ASHRAE Material Properties 2 — Questions and Answers
Question 1: Which value is closest to the thermal conductivity of pure copper at room temperature?
- About 50 W/m·K
- About 205 W/m·K
- About 16 W/m·K
- About 400 W/m·K (Correct answer)
Correct answer: About 400 W/m·K
Pure copper conducts about 400 W/m·K, roughly twice as well as aluminum (~205), which is why it is used for heat exchanger tubing.
Question 2: A radiant barrier works mainly because its surface has which property?
- High specific heat
- High thermal conductivity
- Low emissivity, around 0.05 (Correct answer)
- High emissivity, around 0.90
Correct answer: Low emissivity, around 0.05
Polished aluminum foil has an emissivity of about 0.03-0.05, so it gives off very little radiant heat across an air space.
Question 3: What is the approximate specific heat of liquid water in I-P units?
- 0.24 Btu/lb·°F
- 0.12 Btu/lb·°F
- 0.45 Btu/lb·°F
- 1.0 Btu/lb·°F (Correct answer)
Correct answer: 1.0 Btu/lb·°F
The Btu was originally defined from water, so water's specific heat is about 1.0 Btu/lb·°F.
Question 4: What is the approximate density of water at 60°F?
- 0.075 lb/ft³
- 8.34 lb/ft³
- 62.4 lb/ft³ (Correct answer)
- 1,000 lb/ft³
Correct answer: 62.4 lb/ft³
Water at 60°F weighs about 62.4 lb/ft³, which is the same as about 8.34 lb/gal.
Question 5: Under the International Codes, what is the maximum vapor permeance of a Class I vapor retarder?
- 1.0 perm or less
- 10 perms or less
- Between 1 and 10 perms
- 0.1 perm or less (Correct answer)
Correct answer: 0.1 perm or less
Class I vapor retarders, such as polyethylene sheet, are rated at 0.1 perm or less.
Question 6: Thermal diffusivity of a material is defined as which ratio?
- ρ·cp / k
- cp / (k·ρ)
- k·ρ / cp
- k / (ρ·cp) (Correct answer)
Correct answer: k / (ρ·cp)
Thermal diffusivity α = k/(ρ·cp) tells you how quickly a temperature change spreads through a material.
Question 7: Which insulation typically gives the highest R-value per inch?
- Expanded polystyrene (EPS)
- Fiberglass batt
- Foil-faced polyisocyanurate board (Correct answer)
- Loose-fill cellulose
Correct answer: Foil-faced polyisocyanurate board
Polyiso is rated at about R-6 to R-6.5 per inch, compared with roughly R-3.1 to R-4 for the other three.
ASHRAE Material Properties 2
Which value is closest to the thermal conductivity of pure copper at room temperature?