Did you know that your roof can actually get colder than the air around it? It might sound counter-intuitive, but this can often happen on a clear, cold night.

It’s a phenomenon known as “night sky radiation”, and while most people outside the roofing industry have never heard of it, it can sometimes leave them wondering whether their roof has sprung a leak. Fortunately, the explanation is usually far less dramatic.

Put simply, night sky radiation is the process by which your roof loses heat into the clear night sky. With no cloud cover to reflect warmth back down, the roof continues to radiate heat upwards, becoming colder than the surrounding air. In fact, the roof cladding can cool to several degrees below the outside air temperature.

That’s when condensation comes into play.

Warm, moisture-laden air from inside your home naturally finds its way into the roof space. When that warm air meets the now much colder underside of the roof, the moisture condenses into tiny droplets of water. The result? Your roof appears to “sweat”, even though it hasn’t rained.

It’s easy to mistake this for a leak, but in many cases the water has formed through condensation rather than entering from outside.

Some condensation is normal, and modern underlay is designed to manage this. However, over time, excessive moisture can contribute to mould growth, damp insulation and, in some cases, deterioration of timber roof framing.

The good news is that there are ways to mitigate this risk.

If you’re planning a re-roof, ask your roofing contractor what measures they recommend to help minimise condensation and the effects of night sky radiation. Improved roof-space ventilation, appropriate roof underlay, and effective moisture management within the home can all make a significant difference. (If they can’t answer your question, contact us and we’ll be happy to help.)

And if you’re a roofing contractor, it’s worth discussing ventilation improvements with your clients as part of the re-roofing process. A little extra planning now could help prevent much bigger problems later.