into place.
On some occasions the full window frame does need to be replaced. This is due to the fact that the frame has become rotten, warped or structurally unsound. If this is the case, then the whole window is removed rather than just the glass unit. The new frame is then fitted into the opening in the wall with the glass being fitted into that. Any gaps around the frame are then insulated and sealed.
If the window glazing is structurally bonded to the frame, the glass becomes impossible to remove without destroying the frame. If this happens a full window replacement will be needed.
Taking care of your windows and conducting regular maintenance can help keep them in good condition and prevent issues from developing.
How much does it cost to replace a window?
Replacing a blow window can vary in price depending on the size, type and whether the full window needs replacing or only the glass unit.
Prices can start from £55 to £400 per window, but if the whole frame needs replacing as well, prices can go up to £1,000+.
If you think your window needs replacing, we suggest getting 2-3 quotes from reputable installers. Make sure that they are FENSA or CERTASS registered so that the windows comply with any building regulations that you may have.
Max Kelly
Max is a home security how-to expert with extensive knowledge of securing doors, windows, and entire properties. With years of experience in the field, he specialises in easy-to-follow DIY solutions for common security issues. His practical, step-by-step guides empower homeowners to strengthen their safety measures with confidence and ease.
