ASHRAE Material Properties 5 — Questions and Answers
Question 1: Why does R-13 cavity insulation in a steel-stud wall deliver much less than R-13 overall?
- Steel conducts heat roughly 400 times better than wood and forms thermal bridges (Correct answer)
- Steel absorbs moisture from the insulation
- Steel has a high emissivity
- Steel studs compress the insulation
Correct answer: Steel conducts heat roughly 400 times better than wood and forms thermal bridges
Steel (~50 W/m·K) conducts far better than wood (~0.12 W/m·K), so an R-13 cavity in 16-in. o.c. steel studs works out to only about R-6.
Question 2: What happens to the dynamic viscosity of liquid water as its temperature rises?
- It increases
- It increases up to 100°F and then decreases
- It stays the same
- It decreases (Correct answer)
Correct answer: It decreases
Liquid water gets thinner as it warms, so friction losses in hot water piping are a bit lower.
Question 3: An assembly has a total thermal resistance, including air films, of R-20. What is its U-factor?
- 0.20 Btu/h·ft²·°F
- 0.5 Btu/h·ft²·°F
- 0.05 Btu/h·ft²·°F (Correct answer)
- 20 Btu/h·ft²·°F
Correct answer: 0.05 Btu/h·ft²·°F
U = 1/R_total = 1/20 = 0.05 Btu/h·ft²·°F.
Question 4: About what thermal resistance does ASHRAE list for still air on the inside of a vertical surface with heat flowing horizontally?
- R-2.0
- R-0.68 (Correct answer)
- R-0.17
- R-0.03
Correct answer: R-0.68
ASHRAE lists still air on a vertical surface at about 0.68 h·ft²·°F/Btu, compared with about 0.17 for an outside surface in a 15 mph wind.
Question 5: What is the ozone depletion potential (ODP) of R-410A?
- 0.05
- 1.0
- 0 (Correct answer)
- 0.55
Correct answer: 0
R-410A is an HFC blend with no chlorine, so its ODP is zero, even though its GWP is high (~2,088).
Question 6: How does the thermal conductivity of austenitic stainless steel compare with carbon steel?
- It is higher than copper
- They are about the same
- It is much higher, about 200 vs 50 W/m·K
- It is much lower, about 16 vs 50 W/m·K (Correct answer)
Correct answer: It is much lower, about 16 vs 50 W/m·K
Austenitic stainless steel has about one-third the conductivity of carbon steel, which reduces heat transfer in thin-wall exchangers.
Question 7: Which insulation is most commonly used on refrigerant suction lines to stop condensation?
- Unfaced fiberglass pipe wrap
- Mineral wool pipe sections without jacket
- Flexible closed-cell elastomeric foam (Correct answer)
- Calcium silicate
Correct answer: Flexible closed-cell elastomeric foam
Closed-cell elastomeric foam has very low vapor permeance and needs no separate vapor jacket, so moisture does not get in on cold lines.
ASHRAE Material Properties 5
Why does R-13 cavity insulation in a steel-stud wall deliver much less than R-13 overall?