Chapter 6 (20-38)

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

A buildings components are more vapor permeable because vapor pressure is higher than air pressure making it easier for vapor to move through a component than air. If vapor was to go through an envelope but was not able to leave at the same rate the water would eventually build up causing mold from the built up pressure.

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

A vapor barrier is not a perfect retarder and sealing leaks may be imperfect allowing the passing of vapor into the assembly. This migrated vapor needs to be able to escape the assembly and is why AWB is required to be vapor permeable.

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

Air moves through components by leakage and diffusion, the same as water vapor but the pressure that each exert on an envelope are different. Vapor pressure from outside to inside is higher than that of air pressure.

33 What are the advantages of attic ventilation in warm climates?

Attic ventilation in warmer climates are necessary for heat transmission from the attic to the interior spaces below. With little to no ventilation the temperature in an attic can become higher than the outside temperature.

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

Class I: Perm rating less than or equal to 0.1 - most impermeable Class II: Perm rating between 0.1 and 1.0 Class III: Perm rating between 1.0 and 10.0 - least impermeable

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

Concealed condensation happens when the condensation of vapor collects inside or between an envelopes assembly.Surface condensation exists on window surfaces when the interior air is more humid and warm while the outside air is more dry and cold.

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

Fiberglass batts faced with building paper is a Class II vapor retarder for the perm rating of 1.0

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

Gypsum boards with latex or enamel paint are a Class III

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

In colder climates where water vapor is greater inside compared to outside. Building codes require a vapor retarder be installed toward the interior side of the building envelope.This strategy is used to allow the higher outside water vapor to permeate through the assembly where the HVAC system which is removed form the inside.

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

Polyethylene sheets 4-8 mil thick are a Class 1 vapor retarder

24 What measure should be taken to prevent surface condensation?

Surface condensation can be prevented by increasing the R-value of the assembly which will increase the temperature of the interior surface versus the cooler exterior temperatures.

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

The same material with a difference in thickness means the material is more impermeable. The smaller the perm rating the more impermeable the material is to vapor.