9-Point Survey of a Georgian Bar Window Before Ordering a Pilkington Optiwhite Unit

Rebate readings come first

The depth gauge goes into the frame rebate before the sightline is measured. A Pilkington Optiwhite low-iron pane is usually specified as one leaf of a sealed unit, and a standard double glazed unit runs 24mm overall: a 4mm outer leaf, a 16mm cavity and a 4mm inner leaf.

If the Georgian frame rebate measures only 18mm deep, that 24mm unit cannot seat properly. The specification then narrows to a slimline 14mm or 16mm build with a thinner cavity. On a genuine Georgian bar sash, the rebate is often shallow because it was cut for single 3mm float glass. The depth reading at four points around each light is the survey figure that confirms or stops the order.

Rebate width is logged alongside depth. A unit needs a bead or putty land of at least 3mm on each face, and a warm edge spacer bar sits proud of the glass edge, so the pocket has to take the spacer and sealant as well as the glass.

Out-of-square rebates are common in frames over eighty years old. The widest and narrowest readings both go on the sheet, with the toughener sizing to the tight dimension.

Bar alignment across the transom

A Georgian bar pattern depends on the horizontal bars lining through from one sash to the next. On a six-over-six sash, the survey checks that the meeting rail bars sit within roughly 2mm of alignment when the sashes are closed. Optiwhite is water-white and unforgiving; a step in the bar line catches the eye more sharply than it would behind green-tinted float glass.

The survey also has to name the bar system. True divided lights split the glass into separate panes with structural glazing bars, so each pocket takes its own small Optiwhite unit. Applied bars, also called Georgian bar overlays, are fixed to the face of a single large unit with a spacer grid inside the cavity to match. Between-glass bars sit entirely within the 16mm cavity.

Each system carries a different measuring routine. If the factory drawing mixes them up, the sash can arrive with an internal cavity grid that fails to register with the applied face bar. Photographs of the existing pattern and a pane count for each sash keep the order sheet clear.

Condensation changes the specification

Heavy condensation on the inner pane of an existing unit often leads to a request for a bigger cavity, but that is the wrong specification response for room-side moisture. Condensation on the room-side face is a warm-edge and ventilation issue. The glazing correction is a warm edge spacer bar in place of the old aluminium one.

Aluminium spacers conduct heat straight across the glass edge. That chills the inner pane perimeter below dew point, which is where the misting begins. A polymer or stainless warm edge spacer, such as Swisspacer or the Super Spacer product, lifts that edge temperature by several degrees and moves the perimeter above dew point in most heated rooms.

Moisture between the panes means the unit seal has failed. No spacer choice rescues that sealed unit, so the reading sends the whole unit to replacement. The survey needs to state clearly which face carries the moisture, because room-side condensation and failed-seal misting lead to different work and different cost.

Cill fall and water shedding

Put a torpedo level on the external cill. A Georgian cill should throw water outward, with a fall of around 5 to 9 degrees as the working target on a timber weathering. A fall under 5 degrees is a warning mark on the survey, especially where the frame has already settled.

Dead-flat and back-falling cills appear regularly on period frames. They pool water against the bottom rail and rot the joinery that will be expected to carry the new Optiwhite unit for the next two decades. Where the fall has gone, the survey flags cill repair or a lead-dressed weathering as a precondition, because bedding a sealed unit into a water-holding frame guarantees early edge-seal failure.

The throating needs the same check. This is the groove cut under the cill lip to break surface tension and stop water tracking back to the wall. On many painted-over Georgian cills, the throat is choked with decades of gloss. Clearing it during the survey visit takes minutes; leaving it blocked can turn into a wall-damp problem.

Draught paths around the sash

A smoke pencil held to the meeting rail and staff bead on a breezy day shows the draught path quickly. The back of a wet hand works as a rough check too. Sash window draught proofing is a separate line item from the glazing, yet the survey captures it in the same visit because the two affect each other.

A new heavy Optiwhite double glazed sash changes the weight balance. If the box sash cords and weights are left unbalanced, the sash will fail to sit tight against the parting bead and the draught gap the customer wanted closed will reopen.

The proofing itself is usually a brush pile or a compression seal carried in a routed groove around the stiles, meeting rail and bottom rail. Aftermarket systems such as Reddiseals or the Ventrolla routed brush profile are specified by the gap they must close.

That means the gap is measured at the meeting rail, the parting bead and the two side runs. A 4mm draught gap and a 7mm gap take different pile heights. A single system sized to the average leaves one edge whistling.

Fensa and the paperwork trigger

Replacement of glazed units in an existing frame is notifiable building work in England and Wales, and Fensa certification requirements mean the installing company must be registered to self-certify against the thermal element rules or the job needs a building control notice.

The survey records the frame material and states whether the replacement is unit-only or a full sash, because that distinction decides whether the U-value target of the Approved Document L applies to the whole window.

When the same visit prices a blind

Many Georgian bar surveys become blind measures before the glazier leaves. The reveal readings for blinds follow a different discipline from the glass readings.

Bay window blind measuring is the common trap. A three-sided bay is three separate drops, and each one is measured individually. Corner posts and the angle between facets are recorded so the blinds clear each other when raised.

For a recess measure, the width is taken at the top, middle and bottom because reveals taper. The smallest of the three governs the fabric width to avoid fouling.

A motorised blind track installation adds a power check. A hardwired 240V track needs a fused spur within reach of the headrail. A battery tubular motor such as the Somfy or Louvolite rechargeable range needs clearance for the motor crown at one end, adding roughly 30 to 40mm to the drive-side dimension. That clearance comes off the fabric width and is entered on the survey sheet.

Where the same property has a Velux loft window replacement in the same order, the blind for that rooflight is selected from the coded pane size on the sash itself. The Velux code stamped on the top of the sash is copied down verbatim.

Glass edgework and the low-iron premium

Optiwhite is a low-iron glass, so the green cast of standard float glass is gone and the edges read almost colourless. That clarity makes edge quality more visible.

The survey notes whether the pane will be seen edge-on, as in a frameless or minimally beaded Georgian light. A ground or polished edge is specified there, while an arrissed edge is sufficient behind a full timber bead. Putting the edgework on the order sheet avoids a re-cut, since the low-iron premium already adds cost over a standard clear unit and a second toughening run doubles it.

Reading the survey back

Nine checks sit on the sheet, and the rebate depth at the first check can still override the rest. Perfect bar alignment and a sound cill fall lose value if the frame will only accept a 14mm slimline unit, because a slimline cavity narrows the warm edge options and pushes the edge temperature back toward the dew point the survey was trying to avoid.

The open item is the same constraint exposed by the gauge: a shallow Georgian rebate can make the low-iron glass choice depend on how much timber is available around it.