Calculating the energy savings of an insulated cavity in home wall construction

[ioan]

I am a client who bought a new house and the cavity in the wall was not insulated.

I can calculate the U value of the uninsulated wall and the U Value of the wall if it were insulated correctly.

front the two different U values I want to calculate how much extra energy is being lost through a meter square of the uninsulated wall.

I want this figure is Kw per meter square. Then I can measure the area of the wall and by using a typical heat of 21 degrees C and a typical heating pattern calculate how many additional Kws they have had to purchase since they bought the house.

Any help would be very welcome.

 

[anorlunda]

Start with the definition of R value.
https://en.wikipedia.org/wiki/R-value_(insulation)#R-value_definition.

Then, I found this source for uninsulated walls. Natuarlly, it depends on construction methods and materials.

https://builditsolar.com/References/Calculators/InsulUpgrd/RValues.htm

Concrete and Block Walls:
Above grade concrete wall including inside and outside air films = R 1.2 for 6 inch, R1.4 for 8 inch

Concrete block wall with air in cavities = R 2.0

Concrete block wall with Perlite in cavities = R 2.9

 

[jrmichler]

Then I can measure the area of the wall and by using a typical heat of 21 degrees C and a typical heating pattern calculate how many additional Kws they have had to purchase since they bought the house.

From your U values, you have the heat loss per square meter for both insulated and uninsulated walls. The heat loss has units of Kw per square meter per degree. Carefully check the units of your U values because they may or may not be in Kw. Multiply by 24 to get Kwh per day per square meter.

Then you find the heating degree days (HDD) for your area. Here's a good description of what HDD is in an area that uses metric units: https://safety.networkrail.co.uk/ho...nagement/energy-management-tools/degree-days/. I used search term annual heating degree days to find it.

Then multiply the Kwh per day per degree times the degree days per year to get Kwh per year. Do this for both the insulated and uninsulated walls, then subtract to find the savings. I used this method to estimate the annual gas usage for my house, and it agreed nicely with the total gas consumption. If you want the total heat loss for the house, calculate walls, windows, roof, doors, and foundation separately, then add the results. A good way to check your calculations is to do this calculation for the entire house, and compare to the annual heating bills.

The total heat loss in my house was calculated to be 13,200 BTUH (3.9 Kw) at 70 deg F inside and -20 deg F outside, and measurements confirmed that number. My own house is in an area where the average heating load is 9500 heating degree days per year, based on 65 degree F base and degrees F.