Slim Double Glazing vs Secondary Glazing
Slim-profile sealed units (6–12 mm cavity) look almost like single glazing — attractive for heritage properties, but the technical trade-offs matter.
The slim-DG pitch
A drop-in replacement for a Georgian or Victorian sash, keeping the glazing-bar rhythm and one-pane look while adding a thin argon-filled cavity.
The trade-offs
The 6–8 mm cavity limits thermal performance to around U=1.6 — better than single, worse than a standard modern DG unit. Acoustic gain is marginal. Long-term seal life is shorter than a standard sealed unit due to the tight cavity.
Why secondary is often preferred
It leaves the original window untouched, delivers better acoustic performance, matches or beats slim-DG on U-value, and doesn't require the disposal of historic glass. Conservation officers usually prefer it on this basis.
When slim-DG makes sense
Where the original sash is already beyond restoration, no listed protection applies and the owner wants a single-pane look from inside as well as outside.
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