We know that the primary heat losses for the house come from conductive heat flow (heat moving through the structure molecule by molecule from the hot side to the cool side) and convective heat flow (heat carried around by all the airflows moving in an out of the house). We know that a U value is the ability of an element to conduct heat. We can add up all the conductive U values for the walls and windows and doors and ceilings and such, and we can estimate what the convective U value is for air leaking in and out through all the cracks and holes. And we can add all those U values up to come up with a total U value for the house.
That number is reflected in how much energy your house has used to stay warm and how cold a year it has been (which we know from the Heating Degree Days). This particular house is in Falmouth, Massachusetts. The degree days for this past year are:
| Month | Heating Degree Days |
| January | 732 |
| February | 733 |
| March | 538 |
| April | 419 |
| May | 181 |
| June | 84 |
| July | 3 |
| August | 3 |
| September | 65 |
| October | 215 |
| November | 606 |
| December | 572 |
| Total | 4151 |
We can determine the Total U value for the house from the formula:
Heat Energy Used/(HDD x 24)
It’s pretty simple: There are 24 hours in the day and when it’s colder outside, the house uses more heat! We know from the gas bill, that a total of 37 therms were used in January. Last week, we figured that about 5 therms per month are used for cooking and the heating hot water for the shower. The house used 37 – 5 or 32 therms for heating in January.
We have to step back here for a second and address the fact that the heating system is not 100% perfect. When the gas furnace burns the gas, some of that heat goes up the chimney. Some heat is lost because of an imperfect duct system – missing insulation or leaks in the joints. So in this case the system is about 80% efficient (that’s a reasonable average). So if a therm of natural gas has a potential 100,000 BTUs, only 80,000 are being delivered in this house. So in January, the house used 32 x 80,000 or 2,560,000 BTUs for heat. Dividing that by the HDD times 24 gives us a Total U value for January of 145.7. We can do that for the whole year and come up with an average, Total U value for the house.
| Month | Heating Degree Days | Therms Used (Part 8 Bill) |
U Value |
| January | 732 | 37 – 5 = 32 | 146 |
| February | 733 | 44 – 5 = 39 | 177 |
| March | 538 | 41 – 5 = 36 | 233 |
| April | 419 | 25 – 5 = 20 | 159 |
| May | 181 | 8 – 5 = 3 | 55 |
| June | 84 | 7 – 5 = 2 | 79 |
| July | 3 | 0[1] | |
| August | 3 | 0 | |
| September | 65 | 0 | |
| October | 215 | 0[2] | |
| November | 606 | 16 – 5 = 11 | 61 |
| December | 572 | 29 – 5 = 24 | 140 |
| Total | 4151 | Average: 130 |
The total U value for this house is 130. That is the sum of the heat loss through all the structural components as well as the convective losses. What can you do with that number? If you are planning on making improvements to your house, you can monitor the changes to the total U. If insulation is being added to your attic or you are going to replace some windows, those improvements should reduce the total U.
When you figure out the U value of individual components, you can use that information to determine if an improvement is going to pay off or not.
Next time: Component U Values
Another great article, Paul! One little clarification, though: I think what you’re calling the total U value is better thought of as the average U value times the total area of the building enclosure. In the IECC and other places, you see reference to a UA analysis, and that’s basically what you’re doing here. This is what allows you to do tradeoffs in new homes that have to meet the code’s requirements for heat loss/gain. You meet it through a prescriptive path for each part of the building assembly (e.g., walls have to be R-13, ceilings R-30, floors R-19), or you can make sure the whole building enclosure meets a certain level of efficiency through the type of analysis you discuss here. Right?
Allison – Oh, you techy! Thanks for the kind words and the input. When I started with this stuff, we used to total up all the UA stuff from the windows, doors, walls, etc. and come up with a Total U value. It includes both the conductive and the convective U values therefore it is not just limited to the “total area of the building enclosure”. And since the heat loss from the building which is made up for by the amount of energy used to maintain the temperature in the house over the course of the year, we can use the energy used and the HDD to determine a Total U value. But I could be wrong! It happened once before.