There is some lack of consensus as to the benefits of green roofs in cold, heating dominant climates. There aren't a lot of studies done into this so the rule of thumb has generally been that green roofs provide their energy savings in hot climates where the demand is on air conditioning. That said, we've completed a study on the energy efficiencies brought on by our own green roof in Calgary... and it looks good! Despite being a heating dominant climate, we still saw appreciable energy savings from our green roof.
Our green roof was installed back in 2007 and we've been in the building since 2002, so we have a bit of data to go on. As temperatures got colder outside, we used less natural gas with the green roof. This suggests that the green roof continued to offer improved insulation over the winter months, compared to our old conventional roof.
But the big thing that jumped out was an overall 66% reduction in electricity consumed after the green roof was installed. Some of the savings could be attributed other efficiencies that were implemented at the same time, but the green roof likely also reduced some of the burden on our HVAC system, lowering electrical demand. Because we used less energy, we also required a smaller metre. This reduced our transmission and distribution charges significantly. In all, we estimate that our green roof saves us $3,650 per year in natural gas and electricity bills, based on average energy prices.
Regardless of climate, Green roofs have other financial benefits to building owners. They extend the lifespan of the roof membrane from 15-17 years to upwards of 40 years (on our building that would be about $16,500 every 15). They increase property value, improve tenant enjoyment and insulate from overhead noise.
The big take away: green roofs pay for themselves, even in cold climates. Though the initial upfront cost of a green roof is more than a conventional roof, the energy and roof replacement savings pay for the roof in about 15 years.