or Pa; A unit of measurement of air pressure.


A hole passing through the building envelope in which ducts, pipes, wires, structural elements, and windows are run between inside and outside. Windows are also a penetration.


Unit of measurement typically Perm; A measurement of how much water vapor a material will let pass through it per unit of time OR Semipermeable: speed at which water molecules pass through a material by diffusion (see diffusion).


Relatively large chamber in an air duct or pathway experiencing low air velocity.


(units of energy): Electrical power is usually measured in watt (W), kilowatt (kW), megawatt (MW), etc. Power is energy transfer per unit of time.


A force encouraging flow by virtue of a difference in some condition between two areas.

Pressure drop

The decrease in pressure due to friction of a fluid or vapor as it passes through a tube or duct or/and lift.

Pressure-equalized Rainscreen

A specific type of drained wall system that uses spatial compartmentalization, careful venting to air leakage ratios, and stiff components to encourage short-term equalization of drained cavity (also the air chamber) pressures with exterior wind pressures to reduce the pressure across the screen (or cladding). Such systems rely on effective drained to control rain, and if functional, merely reduce the rainwater that the drained system must accommodate.


The science of the relationship between air and its energy and, water vapor contents.

PV or PhotoVoltaic

Capable of producing a voltage when exposed to radiant energy, especially light.