Hot, damn hot

I know this may not come as a surprise, but the sun is hot.  Really hot.  Even though it’s 150 million km away, its surface is 5,600˚ C (10,000 F) and its rays still have plenty of energy left when they reach the blue planet.  Also no surprise, because we’ve loaded up our troposphere with C02 and made a “greenhouse”, it’s even hotter these days.  As our summers of wildfires, heat domes and droughts worsen, keeping our indoor environments cool with air-conditioning grows ever more energy-intensive, and thus more carbon intensive.  Talk about a vicious cycle. 

So, building a passive house — which by design should consume only 10% of the usual amount of energy for heating and cooling a home — is getting more difficult.  The cooling part, that is. The good news is, not that much more difficult and there is a dead-easy solution. 

Another non-surprise: if you don’t let the sun’s rays penetrate a building envelope, it stays cool, regardless of the temperature outside.  We wear hats on hot, sunny days.  Then why doesn’t the building industry realize this, and continues to make ovens that need to be artificially cooled, pushing up energy consumption?  Er, because they want to shift the burden to the consumer, we rubes living inside those ovens.  (See pic taken recently near my home in British Columbia, of a new condo development cooking in the sun.) 

Our house has a broad south-southwest exposure, which means a slight bias to the west (and thus warmest in the late afternoon).   It is also on a hillside that is parched May through September.  Our steel roof can reach more than 50˚ C; our deck hits 40˚ C (that’s 122˚ F and 104 ˚ F, respectively). 

A passive house has most of its glass on the southern face, for solar gain in the colder months (blue lines, Diagram A.)  But what about when it’s hot, and you don’t want that gain?   Heat travels in three ways: conduction (through solids), convection (through air), and radiation (electromagnetic waves given off by the sun, ultraviolet rays).  We solved conduction with super-insulated walls and high efficiency windows and doors; we solved convection with our heat-recovery ventilation system, or HRV, which manages the flow of air. 

That leaves radiation.  It can only move in a straight line, so the way to control it is to block it.  Blinds?  Window films?   Those methods help a bit, but deal with heat that is already inside the building envelope.  So not much use.  The UV need to be intercepted outside the structure. 

Here it must be confessed we underestimated our home’s solar heating potential.  A lot of climate models have not kept pace with the extreme events we now have.  Moreover, the overhangs designed onto our place were insufficient for the south-southwest summer sun, especially mid- to late-afternoon when it is beyond its apex and thus low enough to strike the windows (see diagram A, which assumes the sun only at solar noon, its zenith.  Blue lines are winter, yellow are summer). 

After living through three hot summers in an overheated space, we are now in comfort.  No air-con is needed.  Double bonus: the sunniest time of the year is of course also the best for photovoltaic solar production, and our most recent two-month electricity bill showed consumption in a very sunny May and June of negative 850 kWh.  That means the power utility pays me.  Because we have net-metering, I’ll use that credit in the winter to run my heat pump.

What’s not to like?