A wall has insulation with R-value of 13. Adding a second layer of the same insulation (R-13) gives a total R-value of:
A13
B26
C169
D6.5
R-values add in series, just like resistances in a series circuit. Two layers of R-13 insulation give a total R-value of 13 + 13 = 26, cutting the heat flow in half compared to a single layer.
Question 2 True / False
A puffy winter jacket keeps you warm by generating heat from its synthetic fibers.
TTrue
FFalse
Answer: False
The jacket does not generate heat -- your body does. The jacket's puffy material traps air in small pockets, creating insulation that slows heat transfer from your warm body to the cold air. The trapped air is the actual insulator; the fibers just hold the air in place.
Question 3 Short Answer
Why does a thermos (vacuum flask) use both a vacuum and a reflective coating?
Think about your answer, then reveal below.
Model answer: The vacuum eliminates conduction and convection (which require a material medium to transfer heat), while the reflective coating reduces radiation (which can transfer heat across a vacuum). Each strategy addresses a different heat transfer mechanism, and together they provide much better insulation than either alone.
This is an example of defense-in-depth in engineering design. A vacuum alone would still allow radiative heat transfer. A reflective coating alone would still allow conductive and convective losses. Combining both strategies minimizes all three heat transfer mechanisms.