Deck 6: Properties of the Envelope-II Air and Water Vapor Control

Question 1

In addition to being a barrier to the passage of air, an air barrier must also function as a barrier to the passage of

A)subsoil water.
B)both noise and water vapor.
C)water vapor.
D)noise.
E)rainwater.

Answer 1

E

Question 2

Air leakage through a building envelope is a function of

A)the area of holes, cracks, and openings in the envelope.
B)wind speed.
C)materials of which the component is made.
D)all of the above.
E)A, B, and the inside- outside air temperature difference.

Answer 2

E

Question 3

The use of membrane or liquid air barriers is required in the envelope of all heated or cooled buildings.

Answer 3

True

Question 4

A wrap- type, membrane air barrier is available in many varieties, but in all of them, it is made from

A)a perforated plastic sheet.
B)an asphalt- coated paper.
C)an unperforated plastic sheet.
D)any one of the above.
E)A or C.

Question 5

During the heating season, the vapor pressure of the inside air in a typical dwelling in the northern United States is

A)greater than the outside air.
B)the same as the outside air.
C)lower than the outside air.

Question 6

Concealed condensation occurs inside an envelope assembly during a heating season if

A)the R- value of the assembly is high.
B)the R- value of the assembly is low.
C)the interior vapor can permeate into the assembly but cannot exit it.
D)A and B.
E)A and C.

Question 7

Although leakage and diffusion are two separate mechanisms, their impact on the performance of the envelope is the same.

Question 8

Which of the following materials has the lowest perm rating?

A)1/16- in- thick aluminum sheet
B)4- in.- thick concrete wall
C)4- in.- thick brick wall
D)1/16- in.- thick plastic sheet
E)All of the above materials have the same perm rating.

Question 9

Breathability of an air barrier refers to its

A)permeability to water.
B)permeability to air, water vapor, and water.
C)permeability to air and water vapor.
D)permeability to air.
E)none of the above.

Question 10

Perm rating is a property that measures

A)how effectively water vapor leaks through the material.
B)how effectively the air diffuses through the material.
C)how effectively water vapor diffuses through the material.
D)A and B.
E)A and C.

Question 11

During the heating season, the vapor flow in conditioned buildings is generally from

A)the outside to the inside of the envelope.
B)the inside to the outside of the envelope.

Question 12

A membrane air barrier is

A)factory- adhered to sheathing panels.
B)adhered to sheathing on site.
C)fastened to sheathing on site.
D)any one of the above.

Question 13

An air barrier is generally used on

A)the ceiling.
B)the walls.
C)the roof.
D)all of the above.

Question 14

The primary purpose of sealants and weatherstripping is to

A)reduce vapor diffusion through the envelope.
B)reduce air leakage through the envelope.
C)reduce air diffusion through the envelope.
D)all of the above.
E)A and B.

Question 15

Liquid- applied air barriers and membrane air barriers can be used interchangeably.

Question 16

An air barrier is generally placed

A)toward the cold side of the envelope assembly.
B)toward the warm side of the envelope assembly.
C)on the exterior face of interior gypsum board.
D)on the exterior face of an exterior sheathing.
E)B or C.

Question 17

The dew point of air is always

A)unrelated to the air temperature.
B)greater than or equal to the air temperature.
C)less than or equal to the air temperature.

Question 18

Building assemblies are generally more vapor permeable than air permeable because

A)vapor is not really a gas.
B)vapor is only a small part of the air- vapor mixture.
C)vapor has only water molecules, while air is a mixture of nitrogen and several other gases.
D)vapor pressure differential between the inside and the outside air is much greater than the corresponding air pressure differential.
E)vapor molecules are heavier than air molecules.

Question 19

Leakage and diffusion of a gas (such as air or water vapor)through a building envelope are two different mechanisms because

A)leakage occurs through holes, cracks, and openings in the envelope, while diffusion occurs through the body of envelope component or its material(s).
B)leakage refers to water leaking through the envelope (such as rainwater or subsoil water)and diffusion refers to the passage of water vapor through the envelope.
C)diffusion does not occur through a building envelope while leakage does.
D)leakage occurs through the body of the component or material(s)of the envelope, while diffusion occurs through holes, cracks, and openings in the envelope.
E)none of the above.

Question 20

In hot, humid climates, the vapor flow in conditioned buildings is generally from

A)the inside to the outside of the envelope.
B)the outside to the inside of the envelope.

Question 21

Explain how air leakage through a building envelope can be reduced.

Question 22

Explain why it is necessary to reduce air leakage and air diffusion through a building envelope.

Question 23

Explain the difference between leakage and diffusion through building envelope assemblies.

Question 24

Does the sealing of cracks and joints in building envelope reduce diffusion?

Question 25

A material whose perm rating in the U.S. system of units is 0.5 perm is a

A)Class I vapor retarder.
B)Class III vapor retarder.
C)Class IV vapor retarder.
D)Class II vapor retarder.
E)none of the above.

Question 26

When gable vents are provided to ventilate an attic space,

A)turbine vents are also necessary.
B)ridge vents are also necessary.
C)soffit vents are also necessary.
D)no other vents are necessary.

Question 27

Soffit vents alone are not adequate to provide attic ventilation.

