Reading a make-up string
An insulated unit is quoted as its layers in order, outside to inside. A double unit written 4 - 16 - 4 is a four millimetre outer lite, a sixteen millimetre spacer and a four millimetre inner lite, which comes to twenty-four millimetres overall. In inch terms a common residential double is 1/8 - 1/2 - 1/8, which is three quarters of an inch. A triple has two spacers and three lites and gets deep fast.
The overall figure is the one that has to fit something. A frame has a rebate — a pocket the unit sits in — and that pocket was sized by whoever specified the frame, often years before anybody thought about replacing the glass. The calculator adds the layers, reports the overall, and subtracts it from whatever rebate depth you measured. Nothing about that subtraction is a verdict: how much clearance a unit needs front and back, how deep it is set and what the glazing system requires are all figures from the frame maker and the glazing system.
Only the glass weighs anything
The air or gas in the spaces contributes nothing to the weight, so a 1/8 - 1/2 - 1/8 unit weighs exactly what a quarter inch of monolithic glass weighs, around 3.25 pounds a square foot. That catches people out in both directions. A three quarter inch unit sounds heavy and is not, while a triple with three quarter inch lites is genuinely heavy and looks no different from the outside.
What the page leaves out is the spacer bar, the desiccant inside it and the edge seal. On an ordinary residential unit that is a pound or two, which is inside the noise on a delivery of twelve and outside it on a delivery of two hundred. The fabricator has a shipping weight for the exact make-up, and a weighed number always beats a calculated one.
Sightline: the glass you paid for and cannot see
The frame or the bead laps over the edge of the unit, and the inner edge of that lap is the sightline. Everything outside it is glass you bought and will never look through. On the figures this page starts with — a 34 by 58 inch unit with a half inch cover all round — the visible opening is 33 by 57 and the visible area is just over 95 percent of what was made.
The proportion gets worse as units get smaller. The same half inch cover on a 14 by 20 inch unit leaves 13 by 19, which is about 88 percent. On a divided-light sash made up of many small units it is worth working out before deciding how the window is put together, because the difference between one large unit and six small ones in the same opening is a surprising amount of daylight.
Edge deletion and where it shows
Coated glass is usually made by coating the whole sheet and then removing a band around the edge, so the sealant bonds to bare glass rather than to the coating. That band is the edge deletion, and its width is set by the fabricator for the coating they are working with. It sits just inside the edge of the unit.
Two numbers therefore have to be compared: how far the frame laps over the glass, and how wide the deletion band is. When the lap is wider, the band is hidden and nobody knows it exists. When the band is wider than the lap, a strip of it is inside the daylight opening, and depending on the coating it can be visible at some angles. The page subtracts one from the other and reports which way round it came out. It reports it as geometry, not as a defect, because whether a visible strip matters is a conversation with the fabricator who set the width and with whoever is going to be looking at the window.
What is deliberately absent
There is no U-factor here, no solar heat gain coefficient, no gas fill performance and no statement about what any make-up achieves. Those depend on the coating, the gas, the spacer type, the frame and the test standard, they are certified figures rather than calculated ones, and they belong on the label of the specific unit. The same goes for what thickness of glass any size or location calls for, which comes from your fabricator, the glazing system and the code your jurisdiction has adopted. This page adds up dimensions.
Questions people ask
How thick is a double glazed unit?
Whatever its layers add up to. Two 1/8 inch lites with a 1/2 inch spacer is 3/4 of an inch overall, which is a common residential build. Metric make-ups such as 4 - 16 - 4 come to 24 mm, a shade under an inch. There is no standard thickness — it is set by the lites and the spacer your fabricator is using, which is why both are fields here rather than assumptions.
Will my new unit fit the old frame?
Only the frame maker and the glazing system can answer that, but the arithmetic is the starting point. Measure the rebate depth on the actual frame, put it in, and the page reports how much of the pocket the unit uses and how much is left. What that leftover needs to be — clearance front and back, setting depth, what the glazing system requires — is a specification from them. A unit deeper than the pocket is a problem best found before it is made, not on the day of installation.
Does an insulated unit weigh more than single glazing?
It weighs what its glass weighs, because the gas in the spaces contributes nothing. Two 1/8 inch lites weigh the same as one 1/4 inch pane, about 3.25 pounds a square foot, even though the unit is three times as thick. A triple with heavier lites is a different matter and genuinely is heavy. The page adds the lites and ignores the spaces, which is the honest way round.
What is the edge deletion band on coated glass?
A strip around the perimeter where the coating has been removed so the sealant bonds to bare glass instead of to the coating. Its width is set by your fabricator for the specific coating. It matters visually only when it is wider than the frame laps over the glass, because then part of it sits inside the daylight opening. The page compares the two and reports which way round it came out on your figures.
How much of the unit can I actually see through?
The unit size less twice the frame cover in each direction. On a 34 by 58 inch unit with half an inch of cover all round, the visible opening is 33 by 57 and that is about 95 percent of the area. The proportion drops as units get smaller — the same cover on a 14 by 20 unit leaves about 88 percent — which is worth knowing before deciding between one large unit and several small ones in the same opening.