Energy efficiency is at the heart of modern Net Zero and Passive House design. Thick super-insulated walls, triple-glazed windows, Energy Star appliances, LED light bulbs, and cutting-edge HVAC systems are all popular topics for reducing energy costs. But often overlooked, your roof systems are a key resource for saving energy.
It is estimated that over 85% of Canadian homes have poorly designed roofs, with dark, non-reflective materials that absorb rather than reflect heat. Rooftop temperatures can get 32 to 38°C hotter than the outside air. With a heat-absorbing roof, this heat transfers into your attic and, ultimately, your home. Good insulation will offer some relief, but if the roof fails, the attic will overheat, and that heat is going to transfer into the home, making the air conditioner work much harder than it needs to.
Designing an energy-efficient roof
The pitch, shape and aesthetics of the roof affect the appearance of the entire building. It could even be argued that the roof makes the house. Many considerations influence the design of your home’s roof, beyond personal preference. Certain municipalities, neighbourhoods or subdivisions have a zoning plan that defines the shape of the roofs that are permitted in the area. There may be heritage and streetscape restrictions you need to investigate.
A significant factor on BC’s West Coast is the weather. Heavy snowfall in Whistler, torrential rainforest downpours and strong winds are common in the Sea to Sky Corridor, and a high bushfire risk (BAL 29 and more) in some wooded areas can affect your roof’s design. In areas of heavy rainfall or big dumps of snow, steeper roof angles help in shedding all that water quickly, while preventing water from collating in puddles, and finding its way into the building envelope. Uncomplicated roofs with simple lines, combined with non-flammable metal or tile materials, can prevent flying embers from igniting the structure.
If you’re building in a rural-urban fringe area that experiences frequent power outages, it may be important to think “periodically off-grid” for one or two seasons of the year. Most people don’t think about it, but if you might experience power outages lasting a few days, the generators at the pumping station may not be available for long. In this situation, you want to design a roof with a large area to collect the most rain – every raindrop matters. The optimal orientation for the highest efficiency of solar panels will also be critical, and that doesn’t always mean a north-facing roof. For example, it could be better to mount the panels facing north-west to handle the highest afternoon loads.
The BC Energy Step Code is pushing local governments to have all new construction Net Zero Ready by 2032. This means that your roof’s orientation is a prudent consideration, as you are likely to opt for roof-mounted solar arrays at some point. Your roof can not only generate renewable power for your family; it can also collect and heat your water.
Cool roof shingles
Asphalt shingles can still potentially deliver energy savings when “cool” shingles are installed. Cool shingles have been engineered to have higher solar reflectance and thermal emittance properties. They are typically white, light grey or light tan. Solar reflectance refers to the fraction of the infrared and ultraviolet solar energy that is reflected by the roof. Shingles with a higher solar reflectance permit less heat to transfer into the home. Solar Reflectance Index (SRI) measures a roof’s ability to reject solar heat. Asphalt shingles with higher solar reflectance values are considered “cool” products, and they can achieve ENERGY STAR certification.
Thermal emittance refers to the fraction of absorbed heat that is released back into the atmosphere. Materials with a high thermal emittance are usually a better choice for hot, sunny regions, while those with a low thermal emittance may help homes lower the heating load in colder Canadian winters.
The US Department of Energy defines a Cool Roof as, “one that has been designed to reflect more sunlight and absorb less heat than a standard roof.” Any energy-efficient roof could therefore qualify as a cool roof.
Metal sheet and standing-seam roofing
Steel roofing is one of the most popular roofing materials in high-performance homes. Permanent metal is very strong and long-lasting, holding up well against rain and wind storms, and big dumps of snow. And it’s non-combustible, providing a barrier against sparks from bushfires. Other metal variations include aluminum, copper, titanium-zinc, galvanized, or coated sheets.
When the phrase “metal roofing” is used, most people envision a roof covered in corrugated galvanized metal. Modern standing seam metal roofing offers some exciting new design options, with greater thickness, a wide range of colours, invisible fastening systems and other performance enhancements.
