Fogged Sealed Units Replaced with Saint-Gobain Planitherm Panes on a Bay Front

Condensation sealed between two panes on a bay front comes from a mechanical failure in the insulated glass unit. The edge seal has broken, argon has escaped, and moist air moves through the cavity as the glass warms and cools. Frames, hinges and sash hardware are often still serviceable. On a three-light or five-light bay, one pane can mist first because it faces the prevailing wind or receives the longest daily sun exposure, so its seal sees harder thermal cycling than the others.

The useful repair is a sealed-unit replacement with the existing frame and beading retained. A modern Saint-Gobain Planitherm unit in a standard 4-16-4 build, with 90 percent argon fill, gives a centre-pane U-value around 1.1 W/m2K. Many failed bays still contain 1990s air-filled double glazing at roughly 2.8 W/m2K. Across the glazed area, that shift cuts heat loss by more than half, and the glazed area is the dominant part of a bay facade.

Why one light fogs first

A bay is made from three or five separate sealed units, each sitting at an angle with its own perimeter seal and local exposure. A centre light on a south or west aspect absorbs the most solar radiation, expands and contracts across the widest daily temperature range, and stresses its edge seal most heavily. The polyisobutylene primary seal and the polysulphide or hot-melt secondary seal fatigue under that cycle.

Once a pinhole path opens, the desiccant inside the spacer bar can saturate within a heating season. Interstitial condensation then becomes visible below about 8 degrees dew point.

Pricing should treat each light separately. Replacing only the fogged centre unit while leaving two clear flankers is common, although the adjacent units are usually the same age and may be within a year or two of the same failure. Many installers give a firm price for the misted light and a discounted batch price for the neighbouring panes, since the glazier is already on site, the beading is out, and extra units add less labour than the first one.

Measuring the reveal before glass is ordered

Take three width readings and three height readings, then use the smallest measurement for the order. Angled mullions on a bay make this awkward because the sight line into each opening is rarely square to the glass. Measure the daylight opening and deduct the edge clearance required by the unit manufacturer, commonly 5mm all round for glass sitting on setting blocks. A 600mm daylight width therefore becomes a 590mm sealed-unit order.

The same visit can produce the measurements for blinds fitted after the glazing work. A recess-mounted roller or Venetian needs the tight reveal reading with a 10mm to 15mm bracket deduction. A face-fixed blind needs the reveal width plus a 40mm to 50mm overlap on each side to control light gaps. Recording both figures while the installer is already inside the bay avoids a second measuring visit and gives a clear line to every mullion.

Settled masonry bays often create out-of-square reveals. If the diagonal measurements differ by more than 5mm, the unit is cut to the smaller rectangle that will fit inside the parallelogram, with packing shims taking up the difference behind the beading. A square order taken from a skewed bay can bind at one corner and rock on its setting blocks.

Spacer bars and edge loss

Older insulated glass units commonly used aluminium spacer bars. Aluminium conducts heat across the cavity edge and lowers the perimeter surface temperature enough for room-side moisture to condense on the glass edge in cold weather. That cold band is why clear double glazing can still stream at the bottom corners.

A warm edge spacer, such as Swisspacer Ultimate or Edgetech Super Spacer, replaces aluminium with structural foam or a thin stainless composite. Edge conduction drops sharply, and the edge-of-glass temperature rises by several degrees.

This matters more on a bay than on a flat window because the total spacer length is greater. Three or five units bring three or five separate perimeters, and bay corner joints sit in one of the coldest, least-ventilated parts of the room. Warm edge should appear as a confirmed line item on the order, since a cheaper unit can underperform at the exposed perimeter. The glass maker’s centre-pane U-value assumes the coating; the whole-unit U-value changes with the spacer, and the gap between those figures can run from 0.2 to 0.4 W/m2K depending on the spacer fitted.

When solar gain becomes a summer problem

A south or west bay fitted with standard Planitherm low-E glass can overheat a room in summer while reducing winter heat loss. The coating reflects long-wave heat back into the room but still allows short-wave solar energy through the glass. A bay also presents a large collector area on aspects that take strong afternoon sun.

Where the room already runs hot, a solar-control specification such as Planitherm 4S or a Pilkington equivalent reduces the solar factor. It trades a little winter gain for a lower g-value, bringing that figure toward 0.4 or below instead of the 0.6-plus of a clear low-E unit.

Orientation and room use decide the specification. Solar control is unnecessary on a north-facing bay and reduces daylight. A west-facing sitting-room bay that bakes from mid-afternoon gains more from the lower g-value than from the marginal winter U-value difference. Cutting solar gain at the glass is cheaper and more permanent than adding reflective film later or keeping blinds closed through the brightest hours. Applied films can fade or peel over five to eight years.

Cost split on a three-light bay

Take a three-light bay with each daylight opening roughly 600mm by 1200mm, with all three units fogged within one heating season. Sealed-unit supply in 4-16-4 Planitherm with a warm edge spacer sits in the region of 90 to 140 per unit at typical trade glass rates. Three panes therefore land around 270 to 420 in glass. Labour to remove beading, lift out failed units, set the replacements on blocks and reglaze is usually a half-day for one glazier, often priced as a flat visit charge plus a per-unit fitting rate.

A full bay replacement in uPVC or timber reaches several thousand because it includes the frame, cill, structural head above the bay, and making-good to plaster and render. Where the frames remain sound and the sealed units have failed, replacing the glass captures nearly all of the thermal benefit at a fraction of the full replacement cost. The frame’s own U-value contribution stays fixed; the glass is the element moving from 2.8 to 1.1 W/m2K.

Frame condition sets the boundary of the job. New glass in rotted timber or a uPVC bay with failed welds wastes the unit, while sound frames can keep the existing cill, head and surrounding finishes in service. If only the centre light has misted, the clear flankers may still be original units of the same age and close to the same seal failure. The finished bay can look settled on installation day while the next weak edge seal is already sitting beside it.