Does secondary glazing need Building Regulations approval?

Secondary glazing does not normally require Building Regulations approval in England and Wales because it is classed as an addition to the existing window, not a replacement. However, in Scotland, any glazing work may require a building warrant. Listed building consent is not usually needed for internal secondary glazing, but check with your local conservation officer.

How effective is secondary glazing for noise reduction?

Secondary glazing can be more effective than replacement double glazing for noise reduction. With an air gap of 100mm or more, secondary glazing can achieve 35-45 dB noise reduction compared to 25-30 dB for standard double glazing. The wider air gap is key. For maximum noise reduction, use laminated glass on the secondary panel and ensure good seals.

What air gap do I need for secondary glazing?

A minimum air gap of 20mm is needed for any thermal benefit, but 100mm or more is recommended for optimal noise reduction and thermal performance. For thermal improvement only, 50-100mm is adequate. The air gap must be sealed but not airtight; a small amount of ventilation prevents condensation in the cavity.

Will secondary glazing cause condensation?

If installed correctly with adequate sealing, secondary glazing should not cause condensation in the cavity. Any moisture must be prevented from reaching the cold primary glass surface. Small silica gel desiccant sachets in the cavity can absorb residual moisture. Ensure the secondary panel has better seals than the primary window.

Magnetic or channel fixings — which is better?

Magnetic strip fixings are the simplest and most removable option, ideal for seasonal use. Aluminium channel systems provide a more permanent, higher-performing seal and are better for noise reduction. Hinged or sliding systems cost more but allow easy opening for ventilation and cleaning. Choose based on your need for removability versus performance.

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Secondary Glazing Calculator — Panel Sizes, Fixings & Noise Reduction

Calculate secondary glazing panel sizes, fixing requirements and estimated noise reduction. Ideal for listed buildings and conservation areas where replacement windows are not permitted.

Number of Windows

Windows of the same size

Window Width (reveal)

Internal reveal width

Window Height (reveal)

Internal reveal height

Reveal Depth (air gap)

Depth from primary glass to secondary panel

Fixing TypeHow the secondary panel attaches

Panel MaterialGlass gives best noise reduction

How We Calculate This

This calculator determines secondary glazing panel dimensions, fixing material quantities and estimates acoustic and thermal performance improvement.

Panel sizing

Panel size = Window reveal size - clearance (2mm magnetic, 5mm channel)

Noise reduction

Noise reduction depends primarily on the air gap between primary and secondary glazing:

20-50mm gap: ~22-28 dB reduction
50-75mm gap: ~28-33 dB reduction
75-100mm gap: ~33-38 dB reduction
100mm+ gap: ~38-42 dB reduction

Thermal improvement

Combined U-value is estimated using the thermal resistance of the air gap and the primary and secondary glazing layers.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.