HERS Building Science Fundamentals 3 — Questions and Answers
Question 1: What is the primary function of a drain-plane (drainage plane) in a wall assembly?
- To reflect solar radiation away from the wall
- To provide a path for water that penetrates the cladding to drain out (Correct answer)
- To increase the thermal resistance of the wall
- To equalize pressure between indoors and outdoors
Correct answer: To provide a path for water that penetrates the cladding to drain out
A drainage plane directs any water that gets past the cladding downward and out of the wall assembly before it can cause damage.
Question 2: In building science, what does 'hygrothermal analysis' evaluate?
- The structural load-bearing capacity of a wall under thermal expansion
- The combined heat and moisture movement through building assemblies over time (Correct answer)
- The acoustic performance of assemblies in humid environments
- The energy efficiency of HVAC equipment under high humidity conditions
Correct answer: The combined heat and moisture movement through building assemblies over time
Hygrothermal analysis models both heat (hygro = moisture, thermal = heat) transport simultaneously to predict condensation risk and durability in assemblies.
Question 3: A homeowner complains of condensation on interior window surfaces in winter. Which condition is the most likely cause?
- The window U-factor is too high relative to indoor relative humidity and temperature (Correct answer)
- The window SHGC is too low, reducing solar heat gain
- The window frame has a high emittance coating
- Exterior storm windows are creating excessive heat at the interior glazing
Correct answer: The window U-factor is too high relative to indoor relative humidity and temperature
Windows with high U-factors (poor thermal performance) have colder interior surface temperatures that fall below the dew point when indoor humidity is elevated.
Question 4: Which climate zone classification system is used by the IECC and referenced in HERS ratings?
- ASHRAE Standard 55 climate categories
- IECC/DOE climate zones 1–8 based on HDD and CDD thresholds (Correct answer)
- Köppen climate classification system
- NOAA regional climate divisions
Correct answer: IECC/DOE climate zones 1–8 based on HDD and CDD thresholds
The IECC uses DOE climate zones 1–8, which are defined by heating and cooling degree-day thresholds, and are the basis for building energy code requirements.
Question 5: What is 'interstitial condensation' and why is it a concern in building assemblies?
- Condensation forming on window interior surfaces, causing sill rot
- Condensation occurring within a wall or roof assembly where water vapor meets a surface below its dew point (Correct answer)
- Moisture migrating through concrete slabs by capillary action
- Condensation on ductwork surfaces in unconditioned attics
Correct answer: Condensation occurring within a wall or roof assembly where water vapor meets a surface below its dew point
Interstitial condensation forms inside a building assembly and can cause structural decay, mold, and reduced insulation performance without visible exterior signs.
Question 6: Which type of heat transfer is dominant in a radiant barrier system installed in an attic?
- Conduction through the foil material
- Convection across the air gap beneath the foil
- Radiation between the hot roof deck and the attic floor (Correct answer)
- Latent heat transfer from moisture evaporation
Correct answer: Radiation between the hot roof deck and the attic floor
Radiant barriers work by reflecting long-wave infrared radiation emitted from the hot roof deck, reducing radiant heat gain to the attic floor and insulation.
Question 7: In the context of building envelope performance, what is 'air washing' of insulation?
- Cleaning insulation surfaces with pressurized air during installation
- Cold air moving through or around insulation, degrading its effective thermal resistance (Correct answer)
- Using air pressure to test insulation continuity
- The process of removing moisture from wet insulation using forced air
Correct answer: Cold air moving through or around insulation, degrading its effective thermal resistance
Air washing occurs when cold air infiltrates around or through insulation, carrying heat away by convection and significantly reducing its effective R-value.
HERS Building Science Fundamentals 3
What is the primary function of a drain-plane (drainage plane) in a wall assembly?