The 30-second answer
Most draughty double hung windows do not need one seal, they need the right seal in each spot. The three you will run into are brush seal, compression seal and V-seal, and each one suits a different kind of gap. Here is the short version before we get into why.
- Brush seal suits surfaces that slide past each other, like the sides of a sash running up and down in the frame, and the meeting rail where the two sashes pass. The soft pile drags across timber without binding the window.
- Compression seal suits gaps that close rather than slide, like the top of the upper sash against the head and the bottom of the lower sash against the sill. It squashes when the window shuts and springs back when it opens.
- V-seal suits narrow, hidden gaps down the sides and in rebates. It folds flat to fit in and springs open to bridge the gap, so it stays out of sight.
The rest of this page explains why each one goes where it does, so you can look at your own window and pick the right seal for each gap.
Brush seal: where it wins
Brush seal, sometimes called pile weatherstrip, is a strip of dense bristle set into a plastic backing. The backing either sits in a slot cut into the timber or clips into a carrier, and some versions come with a self-adhesive back.
What makes brush seal the right choice for a sliding sash is that the pile touches the timber lightly and drags across it without grabbing. A compression seal on a sliding surface would either tear off or make the sash stiff to lift. Brush lets the sash move freely while still closing the gap the whole way up and down.
That is why brush is the seal you will use on the two long sides of each sash where it runs in the frame, and on the meeting rail where the upper and lower sashes overlap in the middle. If you want to see it fitted from start to finish on a real window, the full job is walked through in our guide on how to draught proof a double hung window.
Compression seal: where it wins
Compression seal is a strip of foam or hollow rubber that flattens under pressure and pushes back when the pressure comes off. Because it works by being squashed, it belongs on the gaps that close shut rather than slide.
On a double hung window that means the top rail of the upper sash, which meets the head of the frame when the window is shut, and the bottom rail of the lower sash, which sits down onto the sill. When the sash closes it presses the seal flat and the seal fills the gap. When you open the window the seal lifts clear, so it never rubs while the sash is moving.
The mistake to avoid is running compression seal down a sliding side. It will bind the window or get shredded within a week. Keep it to the surfaces that only meet when the window is closed. Our compression seal comes in short lengths through to a 100m roll, so you can buy the amount that matches how many windows you are doing.
V-seal: where it wins
V-seal is a folded strip, usually plastic, shaped so that it opens into a shallow V when it is in place. Push it into a gap and the two halves spring apart to bridge it. Because it sits inside the gap rather than on top of a surface, it stays hidden and does not change how the window looks.
That makes V-seal handy for the narrow gaps you find down the sides of a sash and along the parting bead, and it works well in door rebates too. It is thinner than brush or compression, so it suits spots where there is not much room to work with. Our V-seal with adhesive back comes in rolls you can cut to length.
V-seal is not the answer for a wide or worn gap. If the sash rattles with a real space around it, brush or compression will do more. V-seal is at its best on a snug gap that just needs the last bit of air stopped.
Mixing seals on one window
Here is the part that surprises people. A single double hung window usually ends up with more than one type of seal, because it has more than one kind of gap. There are four places air gets through: the two sliding sides, the meeting rail in the middle, the top of the upper sash and the bottom of the lower sash.
A typical result is brush seal down the sliding sides and along the meeting rail, compression seal along the top and bottom rails where the sash closes onto the frame, and V-seal tucked into any narrow gap that the other two cannot reach neatly. You are matching each seal to the way that gap behaves, sliding or closing, wide or narrow.
If you are new to all of this and want the plain background first, start with what draught proofing actually is, then come back and plan which seal goes on which gap.
Not sure the seal will suit your window before you commit to a roll? You can order a free sample of the compression seal and try it against your own gaps first.
Frequently asked questions
Can I use compression seal on a sliding sash side?
No. Compression seal is made to be squashed by a gap that closes, not dragged along by a surface that slides. On a sliding side it will either make the sash stiff to lift or tear off. Use brush seal on the sliding sides and keep compression seal for the top and bottom rails.
Which seal lasts the longest?
It depends more on where you fit it than on the type. A seal on a surface that rubs every time you open the window wears faster than one that only presses shut. Matching the seal to the gap, so it is compressed rather than dragged where possible, is what makes any of them last.
Do I have to take the sash out to fit these?
For the sliding sides you usually need the sash out or partly out to reach the surfaces cleanly, which is often the case with brush seal in a carrier. Self-adhesive V-seal and some compression seals can go onto the top and bottom rails with the window in place. The full step by step is in our draught proofing walkthrough.
Can I try a seal before buying a whole roll?
Yes. You can order a free compression seal sample and hold it against your own gaps to see how it fits before you decide how much to buy.
Will sealing my windows cause condensation?
Sealing changes how air moves through a room, so it is worth thinking about ventilation at the same time. If you already get condensation on cold mornings, read up on what causes it and how to manage it before you seal everything tight.