Meeb Chapter 4 - Heat Flow

Front 1

Connectors, Filters, Barriers:
What is the "Connector-dominated" approach?

Back 1

When exterior environment resembles the interior, the building's skin is only changed slightly to control a few outdoor forces.

Front 2

Connectors, Filters, Barriers:
Describe the "Open Frame" design approach

Back 2

Fig. 4.3, pp. 118
A barrier-dominated (separating element) building envelope design approach.

Used in hot humid and temperate climates.

A stick frame.

Front 3

Connectors, Filters, Barriers:
Describe the "Open Frame" envelope in a hot humid and temperate climate

Back 3

Hot humid: the envelope employes a barrier roof of plants, a raides floor to avoid damp soil but permits breezes to flow over and under structure.

Temperate: the envelope is wrapped in light-filtering animal skin.

Front 4

Connectors, Filters, Barriers:
Describe the "Closed Shell" design approach.

Back 4

A barrier-dominated (separating element) building envelope design approach.

Used in arid and cold climates.

Front 5

Connectors, Filters, Barriers:
Describe "Closed Shell" envelope in arid and cold climates.

Back 5

Arid: Mud walls serve as the barrier to block wind and sun while filtering heat by delaying and reducing its impact on the interior.

Cold: The igloo's ice filters light and heat and blocks wind. Holes are added for entry and smoke ventilation.

Front 6

Connectors, Filters, Barriers:
Define "Thermal Sailing"

Back 6

A passive heating or cooling technique whereby the user manipulates a thermal switch (opening a coat in early morning cold) to prepare for environmental changes that will effect their body later.

Front 7

Describe Thermal Switches.

Back 7

1. The designer's way of giving occupants control of their own enviroment.

2. Used for controlling daylighting. (adjusting awnings for glare control during spring time sun - Fig. 4.6, pp. 122)

3. Encourages interaction between user and environment.

4. Controls passive heating and cooling (thermal sailing).

Front 8

Heat Flow and Seasons:
How does a building experience heat gain/loss?

Back 8

1. Convection - cooler air absorbs hear from warmer surfaces, expand in volume, rise, and carry it away.

2. Conduction - heat transferred directly from the molecules of warmer building surfaces to molecules of cooler solids, like contact with the ground.

3. Radiation - heat flow in electromagnetic waves from hotter surfaces to detached colder one through any transport medium - even empty space.

Front 9

Heat Flow and Seasons:
How to heat flow trough a wall cavity?

Back 9

Heat flows from the hotter object to the cooler object.

From conduction, convection/raidiation, back to conduction.

Heat is lost through the assembly of building materials.

Front 10

Heat Flow and Seasons:
What the best way to control radiative heat flow in hot spaces?

Back 10

Fig. 4.7, pp. 123.

By adding an air space and reflective surfaces in the wall cavity when temperature differences are large.

Front 11

Heat Flow-Solids:
What is the Rate of Conductivity (k)?

Back 11

The number of British thermal units per hour (Btu/h) that flow through 1 sq. ft. of material that is 1 " thick, when the temp drop through that material 1 degree F.

Front 12

Heat Flow - Solids:
What is Resistance (R)?

Back 12

Fig. 4.8, pp. 126

How effective a solid is as an insulator.

It is measured in hours needed for 1 Btu to pass through 1 sq. ft. of a given thickness of a solid when the temp drops 1 degree F.

Greater R value = more effective insulator.

It's the recipricol of Conductivity (k); R= 1/k

Sometimes listed as "per inch" of thickness.

Front 13

Heat Flow - Solids:
Define Insulators?

Back 13

They retard heat flow.

Three categories:
1. Inorganic fibrous (glass, rock wool, vermiculite)
2. Organic fibrous (cotton, cork, polystyrene, foamed rubber)3. Metallic Organic Reflective Membrane (must face air space to be effective). When used without being attached to batt insulation, it's called a "radiant barrier" and is used in warmer climate roofs.

Front 14

Heat Flow - Air:
What happens to heat transfer along the exposed faces of solids?

Back 14

Heat transfer occurs by convection and radiation.

Convection is reliant upon air movement.

Since warm air rises and cool air falls, vertical surfaces exchange heat faster than horizontal surfaces unless heat flow direction is upward. See Fig 4.7.

Front 15

Heat Flow - Air:
What is the significance of a pocket of still air near a surface?

Back 15

An insulative layer of air is created when air motion along a surface is minimal. When this air is disturbed, its "Resistance" decreases.

Surface layers of air are rated by their "Conductance" because they help transfer heat between a solid and the air.

Front 16

Heat Flow - Air:
How is radiation influenced by surface characteristics and what is Emittance?

Back 16

Shiny materials radiate less than common materials.

This is called a materials "Emittance": the proportion between radiation emitted by one material compaired to that of a blackbody having the same temperature.

Low emittance = low radiative heat exchange.

Front 17

Heat Flow - Air:
How is Emittance related to Absorptance?

Back 17

A highly absorptive (low reflectance) material will have high emittance (high radiative heat exchange).

Front 18

Heat Flow - Building Envelope:
What factors are needed to calculate heat flow through a wall?

Back 18

1. Rate of heat flow of each material which comprises building envelope assembly.

2. Area of each assembly.

3. Temperature difference between inside and outside.

Front 19

Heat Flow - Building Envelope:
Define U-Value.

Back 19

Measures how well a product prevents heat from
escaping a building.

