What a blower door number actually is
A blower door fits a calibrated fan into a doorway and pulls the house to 50 pascals below outdoor pressure, then reports how much air the fan has to move to hold it there. That is CFM50. Divide it by the volume of the house and multiply by 60 and you get ACH50, the number of times the whole volume is replaced per hour at that pressure.
Fifty pascals is roughly what a twenty mile per hour wind does to one side of a building, applied uniformly to the whole shell. It is chosen because it swamps ordinary weather, which makes the test repeatable, and because it puts the fan in a range where it measures accurately. What it is not is a description of a normal day.
To get from the test to an annual energy figure you need a natural infiltration rate, and the usual approach is to divide ACH50 by a factor. That factor is where the honesty of the whole calculation lives. It depends on how windy and cold the climate is, how tall the building is, how sheltered it is by terrain, trees and neighbours, and how the leaks are distributed between the top and the bottom of the shell. Values in general use span roughly 14 to 24, and this page makes it an input with a sensitivity table rather than burying one number in the code.
The energy arithmetic, which is the easy part
Once you have an airflow in CFM, the heat carried out by that air is 1.08 times CFM times the temperature difference. The 1.08 is 60 minutes times the density of air at standard conditions, about 0.075 pounds per cubic foot, times its specific heat of 0.24 BTU per pound per degree. That gives BTU per hour.
To turn an hourly figure into a yearly one you need the temperature difference summed across the year, and that is exactly what heating degree days are. One degree day is one day at one degree below the base temperature, so degree days times 24 gives degree-hours. Annual infiltration heat loss is therefore 1.08 times CFM times 24 times HDD.
A 12,000 cubic foot house going from 8 to 5 ACH50, at a divisor of 18, drops from about 89 CFM of natural infiltration to about 56, a saving of 33 CFM. Over 5,000 degree days that is about 4.3 million BTU. At 85 percent seasonal efficiency the furnace has to burn about 5.1 million BTU to have delivered it, which is about 51 therms of natural gas, or around 71 dollars at 1.40 a therm. The heating degree day calculator will get you a local HDD figure calibrated against your own past bills.
Where air sealing sits against everything else
| Work | What it addresses | Typical character |
|---|---|---|
| Air sealing the attic plane | The largest bypasses, where warm air exits | Cheap in materials, high in awkwardness, usually the best value in the house |
| Attic insulation top-up | Conduction through the ceiling | Cheap per R-value, but worth much less if done before sealing |
| Sealing the basement or crawlspace | The intake end of the stack effect | Rim joists and the sill plate, often the second best value |
| Window replacement | Conduction and some leakage through the openings | Expensive per unit of energy saved, better justified on comfort or condition |
| Wall insulation retrofit | Conduction through the largest area of the shell | Effective but invasive, and it changes the moisture behaviour of the wall |
The reason air sealing generally leads is that air leaks are concentrated. Most of a house's leakage comes from a small number of large holes, usually where the ceiling plane is interrupted, and closing them is a day of foam and caulk rather than a construction project. The home energy audit guide covers how to find them, and the heat loss calculator shows the conduction side beside the infiltration side so you can see which one is driving your total.
The limit that matters more than the money
A house that ventilated itself through its leaks stops doing so once the leaks are gone, and that transition happens somewhere around 3 air changes per hour at 50 pascals. Below that, outdoor air has to be brought in deliberately, and the moisture generated by cooking, showering and simply breathing has to be taken out deliberately. Skipping that step is how a tightening project produces window condensation, musty smells and worse.
The sharper version of the same problem is combustion safety. An atmospherically vented water heater or furnace draws its air from the room and relies on a chimney to take flue gases away. Tighten the shell, add a range hood and a dryer, and the house can go negative enough to pull those gases back down the flue. This is not a hypothetical and it is not something to reason about from a calculator. If there is any atmospherically vented combustion appliance in the house, a professional with a combustion analyser needs to check draft under worst-case depressurization after the work, and the whole house ventilation calculator is where the deliberate air supply gets sized.
Questions people ask
How do I convert ACH50 to natural air changes?
By dividing by a factor, and the honest answer is that the factor is uncertain. It depends on climate severity, the height of the building, how sheltered it is, and where the leaks sit vertically in the shell. Published divisors in general use fall roughly between 14 and 24, with lower values for tall exposed buildings in cold windy places and higher ones for sheltered single storey houses in mild climates. This page makes it an input and shows what the answer does across a range, because a calculator that picks one divisor and presents the result as fact is hiding the largest uncertainty in the calculation. Treat any number produced this way as an approximation.
What is a good ACH50 for a house?
That depends entirely on what the house is and what code applies, and the number that matters for your project is whatever your jurisdiction has adopted rather than any general figure. As orientation rather than as a requirement: older houses with no air sealing history commonly test somewhere in the high single digits or above, houses that have had serious retrofit work land considerably lower, and purpose-built low-energy construction goes lower still. What is much more useful than a target is the before-and-after pair on your own house, because that measures the work rather than the house. Ask the tester for both readings and the raw CFM50 figures.
Is air sealing better value than adding insulation?
In most existing houses that have not had either done, air sealing the attic plane comes first, and it is not usually close. Air leakage is concentrated in a small number of large bypasses, closing them costs materials rather than a project, and insulation blown over an unsealed ceiling is filtering air on its way out rather than stopping it. There is also a sequencing reason: once there is a foot of loose fill on the attic floor you can no longer find the bypasses. Do the sealing, then the insulation, then look at the rest. Run both sides through the heat loss calculator for your own house and the split between conduction and infiltration will tell you where you stand.
Can a house be too tight?
A house cannot be too tight, but it can very easily be under-ventilated, and the two get confused. The principle in current practice is to build tight and ventilate right: control the air deliberately rather than leaving it to holes, because holes deliver air unpredictably, in the wrong places, and carry moisture into assemblies on the way. What that requires is a ventilation system sized and running, rather than an assumption that the house breathes. Below roughly 3 ACH50 that becomes essential rather than optional. The other hard limit is combustion safety, which needs testing by someone with the right instruments after any significant tightening.
Why is my payback so long?
Usually because the leakage reduction was modest, the climate is mild, the fuel is cheap, or all three. Infiltration is only one of the two heat loss terms and in a well insulated house in a mild climate it is a small annual number in absolute dollars. That is worth saying plainly rather than working the assumptions until the answer improves. It is also worth noting that comfort does not follow the same arithmetic: draughts, cold floors and a room that will not hold temperature go away long before the bill notices, and for most people that is the actual reason the work was worth doing. Do not include any rebate or incentive in the payback unless you have confirmed it applies to you.