Drafts and heat: the window is the weak point

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Cellular (honeycomb) shades are the strongest performer in this category and it is not close. The cells trap still air in a sealed pocket against the glass, which is the same principle as double glazing. Double-cell constructions trap more than single-cell. Manufacturers publish R-values for these shades; treat them as a way to compare products against each other rather than as an absolute promise, because the published figure assumes a fit far tighter than most real installations achieve.

Thermal curtains work through fabric mass plus, usually, a coated backing. A heavy lined panel slows convection and adds a still-air layer, and it costs less than a fitted shade. But a curtain hangs loose: air spills over the top, around the sides and under the hem, and that circulation carries away a lot of the benefit. A thermal curtain that is sealed at the top with a pelmet or box and reaches the floor performs dramatically better than the identical curtain hung loose — the seal is worth more than the fabric.

Window film — the shrink-film kits, not privacy film — creates a genuine second air gap across the whole opening. It is visually unappealing and it is seasonal, and it is also the most cost-effective thermal upgrade for single glazing by a wide margin. Not everything that works looks good.

• Seal the top. An open gap above a curtain lets warm room air flow down the glass and away. A pelmet, a box, or a ceiling-mounted track closes that loop and is the single most effective thing you can do to a curtain you already own.
• Reach the floor. A thermal curtain stopping at the sill leaves the cold air an exit route. Floor length, or a little past it, is the thermal choice — see the curtain length guide.
• Fit shades inside the frame, tightly. A cellular shade's insulation depends on the cell pocket being sealed. A half-inch gap at each side turns a good shade into a mediocre one. Measure in three places and take the narrowest.
• Reverse the logic in summer. Close coverings on sun-facing windows *before* the room heats up, not after. Once the heat is inside, an interior covering mostly traps it there.

Thermal window coverings do reduce heat loss, and the effect is real and measurable. What is not credible is a specific dollar figure attached to your house. Savings depend on your climate, your glazing, how many windows you treat, whether the coverings are actually closed at the right times, and what you heat with. Anyone quoting you a percentage off your bill without knowing any of that is guessing.

The honest framing: this is a comfort upgrade that also saves some energy. The comfort difference next to a treated window in winter is immediate and obvious. The bill difference is smaller, slower, and harder to see — and the reason to do it anyway is that sitting by a cold window is genuinely unpleasant. More at do thermal curtains save energy.