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As we have learned, most heat is lost through a house's walls through conduction. One of the three factors that affect heat loss is a wall's capacity to resist heat loss.

We will now look at how to calculate the rate of heat loss of the walls of a house, using the following formula:

$$\text{HeatLoss}\left(\frac{\text{BTUs}}{\text{h}}\right)\text{=}\frac{\text{Area}\left({\text{ft}}^{\text{2}}\right)\times \text{TemperatureDifference}\left(\xb0\text{F}\right)}{\text{R-Value}\left(\frac{{\text{ft}}^{\text{2}}\text{}\xb0\text{Fh}}{\text{BTUs}}\right)}$$

From the above equation, it can be seen that once the house is built, these two variables will NOT change:

- The area of the walls
- The R-value of the walls

The only variable that will change is the temperature difference between inside and outside.

### Example

**Calculate the heat loss for a 10 ft by 8 ft wall, insulated to R-value 22. The inside temperature is maintained at 70° F. The temperature outside is 43° F.**

Please watch the following 2:25 presentation about Hourly Heat Loss: