HERS Building Science Principles — Questions and Answers

Question 1: Which phenomenon causes warm, moist air to condense on a cold surface inside a wall assembly?

Thermal bridging
Dew point condensation (Correct answer)
Stack effect pressure
Vapor diffusion equilibrium

Correct answer: Dew point condensation

When air cools to its dew point temperature at a cold surface, water vapor condenses into liquid moisture.

Question 2: In a hot-humid climate, where should the vapor retarder be placed in a wall assembly?

On the interior (warm-in-winter) side
In the center of the cavity
On the exterior side, toward the hot exterior (Correct answer)
No vapor retarder is needed in hot-humid climates

Correct answer: On the exterior side, toward the hot exterior

In hot-humid climates, the dominant moisture drive is inward from the hot exterior, so the vapor retarder belongs on the exterior side.

Question 3: What does the term 'thermal mass' refer to in building science?

The total weight of insulation in a wall
A material's ability to store and slowly release heat (Correct answer)
The R-value of a dense-pack insulation product
The ratio of conduction to convection in a building envelope

Correct answer: A material's ability to store and slowly release heat

Thermal mass describes a material's capacity to absorb, store, and release heat energy, moderating interior temperature swings.

Question 4: Which heat transfer mechanism does low-emissivity (low-e) window coating primarily reduce?

Conduction through the glass
Convection at the glass surface
Radiation across the air gap (Correct answer)
Air infiltration around the frame

Correct answer: Radiation across the air gap

Low-e coatings reflect infrared radiation, reducing the radiative heat transfer between the glass panes.

Question 5: A HERS rater measures attic ventilation to ensure balance between intake and exhaust. What is the recommended ratio of net free area to attic floor area when using balanced ventilation?

1:100
1:150 (Correct answer)
1:300
1:600

Correct answer: 1:150

The standard building code ratio for balanced attic ventilation (50% intake, 50% exhaust) is 1:150 net free area to floor area.

Question 6: What is the primary purpose of an air barrier in a building enclosure?

To prevent vapor diffusion through materials
To resist convective air movement through the building shell (Correct answer)
To add R-value to wall assemblies
To control solar heat gain through windows

Correct answer: To resist convective air movement through the building shell

An air barrier controls convective airflow (infiltration and exfiltration) through the building envelope, not diffusion.

Question 7: Which condition best describes a 'negative pressure' house?

Interior air pressure is higher than exterior pressure
Interior air pressure is lower than exterior pressure (Correct answer)
Pressure is equal on all sides of the envelope
The HVAC system is over-sized for the heating load

Correct answer: Interior air pressure is lower than exterior pressure

A negative pressure building has interior pressure below outdoor pressure, causing outdoor air to be drawn inward through any gaps.

Question 8: How does wind-driven rain most commonly enter a wall assembly in a climate with high wind exposure?

Vapor diffusion through exterior sheathing
Capillary action through the sill plate
Forced water entry through gaps and cracks under pressure (Correct answer)
Condensation on the back side of the cladding

Correct answer: Forced water entry through gaps and cracks under pressure

Wind pressure forces liquid water through gaps, cracks, and penetrations in the cladding and air barrier under positive pressure.

Question 9: What is 'interstitial condensation' in a building assembly?

Condensation on interior glass surfaces
Condensation forming within the layers of a wall, roof, or floor assembly (Correct answer)
Condensation on exterior foundation walls below grade
Condensation in ductwork caused by oversized cooling equipment

Correct answer: Condensation forming within the layers of a wall, roof, or floor assembly

Interstitial condensation occurs inside the building assembly layers when vapor migrates to a surface that is at or below the dew point.

Question 10: A building with tight construction and mechanical ventilation exemplifies which principle from ASHRAE 62.2?

Build loose, ventilate naturally
Build tight, ventilate right (Correct answer)
Balanced pressure at all times
Maximum infiltration for indoor air quality

Correct answer: Build tight, ventilate right

ASHRAE 62.2 embraces the 'build tight, ventilate right' approach, using controlled mechanical ventilation instead of uncontrolled air leakage.

Question 11: Which type of foundation is most susceptible to ground moisture intrusion by capillary action?

Elevated post-and-beam foundation
Slab-on-grade without capillary break (Correct answer)
Basement with exterior waterproofing membrane
Crawlspace with closed-cell spray foam on walls

Correct answer: Slab-on-grade without capillary break

Concrete slabs placed directly on soil without a capillary break (gravel layer or poly film) allow moisture to wick upward continuously.

Question 12: In whole-building energy modeling, what does the term 'internal gains' refer to?

