To reseal a window, cut out the old sealant, scrape the joint clean and let it dry, then run a fresh bead of low-modulus silicone along the outside joint and tool it smooth. Work on a dry day above 5°C. Leave it 24 hours before rain reaches it. Inside joints take paintable acrylic instead.
Your windows need resealing when the joint between the frame and the wall has cracked, shrunk or pulled away from one side. That gap is the route air and rain take into the wall. The glass and the frame can both be in good order while the joint around them has given up.
Five signs worth checking on your own windows:
To find the leak, light an incense stick and move it slowly along the joint with the room closed up. Where the smoke pulls sideways, you have found the gap. Incense works better than a candle here, and you are not holding a flame next to a timber frame.
If the draught comes from the opening light rather than the joint around the frame, that is a different repair. Our guide to stopping draughts around windows covers that one.
One symptom is not a sealant problem at all. If the misting sits between the two panes rather than on the room side of the glass, the sealed unit itself has failed. No amount of work on the outside joint will clear it.
Replacing the unit or the whole window is then the question, and double and triple glazing compared sets out what the step up buys you.
Three products to keep away from your windows. General-purpose bathroom silicone has a high modulus and will tear away from the joint as the frame expands. Decorator's caulk belongs indoors and turns to powder outdoors within a season. And silicone will not stick to old silicone, whatever the tube promises.
Backer rod is the item people skip and then regret. A deep joint filled solid with sealant bonds on three sides and tears as soon as the frame moves. The rod gives the bead a defined depth and lets it stretch.
Cut the old bead along both edges first, then lift the middle out in one length. Working the middle first leaves you picking at fragments for an hour. Once the bulk is out, scrape the residue and wipe the joint with methylated spirits so the surface is clean and free of silicone oils.
Use a plastic scraper or a blunted knife. uPVC scratches, and the marks show against white. Take the last of the residue off with a silicone remover rather than abrasive pressure. Skip sandpaper and solvents on the frame itself, since both can dull and damage the surface.
Timber is more forgiving of a knife but less forgiving of what it hides. Once the old bead is out, probe the exposed timber with a bradawl. Soft, spongy wood means rot, and rot needs cutting out and filling before anything gets sealed over it. Keeping the frame painted is what stops it coming back, and looking after timber window frames sets out the routine.
Two details decide whether the joint lasts. The first is one continuous bead. Stopping and restarting leaves a weak point at every join, and that is where water gets in first. Keep the gun moving at a steady pace and push the nozzle into the joint rather than dragging it along the surface.
The second is tooling. Press the bead into the joint with a wetted jointing tool so it makes contact on both faces, then pull the masking tape away while the surface is still wet. Tape removed after the skin forms takes a ragged edge of sealant with it.
Check the tube for cure times. Most silicones skin over in under an hour and cure through over 24 hours, but a deep bead in cold weather takes longer. Keep water off it until it has cured through, not just until it feels dry.
You do not seal it. The glass in a uPVC window sits against a rubber gasket and is held by a glazing bead that clips into the frame. If air or water comes past the glass, the gasket has hardened or shrunk, and the repair is a new gasket rather than a bead of silicone.
Take out a short length of the old gasket and measure the profile before you buy. Gasket sections are not interchangeable between window systems. Silicone squeezed around the glass instead will trap water against the bead, and it makes the eventual proper repair a much worse job.
Rake out the old putty, prime the bare timber rebate, then bed the glass on a thin layer of fresh putty and face it with a bevelled line on the outside. Work the putty in your hands until it is pliable first. Straight from the tub it tears rather than spreads.
Bevel the face putty so the top edge sits just below the sight line of the glazing bar when you look from inside. Get that line right and the putty is invisible from the room. Get it wrong and every window on the elevation looks heavy.
Putty needs one to two weeks to skin before it takes paint, and the paint should lap a couple of millimetres onto the glass to seal the top edge. Rushing to paint soft putty is the usual reason a reglazed timber window fails within a year.
Resealing fixes the joint around a sound window. It does nothing for a window that has reached the end of its life. Three cases where fresh sealant is money and a weekend spent on the wrong job:
If you are reaching that point on most of the windows in the house, the honest comparison is the cost of repeat repairs against a replacement that comes with a warranty. Our uPVC window frames carry a 10-year full warranty, and every product we make is UKCA marked.
You can weigh a replacement in uPVC or Scandinavian pine against another round of resealing, and when to replace your windows walks through the signs that settle it.
A sealed joint lasts longer when the window around it is looked after. On uPVC, wipe dirt and deposits away with a damp cloth and ordinary cleaning agents. Avoid sandpaper and solvents. For heavier soiling, a dedicated plastic cleaner does the job without attacking the surface. The full routine is in cleaning and maintaining uPVC windows.
Alu-clad frames need less. Wash the frame and sash a couple of times a year with pH neutral soap and lukewarm water, and the exterior needs no painting at all.
Once a year, lubricate the hardware and hinges on every window. Clean the dirt off first, because grit in a hinge works as an abrasive, and wipe any spilled oil off the frame before it stains.
One thing not to seal: the trickle vents. They look like a draught and they are not. Blocking them moves the moisture problem from your window to your walls.
Good luck with the job. If you get partway in and find rot rather than a tired joint, that is worth knowing before you buy the sealant.
