A double glazed window is three products in one purchase: the sealed glass unit, the frame it sits in, and the installation that ties it into the wall. Buyers compare the first, salespeople talk about the second, and the third is what decides whether the window performs and whether it leaks. Knowing which part a quote is actually describing is most of the skill in buying them.
The sealed unit
Two panes with a spacer between them and a gas fill, sealed at the edge. The things that vary are the coating on the inner face of the outer pane, which is what reflects heat back into the room, the gas between them, and the spacer material, where a warm edge spacer reduces the cold bridge at the perimeter and with it the condensation that forms at the bottom of the glass. The seal is also the part that eventually fails, which is what a misted unit is.
The frame does more than hold the glass, from uPVC to wooden double glazing
UPVC is the volume product and performs well at the price. Aluminium gives slimmer sightlines and more glass for the same hole, with a thermal break doing the work the material does not. Timber suits period buildings and needs maintenance. Composite combines a timber inner with an aluminium outer. The whole window rating matters more than the glass specification alone, because a good unit in a poor frame is a mediocre window.
What the regulations expect on a replacement
Replacing windows is controlled work. The new windows have to meet a minimum thermal performance, keep the ventilation the room had through trickle vents, maintain any escape window the room relied on, and use safety glass in critical locations. Most installers self certify through a competent person scheme and give you a certificate; otherwise the work goes through building control. That certificate is what a buyer's solicitor asks for, so keep it.
The survey and the fit
A proper survey measures every opening rather than assuming they match, checks the lintel and the state of the reveal, and flags where a frame has been carrying something it should not. Installation is then about packing and fixing the frame square, sealing on the outside with a weather seal and on the inside against air movement, and making good. A window fitted out of square never closes properly and no amount of adjustment later corrects it.
How to compare three quotes honestly
Ask each for the whole window energy rating or U value rather than the glass figure, the frame material and system name, whether trickle vents are included, which openings are escape windows, who certifies the work, what the guarantee covers and whether it is insurance backed, and what making good is included. Written on one sheet, three quotes stop looking similar very quickly.
Questions people ask about double glazed windows
Do replacement windows need building regulations approval?
Yes, replacement windows are controlled work. In practice most installers self certify under a competent person scheme and issue a certificate, which is the document you need when you sell.
Are trickle vents compulsory?
Where the window being replaced had them, the replacement must provide no less ventilation, and current guidance means most replacements now include them. An installer offering to leave them out is offering to fail the requirement.
How long do double glazed windows last?
Frames commonly last decades. The sealed units are the shorter lived part and can mist well before the frame is finished, which is why units can usually be replaced on their own.
Is it worth replacing windows one at a time?
It spreads the cost and it loses the discount that comes with one visit and one scaffold or access set up. Doing a whole elevation at once is usually the better value compromise.