Blown and Misted Double Glazing Window Repair and Replacement Cost Calculator

How this calculator works

This calculator compares typical UK supplied-and-fitted price ranges for the two options, drawn from published 2026 industry cost guides. Your inputs scale those base ranges: the calculator multiplies the per-unit range for each option by the number of units and by adjustment factors for the choices you make below, then shows both totals side by side along with the estimated difference between them. Because real quotes vary, everything is expressed as a range from a realistic low to a realistic high rather than a single figure.

Misting between the panes means the perimeter seal of the double glazed unit has failed, letting moisture into the cavity, and the insulating gas will have escaped, costing up to half the unit’s thermal performance. The fix does not usually require new windows: replacing just the sealed glass unit within the existing frame typically costs around to per unit fitted in 2026, roughly a tenth of the to cost of replacing the whole window. On a typical house where four or five units have misted, an all-in repair commonly lands between and in a single visit.

Number of misted or blown windows

Enter how many windows have misting or condensation between the panes. Seal failure typically starts 15 to 20 years into a window’s life, and once one unit has blown, others of the same age often follow, so count every affected window.

Window size

Size is one of the biggest cost drivers for any glazed unit. A larger opening needs more frame material and a bigger, heavier sealed glass unit, and it usually takes longer to fit. The calculator uses three broad size bands based on common UK window dimensions: choosing the small band reduces the typical range by around 20 percent, while the large band increases it by around 40 percent. If your windows vary in size, run the calculator once per size band and add the results together.

Where you live

Labour rates are not uniform across the UK. Published cost guides consistently show installation prices in London and the South East running roughly 20 to 30 percent above the national average, driven by higher labour costs, travel, parking and congestion charges. The calculator applies an uplift of around 20 percent if you select London and the South East. Product prices themselves are broadly similar nationwide, so the difference mostly reflects fitting costs.

What else affects the price

Repair is the right call when the frames themselves are sound: if the uPVC, aluminium or timber is straight, undamaged and sealing properly, a new A-rated argon-filled unit restores both clarity and insulation. Full replacement makes more sense when frames are cracked, warped or rotten, when the windows are 25 or more years old, or when several components have failed at once. Unit prices rise with size and with glass upgrades such as toughened, laminated, acoustic or self-cleaning glass, and upper floor units may need access equipment. Check your paperwork before paying anything, as many double glazing installations carry 10 to 20 year guarantees that cover seal failure.

Planning permission, building regulations and certification

Like-for-like window replacement on most houses does not need planning permission, but listed buildings always require consent and conservation areas often restrict materials and styles on visible elevations, so check with your local planning authority if either applies. Building regulations apply to every replacement window in a dwelling, setting minimum energy performance and safety glazing standards. Installers registered with a competent person scheme, such as FENSA or Certass, can self-certify compliance and issue a certificate after completion; otherwise you must arrange building control sign-off separately. Keep the certificate safe, as buyers’ solicitors routinely ask for it when you sell.