Comparing windows by U-factor, SHGC and energy rating

What is U-factor?

U-factor is the rate of heat flow through the whole window, frame included, per square metre and per degree of temperature difference. In metric labels the unit is W/m2K. Lower means better insulation. Triple glazing, low-emissivity coatings, gas fills and insulated spacers lower it; so does a well-insulated frame.

A quick calculation shows the effect. Heat loss equals U times area times temperature difference. For 10 m2 of windows on a day when the inside-outside difference is 30 degrees C, a window at 1.6 W/m2K loses 480 W, and one at 1.2 loses 360 W. The difference of 120 W continues for as long as the cold lasts. Ratings on real products vary, so compare the labels of the windows you are quoted.

What is SHGC and why does it matter?

The solar heat gain coefficient is the fraction of the sun's heat that passes through the window, between 0 and 1. A higher number lets in more free heat, which helps on south-facing glass in a Canadian winter. A lower number blocks heat and helps where summer overheating is the problem, such as large west-facing windows.

So the best SHGC depends on the wall. Many designers choose higher values on the south side and moderate or low values on east and west. The passive solar design guide covers the layout side of that decision.

What is the energy rating (ER)?

The Canadian energy rating is a single number calculated from solar gain, heat loss and air leakage under standard conditions. Higher is better, and a positive value means the window gains more heat than it loses over a typical heating season in the test conditions. It is useful for a quick comparison of two windows with the same size and type, though it does not replace looking at U-factor and SHGC when orientation is unusual.

Certified products carry a label showing the rating and, for ENERGY STAR qualified windows, the climate zone in which they are qualified. Ask the supplier which zone applies to your address and check the current requirements.

How do you compare two window quotes?

Ask for the same data on both: size, frame material, glazing makeup, U-factor, SHGC, energy rating, air-leakage class and what the installation includes. Check that the quote covers flashing, air sealing around the frame and disposal. A lower price that omits an insulated rough opening or proper sealing can cost more in heat loss than the saving. Ask each supplier what the warranty covers for the glass seals and for labour, and for how many years. Then compare the total, not the price per window, with a note of the orientation each window sits on.

For a house aiming at net zero, the window schedule is usually fixed early, because every later change to glazing area shifts the size of the heating system and the solar array.

Other features that decide performance

• Air leakage. A well-sealed operable window matters as much as the glass. Fixed windows leak least.
• Frame material. Vinyl, fibreglass, wood and thermally broken aluminum all differ in conductivity and upkeep.
• Installation. A poor install, with gaps or missing air sealing and flashing, wastes the label values.
• Condensation. Warmer interior glass surfaces reduce it, and so do ventilation and heat recovery.

Are new windows worth the money?

Often windows are not the first upgrade. Per dollar spent, air sealing and attic insulation usually save more, which is why the insulation priorities put windows after them. Replace windows when they are failing, leaking or being changed during a renovation, and choose the best level your budget allows then.

For new, very efficient houses the windows are a main design choice. The 52 Drake Landing homes were built to R-2000 and Built Green Alberta Gold levels, where glazing quality was one of several measures that kept the heating load small enough for a low-temperature solar loop to carry.

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