Are Rooflights Cold in Winter?

Why glass feels cool

Glass is thin, and thin things conduct heat. Even a good rooflight has far less insulation than the roof around it, so on a frosty night the inner pane is the coolest surface in the room. Warm air meets it, loses heat and slides downward, which you can feel as a slight draught even when nothing is leaking.

That effect is normal. The question is how strong it is, and that depends on the age and build of the unit.

Why old rooflights feel worse

A single pane has almost nothing between the room and the weather. Early double glazing with metal spacers and no coating does better but still leaves a cold edge. A sealed unit that has lost its gas or let in moisture performs worse again. Add a cracked mastic seal or a frame that no longer closes tightly, and the cold you feel may be coming through gaps rather than glass.

A quick test is to hold the back of your hand a few centimetres from the frame on a windy day. Moving air points to a gap, whereas a steady chill from the pane itself points to the glazing.

What makes a modern unit comfortable

Three features do most of the work, and each has its own page.

• Two or three panes with a sealed, gas-filled gap between them. See double or triple glazed rooflight.
• A low-E coating that sends radiant warmth back inward. See what low-E glass is.
• A warm-edge spacer that stops the cold travelling around the rim of the glass. See warm-edge spacers.

The frame and the upstand matter too

People look at the glass, but the cold often creeps in at the edges. A flat rooflight or lantern stands on a kerb of insulated material, commonly 150mm or more above the finished roof. If that kerb is thin or poorly insulated, the ring around the glass can feel cold and gather condensation. When we fit one, we insulate around the opening with care and photograph it before it disappears under the finish. You get the photos when we hand the job over.

A thermally broken frame, with a barrier that stops heat running straight through the metal, helps as well.

Where the room sits makes a difference

A rooflight over a dining table, where people sit still for an hour, is more noticeable than one above a hallway. A north-facing opening sees less winter sun than a south-facing one, so it has less free warmth to offset the cool surface. Placing the unit sensibly in the first place is part of the answer, and we talk about it at the survey.

Seating is worth a thought too. If a favourite chair sits directly under a large fixed pane, you will feel the sinking cold air more than someone standing at the worktop.

Small habits that help

You can take the edge off a cool rooflight without changing anything. Keep the room heated evenly rather than in bursts, because a glass surface chills fastest when the room has been allowed to go cold. A blind drawn at dusk adds a layer of trapped air. A radiator or underfloor loop placed away from the glass means the warm air rises and meets the pane more gently. None of these replaces good glazing, but together they help.

Walk-on and flat rooflights

Flat glass set into a roof or terrace has its own quirks. A thick, laminated, multi-layer unit can be very well insulated, yet the single large surface faces the sky and chills quickly at night. If the unit is a walk-on type, the build-up below the glass and the upstand beneath it decide much of how warm the room stays, so a well-made upstand counts for more than a thicker pane. Ask for the declared figure for the whole unit, not only the glass in the middle.

Snow, frost and what you can see

A rooflight that stays clear of snow while the roof around it is white is losing heat, because warmth is melting the snow from below. A well insulated unit holds snow for longer, which is a sign that it is keeping the heat in. Frost on the outer pane in the morning is normal for good glazing for the same reason. It is the inner pane that you want to stay close to room temperature.

Condensation is a separate symptom

If the glass is misting or dripping, the cause is usually moisture in the room meeting a cool surface, not a fault in the rooflight. Cooking, drying washing indoors and a lack of ventilation all contribute. Our guide to condensation on rooflights covers the causes and the cures.

What to expect from a replacement

If your present rooflight feels cold, a new one will almost certainly feel better. A swap of the whole unit is notifiable, so we handle that with the council or a registered building control approver, and you get the certificate afterwards. The new unit must reach 2.2 W/m²K as a rooflight, or 1.4 W/m²K, or Window Energy Rating Band B, as a roof window. During the job, we lay covers over floors and furniture and seal the opening before we leave each evening.

You can read more about the whole range on our energy efficient rooflights page.