Thermal Concepts

Question 1

What is radiation?

Answer 1

Transfer of heat from a warmer object to a cooler object through the air, via electromagnetic waves.

Question 2

What is an example of radiation heating?

Answer 2

When skin is warmed near a fire.

Question 3

What is an example of radiation cooling?

Answer 3

When skin is cooled when standing under the cool night sky.

Question 4

What is conduction?

Answer 4

Transfer of heat through a solid object.

Question 5

What is an example of conduction?

Answer 5

Burning your elbow on the stove.

Question 6

What is convection?

Answer 6

Transfer of heat from a warmer object to a cooler object, using a moving stream of air or water as a vessel.

Question 7

What range of the light spectrum does objects on earth radiate heat towards each other by?

Answer 7

Infrared, the invisible part of the light spectrum.

Question 8

When can two objects not exchange radiant energy?

Answer 8

As long as two objects can see each other though an invisible (or transparent to light) medium such as air or a vacuum, they can exchange radiant heat.

If this line of sight is blocked, the radiant flow of heat immediately stops.

Question 9

What is the relationship between object temperatures and radiant heat transfer?

Answer 9

As the gap between two object temperatures increases, the rate of heat transfer rapidly increases.

Question 10

What is emittance?

Answer 10

Emittance is a material’s ability to radiate heat towards another object. A material’s emittance value is equal to its absorbance value.

Question 11

What is reflectance?

Answer 11

Reflectance is the proportion of incoming radiant heat that bounces off the surface of the material, thereby not raising its temperature.

Question 12

What is absorbance?

Answer 12

Absorbance is the proportion of radiant heat that is absorbed by the material, thereby raising its temperature.

Question 13

What 2 types of thermal radiation do buildings have to deal with?

Answer 13

Solar radiation from the sun and terrestrial radiation from other objects. Building materials react quite differently to these different types of radiation.

Question 14

What happens when a metal foil is sandwiched between other layers of construction? Does it still reflect radiation?

Answer 14

It cannot reflect radiation; only conduct heat directly (as metal is an excellent conductor). Metal foils are only useful as insulators if they are installed with an adjacent airspace.

Question 15

If a material resists conduction, what is it useful for?

Answer 15

Insulation.

Question 16

What is the relationship between a segment of building’s loss / gain of heat and air temperature (inside and outside)?

Answer 16

The rate at which a part of a building gains and loses heat is directly proportionate to the difference of outside and inside temperatures of the building.

The rate at which a part of a building gains and loses heat is inversely proportionate to the overall thermal resistance to conduction of that section of the assembly.

Question 17

What is a general rule of thumb about building walls, roofs and floors with thermal resistances?

Answer 17

In order to ensure saved energy costs and thermal comfort; in general, walls, floors and roofs should be designed and built with maximum thermal resistance to conduction as possible.

Question 18

What is the rule of thumb for the rough thermal resistance (to conduction) of materials?

Answer 18

Thermal resistance is roughly inversely proportionate to the density of a material.

Metals – very low resistance
Masonry – moderately low resistance
Wood – high resistance

Question 19

What is the best resistor of heat flow?

Answer 19

Air, provided it can be kept from moving. Otherwise it acts as a convection current, which transfers heat flow from warm to cool surfaces quite efficiently.

Question 20

How does insulation inside walls and roofs act to resist heat flow?

Answer 20

The fine fibres within insulation material are poor thermal resistors themselves, but they act to resist the circulation of air in the wall, and hence keeping it still. Air, is the best natural insulator.

Question 21

Why do double or triple glazed windows exist? Why not regular windows?

Answer 21

Because window glass has terrible thermal resistance to conduction.

Question 22

What does double-glazing entail?

Answer 22

Trapping a thin layer of still air between two sheets of glass.

Question 23

Is the thermal resistance of double-glazing equal to a well-insulated wall?