Solar radiation entering through south-facing windows
Heat generated inside the building by occupants, lighting, and appliances (Correct answer)
Heat conducted inward through the building envelope
Sensible heat produced by the heating system itself

Correct answer: Heat generated inside the building by occupants, lighting, and appliances

Internal gains are heat sources within the conditioned space—occupants, lights, cooking, and plug loads—that contribute to the cooling load.

Question 13: What is the function of a 'drain plane' or 'drainage plane' in a wall assembly?

To direct electrical conduit away from insulation
To channel any water that gets past the cladding back out of the assembly (Correct answer)
To reduce vapor diffusion through the wall cavity
To provide a thermal break between the stud and exterior sheathing

Correct answer: To channel any water that gets past the cladding back out of the assembly

A drainage plane (housewrap, building paper, or similar) sheds water that penetrates the cladding and directs it out of the wall system.

Question 14: Which metric is used to quantify the airtightness of a building in a blower door test?

CFM50 or ACH50 (Correct answer)
MERV rating
Latent heat ratio
Net free area per square foot

Correct answer: CFM50 or ACH50

Blower door results are reported as CFM50 (cubic feet per minute at 50 Pa) or ACH50 (air changes per hour at 50 Pa depressurization.

Question 15: What is a 'thermal bridge' and how does it affect wall performance?

A section of high R-value insulation that spans studs
A highly conductive element like a metal stud that bypasses insulation and increases heat flow (Correct answer)
A radiant barrier that bridges heat across an air gap
A vapor-permeable material that allows moisture to pass through the wall

Correct answer: A highly conductive element like a metal stud that bypasses insulation and increases heat flow

A thermal bridge is a conductive path—such as a metal stud—that bypasses insulation and significantly increases heat transfer through the assembly.

Question 16: During summer in a cooling-dominated climate, which solar orientation typically receives the highest heat gain through unshaded windows?

North-facing windows
South-facing windows
East- and west-facing windows (Correct answer)
Skylights with low-e coating

Correct answer: East- and west-facing windows

East and west windows receive direct low-angle sun in the morning and afternoon, causing high solar heat gain that is difficult to shade externally.

Question 17: What does 'sensible heat' describe in HVAC and building science?

Heat stored in moisture held in the air
Heat that changes the temperature of a substance without changing its phase (Correct answer)
Heat released during condensation of water vapor
The latent load created by occupants and cooking

Correct answer: Heat that changes the temperature of a substance without changing its phase

Sensible heat is thermal energy that causes a measurable temperature change in a substance, measured with a thermometer.

Question 18: In a vented crawlspace, which condition creates the most risk of moisture damage to floor framing?

Warm, dry exterior air entering the crawlspace in summer
Cold, dry winter air entering the crawlspace in January
Warm, humid outdoor air entering a cool crawlspace in summer (Correct answer)
Low humidity in the crawlspace relative to the conditioned space above

Correct answer: Warm, humid outdoor air entering a cool crawlspace in summer

When warm, humid outdoor air enters a cool crawlspace in summer, it cools to its dew point and condenses on framing and insulation.

Question 19: What is the purpose of the 'Solar Heat Gain Coefficient' (SHGC) of a window?

It measures the window's resistance to conductive heat loss
It quantifies how much solar radiation passes through the window into the building (Correct answer)
It rates how well the frame resists air infiltration
It describes the ratio of visible light to UV light transmitted

Correct answer: It quantifies how much solar radiation passes through the window into the building

SHGC is a dimensionless number (0–1) indicating the fraction of incident solar radiation transmitted through a window as heat gain.

Question 20: Which insulation material is classified as an air barrier when installed properly at the correct thickness?

Fiberglass batt
Cellulose dense-pack
Closed-cell spray polyurethane foam (ccSPF) (Correct answer)
Mineral wool batt

Correct answer: Closed-cell spray polyurethane foam (ccSPF)

Closed-cell spray foam is both an insulator and a Class II vapor retarder and air barrier when applied at sufficient thickness (typically 2 inches or more).

Question 21: What is 'hygric buffering' (or moisture buffering) in the context of building materials?

The ability of a material to block all vapor diffusion
A material's capacity to absorb and release moisture, dampening indoor humidity swings (Correct answer)
Chemical treatment applied to wood to prevent rot
The drainage capacity of a wall's drainage plane

Correct answer: A material's capacity to absorb and release moisture, dampening indoor humidity swings

Hygric buffering describes how materials like wood or gypsum board absorb excess indoor moisture and release it later, stabilizing humidity levels.

HERS Building Science Principles

Which phenomenon causes warm, moist air to condense on a cold surface inside a wall assembly?

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