What makes a window energy efficient.

Six parts of a window decide its energy numbers: the frame, the number of panes, the coatings on the glass, the gas between the panes, the spacer at the edge of the glass, and the seals that keep air out. The rating is the sum of all six. That is why two windows that are both sold as triple glazed can rate very differently. The energy guide explains the numbers on the label. This article goes through the parts that produce them.

The frame.

The frame and sash take up a real share of the opening, and more of it on an operable window than on a fixed one. The rated U-factor counts that area along with the glass.

• uPVC. The material conducts little heat. The profile is an extrusion divided by internal walls into chambers. Each chamber is a pocket of still air, and heat has to cross them one after another. A deeper profile with more chambers gives heat a longer path. The steel that stiffens the profile sits inside a chamber, with uPVC and air on both sides of it.
• Aluminum. The metal conducts heat readily, so a modern aluminum window has a thermal break: a strip of low-conductivity material between the inside metal and the outside metal. Aluminum is stiffer than uPVC. For curtain wall, very large spans and the slimmest sightlines it is often the right material, and the glass is asked to do more of the energy work.
• Wood. A natural insulator. It is a sound choice where the look matters and the upkeep of the finish is planned for.

Two certified ratings show the size of the frame's share. With the same standard insulated double-pane glass and the same aluminum spacer, a Vi5 casement is rated U 0.30 and a Vi6 casement is rated U 0.27. The glass and the spacer did not change. The frame profile did.

The number of panes.

Each extra pane adds one more sealed space, and two more surfaces that can carry a coating. On a Vi5 casement, the certified U-factor goes from 0.30 with the standard double to 0.20 with insulated triple-pane glass and a warm-edge spacer.

The third pane costs more than money. The triple is noticeably thicker than the standard double, and heavier. The frame has to be deep enough to take it, and the hinges have to carry the sash.

Low-E coatings.

Gas and still air slow conduction. They do nothing to stop radiant heat, and plain glass radiates heat freely. A low-E coating is a very thin metallic layer that lowers the emissivity of the glass surface, so far less heat is radiated across the space to the next pane.

There are two broad families. One keeps room heat in and lets most of the sun's heat through. The other, solar control, also turns away the part of sunlight that carries heat without visible light. The family, and the surface it sits on, set the balance between U-factor, SHGC and visible transmittance.

Gas fill and spacer.

The space between the panes is filled with a gas that conducts less heat than air, usually argon. Each gas has a gap width where it works well. Too narrow, and heat conducts straight across. Too wide, and the gas starts to circulate and carry heat with it.

The spacer is the bar around the edge that holds the panes apart. A plain aluminum spacer is a thermal bridge past the gas. A warm-edge spacer conducts far less. It lowers the whole-window U-factor and keeps the bottom edge of the glass warmer, which is where condensation shows first.

Air tightness.

U-factor describes heat passing through materials. It says nothing about air passing through joints. Three things decide air leakage:

• How the sash closes. Casements and hoppers close against gaskets and are pulled tight by the locks. A sliding panel has to move past its seal, so it is harder to make as tight.
• Gaskets and locking points. Continuous gaskets, and enough locking points to keep them compressed along the whole sash.
• Welded corners. uPVC frame corners are fused, so there is no mechanical corner joint to open up over time.

In the Vi5 series, the casement tested at less than 0.01 cfm/ft² and the sliding door at 0.07 cfm/ft². Both figures come from structural test reports at class CW-PG50. Test pressure changes with performance class, so compare air leakage within one class.

The laboratory figure describes the product. The joint between the frame and the wall belongs to the installer and the envelope consultant, and it deserves the same care.

R-value and U-factor.

R-value is resistance to heat flow and is used for walls and roofs, while U-factor is the rate of heat flow and is used for windows. One is the inverse of the other, so a window rated U 0.20 is R-5.

What the energy code asks for.

The United States has no single national energy code. States and cities adopt a model code, the IECC or ASHRAE 90.1, and amend it. Specific limits change with every code cycle and every jurisdiction, so none are printed here. The shape of the requirement is steady:

• Limits by climate zone. Colder zones get a lower maximum U-factor. Warmer zones get a lower maximum SHGC. There is an air leakage limit as well.
• Residential or commercial. An apartment building falls under one set of provisions or the other depending on its height, and the limits differ.
• Rated values. The code expects NFRC-rated numbers. A product without them is given default values from a table in the code, and the defaults are poor.

What to ask about each part.

Part	What it moves	What to ask for
Frame	U-factor	The whole-window rating for this operating type, not a glass value
Panes	U-factor, weight	Unit thickness, sash weight, and hardware rated to carry it
Coating	U-factor, SHGC, VT	SHGC and VT from the same certified line as the U-factor
Gas and spacer	U-factor, edge condensation	The gas and spacer named in the certified listing
Seals and locks	Air leakage	The test report figure for this operating type

Certified values for every Vistaza type and glass are on the energy page. To confirm that a number on a submittal is certified, see how to check a U-factor.