Buyer's Guide
How Windows Lose Heat (and How Blinds Help)
Glass is the weakest link in almost every home's building envelope — a single-pane or even double-pane window loses heat far faster than an insulated wall. Understanding exactly how that heat escapes makes it obvious why some window coverings barely help and others make a real difference. Here's the physics in plain language, and what it means for choosing coverings in a Calgary winter.
The 3 ways heat moves through a window
Heat always moves from warm to cold, and it does that through three physical mechanisms. A window rarely loses heat through just one — usually all three are happening at once, which is why window heat loss feels worse than the R-value on a spec sheet suggests.
- Conduction — heat travels directly through a solid material. Glass conducts heat far more readily than an insulated wall, so warmth inside your home moves straight through the pane to the cold outdoors.
- Radiation — every surface radiates infrared heat toward cooler surfaces nearby, even with no air movement at all. Your body, furniture, and walls radiate heat toward the cold glass, and that glass radiates cold back at you — which is why you feel a chill sitting near a window even if there's no draft.
- Convection (air movement) — cold air sinks. Air touching the cold glass surface cools, drops, and flows into the room as a draft along the floor, while warmer room air rises to replace it near the window. Add any gap around the frame or sash and outside air infiltrates directly, which is technically air leakage rather than convection but produces the same drafty result.
Why glass is the weak point in the wall
A typical insulated exterior wall has an R-value in the range of R-15 to R-20+. A standard double-pane window is closer to R-2 to R-3, and older single-pane glass can be under R-1. That gap means a wall-sized window is losing heat many times faster per square foot than the drywall beside it — no coating or gas fill fully closes that difference. Add a Calgary winter with regular nights well below -20°C, and even a small area of glass becomes a significant heat drain and a source of cold-surface discomfort near seating and workspaces.
How window coverings interrupt each mechanism
A covering doesn't change the glass itself — it works by creating a still-air buffer between the room and the cold pane, and it can address each heat-loss path differently.
- Slowing conduction: any covering with trapped air (rather than a single flat layer) adds an insulating buffer between the warm room and the cold glass, reducing how quickly conducted heat reaches the window surface.
- Blocking radiation: a covering placed close to the glass intercepts the radiant cold coming off the pane before it reaches your skin or furniture, and reflects some of your room's radiant heat back rather than letting it hit the glass and dissipate.
- Cutting convection and drafts: a covering that fits snugly to the frame — sealed at the sides, not just hanging loose — stops the cold-air sink-and-drop cycle at the glass and blocks stray airflow finding its way around the edges.
Not all coverings interrupt heat loss equally
This is the key distinction: a covering only helps as much as it traps still air and seals to the frame. A thin fabric hanging a few inches off the glass with open sides looks like insulation but does very little — air still convects around it and radiant cold still passes through a single layer easily. A covering built with trapped air cells and a snug, edge-sealed fit does meaningfully more, because it addresses all three mechanisms at once instead of just adding a thin barrier.
- Loose, open-sided coverings mainly reduce radiant discomfort a little — drafts and conduction pass through mostly unaffected.
- Coverings with dead-air construction reduce conduction more effectively by adding a genuine insulating layer.
- Track- or side-channel-mounted coverings that seal at the edges cut the convective draft loop, which is often the difference people feel most on a cold night.
What this means before you choose a covering
If your goal is comfort near windows and lower heat loss (not just light control or looks), the fit matters as much as the fabric. Ask how close the covering sits to the glass, whether it seals at the sides, and whether the construction traps air rather than hanging as a single flat panel. A free in-home measurement lets a consultant check your actual window openings, drafts, and glass condition, and recommend coverings sized and mounted to actually close the gap — rather than guessing from a showroom sample. For a deeper comparison of specific insulating covering types and expected performance, see our energy-efficient blinds guide.
