Windows and glazing

The two numbers

U-factor is the rate of heat transfer through the whole window assembly, glass and frame together. Lower is better, and it is the inverse of the resistance idea used for insulation, which is a persistent source of confusion: for insulation a bigger number is better, for windows a smaller one is.

Solar heat gain coefficient is the fraction of incident solar radiation that gets through into the building. Lower means less solar heat admitted. Unlike U-factor, there is no universally better direction: a low coefficient reduces cooling load and rejects free winter heat, and a high one does the reverse.

The pair is the specification. A window selected on U-factor alone, in a climate with a serious cooling season and a south or west exposure, can meet its thermal specification and make the room unusable in August.

Orientation decides which number matters

A window facing the equator receives strong, high-angle sun in summer and lower-angle sun in winter, which makes it the elevation where shading works. A modest overhang can exclude summer sun and admit winter sun, because the geometry does the switching for you.

East and west are the difficult ones. Sun arrives at those elevations low in the sky, in the morning and the late afternoon, which is exactly when an overhang cannot intercept it. West-facing glazing in a warm climate is the most reliable way to overheat a room, and the answer is usually glass specification and external shading rather than the mechanical system.

The poleward elevation receives little direct sun and is dominated by heat loss, so it is a U-factor question and not a gain question.

None of this is exotic and all of it is free at design stage. It costs nothing to put the glass where it works and something to correct it afterwards with equipment.

The frame is usually the weak point

Modern insulated glazing units perform well. The frame surrounding them frequently does not, and the whole-assembly U-factor is the number that includes it, which is why comparing centre-of-glass figures between products is misleading.

Aluminium without a thermal break conducts heat straight through the assembly and is the clearest example. Thermally broken aluminium, timber, fibreglass and various composites each behave differently, and the difference between two windows with identical glass can be substantial.

Then there is the installation. A window is only as airtight as the joint between the frame and the rough opening, and that joint is made on site by whoever is fitting it. It has to control water, air and heat, and it is the reason a window schedule that stops at the product specification is incomplete without an installation detail beside it.

Condensation is a surface temperature problem

Water appears on the inside of a window when the interior surface is colder than the dew point of the air touching it. That is not a defect in the glass. It is a statement about three things at once: the humidity in the room, the temperature of the inner surface, and how well air is moving across it.

It follows that condensation is diagnostic rather than merely annoying. Persistent water on the inside of windows in winter means the interior humidity is high relative to the surface temperature, which usually means the ventilation is inadequate, the glazing is underperforming for the climate, or both. Wiping it up addresses none of that, and the water eventually reaches the frame and the sill, which is where the damage accumulates.

The design responses are ordinary. Better whole-assembly performance raises the inner surface temperature. Ventilation lowers the interior humidity. And avoiding deeply recessed sills, heavy drapes and blocked supply air keeps room air moving across the glass. Which of those applies is answerable by measuring the humidity in the room, which costs almost nothing and settles the argument.

What to ask for, and what governs the minimum

Ask for whole-assembly U-factor and solar heat gain coefficient, per orientation, on the schedule rather than a single specification applied to every opening. Ask what the installation detail is at the head, jamb and sill, and who is responsible for it. And ask whether the glazing specification differs on the west elevation, because if it does not, nobody has thought about the west elevation.

The minimum performance is set by the adopted energy code, which varies by jurisdiction, edition and amendment, so the required values come from your building department rather than from any published table. [1] The reasoning for specifying by elevation rather than by product is the same continuity argument that runs through The Build Sheet under envelope.