Do Sun Tunnels Let Heat Out?

Warm air inside a house rises and presses against the ceiling. Anything that breaks the ceiling and the roof above it gives that warmth a path to cool outside air. A sun tunnel breaks both, so it is fair to ask whether it leaks heat.

Heat leaves a tunnel in three ways. It passes through the glazed dome by conduction. It slips round the tube if the insulation is not packed tight. And it moves through the air column inside the tube, which can circulate warm and cool air between the diffuser and the dome.

Each of these can be reduced, and a good installation deals with all three.

The honest framing is that a sun tunnel trades a small amount of warmth for daylight where there was none. In a hallway or windowless bathroom the alternative is a light bulb burning through the daytime, and the room may be no warmer without the tunnel. In a bedroom, where the room is used at night and the heating is on, the diffuser is the place to look at first when someone asks about comfort.

The question to ask at survey is how the tube will pass through whatever insulation is already in the roof. We look at the loft depth, the type of insulation in place and how it ends at the tube, then tell you what we would do about it. Nobody can promise a figure for how much warmth a given tunnel will lose, since the house, the weather and the heating habits all change the result, which is why we leave the figure out.

The dome on top is usually two layers of clear plastic or glass with a gap between, much like double glazing. The gap slows the passage of heat. Under it, many tubes have a second sealed lens or a diffuser with a thermal break at the ceiling, which stops warm room air from rising up into the tube at all.

That lower barrier is the quiet hero. If warm air cannot enter the tube, it cannot cool against the dome and fall again. The tube stays a still column, and the diffuser stays close to room temperature.

If you want to know what each of these adds to the price, see how much a sun tunnel costs.

In a pitched loft the tube runs through cold air. The tube itself is a mirror-lined surface, not an insulator, so on a cold night its outer face can be cold. A rigid metal tube is a good conductor, which means it can carry cold toward the ceiling if it touches nothing warm.

For this reason we wrap or pack insulation round the tube where it crosses the loft, and keep the loft insulation continuous up to its edge. Compared with a flexible tube, a rigid one benefits most from the wrap, because its metal walls carry cold more readily.

The heat loss of a rooflight or roof window is measured in a number called the U-value. Under Part L of the Building Regulations for existing homes, a roof window is held to 1.4 W/m²K (or Window Energy Rating Band B), where a rooflight is allowed 2.2 W/m²K. Both numbers describe the finished unit, frame included.

A sun tunnel opening is far smaller than a roof window, so the area through which heat can pass is small. That small area is a large part of why it can be competitive. A big opening with excellent glazing can still lose more total heat than a small opening with ordinary glazing, just by size. Read the sun tunnel vs rooflight comparison for how the two differ in other ways.

If winter warmth matters more than daylight in the room, the energy-efficient rooflights range lets you pick glazing for the best thermal result.

The same sun tunnel can feel warm in one house and chilly in the next. The cause is usually the fitting rather than the product.

• Gaps. A tube that is not sealed at the ceiling lets room air climb up into the loft.
• Loft insulation. If the quilt stops short of the tube, there is a cold ring around it.
• Tube length. A short tube has less surface to lose heat from.
• Room use. A kitchen or bathroom adds moisture, so the diffuser surface needs to stay warmer.

We pack the insulation to the tube on the day, and we mention any gaps we find in the loft at the same visit. Our sun tunnel installation hub explains how a visit runs from survey to finish.