Windows are holes in the wall, and every one of them depends on a few small details to keep driving rain out. Those details are easy to miss from the inside, and by the time the evidence shows on a reveal or a curtain the wall around the frame has usually been taking water for a while.
The sill has to throw water away
An outside sill does two jobs. Its top slopes away from the frame so rain runs off rather than pooling, and its underside carries a drip groove, a small channel cut along the front edge, that stops water running back underneath and down the face of the wall.
Run a finger under the front of each sill. If the groove is full of paint, moss or old sealant, water creeps back along the underside and soaks into the brickwork below. Clearing it with a screwdriver takes a minute.
Sealant is a short-term part
The joint between the frame and the wall is usually sealed with mastic. It does not last for ever. Look for sealant that has cracked, pulled away from the brick on one side, or gone hard and shiny. Cut out the old bead completely and replace it rather than running new sealant over the top.
What the inside is telling you
Damp around a window shows first in the reveal, the short return of plaster between the frame and the room. Bubbling paint low on the reveal, a tide mark below the sill board, or black spotting in the bottom corners all point to water getting past one of the details above. The spotting itself can be condensation, especially on cold single glazing, so it is worth knowing which you have before spending money. When readers in Ohio have described that kind of patch, the advice has been to arrange mold testing in Columbus or locally, because a test shows whether growth sits in the wall or only on the surface.
Weep holes and soft timber
Never seal the bottom of a frame shut. Many windows rely on small weep holes along the base to let water drain out of the frame itself, and blocking them sends that water inwards.
On wooden frames, push a small screwdriver into the bottom rail and the lower corners of the sill. Firm wood resists. If it sinks in, rot has started, usually where paint failed on a joint. A patch the size of a thumb can be dug back to sound wood and rebuilt with resin filler; a soft corner or a soft rail means calling a joiner.
A routine worth keeping
Twice a year, alongside the gutters, walk round every window. Clear the drip grooves, check the sealant, press the timber and look at each reveal from inside. It takes half an hour and saves a frame.