Why We Need Passivhaus in Hot Climates

One of the many myths about the Passive House or Passivhaus standard of building is it was designed for temperate Germany and is not really suitable for really cold or hot climates.

The Passivhaus concept is a particularly tough sell in hot climates, especially in places with big diurnal swings in temperature, where one would traditionally design houses with thick walls for thermal inertia and natural ventilation at night, and nobody thinks much about insulation. But what once worked in the desert doesn't anymore.

At the time of writing, there is a heat wave in the southwest United States. The Guardian titles its article "As Phoenix swelters, the nights are even worse than the boiling days," reporting: "Night-time temperatures have risen twice as fast as daytime highs over the past three decades, according to NWS data." It's not even summer yet, but they are not getting those diurnal swings.

Oliver Style has an energy consultancy in Barcelona, Spain, designing Passivhaus buildings for the Mediterranean climate. He presented some of the work of his firm, Praxis Resilient Buildings, on the first virtual day of the Passive House Network Annual Conference in a session titled "Keep Cool and Carry On."

There were lots of basic tips that reduce temperatures and could work passively, or without active systems like air conditioners, including:

• Pay close attention to local climate–something we don't see much of in North America where you can see the same house design in California or New England.
• Use cool colors on the exterior with high reflectance and high emittance.
• Ground coupling: using less insulation on the ground floor so that the cool earth absorbs heat.
• Lots of roof insulation and radiant barriers can be useful.
• Thermal Inertia and natural night ventilation—the traditional way of keeping cool.
• Ceiling fans, but remember they cool people, not the room.
• Watch internal gains, especially from domestic hot water systems! This is an important one, everything puts out heat, even the ceiling fan. As Allison Bailes of Energy Vanguard taught us, even ceiling fans heat the room.
• Careful opening design with shading systems. This is critically important and often ignored. We have covered this in Nice Shades posts, how you keep the sun out before it hits the glass.

But we are getting to the point that the passive techniques are not enough, and you need active cooling or air conditioning/heat pumps. And this is where Passivhaus design becomes even more crucial. In Texas, electric power use just broke its all-time record due to air conditioning demand in the heat wave. And because of the current energy crisis, electricity prices are up by 70%.

We wrote last year in the Texas freeze that every home should be a thermal battery to keep the heat in, and the same thing applies to keeping the heat out. Style told Treehugger that in a Passivhaus design, the thermal battery shaves the peaks of power demand and raises the troughs. It reduces the stress on the electrical grid.

Style also told Treehugger, "Thermal inertia is not enough anymore, and people are suffering from sleep deprivation, tiredness, stress, and are on edge." He sells the idea of Passivhaus on the basis of comfort: There are no hot spots on the walls, the windows don't let in much heat, and even if the windows are closed to keep out the hot air, there is still fresh air from the ventilation system which is separate from the cooling system.

Passivhaus design also reduces air infiltration—a significant source of heat gain as well as heat loss. It is not as big a deal in hot climates as it is in cold, because the differences in temperature are not as big. But on his blog, Style notes "reducing air infiltrations in coastal Mediterranean climates with high humidity can help reduce latent cooling loads when active cooling is on and windows are shut."

It's just too hot to rely on traditional techniques and live without air conditioning. The climate has changed and so have we. I have often quoted William Saletan, who wrote in Slate many years ago:

"Air conditioning takes indoor heat and pushes it outdoors. To do this, it uses energy, which increases the production of greenhouse gases, which warm the atmosphere. We're cooking our planet to refrigerate the diminishing part that's still habitable."

This is why we need Passivhaus as much in hot climates as in cold: to keep out the heat, reduce the need for air conditioning and the size of the units, and shave those peaks in demand. It may be called passive, but it is necessary to reduce the active.