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Chapter 1: Q. 1.58 (page 40)

According to a standard reference table, the R value of a 3.5 inch-thick vertical air space (within a wall) is 1.0 R in English units), while the R value of a 3.5-inch thickness of fiberglass batting is 10.9. Calculate the R value of a 3.5-inch thickness of still air, then discuss whether these two numbers are reasonable. (Hint: These reference values include the effects of convection.)

Short Answer

Expert verified

The value of 3.5 inch thickness of still air is Rair= 0.176 m2. oK.s.J-1

Step by step solution

01

To Calculate R

Rvalue is,

R=螖kkt

For

localid="1651328451771" airkt=0.026Wm-1k-1

So,

3.5inch=3.5脳25.6mm

=89.6脳10-3m

Rair=89.6脳10-3m0.026Wm-1K-1

=3.45蝇-1m2k

02

After conversion.

Then,

Rair=3.450.58role="math" localid="1651327954414" offt2hr/btu

Rair=5.95role="math" offt2hr/btu

As a result, the value does not match the reference value (f=1).

Because of the convection process, reference values incorporate the impacts of convection.

The air's thermal conductivity rises.

As a result, the value of Rdecreases.

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