Heat Transfer Fundamentals

Before the three modes, one law underlies all of them: heat always flows down a temperature difference, from warmer to cooler, and never the other way on its own. That is the second law of thermodynamics stated for buildings, and it is why orientation, insulation and shading matter at all. The bigger the temperature gap between inside and out, the harder heat pushes across the envelope - which is exactly why the same wall loses far more heat on a 4 degC Delhi winter night than on a mild 18 degC evening.

A simulation never asks 'how hot is it' in isolation; it asks 'what is the temperature difference across each surface, right now'. Multiply that difference by how easily heat crosses (the conductance) and by the area, and you have the instantaneous heat flow in watts. A concrete feel for the arithmetic: a 10 m2 window with a U-value of 3 W/m2K, on a night 15 degC colder inside than out, loses 10 x 3 x 15 = 450 watts - the output of several people, leaking away through one window. Do that for every wall, window, roof and floor, add the sun and the people, and you have the room's heat balance for one hour. EnergyPlus repeats that balance 8,760 times - once per hour of a typical year - which is all a whole-building energy model really is.

One further distinction underlies everything that follows. A steady-state view assumes the temperature difference holds constant and asks only for the resulting flow - fine for sizing a peak heat loss. A dynamic view lets the temperatures, the sun and the occupancy change hour by hour, which is what real days actually do and what simulation captures. Heat transfer is the physics; the weather file supplies the ever-changing temperature differences that drive it.

Conduction is heat travelling through a material without the material itself moving - vibrating molecules passing energy to their neighbours. It is how heat crosses a brick wall, a concrete slab or a pane of glass. Every material has a thermal conductivity (the symbol is k, units W/mK): a number for how readily it passes heat. Copper and aluminium are enormous conductors; that is why a metal window frame is a weak point. Dense concrete and brick are moderate conductors. Still air, and the trapped-air foams and fibres we call insulation, are very poor conductors - which is the whole point of insulation.

Conduction is the mode that U-values describe, and it is the star of the next lesson. For now hold the intuition: a thick, low-conductivity layer resists heat; a thin, high-conductivity one barely slows it. A 50mm slab of insulation (k around 0.035) can resist heat better than 600mm of dense concrete (k around 1.5). This is why what you build a wall from, and in what order, changes its performance far more than how thick it merely looks.

One subtlety worth holding early: conductivity is not always a fixed number. It rises when a material gets wet - water is a far better conductor than the trapped air it displaces - which is one reason damp insulation performs badly and moisture (lesson 3.4) is a thermal problem as well as a durability one. It also varies with temperature and, for gases and foams, with the size of the trapped pockets. Simulation engines carry these material properties in their construction libraries so you rarely set them by hand, but knowing that a soaked wall conducts more helps you read a strange result rather than trust it blindly.

Convection is heat carried by a moving fluid - in buildings, almost always air (and the water in pipes and coils). Warm air is less dense, so it rises; cool air sinks; the resulting circulation moves heat around a room and scrubs it off surfaces. There are two flavours. Natural convection is buoyancy-driven - the gentle plume rising off a warm window, the pool of cold air sliding off a cold pane onto your feet. Forced convection is driven by fans and wind - a ceiling fan, an air-conditioner's supply, a breeze through an open window.

Convection is why a room is not a single temperature and why air movement feels cooling even when the air is warm: moving air strips heat off your skin faster. In a model, convection at every surface is bundled into a surface heat transfer coefficient (roughly, how fast that surface swaps heat with the air touching it). Outdoors, wind makes this large; indoors, still air makes it small. This is also the physics behind the two 'surface resistances' (Rsi and Rso) you will add to a U-value calculation - they are the thin, still air films clinging to each face of a wall. It is why a ceiling fan lets you stay comfortable a few degrees warmer, saving cooling energy: it changes nothing about the air temperature, only the rate your skin sheds heat to it. And it is why an unventilated cavity insulates while a windswept surface barely does - the trick with air is always whether it is trapped and still, or moving and scrubbing heat away.

Radiation is heat carried by electromagnetic waves; it needs no material at all, which is how the sun's heat crosses 150 million km of vacuum to warm a wall. Two kinds matter in buildings. Shortwave (solar) radiation is the sun's direct and diffuse beam - the single biggest heat gain in most Indian buildings, pouring through glass and soaking into surfaces. Longwave radiation is the invisible infrared every warm surface emits: a hot roof radiating to the sky, warm walls exchanging heat with cooler ones, your own body losing heat to a cold window.

Radiation explains effects that pure air-temperature thinking misses. A building can lose heat to a clear night sky by longwave radiation and end up colder than the air around it - the basis of night-sky radiant cooling. Sitting beside a sun-baked wall feels hot even in a cool room because you are absorbing its longwave radiation. This is why comfort depends on mean radiant temperature, not just air temperature, and why simulation tools such as Ladybug compute the radiant environment separately. Get radiation wrong - especially solar - and a hot-climate energy model is simply wrong.

In a real building the three modes never act alone; they hand heat off to one another in a chain. Trace a summer afternoon: the sun radiates onto the roof (shortwave); the roof conducts that heat down through its layers; the warm ceiling convects heat to the room air and radiates it to the occupants; the air-conditioner's coil then convects it away again. Every step is one of the three modes, in series and in parallel.

The powerful move - and the mental model this whole course leans on - is to draw the building as a network of resistances and stores. Each path heat can take is a resistance (a poor path resists more); each heavy element that soaks up heat is a capacitance (a thermal store, the subject of lesson 3.3); the sun and the occupants are heat sources. Solve that network hour by hour against a weather file and you have predicted the building's temperatures and loads. That is precisely what EnergyPlus does under the hood - which is why understanding these three modes, and how they connect, is the foundation everything else in the course is built on.