PÉCLETBuilding Physics
Buildings asClimate systems.
A building is not a static object. It is a system exchanging heat, light, air and moisture with its climate, every hour of its life.
An interactive cross-section through a wall assembly. Reading from the outside in: a rainscreen, a ventilated cavity, insulation, structure and an internal finish. Particles carrying heat cross the section from the hot exterior toward the cooler interior. They move quickly through the air layers and the cavity, and slow markedly inside the insulation, where most of the temperature drop occurs. Moving the pointer adds a local heat source and changes the exterior condition.
Two ways to move
Heat, air and moisture reach a building in two ways. They are carried — pushed along by a flow. Or they spread — working their way through a material without going anywhere in particular.
The Péclet number is the ratio between the two. Below one, spreading wins and the material decides the outcome. Above one, the flow does. Almost every question in building physics is really a question about which side of that line you are standing on.
A field of particles whose motion can be shifted between two transport modes. At a low Péclet number the particles wander individually as a spreading cloud — diffusion. At a high Péclet number they resolve into ordered horizontal lanes streaming in one direction — advection. The same particles are shown throughout; only the balance between random and directed motion changes.
Six ways a building meets its climate
An atlas of problems
Five climates,
one country
A wall build-up that is correct in Jaisalmer can be a defect in Kochi. The atlas sets out what governs each Indian climate zone, what resource each one offers, and the order in which decisions should be taken.
Open the climate atlasDirect solar gain
Moisture
Seasonal reversal
Nothing, severely
Fabric heat loss