Current Glass Technology - June 2013
There is a strong move toward two Low-E coatings in the IGU (insulating glass unit) industry right now, probably driven by the effort the achieve R5 insulation value for the energy rating regulators in the U.S. Guardian Glass has a product that achieves R5.02 using a soft coat on surface 2 plus a new hardcoat on surface 4 - the interior surface! It is not thermally effective to have the Low-E coatings on surfaces 2 and 3, so they’ve come up with a smooth, scrubbable product which they call IS-15 for the inside surface. This unit gives R5.02 with an argon gas fill. With krypton, it becomes R5.7. Currently my company is charging about $3.10 per litre plus a handling charge of $100 to use krypton instead of argon in a unit. If we are using a 3/8” spacer, this works out to about $2.75 per square foot extra, over the cost of an argon filled unit.
However, the dramatic increases in R-value don’t occur until triple glazing is used. Using three sheets of glass with two Low-E coatings, plus krypton gas, we can build a one inch thick IGU with R9 performance. I’m guessing at a selling price of just over $30 per square foot for this unit. The one inch overall thickness means that the gaps are each 5/16”, which is ideal for krypton. Argon and air perform better with larger spaces.
And then going to the next step, a quadruple unit could probably get up to about R12 using krypton. It would be heavy- 6 pounds per square foot- thick, at 1 7/16”, expensive at around $40 per square foot, and would keep out at least half the visible light. However, if we use low-iron glass on the two clear lites, this can be kept to a minimum.
Next project when there’s some time: design and price an outswinging casement window with a quad unit, and provide technical specs on the glass performance. Then build one and see how smoothly it actually works. I’d probably use butt hinges rather then conventional concealed sliding hinges. My hunch is that quad units would be required in the elusive zero-energy house design, at least for climates similar to Ottawa, Canada.
Why do we put small bars, known as muntins, or glazing bars, in our windows? We believe it reminds us of
the past, of a time when architecture was more human-scaled, but also that a well done bar pattern has
a beauty of its own, in repeating the moulding detail of the sash. Perhaps too, we feel more comfortably
protected from the outdoors by a grillwork across the opening.
A hundred years ago the muntins were strictly functional. Glass was available in relatively small sizes,
so filling a large opening meant repeating these small units horizontally and vertically. The bars were
kept narrow in most residential windows, to impede the view as little as possible.
In the last ten years bars have made a huge re-appearance in thermal glazed wood windows. In a modern
window they make no sense structurally or for energy efficiency reasons, but they can be justified for
probably the most important of all considerations: they are beautiful and people like them. In our shop
we usually create muntin bars with a wood cladding pattern on both faces of the glass. The interior pattern
is fixed to the sash and carries the edge-moulded shape; it stays intact if the thermal unit is replaced.
The exterior pattern is taped to the glass surface and is meant to copy the look of the putty lines of
single glazed windows; it must be replaced if the glass is changed. This system is known as simulated
divided lites or SDL. If we build a thicker sash, the moulded bars on the interior can be made realistically
deep to give dramatic shadow lines.
You might ask if all this effort is justified to make "fake" bars on a window. We believe it is, because
it all contributes to the aesthetic relationship between a person and the space they occupy. It is no more
fake than building a window out of high quality woods in the first place, or of considering window proportions
relative to the building’s architecture.
Low-E/argon or not?
Many companies offer ‘free Low-E', so why not use it if it's better? Also referred to as high
performance thermal glazed units, Low-E/argon (low emissivity coated, argon gas filled) units
have advantages and drawbacks. They have a slight colouration which is usually visible from the
outside of a building. They allow less light through, but this is almost imperceptible to the eye.
They have some negative effect on plant growth because the UV spectrum is largely filtered out. On
the positive side, they are about 1/3 better insulators than clear thermal units, they keep out
much of the radiant summer heat from warm outdoor surfaces, and they protect interior furnishings from UV damage.
The argon gas which is pumped in to replace the air acts as a better insulator simply because it
is more dense than air. Spend more money and you can get krypton, which is denser still. The cynics
in the business say that these gases have pretty much leaked out through the edge sealant within a
few years. Unfortunately we don't yet have a simple gas-content testing device. You can see the Low-E
coating by shining a flashlight at an angle against the glass surface. One of the multiple reflections
will have a purple tint. It should be on the inner surface of the interior sheet of glass.
If you have to replace a thermal unit it won't be ‘free Low-E'- we currently charge $2.00 per square
foot extra for the Low-E/argon option. To answer the question, in many residential applications it is
advantageous to use Low-E/argon glazing because of the reduced heating and cooling loads, especially
where there is a lot of glass area.
Why do crank-out windows become hard to close after a few years?
Usually the problem is due to the bottom corner of the latch side of the sash dragging against the
frame. If the crank can't pull the sash in, the latch won't reach far enough to engage the keeper.
It's most annoying to have to push the sash in from outside.
Two things can cause the sash to drag against the window frame: the sash may sag out-of-square because
of improper glazing, or the frame may settle out-of-square because of poor installation. Let's look at
sash sag first. When the window is built, the thermal glazing unit is put into the sash with a 1/8" gap
around the whole perimeter, sitting on a couple of EPDM shims at the bottom. The sash is held up at only
one corner though (where the hinge is), so the glass weight wants to pull the sash down a bit at the other
corner, until the unit tips over and touches the side of the sash. For moderate sized units, silicone
bedding in the glazing rabbet will prevent this small movement. For very large swinging sash however,
the glass weight must be supported at the bottom hinge corner of the sash. A heel shim is put in at the
bottom hinge side, and a third shim at the top latch side. Thus, the weight of the glass is concentrated
over the hinge point and centred in the sash rabbet by the other two shims. A very large sash should swing
on butt hinges rather than the standard concealed sliding hinge of an outswinging-casement style window.
This puts extra stress on the bottom hinge-side sash joint, which is now carrying all the glass weight.
The more likely cause of the dragging problem is a small movement of the window frame after installation.
We design for a 1/8" uniform gap between sash and frame, so even a sag of half this amount will cause
problems as the weatherstrip has to compress more. Why does the window frame move? Again, it's because
all the glass weight is transferred down through the hinge side of the frame. There must be a shim under
the frame at this point, basically taking the full weight of the window. It is not enough to stick a
casement window in a rough stud opening, then nail it through the brickmold to the sheathing.