Condensation between the two panes of a double glazed window means the seal of the unit has failed. Moist air has got into the gap, and it will not dry out again. The good news is that you can usually replace just the sealed glass unit, not the whole window, as long as the frame is in good condition.
Why sealed units fail
A double glazed unit is two panes held apart by a spacer and sealed around the edges. Over the years, sunlight, temperature changes and water sitting in the frame wear the seal. Once moisture gets past it, the desiccant in the spacer fills up and the unit starts to mist on cold mornings.
How to measure a replacement unit
The size you can see from inside the room is not the size of the unit. Part of the glass sits hidden in the frame. The most reliable method is:
- Remove the glazing beads on one side (usually inside on modern windows) and take out the unit, or ask a glazier to do it.
- Measure the glass unit itself, width first, then height, in millimetres, at both ends and in the middle. Use the smallest figure.
- Measure the thickness of the whole unit from outer face to inner face. Common thicknesses are 24 mm and 28 mm, but older windows vary.
- Check for markings in the corners of each pane. A BS EN 12150 mark means toughened glass; a laminated mark means laminated. The replacement should be at least as safe.
If you cannot remove the unit, measure the frame opening from rebate to rebate and tell us so. We will make a careful allowance, but a measured unit is always safer.
What to tell us
- Width, height and overall thickness
- Which panes are toughened or laminated (or where the window is, so we can advise)
- Any obscure or patterned glass, Georgian bars or leaded detail
- Whether you want a Low-E coating and gas filling for better insulation
When glass in a window must be safety glass
In England, Approved Document K asks for safety glass in critical locations, such as glazing in or beside doors and low-level glazing. If the window is in one of these places, the replacement unit should use toughened or laminated panes. Your installer or building control can confirm.
Which units actually need replacing
Condensation on the room side of the glass is a ventilation problem, not a failed unit; it wipes off. Condensation on the outside face on cold mornings is normal with modern, well insulated glass. Only misting between the panes, which you cannot wipe from either side, means the seal has failed. Check each window on a cold morning and make a list before you order, so all the failed units can be made at once.
Low-E and gas: worth adding?
Many older units were made with plain glass and air in the gap. A replacement unit with a Low-E coating and argon filling loses noticeably less heat, at little extra cost, and fits the same frame as long as the overall thickness is the same. It is usually worth asking for.
Do you need building regulations approval?
Replacing only the glass unit in an existing frame is usually treated as a repair and does not need building regulations approval. Replacing the whole window, frame and all, is different: in England it must meet the energy and safety rules, and is normally done by an installer registered with a competent person scheme such as FENSA or Certass, who can certify the work. If you are not sure which you are doing, ask your installer.
Fitting it yourself or using a glazier
Swapping a unit in a modern window with internal beads is a job many confident DIYers do, but the units are heavy and need the right packers under them to sit correctly in the frame and keep the window working. Older windows with external beads or putty, and large or high windows, are best left to a glazier. Either way, wear gloves and lift with two people.
Questions we are asked
Can a misted unit be repaired instead?
Some companies drill and dry units, but the seal stays broken and the insulating gas is lost. Replacing the unit is the lasting fix.
Will a new unit fit my old frame?
Yes, if it is made to the same size and thickness. Thinner units can sometimes be fitted with different beads; tell us if you want to change the thickness.
How long do double glazed units last?
Many last for decades, but life depends on the quality of the seal, how much sun the window gets and whether water sits in the frame. Misting between the panes is the sign that a unit has reached the end.
Can I upgrade to triple glazing in the same frame?
Only if the frame can take the thicker, heavier unit. Most frames made for double glazing cannot. Tell us the frame type and the thickness it accepts.
Send your sizes for double and triple glazed units made to measure, or read double or triple glazing: what the numbers mean.