Understanding roof heat.
Sir Isaac Newton created a formula to calculate the temperature of an object as it loses heat. The heat moves from the object to its surroundings. The rate of the temperature change is proportional to the temperature difference between the object and its surroundings. The formula can be used to find the temperature at a given time. The SI unit of temperature is the Kelvin (K), but degrees Celsius () is common.
T(t) = Ts + (T0 - Ts ) e(-kt)
T(t) = temperature of an object at a certain time (Kelvin, K)
t = time (s)
Ts = temperature of the surroundings (Kelvin, K)
T0 = starting temperature of the object (Kelvin, K)
k = a cooling constant, specific to the object (1/s)
(10 Celsius = 283.15 Kelvin)
So, if your roof sheet or tiled area (whole surface of roof) heats up throughout the day at up to 900W/sq.m (approx to 55C-65C), this in turn heats the roof air above surface and beneath internally(cooler). If this air is not evacuated at the constant cooling rate, then this (accumulative) heat then transfers to the internal air and also the structures. This is why you ‘feel’ the heat in the latter part of the day. For greater efficiency, the cooling air should be of greater difference to the object. This can only be done by using the coolest air possible (<22C) - from the shaded South side at a specific flow rate. The best way to do this is using negative pressure with a Solar Whiz roof extraction system. Same rules apply to commercial roofs and buildings also, but with the added bonus of greatly increased WHS due to dust, chemical and process gases being removed. With foodstuff storage warehouses, this system eliminates high product disposal/loss due to early expiration before ‘sell by dates’ due to excess heat and the baking or stale effect.
