Calculate Heat Loss Through House Wall - R19 & Brick Layer

In summary, the total rate of heat loss through a wall made of a 4.0-in. layer of brick and an R-19 layer of insulation, with a total area of 195 ft^2 and a temperature difference of 15 degrees F, can be calculated using Q/t = kA ( (T1 - T2)/L ) and R = L/k, regardless of the thickness of the R-19 layer.
  • #1
nightwing973
4
0
1. Suppose the insulating qualities of the wall of a house come mainly from a 4.0-in. layer of brick and an R-19 layer of insulation. What is the total rate of heat loss through such a wall, if its total area is 195 ft^2 and the temperature difference across it is 15 degrees F.


2. Q/t = kA ( (T1 - T2)/L )
R = L/k



3. Okay so the R-19 thing just means that R = 19 (ft^2 * hour* degreeFahr)/Btu. R = 19, so L/k = 19, I can inverse that and substitute it into the first equation, but it says the temperature difference across the WHOLE wall is 15 degrees fahrenheit. My main problem is that they ask you for the rate of heat loss through the whole wall, but the wall is made of two different materials, and you don't know how thick the R-19 layer is.
 
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  • #2
Welcome to PF!

Hi nightwing973! Welcome to PF! :smile:
nightwing973 said:
1. Suppose the insulating qualities of the wall of a house come mainly from a 4.0-in. layer of brick and an R-19 layer of insulation.

3. Okay so the R-19 thing just means that R = 19 (ft^2 * hour* degreeFahr)/Btu. R = 19, so L/k = 19, I can inverse that and substitute it into the first equation, but it says the temperature difference across the WHOLE wall is 15 degrees fahrenheit. My main problem is that they ask you for the rate of heat loss through the whole wall, but the wall is made of two different materials, and you don't know how thick the R-19 layer is.

You don't need to know the thickness of the R-19 layer …

eg, if was twice as thick, it would R-38. :wink:
 
  • #3


I would approach this problem by first calculating the thermal conductivity (k) of each material, in this case, the brick and the R-19 insulation layer. Once we have the thermal conductivity values, we can use the formula Q/t = kA ( (T1 - T2)/L ) to calculate the rate of heat loss through each material separately.

To find the thermal conductivity of the brick layer, we can consult a table or use a thermal conductivity meter to measure it. Let's assume it is 0.7 W/mK. The R-19 insulation layer has a thermal conductivity of 0.036 W/mK.

Next, we need to determine the thickness (L) of the R-19 insulation layer. This information is usually provided by the manufacturer, but if it is not available, we can assume a standard thickness of 6 inches (0.15 m).

Now we can plug in the values into the formula Q/t = kA ( (T1 - T2)/L ) for each material. For the brick layer, Q/t = (0.7 W/mK)(195 ft^2)(15°F)/(0.10 m) = 204.75 W. For the R-19 insulation layer, Q/t = (0.036 W/mK)(195 ft^2)(15°F)/(0.15 m) = 23.4 W.

Finally, we can add the two values to get the total rate of heat loss through the wall: Q/t = 204.75 W + 23.4 W = 228.15 W.

So, the total rate of heat loss through the wall is 228.15 watts. This information can be useful in determining the energy efficiency of the house and making improvements to reduce heat loss.
 

1. How do I calculate the heat loss through a house wall with R19 insulation and a brick layer?

To calculate the heat loss through a house wall, you will need to use the formula Q = U x A x ΔT, where Q is the heat loss in BTUs per hour, U is the overall heat transfer coefficient, A is the area of the wall, and ΔT is the temperature difference between the inside and outside of the wall. To find the overall heat transfer coefficient, you will need to add the R-values of each layer of the wall (in this case, the R-value of the R19 insulation and the R-value of the brick layer) and then divide that number into 1. Once you have the overall heat transfer coefficient, you can plug it into the formula along with the other variables to calculate the heat loss.

2. What is the R19 insulation value and how does it affect heat loss?

The R19 insulation value refers to the thermal resistance of the insulation, which is a measure of how well it resists the transfer of heat. The higher the R-value, the more effective the insulation is at reducing heat loss. In this case, R19 insulation has a higher R-value compared to other types of insulation, so it can help reduce heat loss through the wall.

3. How does the brick layer impact the heat loss through a wall?

The brick layer can act as a thermal mass, which means it has the ability to absorb and store heat. This can help regulate the temperature inside the house, as the brick will slowly release the stored heat into the house as the temperature outside drops. However, if the brick layer is not properly insulated, it can also contribute to heat loss through the wall.

4. Are there any other factors that can affect the heat loss through a wall?

Yes, there are several other factors that can affect the heat loss through a wall, such as the type and quality of insulation, the thickness and condition of the walls, the type and number of windows, and the overall air tightness of the house. These factors should also be taken into consideration when calculating heat loss through a wall.

5. How can I reduce heat loss through my house wall?

To reduce heat loss through your house wall, you can consider adding more insulation or upgrading to a higher R-value insulation, sealing any air leaks, installing energy-efficient windows, and using thermal mass materials like brick or stone. It is also important to regularly maintain and inspect your walls to ensure there are no gaps or damage that could contribute to heat loss.

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