ASHRAE Material Properties 5

Question 1

Why does R-13 cavity insulation in a steel-stud wall deliver much less than R-13 overall?

Accepted Answer

Steel conducts heat roughly 400 times better than wood and forms thermal bridges

Answer

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?

Accepted Answer

It decreases

Answer

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?

Accepted Answer

0.05 Btu/h·ft²·°F

Answer

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?

Accepted Answer

R-0.68

Answer

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?

Accepted Answer

0

Answer

R-410A is an HFC blend with no chlorine, so its ODP is zero, even though its GWP is high (~2,088).

ASHRAE - American Society of Heating, Refrigeration and Air Conditioning Engineers Certification Practice Test