White roofs in winter in Winnepeg
After posting about the benefits of white roofs, (as Mike did here on TreeHugger), Andrew Sullivan published a couple of letters from readers questioning their benefits. I had a look at this issue two years ago, and wrote that Arguments Against White Roofs in Northern Cities are Specious. But one commenter hit a new level of speciousness with his argument:
Winter is when you actually want a black roof, in order to suck up the sun's radiant heat and melt as much of the snow as possible. Snow on the roof certainly looks picturesque, but it frequently leads to all sorts of problems, such as ice dams that force water under your shingles, through your roof and into your house.
First of all, the white roof idea is for flat, unventilated roofs. Almost all shingled roofs are over uninsulated, vented attics and the colour of the roof is not going to make much difference at all. I am having almost-white shingles put on my roof as I write this, but I have a finished attic and it probably will make a bit of difference. But in most houses it won't.
Image credit CMHC
Secondly, ice dams are not caused by snow on the roof, they are caused by poor insulation and roofing. Heat loss through the roof melts the snow, and the water refreezes when it gets to the overhang of the roof, creating a dam, which causes water to back up under the shingles. The commenter is describing a lousy roof with insufficient insulation and ventilation and no ice and water shield. It has nothing to do with the colour of the roof.
Another commenter quoted a study and claimed that "if white roofs became ubiquitous [worldwide], the extra energy needed for heating in the winter would exceed the energy savings in the summer." I cannot read the study without paying $ 25, but as I noted in my post two years ago,
1. A house in Minneapolis or Detroit probably doesn't need air conditioning in the summer if it has a white roof, it is that much cooler. It certainly will have a shorter air conditioning season. So just using degree day tables to figure out how many days require heating vs cooling doesn't work.
2. Air conditioning is powered by electricity which for the most part comes from coal and is very inefficient; heating mostly comes from cleaner burning natural gas at far higher efficiency. The peak load of the electrical season is the hottest day of the year, so any air conditioning load taken off the top has a lot more impact than adding a bit of natural gas consumption in the shoulder seasons.
3. A properly insulated house in Detroit or Minneapolis has a white roof for most of the winter- it's called snow.