The blind lives inside the sealed unit. No cords, no dust, nothing to clean. Here is how it works, where it belongs, and what you give up for it.
Every blind in a New York apartment is fighting the same three enemies: dust, cords, and the window itself. The slats collect the street; the cords tangle, fray and are a documented hazard to small children; and the whole assembly sits in front of the glass, in the way of cleaning it, opening it, or looking through it. Blinds between the glass answer all three by moving the blind to the one place none of that can reach — inside the sealed cavity of the insulated glass unit.
This is the first in a Tuesday series on glass. We start here because it is a product people ask about after seeing it once, usually in a bathroom or a patio door, and because it is easy to specify wrong. The spec sheet in our library has the sizes and options; this article is about how to think about it.
A standard double-glazed unit is two lites of glass with a spacer around the edge and a sealed cavity between them. In a blind unit the cavity is made wide enough — the finished unit is 7/8 inch or 1 inch thick — to hold a set of thin aluminum slats on a ladder, with a headrail at the top. The slats raise, lower and tilt exactly like a conventional Venetian blind; the difference is how you reach them.
Nothing passes through the glass. On the room-side lite there is a small operator, a slider a few inches long, with a magnet in it. Inside the cavity, a matching magnet is coupled to the blind mechanism. Slide the operator and the blind follows. The seal of the unit is never breached, which is what lets the manufacturer certify it as an insulated glass unit to the same standard as any other, and warrant it for ten years.
Because there are no cords, the unit meets the window-covering safety standard, ANSI/WCMA A100.1, and the units we supply carry the Best for Kids certification. For a nursery, a school, a medical office or a rental, that is not a detail.
We install these and we like them, so it would be easy to leave this out. It should not be left out.
A conventional high-performance IG unit is sealed and filled with argon, which insulates better than air. A blind unit cannot be argon-filled: the cavity is vented to atmosphere through a capillary tube so the wide unit can equalize pressure with altitude and weather without the glass bowing. That means the cavity holds air, and a blind unit with a given Low-E coating will have a somewhat higher U-factor than a sealed argon unit with the same coating. It is still a Low-E double-glazed unit and a large improvement on single glazing or clear glass; it is not the best thermal unit you can put in the opening. On a north-facing bathroom that does not matter. On the south wall of a passive-house project it might.
Two smaller points. The blind is part of the sealed unit, so if the mechanism ever fails the IG unit is replaced, not the blind; the mechanism is simple and this is uncommon, but it is a different repair model from a $40 blind. And the slat colours are a short list — white, tan, espresso, silver and slate — because the manufacturer keeps the program tight to keep the price and the lead time down.
The product comes in four programs matched to openings. A 7/8-inch unit with a top-centre operator for single and double-hung windows, up to 54 inches wide and 9 square feet. A 7/8-inch and a 1-inch program for casements, fixed lites, awnings, sliders and patio doors, with the operator on the left or right, up to 80 inches tall and 17 to 18 square feet. And a 1-inch XL program for tall patio doors, to 90 inches and 28 square feet, tempered only. Only one dimension of a unit may exceed 36 inches (48 on the XL).
The glass is annealed or tempered to the program limits; tempered is safety glass to ANSI Z97.1 and is what a door needs. Low-E is Cardinal 270, 272, 340 or 366 on the second surface, with the option of an i89 hard coat on the fourth surface to bring the U-factor down further. Solar Gray or Solar Bronze tint is available on the outer lite. Units are made to any size in sixteenth-inch increments, minimum order one, in about eight working days for annealed and twelve for tempered. All of this is on the spec sheet.
The blind unit is a complete IG unit, so there are two ways to get one. In a new window or door, the frame is ordered to take it; Pella and Andersen both offer between-glass blinds as options in their own lines, and any frame with a 7/8-inch or 1-inch glazing pocket can take the units we supply. In an existing window, the question is whether the frame’s glazing pocket accepts the thickness. Many aluminum and vinyl frames from the last twenty years will; many older wood sashes will not. We measure the pocket before quoting, and if it fits, the blind arrives as a glass replacement with no change to the frame.
Spec sheet: Blinds Between the Glass Insulated Glass Units Low-E Glass Tempered Glass Sliding Doors Pella
Send us the window or door — a photo and the rough size is enough. We will tell you whether the existing frame can take the unit or whether it is a new window, and quote either way.