Question 28

Explain how wind affects air leakage in a building.

Question 29

What is the difference between air leakage and ventilation of buildings?

Question 30

Explain how inside- outside air temperature difference affects air leakage in a building.

Question 31

An ice dam is likely to occur

A)at the eave of a sloping roof.
B)at the edge of a flat roof.
C)in the middle of a flat roof.
D)in the middle of a sloping roof.

Question 32

Ventilation of an attic space is generally considered important in

A)moderately cold climates only.
B)hot climates only.
C)all climates.
D)cold climates only.
E)none of the above.

Question 33

A vapor retarder is perforated so that

A)water vapor may pass through the pores.
B)water may pass through the pores.
C)air may pass through the pores.
D)both air and water vapor may pass through the pores.
E)none of the above.

Question 34

The greater is the thickness of a given material, the greater is its perm rating.

Question 35

In warm climates, such as Climate zones 1 to 3, the vapor retarder may be omitted in wood or cold- formed steel (CFS)framed assemblies if the interior wall finish is a latex painted gypsum board.

Question 36

Is the permeability of a material for air the same as its permeability for water vapor? Name some building materials that are impermeable to both air and water vapor.

Question 37

Per the International Building Code, Class I vapor retarder should only be used in wood or cold- formed steel (CFS)frame assemblies in

A)Climate zones 3, 4, 5, and 6, and 7.
B)Climate zones 3, 4, 5, 6, and marine 4.
C)Climate zone 5, 6, 7, 8, and marine 4.
D)Climate zones 4, 5, 6, 7, and marine 4.
E)Climate zones 1, 2, 3, 4, and 5.

Question 38

Does diffusion, as discussed in this chapter, apply to the diffusion of water or air through the building envelope?

Question 39

List three factors that affect air leakage through building envelope.

Question 40

In referring to the permeability of a material to air (or water vapor), do we refer to leakage, diffusion, or both?

Question 41

Which material is commonly used as Class II vapor retarder (VR)in stud walls?

Question 42

What are building code recommendations for the provision of vapor retarders (VR)for various climate zones?

Question 43

What measure should be taken to prevent concealed condensation?

Question 44

Why is an air- weather barrier (AWB)not required on sloping roofs with an attic space?

Question 45

Why are building components generally more vapor permeable than air permeable?

Question 46

In which climates zones do the building codes mandate the use of vapor retarder (VR), and in which climate zones do they not?

Question 47

What measure should be taken to prevent surface condensation?

Question 48

Explain how air diffusion through a building envelope is reduced.

Question 49

Name three classifications of vapor retarders and their vapor permeability values (perm ratings).

Question 50

Which material is commonly used as Class III vapor retarder (VR)in stud walls?

Question 51

Where in the envelope is an air barrier installed?

Question 52

How does the thickness of a material affect its vapor permeability (perm rating)?

Question 53

What is concealed condensation and how does it differ from surface condensation?

Question 54

Why is an air- weather barrier (AWB)not required on flat roofs?

Question 55

What is the function of an air barrier and what is it made of?

Question 56

Explain why an air barrier is called an air- weather barrier (AWB).

Question 57

To be recognized as an air barrier, the building code requires its maximum air permeability to not exceed that of a commonly used building material. What is that material and its thickness?

Question 58

Which material is commonly used as Class I vapor retarder (VR)in stud walls?

Question 59

In a building enclosure, such as a room, the air and water vapor are thoroughly mixed together. In terms of their diffusion (transmission)through the enclosure surfaces, do they behave as one gas? Explain.

Question 60

Explain why the permeability values of air and water vapor through a component are not equal to each other.

Question 61

Explain why it is much easier for vapor to flow through building assemblies than air.

Question 62

Using a sketch, explain why an air barrier is called an air- weather barrier.

Question 63

Explain why the micro- perforations in a membrane- type air- weather barrier allow water vapor to pass through but neither (liquid)water nor air.

Question 64

What are the advantages of attic ventilation in warm climates?

Question 65

The vapor pressure (VP)of air at 60°F and 100% RH = 37 psf. Will the VP of air at 70°F and 100% RH be the same, higher or lower than 37 psf?

Question 66

Explain the difference between various classes of vapor retarders. In which climates does the building code require the use of a vapor retarder, and its class? Would you specify a separate vapor retarder material in warm climates (Climate zones 1 to 3)? Give reason(s)for your answer.

Question 67

If the air is saturated, what is its relative humidity (RH)?

Question 68

The vapor pressure (VP)of air at 60°F and 100% RH = 37 psf. What is its VP if RH = 40%?

Question 69

Describe the types of air- weather barriers currently used and their suitability for different situations. Explain why air- weather barrier is used only in exterior walls, not the roof.

Question 70

What are the advantages of attic ventilation in cold climates?

Question 71

An air- weather barrier (AWB)is required to be vapor permeable. Why?

Question 72

Explain why an attic space must be ventilated.

Question 73

Explain the difference between surface condensation and concealed condensation. Explain in which situations these two types of condensation would occur and why.

Question 74

Using a sketch, explain the difference between leakage and diffusion. To which material (solid, liquid, or gas)does the concept of diffusion apply?

Question 75

List various methods of providing attic ventilation.