Question

1. How many Btu/hr are lost through a 100-square foot wall that is made up of an 8-inch thick brick wall with an R value of 1.6 and 2 inches of foam insulation with an R value of 10.5? The outside temperature is 40 degrees F and the inside temperature is 65 degrees F. Do not enter units. If the answer is one million Btu/hr, enter 1.E6.

Answer 1

Dear Student,

Following is the answer to your question:-

Energy loss through a wall is given by:

Qe = (kA/L)*Td

where

• Qe = energy lost in units of BTUs per hour
• A = area of the wall
• l = wall thickness
• k = conductivity of the wall which depends on the material in the wall
• Td = difference in temperature between inside and outside

Td is defined in units of degree days.

A degree day = Tin - Tout where in and out refer to the inside and outside temperatures

R = (1/k)*L

R-values of materials just add. In other words, the total insulation in a wall would be

Rt = R1 + R2 + R3 + ...

, where the individual R values are associated with different kinds of materials in your wall.

So, Qe = (kA/L)*Td = A/Rt*Td

So, Answer = 86.E7 Btu/hr.