HERS Energy Modeling and Analysis 3 — Questions and Answers
Question 1: What is the RESNET-defined default internal gains assumption used in HERS energy modeling for occupancy?
- Occupancy is calculated as 1 person per 300 square feet of conditioned floor area
- Standard occupancy is set at 0.5 + (CFA/100)^0.43 persons as defined in ANSI/RESNET/ICC 301 (Correct answer)
- Internal gains are fixed at 2,400 BTU/hr regardless of home size
- Occupancy defaults to 4 persons for all homes under 3,000 square feet
Correct answer: Standard occupancy is set at 0.5 + (CFA/100)^0.43 persons as defined in ANSI/RESNET/ICC 301
ANSI/RESNET/ICC 301 specifies occupancy as 0.5 + (CFA/100)^0.43, ensuring larger homes have proportionally more occupants in the reference calculation.
Question 2: Which climate zone designation system is used by RESNET's ANSI/RESNET/ICC 301 standard for HERS energy modeling inputs?
- ASHRAE 169 climate zones
- IECC climate zones (1-8 with moisture designations A, B, C) (Correct answer)
- EPA Energy Star climate zones (North/South/Marine)
- DOE Building America climate zones
Correct answer: IECC climate zones (1-8 with moisture designations A, B, C)
HERS ratings use IECC climate zones 1–8 with moisture designations (A=moist, B=dry, C=marine) as defined in the IECC and referenced by ANSI/RESNET/ICC 301.
Question 3: A home has a cathedral ceiling with R-30 insulation between rafters. How should this be entered differently from a standard attic assembly in most energy modeling software?
- Cathedral ceilings are modeled identically to attic ceilings and require no special entry
- Cathedral ceilings are entered as a roof/ceiling assembly without an attic air space, using the assembly R-value directly (Correct answer)
- Cathedral ceilings must use the total R-value plus an automatic 10% thermal bridging penalty
- Cathedral ceiling insulation is always entered as R-0 because rafters create full thermal bridging
Correct answer: Cathedral ceilings are entered as a roof/ceiling assembly without an attic air space, using the assembly R-value directly
Cathedral ceilings lack an attic buffer zone, so they are entered as a direct roof/ceiling assembly with the actual installed R-value and no attic air space node.
Question 4: In HERS modeling, how is a home with a walkout basement partially below grade typically handled for foundation inputs?
- The entire basement is modeled as a slab-on-grade foundation
- Above-grade portions are modeled as walls and below-grade portions as basement walls with appropriate soil contact fractions (Correct answer)
- Walkout basements default to the same U-factor as conditioned crawlspaces
- The basement is always excluded from conditioned floor area calculations
Correct answer: Above-grade portions are modeled as walls and below-grade portions as basement walls with appropriate soil contact fractions
Software requires separate modeling of above-grade and below-grade wall fractions because heat loss rates differ significantly based on soil contact depth.
Question 5: What effect does increasing the number of bedrooms have on the Reference Home's water heating energy in HERS calculations?
- More bedrooms increase the assumed number of occupants, which increases hot water consumption in the Reference Home (Correct answer)
- Bedrooms have no effect on water heating; only bathroom count matters
- More bedrooms decrease water heating energy because of assumed efficiency improvements
- Water heating energy is fixed at a national average regardless of bedroom count
Correct answer: More bedrooms increase the assumed number of occupants, which increases hot water consumption in the Reference Home
ANSI/RESNET/ICC 301 ties hot water usage to bedroom count as a proxy for occupancy, so the Reference Home's water heating load increases with bedroom count.
Question 6: A rater is modeling a home with a ground-source heat pump. Which COP value should be used in the energy model?
- The manufacturer's rated COP at standard ARI test conditions
- The EER rating converted to COP using the formula EER/3.412
- The site-specific COP determined by ground loop entering water temperature and manufacturer performance data (Correct answer)
- A default COP of 3.0 as specified in ANSI/RESNET/ICC 301 for all ground-source systems
Correct answer: The site-specific COP determined by ground loop entering water temperature and manufacturer performance data
Ground-source heat pump performance varies with entering water temperature, so the model should use manufacturer performance data matched to the site's ground loop conditions.
Question 7: Which metric does energy modeling software use to quantify thermal bridging through wood-framed walls?
- Framing fraction combined with stud and cavity R-values to calculate an assembly U-factor (Correct answer)
- A fixed 10% penalty applied to the nominal insulation R-value
- The steel stud correction factor from ASHRAE Fundamentals
- The ratio of wood volume to total wall volume
Correct answer: Framing fraction combined with stud and cavity R-values to calculate an assembly U-factor
Software calculates an assembly U-factor by combining the framing fraction (typically 0.25 for 16" o.c. framing), stud R-value, and cavity insulation R-value in a parallel-path calculation.
HERS Energy Modeling and Analysis 3
What is the RESNET-defined default internal gains assumption used in HERS energy modeling for occupancy?