Replacement Window GuideIn a cold climate the priority is keeping heat inside, which makes U-factor the rating to watch. The NFRC states that the U-factor of a window determines how much heat will escape from a home, and lists ENERGY STAR U-factor thresholds as low as 0.27 in the Northern zone. This guide explains how to pick a cold-climate window, with every figure tied to its source.
Why U-factor leads in the cold
The NFRC cold-climate guidance states that the U-factor of a window determines how much heat will escape from your home through that window, and that a lower number is better.
In a heating-dominated climate, every unit of heat that escapes through the glass is heat your furnace has to replace. That is why U-factor, not solar gain, is the first number to check on a cold-climate window label.
The ENERGY STAR U-factor thresholds
The NFRC cold-climate guidance lists the ENERGY STAR U-factor thresholds by zone: the Northern zone requires a U-factor at or below 0.27, the North-Central and South-Central zones at or below 0.30, and the Southern zone at or below 0.40.
A colder zone demands a lower U-factor. For a northern home, targeting 0.27 or below is how you match the certification bar built for that climate.
The glass package that keeps heat in
Hitting a low U-factor takes more than a single pane. The NFRC glazing guide describes double glazing as two layers of glass separated by a layer of inert gas that significantly reduces heat transfer, and triple glazing as three layers with two gas layers providing outstanding insulation.
The same guide notes that a study found thin triple-pane windows can increase a home's window energy performance by 40 percent. In the coldest zones, the third pane is often what brings U-factor down to the target.
Solar gain can help in the cold
Unlike hot climates, cold climates can benefit from some solar heat. The NFRC hot-climate guidance lists the ENERGY STAR SHGC threshold for the Northern zone at 0.32, higher than the 0.25 allowed in southern zones.
The higher allowance reflects that winter sun through a south-facing window offsets heating load. In a cold climate you are not trying to block all solar heat, you are balancing a low U-factor against useful winter gain.
Certification and savings
ENERGY STAR requires that windows meet U-Factor and, where applicable, SHGC requirements based on climate zone, and directs buyers to determine their climate zone first.
ENERGY STAR reports that replacing old single-pane windows with certified windows lowers household energy bills by an average of up to 13 percent nationwide. In cold climates, where heating dominates the bill, a properly specified low U-factor window is central to reaching that saving.