Secondary Glazing vs Shutters and Heavy Curtains
Original working shutters and heavy interlined curtains are not a myth: both measurably reduce heat loss. Their limitation is simple and often overlooked — they only work when they are closed, which is when you are not using the room.
What soft furnishings actually deliver
Published retrofit research on traditional buildings has found that closing well-fitted original shutters can reduce heat loss through the window substantially, and interlined curtains fitted close to the reveal do similar work. Both figures depend heavily on how well the edges seal — a curtain hanging over a radiator or stopping short of the sill leaks a convective loop straight past itself.
- Effective only while closed
- Edge sealing dominates performance
- Almost no acoustic benefit
- No effect on condensation during the day
What they cannot do
They do nothing for noise — fabric absorbs a little high-frequency energy and no low frequency at all. They do nothing for daytime comfort in a room you are sitting in. And they do not raise the glass surface temperature, so condensation and the mould that follows it carry on regardless.
Use them together
This is not an either/or. In a Georgian room with working shutters we fit the secondary unit inboard of the shutter box wherever the reveal allows, so the shutters still close over it. You then get a warm room by day and an additional layer at night.
Frequently asked questions
Not if it is planned. We measure the shutter fold at survey and set the panel line to clear it, or fit within the shutter box.
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