As the world moves toward more alternative forms of electricity production and as appliances and other equipment being installed into “smart” homes become more efficient and connected, reducing energy consumption through building design is another aspect of conservation for roofing contractors and homeowners to consider. Below, we’ll look at how the properties of heat convection, conduction and radiation point to opportunities to conserve energy.
Heat Convection
Though we may not recognize it immediately, convection is something we encounter every day. It’s more than new ovens that move hot air to enable us to cook things faster but an oven is a good example. Think of an attic as an oven. During the day, the sun causes the air temperature in the attic to increase. When the air temps increase, the air wants to rise and transfer heat to everything that isn’t as warm as it is. This is why you can take a hot shower – without intending to – in the middle of the day in old houses. The copper lines that run through the attic are heated by the hot air in the attic and then the water inside the lines heats up. Turn on the “cold” water and, presto, hot shower.
Ventilation was developed to reduce heat in the attic. Vents at the lowest points of the attic air space allow cooler outside air to enter. It works much better when there are also vents at the higher end of the attic to allow the rising hot air a means of exiting. You don’t even need to power the air flow with electric fans, though that certainly helps: when there are proper low inlet and high outlets, the temperature difference between the ambient air and the attic air can create enough movement to cool the attic.
Heat Conduction
Conduction involves moving energy between materials that are touching. Shingles, tile, metal etc. all have the capacity to conduct. These materials absorb heat from the sun throughout the day and they look to pass this heat to lower temperature materials that they are touching so that temperatures can equalize between the materials. Heat absorbed by the roof cover conducts well through metal. Roofers understand that metal nails, tin tags and caps are very hot to the touch on a hot day. Each of these hot nails punch the underlayment and roof deck and, after conducting some heat to them, turn into a little heaters in the attic. This heat transfer can be addressed with an insulation layer like Polyiso or EPS boards. Using adhesive instead of nails can also reduce the heat transfer and so help keep the attic cooler.
Along the same line, if you ever vacation up north in the winter, you can see warm screws and plates on a low-slope roof melting snow. The heat originates from the heated space inside the building and is conducted through the screws and plates to the roof surface. In the summer, the outside heat gets conducted into the building along the reverse path. Using an insulating roof sandwich panel instead of plywood to deck will reduce energy conduction into the attic. The real challenge here is to get the insulation layer closer to the sky because everything underneath it will be cooler and will also last longer. How about some metal roof systems that add a layer of insulation between the underlayment products and the final metal panels? Or a tile system that adds a layer of insulation between the underlayment products and the tile? These help keep heat out of the underlayment products, roof deck and attic and represent a considerable energy savings to home. Ultimately, if the attic doesn’t heat up the conditioned air, the air conditioning will not have to work as hard to keep the home cool.
Heat Radiation
Radiation of heat energy is the last path to consider. This is how the sun heats the Earth – including the surface of a roof. The heat energy travels through the heavens and the Earth’s atmosphere and heats whatever it lands on. However, the energy is absorbed by darker surfaces and reflected by lighter ones. When someone selects a white or lighter color roof compared to a darker or black roof, the radiated heat is reduced: a highly reflective roof covering works well to reduce the amount of heat from the sun getting into a home. Other options are to add a reflective foil layer somewhere in the attic or roof assembly. The catch is these layers work better when they have an air gap on at least one side of the foil. This can be accomplished in many ways, like a foil sheet stapled to the underside of the roof trusses in the attic. The same principle of heat transfer applies here also. If you can get the radiative barrier closer to the sky, the materials underneath the barrier will benefit by not getting as hot during the day.
These methods of heat transfer can be tackled on the roof for the benefit of not only our homes interior, but also lower the demand for electricity on the grid for all of us. The lower the demand for electricity, the less we must produce. The best kind of energy conservation is to reduce the demand. That has benefits for all of us.
FRM
Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years working for Polyfoam Products, which was acquired by 3M. When Polyfoam Products was divested by 3M, the name was changed to ICP Building Solutions Group. Riku worked at ICP as Technical Service Manager until March of 2023, when he left to begin his own company. Riku is an engineer and is working as a consultant with FRSA. He is a member on FRSA’s Codes Committee, Codes Subcommittee, Tile Committee and on the FRSA-TRI Manual Rewrite Committee. Riku is also a member of the American Society of Civil Engineers (ASCE) and the Roofing Technical Advisory Committee (TAC) of the Florida Building Commission.
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