Spacer System for IGU

The Least Discussed Component in a Window

The spacer is the strip running around the perimeter of a sealed unit, holding the two panes apart. Nobody asks about it and it is the single most common weak point in a double glazed window.

It does three jobs at once. It fixes the cavity width, which determines how the gas fill performs. It holds the desiccant that absorbs the small amount of moisture sealed in during manufacture. And it forms the substrate for both seals that keep the gas in and the water out.

The Thermal Problem With Aluminium

The traditional spacer is an aluminium box section. It is cheap, dimensionally stable, easy to fabricate and an excellent conductor of heat — which is exactly what you do not want bridging the two panes of an insulating unit.

The consequence is a thermal bridge running around the entire perimeter of every window in the building. Heat takes the path of least resistance, and an aluminium spacer offers a very easy one. This is why the centre-of-glass U-value and the whole-window U-value differ, and why the difference is larger on small panes, where the edge is a bigger proportion of the total area.

It is also why condensation appears at the bottom edge of a window first. The glass there is colder than the glass in the middle, because the spacer is conducting heat straight out of the building. People assume the unit has failed. Usually the unit is fine and the spacer is doing what aluminium does.

Warm-Edge Spacers

Warm-edge spacers replace the aluminium with materials that conduct far less — structural foam, composites, or thin-walled stainless steel formed so the conductive path is long and narrow rather than short and wide.

The improvement shows up in three places. The whole-window U-value improves, sometimes substantially on smaller units. The interior glass edge runs warmer, which pushes back the temperature at which condensation forms — a comfort and a mould issue as much as an energy one. And because these materials flex rather than being rigid, the seal is under less stress as the unit expands and contracts through the year, which tends to mean a longer service life.

The cost difference at manufacture is modest. On any specification where the energy performance matters, warm edge is worth asking for by name — and it is frequently not offered unless you do.

The Two Seals

A sealed unit has two seals and they do different jobs.

The primary seal is a thin bead of polyisobutylene between the spacer and the glass. It is the gas barrier — it keeps argon in and water vapour out. It is not structural.

The secondary seal fills the channel outside the spacer, usually silicone or polyurethane. It is the structural bond holding the unit together and protecting the primary seal.

Both have to be right. Silicone secondary seals resist ultraviolet well, which matters in structurally glazed and butt-jointed applications where the edge is exposed to daylight. Polyurethane retains gas well but degrades under UV, so a unit with a polyurethane secondary seal must have its edge covered by the frame. Specifying a unit with an exposed edge and the wrong secondary seal is a genuine error and it shows up years later.

What to Ask For

When you are comparing quotations, the questions that separate them are: is the spacer warm-edge or aluminium, is the unit argon filled, what is the secondary seal, and is the quoted U-value centre-of-glass or whole-window. Two units that look identical on a price list can differ meaningfully on all four.

Where It Makes the Most Difference

These describe the kind of work each area brings rather than any specific client project.

Small panes and divided lites — historic districts citywide. The edge is a large proportion of a small unit, so the spacer choice matters most exactly where units are smallest. On a true divided lite sash the perimeter length is multiplied several times over, and warm edge is the difference between a decent whole-window number and a poor one.

Condensation complaints — pre-war apartment stock in Manhattan and the Bronx. Where a building has moisture problems and windows are running wet at the edges, the spacer is often the cause and is almost never the thing that gets investigated.

Structurally glazed and butt-jointed work — Midtown, Hudson Yards, Williamsburg. Where unit edges are exposed to daylight, the secondary seal must be silicone. This is a specification item, not a preference.

Colder inland areas — Bergen County, the northern Bronx, the Lehigh Valley. Larger indoor to outdoor temperature differences mean the edge effect is more pronounced and warm edge returns more.

If you are being quoted for sealed units, ask what spacer is in them. We will tell you what is in ours. We supply and install across Manhattan, Brooklyn, Queens, the Bronx and Staten Island, and throughout Northern New Jersey, the Jersey Shore, Philadelphia and the Lehigh Valley.