Answer 23

It has significantly increased thermal resistance, though not resistance comparable to a well-insulated wall.

Question 24

What happens when the airspace of double-glazing is widened beyond an inch?

Answer 24

Within an inch, it has optimum efficiency. Beyond that, air is able to circulate with relative ease.

Question 25

What happens when the airspace in double-glazing is too thin?

Answer 25

The very thinnest of the airspace means whatever efficient thermal resistance it has is too little.

Question 26

How can you raise the thermal resistance of double-glazing?

Answer 26

By substituting air with a gas with lower thermal capacity or by introducing tangled glass fibres into the airspace, you can raise the resistance of double-glazing.

Question 27

Regardless of the airspace in double-glazed windows, what part of the window can increase heat transfer through the window?

Answer 27

The thermal resistance of any still air-space is undermined by the emittance of its enclosing surfaces, regardless of any convective circulation of air itself.

Question 28

How are the emittance of the sheets of glass in a double-glazed assembly countered?

Answer 28

Something called ‘low emissivity coating’, which is a thin metallic coating, is deposited onto the inward facing sides of the glass sheets in the assembly.

Low-e coatings improve the whole assembly resistance to heat transfer.

Double-glazing is then as effective as triple-glazing.

Question 29

Besides the internal mechanics of insulation, what are two other factors affect insulating effectiveness?

Answer 29

1. The position of the assembly.

2. The direction in which convective heat flows.

Question 30

What direction does convective heat flow in a window or wall assembly?

Answer 30

Heat flow moves in a horizontal direction and is encouraged by vertical convective currents.

Question 31

What direction does convective heat flow in a roof assembly in cold climates?

Answer 31

In cold climates, heat flows upward though the cold and top surface of the roof, where it disappears. Heat in the roof is lost.

Question 32

What direction does convective heat flow in a roof assembly in warm climates?

Answer 32

In warm climates, air heated by the hot top of the roof surface tends to remain stratified against the roof. Its movement down to the interior is ‘sluggish’.

Question 33

How would convective heat transfer affect a building with identical airspaces for roof, walls, and floor in winter?

Answer 33

Roof most rapidly, walls moderate rapidity, and floor least rapidly.

Question 34

What direction does radiant heat transfer move towards?

Answer 34

None, radiant heat transfer moves independent of direction.

Question 35

What proportion of heat flow does reflective foil cut down in walls, roofs and floors?

Answer 35

Reflective foil cuts down 1/2 the total heat flow through roof and walls and 1/3 though floors.

Question 36

What does reflective foil attached to roof rafters achieve in summer?

Answer 36

Assuming it has airspace adjacent, it significantly reduces the transmission of heat flow downwards towards the interior of the building. As a result, attics are cooler and air-conditioning costs are saved.

Question 37

What do thermal bridges cause?

Answer 37

Walls and roofs that do not have uniform thermal resistances across their surfaces.

Question 38

What causes thermal bridges?

Answer 38

Components such as framing members transfer heat more rapidly than the insulated parts of the wall and roof.

Question 39

In which materials are thermal bridges not particularly worrisome?

Answer 39

In wood, because wood in itself is a fairly good insulator.

Metal studs and masonry construction on the other hand are filled with thermal bridges, which can result in heat loss from an otherwise well insulated building.

Question 40

How can you detect thermal bridges?

Answer 40

Through ‘pattern staining’ whereby cooler areas of wall surfaces (thermal bridges such as metal studs) attract more dust due to the higher electrostatic charge they carry.

Question 41

What is cloud gel?

Answer 41

A fluffy, stable material. The lowest density solid substance created by man.

Question 42

Is cloud gel an effective material?

Answer 42

Yes, more than tangled glass fibre insulation.

Question 43

What is an example of cloud gel in use?

Answer 43

In double glazed windows, filling the ‘airspace’.

This provides a very high resistance to heat flows. The gel appears to be clear.

Question 44

What is thermal capacity?

