HERS Energy Modeling and Analysis — Questions and Answers

Question 1: In REM/Rate energy modeling software, what does the 'Reference Home' represent?

The most energy-efficient home in a neighborhood
A code-minimum baseline home built to IECC specifications used to calculate the HERS Index (Correct answer)
The average energy consumption of homes in the same climate zone
The builder's own home used as a comparison benchmark

Correct answer: A code-minimum baseline home built to IECC specifications used to calculate the HERS Index

The Reference Home is a code-minimum baseline constructed to IECC specifications against which the Rated Home is compared to calculate the HERS Index.

Question 2: Which input parameter has the greatest single influence on modeled heating energy consumption in a cold climate home?

Window solar heat gain coefficient (SHGC)
Envelope air leakage (ACH50) (Correct answer)
Wall assembly R-value
Heating system efficiency (AFUE)

Correct answer: Envelope air leakage (ACH50)

Envelope air leakage measured at ACH50 typically dominates heating load in cold climates because infiltration losses scale directly with the temperature difference.

Question 3: What is the purpose of the 'normalized modified end-use loads' (nMEUL) calculation in HERS rating?

To estimate occupant utility bills
To adjust end-use loads to account for equipment efficiency and fuel type for Index calculation (Correct answer)
To compare homes of different square footages on a per-square-foot basis
To normalize energy use across different climate zones

Correct answer: To adjust end-use loads to account for equipment efficiency and fuel type for Index calculation

nMEUL adjusts each end-use load by equipment efficiency and fuel-specific factors so that the HERS Index fairly represents source energy consumption.

Question 4: A rater models a home with R-38 attic insulation in a mixed-humid climate. The software shows cooling energy is higher than expected. Which input should be checked first?

Attic insulation R-value entry
Duct location and leakage values (Correct answer)
Window U-factor
Foundation wall insulation

Correct answer: Duct location and leakage values

Ducts located in an unconditioned attic with high leakage dramatically increase cooling loads in mixed-humid climates and are a common modeling oversight.

Question 5: In energy modeling, what is 'latent load' and why does it matter in humid climates?

The structural weight load on HVAC equipment
The energy required to remove moisture from indoor air, which drives cooling equipment sizing and runtime (Correct answer)
The delayed response of a thermal mass wall to temperature changes
The standby energy consumed by HVAC controls

Correct answer: The energy required to remove moisture from indoor air, which drives cooling equipment sizing and runtime

Latent load is the energy needed for dehumidification; in humid climates it can equal or exceed sensible cooling load and significantly affects equipment sizing and energy use.

Question 6: When entering window properties into energy modeling software, which combination of values most completely describes thermal and solar performance?

R-value and visible transmittance (VT)
U-factor and solar heat gain coefficient (SHGC) (Correct answer)
SHGC and emissivity
Glazing area and frame material

Correct answer: U-factor and solar heat gain coefficient (SHGC)

U-factor captures conductive/convective heat transfer while SHGC captures solar heat gain; together they fully define window energy performance in models.

Question 7: A HERS rater discovers that the modeled home's mechanical ventilation rate is set to zero in the software. What is the most likely consequence for the HERS Index?

The Index will be artificially low because ventilation energy is not counted (Correct answer)
The Index will be artificially high because the model assumes more infiltration to meet IAQ needs
There is no effect because ventilation is not part of the HERS rating
The Index will be accurate if the home's air leakage is below 3 ACH50

Correct answer: The Index will be artificially low because ventilation energy is not counted

Omitting mechanical ventilation energy undercounts the home's actual energy use, resulting in an artificially low HERS Index.

Question 8: 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 9: 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 10: 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 11: 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 12: 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 13: 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 14: 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.

Question 15: What is the primary purpose of TMY (Typical Meteorological Year) weather data files in HERS energy modeling?

To provide the worst-case design temperatures for Manual J load calculations
To represent a statistically typical year of hourly weather data used to simulate annual energy consumption (Correct answer)
To document actual weather during the year the home was rated
To establish utility rate schedules based on regional energy prices

Correct answer: To represent a statistically typical year of hourly weather data used to simulate annual energy consumption

TMY files contain representative hourly data assembled from multiple years to reflect typical (not extreme) conditions, enabling accurate annual energy simulation.