U-Factor ratings fall between 0.20 and 1.20. The lower the U-Factor, the better a product is at keeping heat in. U-Factor is important during the winter heating season.

U=1/ΣR (Btu/h x ft² x ºF)

Front 20

Heat Flow - Air:
Still air near the surface or inside the wall cavity have these four characteristics:

Back 20

1. Resistance
2. Conductance
3. Emittance
4. Insulation

Front 21

Heat Flow - Building Envelope:
What is Sol-air temperature?

Back 21

Temperature of outdoor air just above the roof surface that is produced by solar radiation.

Front 22

Heat Flow - Building Envelope:
Define Thermal Gradient.

Back 22

The change of temperature through the building.

It can be determined by the ratio of temperature change and thermal resistance at any point. (Fig 4.12, pp. 137).

Front 23

Heat Flow - Windows:
Describe Solar Heat Gain Coefficient (SHGC).

Back 23

Measures how well a
window blocks heat from the sun.

It is expressed as a
number between 0 and 1.

The lower the SHGC, the better a window is at blocking unwanted heat gain. Blocking solar heat gain is important during the summer cooling season.

Based on performance of entire window assembly: depends on type of glass, tinting, relective coating, blinds, frame.

Front 24

Heat Flow - Windows:
How is heat flow through a window calculated?

Back 24

Btu/h = SHGC x Area (ft²) x SHGF (Btu/h ft²)

Front 25

Heat Flow - Windows:
What is Solar Heat Gain Factor?

Back 25

SHGF is a predefined factor based upon latitude, month, exposure.

Front 26

Heat Flow - Windows:
What is Visible Transmittance (VT)

Back 26

VT measures how much light
comes through a window.

VT is expressed as a number
between 0 and 1. The higher the VT, the higher the
potential for daylighting.

Ligher colored glazing will yeild the heighest VT.

VT is influenced by glazing coatings and number of glazings.

Front 27

Heat Flow - Windows:
What is Light to Gain Ratio (LSG)?

Back 27

LSG is the relationship between SHGC and VT.

Determined by dividing VT by SHGC.

You can reduce SHGC without necessarily reducing VT by using varied types of glazing and coatings.

Front 28

Heat Flow - Windows:
Define Air Leakage (AL).

Back 28

Measures how much outside air comes into a building through a window.

AL rates fall in a range between 0.1 and 0.3. The lower the AL, the better a product is at keeping air out.

Front 29

Heat Flow - Windows:
Define Condensation Resistance (CR)

Back 29

Measures how well a
product resists the formation of condensation.

CR is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation.

Front 30

Heat Flow - Windows:
Describe Low-emittance coatings (low-e).

Back 30

A coating that is applied to the air gap side of one sheet of glazing.

It blocks radiant heat from transferring between glazings.

One application is as effective as adding an extra layer of glazing.

There are two categories:
1. Hard coat: durable, inexpensive, not as thermally effective.

2. Soft coat: better thermally, expensive, and will degrade by from the oxidation that occured during manufacturing.

Low-e reduces ultraviolet transmission, thus keeps furniture from fading.

Front 31

Heat Flow - Windows:
Name the three types of Low-e coatings.

Back 31

1. High-transmission: used for passive solar heating applications where a low U-factor is combined with high SHGC.

2. Selective-transmission: used when winter heating and summer cooling are needed along with low U-factor and low SHGC. Here, the coating is on the outer glazing, so it can trap incoming infrared radiation which is convected away by outside air.

3. Low-transmission: used in combiniation with low U-factor, low SHGC, and low VT where sun is intense. Coating goes on the outside of glazing.

Front 32

Heat Flow - Windows:
Describe the use of Argon or Kypton gas in windows.

Back 32

A less-conductive gas that is used to reduce heat transfer by convective currents within the air gap between glazings. This produces less U-factor resulting in the inner glass surface staying at inside room temperature.

Gas reduces the chance of condensation on inside surface.

Requires a good seal to preserve life of the gas.

Front 33

Heat Flow - Windows:
Describe Superwidows.

Back 33

A window that has all the latest energy conservation products.

Expensive and can have a better heating season thermal performance than that of an insulated opaque window.

Front 34

Moisture/Infiltration:
How do you control moisture in cold climate?

Back 34

1. Install vapor barriers (plastic film with few holes) close to the warm side of the of the building, typically just behind interior finishes (gyp bd., wood floors).

2. Vapor barrier can be installed within the insulation at about 1/3 the distance between inside and outside. This allows the inner third to be used for service runs w/o puncturing the barrier and enough space beyond this point to prevent condensation.

3. Vapor barriers help block airflow through the building.

Front 35

Heat Flow - Windows:
What is Spectrally Selective Glass?

Back 35

A type of glass that can be either tinted or coated, having special properties that actually block or re-radiate the
energy from the sun, reducing solar heat gain through the windows. This type of product is also available for use in residential windows, typically with a spectrally selective low-e coating on the surface of the glass.

Front 36

Moisture/Infiltration:
Why is it good to use a Drainage Plane?

Back 36

1. Used primarily in hot humid climates to control moisture, a drainage plane (building felt) is installed just behind exterior surface materials. Moisture that gets behind it should be wicked to the interior, thus can't use vapor barriers on interior surface.

Front 37

Moisture/Infiltration:
What is Infiltration?

Back 37

The accidental influx of outside air due to leaks in building skin.

Front 38

Moisture/Infiltration:
What is Ventilation?

Back 38

The intentional introduction of outside air.

It's good to allow a small amount of fresh air into a building for comfort and health.