Answer 44

A material’s ability to store heat.

Question 45

What is the relationship between mass and thermal capacity?

Answer 45

A large amount of dense material can store a lot of heat.

A small amount of fluffy material can only store a little heat.

Question 46

Describe the thermal capacity of water.

Answer 46

Assuming ‘normal’ air temperatures, water has a higher thermal capacity compared to any other material.

Question 47

How is thermal capacity measured?

Answer 47

The amount of heat required in order to raise the temperature of a unit volume or unit weight of a material by 1 degree.

Question 48

What happens when a high capacity thick material wall is subjected to a difference between indoor and outdoor temperature?

Answer 48

One side of the wall is slowly warmed.
Through each internal layer of the wall, heat is absorbed.
Heat does not transfer at a rapid rate through the wall until a stable condition is reached – the warm side of the wall reaches the temperature of the warm air and the cool side of the wall reaches the temperature of the cool air. Until then, heat transfers at a low rate.
Before too much heat can penetrate through to the inside, the sun sets.
When the exterior is cooled, the absorbed heat does a U-turn and heads back towards the exterior instead of heading inside.

Question 49

What happens to the temperature of a thick, high-capacity wall when here are temperature fluctuations?

Answer 49

The high-capacity wall resists and delays the fluctuations in temperatures in air on the other side of the wall.
The result is that the house is cooler than outside during the day and warmer at night, which is optimal.

Question 50

How to improve thermal mass of walls?

Answer 50

The thermal mass of walls can be improved by whitewashing building surfaces. The white wash reflects most solar infrared radiation and also efficiently emits the long wave radiation from the heated building’s surfaces back to the cool night sky.

Question 51

What combination of materials can help achieve desired thermal control in a building.

Answer 51

In some circumstances, a combination of high thermally resistant materials and high thermal mass materials can help to achieve desired thermal conditions in a building.

Question 52

How can heavy, warm-climate buildings be thermally insulated?

Answer 52

Heavy masonry surfaces in warm-climates can be thermally improved by providing an additional layer of insulation outside the enclosure. Note: high thermal resistance in front of high thermal capacity.

The insulation even further delays the fluctuation of outside the temperatures, so temperatures inside the enclosure remains very stable.

The insulation also protects the masonry from being damaged by excessive expansion and contraction from heat.

Question 53

Are high capacity, high resistance combination calls useful in temperate climates?

Answer 53

Yes. In a relatively mild summer, higher daytime temperatures are moderated. In relatively milder winters, solar heat gains though windows, heat loss through windows, internal heat gains from internal activities such as cooking etc.

Question 54

Are high capacity, high resistance combination walls useful in hot, damp climates where night temperatures remain high?

Answer 54

No, it is better to reflect solar heat and for the building to react as quickly as possible to cooling breezes and small downward turns in temperature.

Question 55

What climactic conditions is high thermal resistance appropriate? Does high thermal resistance benefit any one building element in particular?

Answer 55

High thermal resistance is appropriate in nearly all climactic conditions, particularly in roofs which are subject to high solar gains in summer and larger heat losses in winter.

Question 56

In what situation is high thermal capacity suitable?

Answer 56

High thermal capacity is suitable in situations where cyclical daily temperature fluctuations need to be smoothed out.

Question 57

Why do building surfaces that are in contact with soil below the reach of frost penetration have relatively stable temperate interiors?

Answer 57

Soil has a high thermal capacity.
It remains in a narrow temperature range throughout the year (dependent on the temperature range in that particular locale), hence well-constructed basements are inexpensive enough to cool in summer and warm in winter.

Question 58

What are three things required for an underground structures to take advantage of thermal stability?

Answer 58

1. Underground surfaces need insulating to the degree that the above ground surfaces would be.
2. Protection from frost penetration by burying horizontal sheets of foam plastic insulation horizontally just below the surface of the soil.
3. Preventing ground water from getting into the building.