calculating heat loss or gain

8/13/2019 Calculating Heat Loss or Gain

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Calculating U-Values

Since the U-value is a measure of the heat transmission properties of the entire wall

section when exposed to air on both sides, this is the property that should be determined

when figuring the heat loss through walls, floors, ceilings, etc.

The U-value for a wall can be determined from the following relationship:

U = 1

R1+ R1+ R2+ . . .Ro

where: Ri = the resistivity of a "boundary layer" of air on the inside surface.

R1, R2= the resistivity of each component of the walls for the actual

thickness of the component used. If the resistance per inch

thickness is used, the value should be multiplied by the

thickness of that component.

Ro= the resistivity of the "air boundary layer" on the outside surface of

the wall.

For example, to determine the U-value for the following wall section composed of three

materials:

1. Hardboard, 1/4-inch thick

2. Expanded polystrene, 2-inches thick

3. Plywood, 3/4-inch thick

The R-values from Table E1.1 are:

Ri= 0.68 ("still" air)

R1= 0.18

R2= 2" X 4.00 = 8.00

R3= 3/4 X 1.25 = 0.94


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Ro= 0.17 (15 MPH wind, winter conditions)

The U-values then is:

U = 1

Ri+ R1+ R2+ R3+ Roo

= 1

0.68 + 0.18 + 8.00 + 0.94 + 0.17

= 1 = 0.10 BTU

9.97 hr. - sq. ft. - oF

Section Break (Next Page)


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Table E1.2 gives several typical wall, ceiling, floor, and roof sections and the

corresponding U-values, overall R-values, and other data tabulated like the proceeding

example and explanation.

Calculating Heat Loss or Gain

The U-value is used to determine the heat loss or gain through a wall, etc., by the

following relationship.

Q = (U) (A) (T)

where: Q = the heat flow through the walls, etc., in BTU per hour.

U = the U-value in BTU per (hour) (square feet) (oF).

A = the area of the wall, etc., in square feet.

T = Difference in outside and inside temperatures in oF.

In the previous example the heat loss for 100 square feet of wall with a 70 oF temperature

difference would be:

Q = (.10) (100) (70) = 700 BTU/ HR

For any room or building, the heat loss for each portion -- walls, ceiling, windows, doors,

floors, etc., must be calculated and added for the total.

When calculating the heat flow through a floor slab resting on the ground, there will not bean air boundary-layer resistance underneath (Ro= 0) and the temperature (to) will be the

ground temperature (not the outside air temperature). The " deep-well" or deep-soil

temperature in Alabama averages about 60oF the year around. This temperature can be

used for all of the floor slab area except the outer three feet, unless perimeter insulation is

used. For the outer three feet of an uninsulated slab, the outside air temperature should

be used as the heat loss is to the outside foundation wall.

If the sun shines on a wall or roof of a building and heats the surface much hotter than the

air (as typical in the summer), the heat flow through the wall or roof would be greatly

influenced by the hot surface temperature; hence, use a surface temperature rather than air

to obtain a more realistic heat flow rate.

Direct radiation through large glass sections is an important heat gain in summer and must

be considered by procedures other than described here.

calculating heat loss or gain

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