Sash Windows: Compression Seals vs Brush Seals: Myths Debunked!
Why a Brush Seal is Far Superior to a Compression Seal for Draught-Proofing Sash Windows
One of the most common questions we hear from homeowners looking to improve the energy efficiency of their timber sash windows is simple: should I use a brush seal, or a compression seal? It's a great question, and the answer is clear. When it comes to draught-proofing double hung sash windows, brush seals are far superior to compression seals. Here's why.
"Should I use a brush seal or a compression seal?"
— The question we hear most often
When it comes to draught-proofing double hung sash windows, brush seals — like Timber Brush Seal with Fin — are far superior to compression seals, such as Schlegel Aquamac and similar products. It comes down to one simple fact: sash windows slide, they don't compress. Once you understand what each seal type is actually built to do, the choice becomes obvious.
Compression vs brush seals: what's the difference?
To choose the right seal, it helps to understand what each type is actually designed to do.
Built for closing surfaces
Compression seals are made to be squeezed between two closing surfaces, like a casement window or a door. They work best when fully compressed, using that squeeze to block air and water at the point of contact.
Built for sliding surfaces
Brush seals are designed for sliding applications, such as double hung sash windows. They use flexible bristles to maintain constant contact with moving timber surfaces, sealing the gap while still allowing smooth, easy operation.
Why compression seals don't work for sash windows
Double hung sash windows slide, they don't compress shut. Using a compression seal in this setting causes a predictable set of problems.
Too much friction
Compression seals create resistance against the sash every time it moves, making the window difficult to open and close, especially on older or heavier windows.
Quick deterioration
The constant movement of the sash against the seal wears it down fast. What's designed to compress and hold still gets dragged back and forth instead, so it breaks down far sooner than it should.
An ineffective seal from day one
Compression seals rely on being squashed to form a seal, something a sliding sash window simply never does. The seal is fighting against its own design from the moment it's installed.
Brush seals are made for the job, and ours go even further
Brush seals, like our trusted Timber Brush Seal with Fin, are purpose built for sliding sash windows. They're lightweight, flexible, and keep the sash operating smoothly with none of the resistance a compression seal creates. Timber Brush Seal with Fin comes in both grey and white, so it's easy to match to your existing frames and beading.
But not all brush seals are created equal. Unlike many brush seals on the market, every Lock & Latch draught proofing product features a central plastic fin embedded within the brush hairs. This fin is the key differentiator, and it's genuinely important to understand: a brush seal without a fin doesn't form a full seal. As long as the fin stays in contact with the sash, it forms a complete, continuous seal on its own. The brush pile around it then adds extra sealing, plus smooth, friction-free movement as the sash slides past.
"Brush seals don't form a full seal" is a myth. Ours do, and they do it better than most.
It's this fin design, combined with genuine brush pile performance, that's made our draught seals the gold standard in the sash window restoration world.
Common questions
Will a brush seal really block air as well as a compression seal?
A standard brush seal alone can fall short, which is exactly why the central plastic fin matters. It's the fin, not just the bristles, that forms the airtight barrier, giving you sealing performance a plain brush seal can't match, without the friction problems a compression seal brings.
Can I retrofit a brush seal to an existing sash window?
Yes. Brush seal systems like Ecobead are designed specifically for retrofitting into existing timber sash windows, without needing to replace the sash or frame.
How long do brush seals last compared to compression seals?
Because brush seals are designed to flex with the sliding motion rather than fight against it, they hold up far longer than compression seals in the same application, which wear down from repeated compression and release against a surface they were never designed to slide across.
Does the plastic fin wear out over time?
The fin is designed to sit protected within the brush pile itself, rather than taking the direct wear a compression seal surface would. This is part of why it holds its sealing performance for years rather than needing frequent replacement.
Ready to draught-proof the right way?
Shop the full range of Ecobead brush seal beading, featuring our central fin design for a genuinely complete seal. Timber Brush Seal with Fin is available in grey and white to match your frames.
