HERS Building Science Fundamentals 2 — Questions and Answers
Question 1: In a mixed-humid climate, what is the primary moisture concern during summer months?
- Condensation on cold water pipes only
- Warm humid outdoor air infiltrating and condensing on air-conditioned surfaces (Correct answer)
- Ice damming on the roof
- Vapor diffusion from the soil through the slab
Correct answer: Warm humid outdoor air infiltrating and condensing on air-conditioned surfaces
In mixed-humid climates, warm humid outdoor air infiltrating a cooled building can condense on surfaces below the dew point, causing moisture damage.
Question 2: Which term describes the total heat flow through a building assembly per unit area per unit temperature difference?
- R-value
- U-factor (Correct answer)
- Thermal mass
- Emittance
Correct answer: U-factor
U-factor (thermal transmittance) measures heat flow rate through an assembly per square foot per degree Fahrenheit of temperature difference.
Question 3: What is 'stack effect' in a building context?
- Heat accumulation on roof surfaces due to solar gain
- Pressure-driven airflow caused by buoyancy differences between indoor and outdoor air (Correct answer)
- Thermal bridging through structural framing members
- Convective loops within wall cavities
Correct answer: Pressure-driven airflow caused by buoyancy differences between indoor and outdoor air
Stack effect occurs when indoor-outdoor temperature differences create buoyancy-driven pressure differences that drive air infiltration and exfiltration.
Question 4: A home in Climate Zone 5 has a cathedral ceiling. Which insulation approach best controls both thermal performance and moisture?
- Ventilated attic with batt insulation at the ceiling plane
- Unventilated assembly with sufficient vapor-impermeable insulation above the roof deck (Correct answer)
- Single layer of open-cell spray foam filling the full rafter depth
- Fiberglass batts between rafters with a polyethylene vapor barrier on the cold side
Correct answer: Unventilated assembly with sufficient vapor-impermeable insulation above the roof deck
An unventilated cathedral ceiling in CZ5 requires enough rigid foam above the deck to keep the sheathing above the dew point, preventing condensation.
Question 5: What does the term 'thermal bridging' refer to in wall assemblies?
- Heat transfer across an air gap via radiation
- Conductive heat loss through high-conductivity elements that bypass insulation (Correct answer)
- The movement of heat through convective air loops in wall cavities
- Solar energy absorbed by dark exterior cladding
Correct answer: Conductive heat loss through high-conductivity elements that bypass insulation
Thermal bridging occurs when conductive materials like steel or wood studs create a path for heat to bypass the insulation in a wall cavity.
Question 6: According to building science principles, where should the air barrier be located relative to the insulation layer?
- Always on the exterior surface of the insulation
- Always on the interior surface of the insulation
- At or near the insulation layer but its exact position depends on climate (Correct answer)
- In the middle of the insulation layer for maximum effectiveness
Correct answer: At or near the insulation layer but its exact position depends on climate
The optimal air barrier location depends on climate — it should be positioned to prevent moisture-laden air from reaching surfaces where condensation could occur.
Question 7: Which mechanism is responsible for the majority of heat transfer through a well-sealed, well-insulated attic floor on a cold winter night?
- Convection through the insulation fibers
- Radiation from the ceiling surface to the cold attic floor
- Conduction through the insulation material (Correct answer)
- Air leakage past the insulation
Correct answer: Conduction through the insulation material
Conduction through the insulation material is the dominant heat transfer mechanism in a properly sealed and installed fibrous insulation layer.
HERS Building Science Fundamentals 2
In a mixed-humid climate, what is the primary moisture concern during summer months?