Glass is somewhere between a third and a half of the cost of a large glazed door, and it is the part of the specification most likely to be quietly downgraded to win a job. It is also the part homeowners are least equipped to interrogate. Worth twenty minutes.
Ug, Uw, and which one you are being sold
A U-value measures how fast heat passes through something, in watts per square metre per kelvin. Lower is better.
Ug is the glass alone. Uf is the frame alone. Uw is the whole window or door, glass and frame and the edge of the unit combined, weighted by area. Only Uw describes the thing you are buying.
This distinction is where a lot of quiet misleading happens. A quote proudly stating a U-value of 1.0 has almost certainly quoted you Ug. The whole-door figure will be higher, because the frame and the spacer are worse insulators than the middle of the glass. If a quote states a figure without saying which one it is, ask.
Ask for the Uw figure for your exact configuration and size, not a headline number for the system. Whole-door U-values change with the ratio of glass to frame, so a small door with the same specification performs worse than a large one. Any fabricator can produce this figure; the willingness to do it tells you plenty.
What is actually in a sealed unit
A standard double-glazed unit is two panes separated by a spacer bar, with the cavity sealed and usually gas-filled. Four things inside it move the performance:
The low-emissivity coating
A microscopically thin metallic coating on one internal face that reflects long-wave heat back into the room while letting daylight through. This is by far the biggest single contributor. Coatings vary in quality and in how neutral they look; cheaper ones can give glass a faint tint, most noticeable when you stand outside on a grey day.
The gas fill
Argon in the cavity conducts heat less readily than air and is standard on any decent unit. Krypton performs better in narrow cavities and costs considerably more; it appears mainly in triple glazing where cavity width is constrained.
The cavity width
There is an optimum, typically around 16 mm for argon in a double-glazed unit. Wider is not automatically better — past the optimum, convection currents inside the cavity start moving heat across it.
The spacer bar
The strip holding the panes apart around the perimeter. Traditional aluminium spacers conduct heat well, which is exactly what you do not want at the edge of the glass. A warm edge spacer — composite or stainless steel — reduces that bridge, improves the whole-unit figure and, more visibly to you, reduces condensation forming around the edges of the glass on cold mornings. It is a modest cost and one of the better-value upgrades on the list.
Solar control, and which elevation you are on
A low-emissivity coating keeps heat in. A solar control coating keeps heat out. These are different jobs and on a large glazed opening you may want both.
The measure is g-value: the proportion of solar energy that gets through. Lower g means a cooler room in July and, unavoidably, less useful free heat in January.
Rough guidance for our part of the world:
- North-facing: maximise solar gain, prioritise the U-value. High g.
- East-facing: generally comfortable; standard specification.
- South-facing, unshaded: solar control is worth serious consideration, especially with a glass roof or large rooflights nearby.
- West-facing: the one people underestimate. Late afternoon sun in a kitchen extension with a west-facing garden — the standard arrangement across most of Hertfordshire — makes the room unusable from four until seven on a hot day.
A four-panel set on a west elevation in Welwyn Garden City, specified without solar control, ended up with external blinds fitted the following summer at more than the cost of upgrading the glass would have been. Solar control is a decision to make once, at the quotation stage.
Acoustic glass
Sound reduction comes from asymmetric pane thicknesses and from laminated glass with an acoustic interlayer, not from the gas fill. A unit with 6 mm on one side and 4 mm on the other performs better acoustically than 4 mm and 4 mm, because the two panes resonate at different frequencies.
Worth specifying if you back onto the A10, the A1(M), the East Coast Main Line through Stevenage and Hatfield, or the West Anglia route through Cheshunt and Broxbourne. The upgrade is not large and the difference is immediately noticeable. It also gets you laminated glass, which is better for security.
Toughened or laminated
Both count as safety glass. Toughened is heat-treated and breaks into small fragments; laminated bonds two panes to an interlayer and holds together when broken. Laminated is generally the better choice on doors: better acoustically, harder to break through, and it stays in the frame if it does fail. It is heavier, which matters when panel weight is close to a system limit.
Reading a glazing line on a quote
A properly written specification looks something like: 28 mm unit, 4 mm toughened low-E outer, 20 mm argon-filled cavity, warm edge spacer, 4 mm toughened inner. If your quote says "double glazed unit", you have no idea what you are buying, and neither do the other quotes you are comparing it against.
Manufacturers publish the glazing options and the resulting figures for their systems — Schüco window systems and door systems both list unit thicknesses and the performance they produce, which lets you check that the numbers on your quote are ones the system can actually achieve. If they are not, someone is quoting optimistically.
The same logic applies across the range. The glazing is what makes the difference between two otherwise identical quotes for ASE 60 sliding doors, and it is where you should spend an extra few hundred pounds if you have it.
Common questions
What U-value should Schüco bifold doors have?
A whole-door figure somewhere around 1.2 to 1.6 W/m²K is typical for a good aluminium door with a well-specified double-glazed unit. Always check whether the number quoted is Uw for the whole door or Ug for the glass alone.
Is triple glazing worth it on a bifold door?
Rarely in a house of this type. The weight penalty is significant, the panel size limits tighten, and the improvement over a well-specified double-glazed unit with a warm edge spacer is modest for the money.
What is a warm edge spacer?
The strip separating the panes around the perimeter of a sealed unit, made from a composite or stainless steel rather than aluminium. It conducts less heat, improves the whole-unit U-value and reduces condensation at the edges of the glass.
Do I need solar control glass?
On a south or west facing elevation with little shading, seriously consider it. On a north-facing opening it is usually the wrong choice, because you lose useful winter heat gain for no summer benefit.