HERS HERS Thermal Envelope Insulation and Air Sealing 2 — Questions and Answers
Question 1: During a HERS pre-drywall inspection, a rater identifies missing insulation in a rim joist cavity. Why is this location particularly important?
- Rim joists are required to be spray-foamed by code
- Rim joists are a significant thermal bridge and air leakage point at the foundation-to-wall junction (Correct answer)
- RESNET requires rim joist R-value to match wall R-value exactly
- Rim joists directly affect duct leakage test results
Correct answer: Rim joists are a significant thermal bridge and air leakage point at the foundation-to-wall junction
Rim joists are a major source of both heat loss and air infiltration at the critical junction between the foundation and above-grade framing.
Question 2: What does the term 'thermal bypass' refer to in a HERS field inspection context?
- A mechanical damper that bypasses the air handler
- Air movement around or through insulation that reduces its effective R-value (Correct answer)
- A duct section that bypasses the heat exchanger
- An alternate refrigerant circuit in a heat pump
Correct answer: Air movement around or through insulation that reduces its effective R-value
A thermal bypass is any air movement pathway that allows air to flow around insulation, significantly reducing its effective thermal resistance.
Question 3: Which insulation material has the highest R-value per inch, making it useful where space is limited in a HERS-rated home?
- Cellulose (dense-pack)
- Fiberglass batts
- Closed-cell spray polyurethane foam (Correct answer)
- Mineral wool batts
Correct answer: Closed-cell spray polyurethane foam
Closed-cell spray polyurethane foam achieves approximately R-6 to R-7 per inch, the highest of common residential insulation materials.
Question 4: A HERS rater is evaluating a home with a vented crawl space. Under current best practice, what is the preferred energy-efficient crawl space configuration?
- Vented with fiberglass batts between floor joists
- Sealed and conditioned with insulation on the crawl space walls (Correct answer)
- Vented with rigid foam on the ground
- Open with no insulation to prevent moisture issues
Correct answer: Sealed and conditioned with insulation on the crawl space walls
A sealed and conditioned crawl space with insulation on the perimeter walls is the preferred configuration for energy efficiency and moisture control.
Question 5: During HERS verification, a rater checks that window U-factors meet IECC requirements. What does a lower U-factor indicate?
- Greater solar heat gain
- Lower thermal resistance (worse insulation)
- Higher thermal resistance (better insulation) (Correct answer)
- Higher visible light transmittance
Correct answer: Higher thermal resistance (better insulation)
U-factor is the rate of heat transfer; a lower value means less heat flows through the window, indicating better insulating performance.
Question 6: What is the primary purpose of a vapor retarder in a wall assembly from a HERS building science perspective?
- To increase the wall's R-value
- To control moisture diffusion and prevent condensation within the assembly (Correct answer)
- To serve as the primary air barrier
- To reflect radiant heat away from the framing
Correct answer: To control moisture diffusion and prevent condensation within the assembly
A vapor retarder controls the diffusion of water vapor through the wall assembly to prevent condensation and moisture damage within the building envelope.
HERS HERS Thermal Envelope Insulation and Air Sealing 2
During a HERS pre-drywall inspection, a rater identifies missing insulation in a rim joist cavity.
Why is this location particularly important?