Do Metal Roofs Keep a House Cooler? What Happens in the Attic

The worry usually goes like this: metal gets hot in the sun, so a metal roof must cook the house. It is a fair instinct. Touch a steel barn roof in July and you will not do it twice. But a house roof is not a sheet of steel over an empty barn. It is an assembly, and in most cases a metal roof leaves the attic cooler than an asphalt shingle roof of a similar colour would.

The reasons are worth knowing, because they also tell you which choices make the biggest difference.

The short answer

Yes, usually, on two conditions. The colour has to suit what you want from the roof, and the panels have to sit on an air gap rather than directly on the deck. Get those right and a metal roof puts less of the sun's heat into the attic than asphalt shingles do. Get them wrong and a dark metal roof can do no better than a dark shingle one.

Where roof heat actually goes

Sun lands on the roof covering and some of it is absorbed. The covering warms up and passes heat into the deck, the deck warms the attic, and the attic works its way through the ceiling into the rooms below. Each of those steps is a place where the flow can be interrupted.

Asphalt shingles are heavy and sit directly on the deck. They soak up heat through the afternoon and keep releasing it into the attic well into the evening. Steel panels are thin and light: they heat up quickly, but they also give that heat up quickly, to the air around them and back to the sky, as soon as the sun moves off them. That is why a metal roof cools down fast after sunset while a shingle roof stays warm for hours.

Colour matters more than the metal

How hot any roof gets in the sun comes down mostly to two properties: how much sunlight the surface reflects, called solar reflectance, and how readily it gives off the heat it does absorb, called thermal emittance. The U.S. Department of Energy's guide to cool roofs rates both from 0 to 1, with higher meaning cooler. Colour drives the first. ENERGY STAR's own example is a clean white roof that reflects 80 percent of sunlight running about 50°F cooler than a grey roof that reflects 20 percent.

Most people choosing a metal roof in Ontario are not choosing white, and that is fine, but it is worth being honest about what a dark roof does. Researchers at Lawrence Berkeley National Laboratory have spent years on "cool colour" pigments that reflect invisible near-infrared light, so that a dark surface stays cooler than an ordinary dark surface, though still not as cool as a white one. If keeping the house cool is the priority, a mid or lighter shade will do more than any coating claim. Choosing a metal roof colour goes through the trade-offs.

The air gap does more than the colour

The part of a metal roof that does most for attic temperature is one you never see. On a properly built metal roof the panels are fixed to strapping rather than laid flat on the deck, which leaves a ventilated space between the metal and the underlayment. Heat that gets through the panel warms the air in that space, and the moving air carries it away before it reaches the deck. Why a metal roof needs an air gap explains how the strapping is set out.

This has been studied properly. Oak Ridge National Laboratory tested metal roofs with ventilated space beneath them alongside an asphalt shingle roof for several years, through heating, cooling and shoulder seasons. The metal roofs were highly effective at cutting the heat flowing through the roof and ceiling, and they reduced the daily swing in attic temperature. In ORNL's earlier tests on metal shakes, most of the improvement over shingles was attributed to the ventilated space rather than to the metal's reflectance.

Shingles nailed straight to the deck have no equivalent. Whatever heat they absorb goes straight into the wood.

What sets your attic temperature

The roof covering is one of three things, and the other two matter at least as much.

• Attic ventilation. Air should come in low at the soffits and leave high at the ridge. If the intakes are blocked, very often by insulation pushed into the eaves during an upgrade, hot air has nowhere to go whatever is on the roof.
• Insulation on the attic floor. A hot attic only heats the house if the heat gets through the ceiling, and insulation is what stops it. ENERGY STAR notes that cool roofs save the most on buildings with little roof insulation, which is another way of saying a well-insulated ceiling is already doing much of the work.
• The roof covering. Colour, weight and the air gap, as above.

Attic temperature on its own can mislead. Even a well-built metal roof over a ventilated attic will run warm on a July afternoon. What matters for comfort downstairs is how much of that heat crosses the ceiling, and how quickly the attic cools once the sun goes.

Is it worth it in Ontario?

Ontario homes use far more energy for heating than for cooling, so a roof's effect on the yearly bill is smaller here than in the hot, sunny climates where most cool-roof research is done. We do not put a percentage on it, because the honest figure depends on your attic, your insulation, your colour and which way the roof faces. Metal roofs, energy and sustainability covers which energy claims hold up.

Where you notice the difference is comfort: upstairs bedrooms under the roof, finished attics, rooms with sloped ceilings, and cottages that sit closed up in the sun all week. Those are the spaces where less heat coming through the deck is something you can feel.

In winter the roof covering is not what keeps the heat in; insulation and air sealing do that. Ventilation still matters, because a cold, well-ventilated roof deck is what stops ice dams forming at the eaves.

Where the “hotter house” idea comes from

Mostly from buildings that are nothing like a house: uninsulated steel sheds, barns and carports where a single sheet of metal is all there is between you and the sun. Standing under one of those on a summer afternoon, the radiant heat is real. Add a ventilated air gap, underlayment, a deck, an attic and a layer of insulation, and it is a different situation entirely.

The other source is touch. At the same temperature, metal feels hotter to the hand than shingles because it conducts heat into your skin faster. That says nothing about how much heat is going into the attic.

What to ask before you choose

• Will the panels sit on strapping, and how is that air space vented at the eave and the ridge?
• Where does air enter and leave the attic, and are the soffit intakes clear?
• Is there enough insulation on the attic floor, and will anyone check?
• If staying cool matters to you, which of the available colours is the most reflective?

A Highland roof includes strapping and ventilation as standard, and replacing the roof is the right time to look at the attic too. To talk about your own house, request a quote or get in touch.

Sources

• U.S. Department of Energy, Guide to Cool Roofs.
• ENERGY STAR, Cool Roofs.
• K. Biswas, P. W. Childs and J. A. Atchley, Performance Evaluation of Advanced Retrofit Roof Technologies Using Field-Test Data, Phase Three Final Report, Oak Ridge National Laboratory, 2014.
• Earlier ORNL metal-shake results as summarised in Building Design + Construction, 2015.
• Lawrence Berkeley National Laboratory, research news on cool-colour roof pigments, 2016.