TradeCalculator.co.uk
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.
Windows of the same size
Internal reveal width
Internal reveal height
Depth from primary glass to secondary panel
Next up: related pages
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. The figures below are an indicative estimate, broadly in line with Historic England guidance (HEAG085, which cites up to ~45 dB at a 100mm gap) and published acoustic testing — not a guaranteed laboratory rating:
- 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
Glass and acrylic panes perform comparably for noise once the air gap and pane thickness are accounted for — the air gap is the dominant factor, not the choice of material.
Thermal improvement
Combined U-value is estimated from the thermal resistance of the air gap and the primary and secondary glazing layers. The unventilated air-gap resistance follows BS EN ISO 6946 (0.18 m²K/W for any gap of 25mm or more, where the value caps because convection sets in). The result is an estimate and should be treated as indicative rather than a certified system value.
Frequently Asked Questions
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.
For noise, a wide air gap usually beats replacement double glazing, whose narrow sealed cavity does little acoustically. Historic England guidance (HEAG085, "Secondary glazing for windows") notes that a gap of around 100mm between primary and secondary panes can give roughly 45 dB of noise reduction. These are indicative figures, not a guaranteed laboratory rating — actual performance depends on the air gap, pane thickness, seals and the existing window. The air gap is the dominant factor; glass and acrylic panes perform comparably once gap and thickness are accounted for. For maximum reduction, use a wide gap, a thicker or laminated pane, and good perimeter seals.
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.
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 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.
Related Calculators
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.