Question 16: A HERS rater is modeling a home with an ERV (Energy Recovery Ventilator). Which additional performance value beyond airflow rate must be entered to accurately model energy recovery?

The motor wattage only
The sensible recovery efficiency (SRE) and total recovery efficiency (TRE) or sensible and latent effectiveness values (Correct answer)
The outdoor air temperature at which the ERV operates
The filter MERV rating

Correct answer: The sensible recovery efficiency (SRE) and total recovery efficiency (TRE) or sensible and latent effectiveness values

Accurate ERV modeling requires sensible and total (or latent) recovery efficiencies to quantify how much heating and cooling energy is recaptured from exhaust air.

Question 17: In the context of HERS ratings, what is 'source energy' and how does it differ from 'site energy'?

Source energy is the energy delivered to the home meter; site energy includes transmission losses
Site energy is measured at the home; source energy accounts for generation and transmission losses back at the power plant (Correct answer)
Source energy applies only to on-site renewables; site energy covers grid electricity
They are identical metrics measured at different time intervals

Correct answer: Site energy is measured at the home; source energy accounts for generation and transmission losses back at the power plant

Site energy is measured at the meter, while source energy traces consumption back through generation and transmission, using fuel-specific site-to-source multipliers.

Question 18: How does HERS modeling handle the energy contribution of a photovoltaic (PV) system when calculating the HERS Index?

PV energy is ignored in all HERS Index calculations
PV generation offsets the home's rated energy use, potentially resulting in a HERS Index below zero for net-zero or net-positive homes (Correct answer)
PV can only reduce the HERS Index to a minimum of 50
PV output is added to the Reference Home to create a fair comparison

Correct answer: PV generation offsets the home's rated energy use, potentially resulting in a HERS Index below zero for net-zero or net-positive homes

Per ANSI/RESNET/ICC 301, PV generation is credited against the Rated Home's energy use and can produce a HERS Index of zero or below for homes that generate as much or more than they consume.

Question 19: What modeling approach is used for mass walls (e.g., concrete masonry) that is different from light-frame wood walls?

Mass walls use a higher framing fraction to account for mortar joints
Mass walls require dynamic thermal mass properties (heat capacity) in addition to R-value to accurately model time-delayed heat transfer (Correct answer)
Mass walls are always assigned an automatic R-5 bonus regardless of actual mass
Mass walls use the same steady-state U-factor calculation as wood-frame walls

Correct answer: Mass walls require dynamic thermal mass properties (heat capacity) in addition to R-value to accurately model time-delayed heat transfer

Thermal mass stores and releases heat with a time delay, so accurate modeling requires both R-value and heat capacity inputs to capture the mass wall's dynamic buffering effect.

Question 20: A rater models two identical homes except one has east/west-facing glazing and the other has south/north-facing glazing. In a heating-dominated climate, which will likely have a lower HERS Index?

The east/west-facing home because morning sun reduces heating demand
The south/north-facing home because south windows maximize passive solar gains in winter (Correct answer)
Both homes will have identical HERS Index scores because total window area is the same
The north-facing home because it avoids summer overheating penalties

Correct answer: The south/north-facing home because south windows maximize passive solar gains in winter

South-facing windows capture winter solar gains more effectively than east/west windows, reducing heating energy and lowering the HERS Index in heating-dominated climates.

Question 21: Which RESNET standard governs the technical requirements for HERS energy modeling software to be approved for use in ratings?

ANSI/RESNET/ICC 301
RESNET Publication 002 (Correct answer)
ASHRAE Standard 140
IECC Section R405

Correct answer: RESNET Publication 002

RESNET Publication 002 (Mortgage Industry National Home Energy Rating Standards) establishes software approval criteria and testing protocols for HERS rating tools.

HERS Energy Modeling and Analysis

In REM/Rate energy modeling software, what does the 'Reference Home' represent?

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