Can I just use self-adhesive draught strip from a DIY shop?

On a timber window, yes, and it works well. On a modern uPVC or aluminium window it usually stops the sash closing properly and does more harm than good, because those systems are designed around a specific gasket in a specific groove. Get the matching gasket instead.

The draught is coming from the bottom of the frame, not the opening part.

That is the frame-to-wall junction or the cill, not the window. It is a sealing job on the outside and often one of the cheapest fixes available, but it has to be done from outside with the right mastic rather than filled from indoors.

Is a draught the same as a cold spot?

No, and confusing the two leads people to replace windows that were fine. A draught is moving air you can feel on skin. A cold spot is radiant: the glass is cold, your body loses heat to it, and it feels like a draught without any air moving. Only the second is solved by better glass.

Living with them · 16 of 19

Why your windows are draughty, and what actually fixes it

A draught from a window is nearly always one of four things: a perished rubber gasket, hinges and a locking mechanism that have stopped pulling the sash up tight, a gap between the frame and the wall hidden behind the plaster or render, or a trickle vent doing exactly what it was fitted to do. Only the third, and the worst cases of the second, point to replacing the window.

A draught is a symptom, and the four things that cause it need four different trades and four very different amounts of money. It is worth ten minutes finding out which one you have before anyone quotes you for a window.

Find out where the air is actually coming in

Do this on a windy day, with the heating off for an hour so the house is not pushing air out of its own accord.

Run the back of a wet hand slowly around the opening sash, then around the outside edge of the frame where it meets the wall. Wet skin registers moving air that dry skin misses.
Close the window on a strip of paper and pull. If it slides out easily, the sash is not compressing the seal at that point. Repeat at the top, bottom and both sides -- where it is loose tells you whether it is the whole seal or one corner.
Check whether it stops when you lock the handle. On most modern windows the handle drives cams that pull the sash in. A draught that disappears when locked is a mechanism out of adjustment, not a worn seal.

The four causes, and what each one means

In rough order of how often they turn out to be the answer:

A perished gasket. The rubber seal around the sash goes hard and stops springing back. It is a strip of rubber that pulls out of a groove and a new one pushes in. The cheapest outcome on this list by a wide margin.
Hinges and mechanism out of adjustment. Friction hinges and the locking cams can be adjusted so the sash pulls up tight again. Where the hinges have worn rather than drifted, they are a replaceable part. See window hardware.
A gap behind the frame. The frame is sealed to the wall with foam and mastic, and on a poor installation that seal is incomplete or has failed. The air is bypassing the window entirely. Raking out and resealing the perimeter is a small job; a badly fitted window that was never packed or sealed properly may be worth taking out.
A trickle vent. If the draught is a steady stream from a slot in the head of the frame, that is a ventilator, and it is there on purpose. See trickle vents before you tape it shut, because closing it permanently moves the problem to the glass as condensation.

When it really is the window

Three cases where a replacement is the honest answer rather than the expensive one:

The frame has moved. A uPVC outer frame that has bowed, or a timber one that has rotted at the cill, cannot be made to seal against anything.
The sash no longer sits square. If the paper test is loose along a whole diagonal and adjusting the hinges will not bring it back, the sash itself has dropped out of shape.
It is single glazed. Draught-proofing a single-glazed window is worth doing and will help, but the cold you feel standing next to it is mostly the glass, not the gaps.

A draught in a timber sash window is a different job again, and usually a very satisfying one: brush-pile draught-proofing routed into the staff and parting beads transforms them without touching the original joinery.

Questions

Can I just use self-adhesive draught strip from a DIY shop?: On a timber window, yes, and it works well. On a modern uPVC or aluminium window it usually stops the sash closing properly and does more harm than good, because those systems are designed around a specific gasket in a specific groove. Get the matching gasket instead.
The draught is coming from the bottom of the frame, not the opening part.: That is the frame-to-wall junction or the cill, not the window. It is a sealing job on the outside and often one of the cheapest fixes available, but it has to be done from outside with the right mastic rather than filled from indoors.
Is a draught the same as a cold spot?: No, and confusing the two leads people to replace windows that were fine. A draught is moving air you can feel on skin. A cold spot is radiant: the glass is cold, your body loses heat to it, and it feels like a draught without any air moving. Only the second is solved by better glass.

Note This guide is general information about England and Wales, not advice about your specific property. TrustedGlazing is an introducer and does not install, survey or quote. Rely on your installer's survey and your local authority for decisions about your own home.