Part II: Conservation FirstChapter 2. Find the Leaks
Home Heat-Loss Basics: Where Your Energy Goes
A map of conduction, air leaks, and hot water so the biggest losses are obvious before you build.
- Difficulty
- Beginner
- Cost
- Free
Heat leaves a house by conduction through the walls, roof, windows, and floors, and by air that leaks out and is replaced by cold air. Hot water is a separate bill, and it is often large enough to matter before you build a collector. You can rank those paths with a tape measure and a year of utility bills.
The ranking is a sketch. A full room-by-room load calculation accounts for thermal bridges, sun, wind, and the way rooms are used. You do not need that to notice that a ceiling at R-11 and a wall of single-pane windows are doing different jobs. You do need to avoid the common mistake of adding a solar heater to a house that is losing most of its heat through an unsealed attic.
Difficulty
- Level
- Beginner
- Time
- An evening with the bills, plus an hour with a tape
- Skills
- Reading a utility bill and measuring rooms
Cost estimate
Free
The calculator on this page runs in the browser. A thermometer is useful and most houses already have one. An infrared camera is optional and not required for the ranking.
Materials
| Item | Qty | Notes |
|---|---|---|
| A year of heating bills | 12 months | Gas, oil, propane, or electric heat. One winter month is not a year. |
| Electric bills, if hot water is electric | 12 months | Used to separate water heating from space heating. |
| Tape measure | 1 | Walls, ceilings, windows, and a rough volume. |
| Notebook | 1 | Write areas and the R-values you believe, and mark which ones are guesses. |
Two equations that are enough to rank a house
Conduction through a layer of insulation is proportional to area and to the temperature difference, and inversely proportional to the R-value. In US units, BTU per hour is roughly the area in square feet times the temperature difference in degrees Fahrenheit, divided by R. A window is usually quoted as a U-factor instead, and U is 1 divided by R, so the same heat flow is area times U times the temperature difference.
Air leakage is the other large term. A common estimate for the heat in the air you exchange is 0.018 times the air changes per hour times the heated volume in cubic feet times the temperature difference. That 0.018 is a property of air, not a fudge factor for your house. The number you do not know is the air changes per hour. A tight new house may be near 0.2 to 0.4 under natural conditions. An older unsealed house can be 0.8 to 1.5 or more on a windy day. If you have never measured, try the calculator at 0.35 and again at 1.0 and see whether the ranking changes.
Neither equation includes the sun shining in, the heat from people and cooking, or the wood framing that bypasses the insulation. Use them to compare surfaces, not to promise a furnace size.
How to fill the calculator without inventing precision
Measure the ceiling under the attic, the exposed walls, and the windows. Ignore interior partitions. If a floor sits over a heated basement, leave the floor area at zero. If it sits over a vented crawlspace or an open garage, include it.
Use R-values you can defend. An insulated 2x4 wall is often called R-13 in the cavity and performs closer to R-10 once the studs are included. A bare ceiling with 3.5 inches of fiberglass is about R-11. A foot of blown cellulose is roughly R-40. Single-pane glass is near U-1. A common double-pane unit is near U-0.5. Newer low-e double pane can be near U-0.3. If you do not know, pick the worse plausible value and write "guess" next to it.
For the temperature difference, 50 or 60 degrees Fahrenheit is a cold design night in a lot of the middle of the United States, not an average winter day. The sketch scales in a straight line, so the shares stay similar if you change the difference, as long as every path sees the same outdoor temperature. A floor over a 50 degree crawlspace does not see the full outdoor difference. If that matters, lower the difference in your head for that one line rather than trusting the floor number on a design night.
What the bills tell you that the sketch cannot
Add up a full year of heating fuel. The calculator's BTU per hour is a rate on a cold day. The bills are the actual energy. They will not match a single multiplication, and they should not. The useful comparison is the shape of the year: a sharp winter peak is space heat, a flat monthly use that never goes away is more likely hot water, refrigeration, and other base load.
If heat and hot water share one gas meter, look at the summer months, when the heating system is idle. That summer gas is mostly water heating and cooking. Subtract a typical summer month from each winter month for a rough picture of space heat. It is rough because you also take longer showers in January and because a pilot light runs all year.
Electric hot water often shows up as a large, steady share of the electric bill. It is a poor target for a photovoltaic array until you have cut the waste: a leaking faucet, a tank set to scalding, or a recirculation loop that runs all night. Those are conservation jobs, not reasons to buy more modules.
Where projects usually sit in the ranking
In a house with a thin attic and loose hatches, ceiling conduction plus attic air leaks often beat the walls. Windows are a small area with a terrible U-factor, so they show up far above their share of the wall. A south window is also a heater on sunny days, which this sketch ignores on purpose. Do not "fix" a south window by covering it in a way you cannot open to the sun.
Air sealing and insulation work together. Insulation on top of an unsealed ceiling does less than the R-value suggests, because the air is leaving through the hatch and the wire holes. The air sealing weekend is the usual next page when the leak line or the ceiling line is near the top of the list.
A solar air heater or a sunspace is a supply project. It belongs after the largest losses are identified, and often after the cheap seals are done. Building it first can still be a good way to learn, as long as you do not expect it to erase a ceiling that the calculator just flagged.
Spread out a year of bills
Separate space heat from hot water as well as the meters allow. Note the units: therms, gallons, kilowatt-hours. Do not rank the house by the dollar column alone, because rates change.
Measure the envelope
Ceiling, exposed walls, windows, and any floor over unconditioned space. Multiply floor area by ceiling height for a volume if the rooms are simple. Subtract nothing for closets. This is a sketch.
Assign R-values you can explain
Use the labels in the attic if the insulation is marked, or a thickness and a typical R per inch. Mark guesses. For windows, use U near 1 for single pane and near 0.5 for older double pane.
Run the sketch twice
Once with a tight air-change guess and once with a leaky one. If the winner changes, your next measurement is leakage, not another decimal place on the wall area.
Pick one action
Seal and insulate the worst surface, or accept that the house is already tight and move on to a supply project. Write the decision in the notebook so the next guide starts from a number.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/Conservation/conservation.htm
- https://www.builditsolar.com/Projects/Conservation/BlowerDoor/BlowerDoor.htm
- https://www.builditsolar.com/Projects/Conservation/BlowerDoor/BuildingBlowerDoor.htm
- https://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
- https://www.builditsolar.com/Experimental/IRCamera/IRCamera.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.