Common DIY Secondary Glazing Mistakes

Assuming the opening is square

Openings in houses more than fifty years old rarely are. Measure width at top, middle and bottom, and height at left, centre and right, then work to the smallest dimension and pack the difference. A frame cut to the largest measurement will not go in, and a frame cut to an average will leave a tapering gap that no amount of sealant will make airtight.

Sealing the outer side instead of the inner

The instinct is to seal the cavity against the outside world. It is the wrong way round. The inner frame, facing the room, needs the continuous airtight seal, because room air carries the moisture. The cavity should be able to breathe gently outward through the original window's natural leakage. Reverse this and you create the cavity condensation people then blame on the product.

• Seal the room side continuously
• Leave the original window's leakage paths
• Never seal both sides completely

Choosing thin glass or acrylic for a noise problem

Mass and damping do the acoustic work. A 3mm acrylic panel weighs a fraction of a 6.8mm acoustic laminate and has no damping interlayer. Fitting the light option and then concluding that secondary glazing does not work for noise is the single most common false conclusion in this trade.

Fitting too close to the existing glass

Kits typically fit into the reveal face, giving perhaps 20mm to 40mm of cavity. That is enough for draughts, not for noise. If acoustic performance is the goal and the reveal is shallow, the frame has to come forward on a subframe, which most self assembly systems do not provide for.

Fixing into the original joinery

Screws into sash boxes can foul cords and pulleys, and screws into original sashes damage a feature you were trying to protect. Fix into the lining, reveal or architrave. On a listed building, fixing into historic fabric can be a genuine consent problem rather than merely bad practice.

Blocking ventilation entirely

A perfectly sealed room with no background ventilation gets humid, and humidity turns into mould in the coldest corner. If you seal the window, provide ventilation deliberately, whether that is trickle vents in the new frame, an existing background vent kept clear, or a routine of airing the room. Sealing plus no ventilation is how a draught problem becomes a damp problem.

Forgetting how you will clean it

Four surfaces need cleaning instead of two, and two of them are inside the cavity. If the panel cannot be removed or opened easily, that job becomes a chore you avoid, and the cavity slowly accumulates dust and cobwebs behind glass you cannot reach. Decide the access method before you buy.

Ignoring means of escape

In a bedroom that relies on the window as an escape route, a fixed or awkwardly removable panel is a genuine safety issue, not a technicality. Fit an openable system on any window that forms part of your escape strategy, and make sure everyone in the house knows how it operates in the dark.