Like corrugated roofs, standing seam roofs are also designed with metal panels fabricated by roll forming. Standing seam systems consist of wide, relatively flat metal panels that have vertical legs (ribs) on each end. The panels lock together and are fastened to the sheathing below with hidden clips. Standing seam roofs are designed to expand and contract with temperature changes, The raised metal seams remain locked and the entire system floats on its clip mounts.
Metal roofs are extremely durable and require virtually no maintenance, with a life expectancy of 40 to 70 years. They can withstand up to Category 4 hurricane-force winds, they’re pest-resistant, and they don’t break down or decay in the weather like asphalt shingles.
Terracotta and concrete tile roofing
Tiles offer very low maintenance with a life expectancy of 50 to 100 years for terracotta and 20 to 50 years for concrete. Both terracotta and concrete tiles are significantly heavier than sheet metal roofing, which can pose some design challenges where thermal bridging has been minimized by using foam insulation structurally.
Terracotta tiles continue to be a popular choice for high-end homes. The tiles are made from kiln-fired natural clay. They are finished with a glaze to lock in the colour, and they age very well. On BC’s coast, rounded tiles are the best choice for shedding frequent rainfall. Flying debris or hail during a storm or heavy periods of rainfall can damage terracotta tiles, causing them to crack; but damaged tiles are relatively easy to replace.
For homeowners who want to create a rainwater catchment system, natural terracotta tiles are a very good choice. They won’t leach heavy metals or other harmful contaminants into the water you catch and store in your cistern. Clay tiles are made from one of the most abundant resources on earth, subsoil. The lighter colour of clay roof tiles can reflect the sun’s heat, keeping your home cooler during the summer; thereby reducing the need for supplemental air conditioning.
Clay tiles retain heat well, providing excellent thermal performance. Combined with super-insulation, tiles can be an excellent material for a net zero home or passive house.
Concrete tiles are coloured with pigment, either as a coating to the top or throughout. Strong UV rays and pollution can dull concrete tiles over time, but they can be revived with a coat of spray enamel. Concrete tiles can weigh five times as much as asphalt shingles and can be 10 times heavier than permanent metal.
Like metal, concrete tiles are fire-resistant, wind resistant and they won’t rot or decay under wet conditions. Weight remains their biggest downfall, and a house needs to be engineered to support that weight. Another consideration is the large quantity of cement used, which significantly increases its embodied energy footprint. The primary benefit of concrete tiles is price.
Slate tiles or shingles
A genuine slate roof is a classic. Slate is known for its durability and resilience. A hard slate, like a grayish-black Buckingham slate, can last 150 to 200 years; and even softer slates have a life span of 75 to 90 years. Slate shingle roofs hardly absorb any water and they do not crack or decay in freezing temperatures. Slate tiles are fireproof and environmentally friendly. Slate is mined – mostly in Italy – and cut into square tiles or shingles that are installed on your roof one at a time. Like terracotta and concrete tiles, the home must be designed to bear the weight of the slate roof.
Synthetic slate shingles are a more recent option. The shingles are manufactured from recycled materials and have been designed to look exactly like slate. Most have a completely authentic look, right down to the chisel marks. A stone mason can tell the difference up close, by picking up a tile, but to the neighbours and passers-by, they look identical to the real thing. One of the biggest advantages of synthetic slate shingles is that they are very lightweight, weighing less than some of the premium asphalt shingles. This makes them a good fit with roof systems that are focused on eliminating thermal bridging, and consequently provide less structural support for heavy roofing materials.
If your natural slate roof is flashed properly and installed correctly, it should be the last roof you’ll ever put on your home, lasting a minimum of 75 years. Synthetic slate won’t last that long, but if installed properly, it should provide 40 to 50 years of service.
When building a passive house or net zero home, it is important to consider energy transfer as materials lose their insulation, and the energy consumed to maintain and repair degraded materials. Choosing materials designed to deliver a long, reliable service life becomes even more important in a high-performance home, so the integrity of the building envelope is never compromised.