Definitions and Units

In keeping with the standards of the US, this website uses the archaic British units rather than the more widely used metric ones. Apologies to anyone reading this from outside the US...someday metric conversions will be added (after I become familiar with them again).

U value - the fundamental unit of heat flow, given in BTU/hr/ft²/°F.  The metric equivalent is W/m²/°K.  To convert to metric, multiply by .17611, and from metric multiply by 5.5782.  Note that there is some confusion in listed value between the overall U value of a material, and the value per unit thickness (ie per inch, per meter).  Materials with greater insulating values have lower U-values.

R value - the inverse of U value.  Materials of greater insulating values, have higher R values.

Note that R values (and the corresponding U value) are measured in a specific steady state condition, which typically minimized radiant and convective heat transfer.  Actual conditions may vary the effective R value quite a bit.  Wind on a wall can increase heat loss by increasing convective transfer, while sun shining on a wall may raise its temperature to much greater than outdoor air temperature, either slowing the rate of heat loss from inside (winter), or increasing the rate of heat gain from outside (summer). The key is that R value determines the heat loss based on the surface temperature of the two sides, not the air temperature of the sides, which may not be the same.

The R-value of materials will change somewhat with temperature, and in some materials will decay over time (for example foam blown with CFCs, where the CFCs leak out over time).  R values of insulation material, especially lose fill or soft batts are specified for a specific density.  Up to a point, packing them more dense will increase the R-value, but beyond that point conduction increases and the R-value goes down.

In some situations radiant transfer or the effect of thermal mass can make the effective R-value very different from the actual one. Radiant barriers work by reflecting radiant heat while also reducing the heat radiated in the opposite side (but since they're metal, have high conduction rates).  Walls of high thermal mass that absorb solar heat during the day have higher effective R values because the hold heat throughout the night, reducing the heat loss from the interior air. (more detail in the insulation section)

BTU - the amount of heat necessary to raise 1Lb of water, 1 degree F.  Btu's are a total amount of heat, not a flow rate.   The metric equivalent is watt-hours, or kilowatt hours.  To convert BTU to kilowatt hours, divide by 3413.

BTU/hr - a heat flow rate.  The metric equivalent is watts, where 1 watt=3.413 Btu/hr.

Kilowatt - a rate of energy flow, equal to 1000 watts.  In the US, electricity is the only energy measured in these units.

Killowatt-hour (Kwh), is a total amount of energy used.  A light bulb is measured in watts (flow rate it uses electric), but your bill is in kilowatt-hours, which is a total amount of electric used, accounting for how long the light bulb is turned on for.  In the scientific world, total energy is often expressed as Joules, which is one watt used for one second, the Joule is too small a unit, so Kwh is used.

Therm - one therm is equal to 100,000 BTU. Most Natural gas companies in the US sell gas by the term.

Degree-day - this typically refers to a heating degree day (HDD), which is calculated as the average difference between daily average outside temperature and an inside temperature of 65F, summed up over all the days of the heating season.  Only days (or the part of the day) where the temperature is colder than 65 outside are counted. These values are calculated from historical weather data, and so are averages.  In any given year, the actual heat need can vary by 10% or so.   If the average outside temperature on a given day is 40F, then that day represents 25HDD. If there were 100 days in the heating season at this temperature, this climate would have 2500HDD.  Typical values: LA is around 2000HDD, Seattle around 4500HDD, NYC is around 5000HDD, and Fargo is around 10000HDD.

Degree days are used to estimate annual energy use, but because they're based on an inside temperature of 65, and most people keep their houses at 68-70, these estimates will be low.

Cooling degree days (CDD), work the same way, but for the cooling season.  Alas, they also use a base temperature of 65F, which means the values are even less realistic if the interior is kept at the suggested 75-78F.

CFM -Cubic feet per minute. This is a rate of airflow, and is how exhaust fans and a home's air leakage are often measured. To calculate heat loss due to air flow you need to know how many cubic feet of warm air escapes, how much heat a cubic foot holds per degree of temperature (.018BTU) and the difference in temperature between the indoor air, and the cold outdoor air that will be replacing it.

ACH - Air Changes per Hour. This tells you how many times per house all the air in a house leaks out. Like CFM, this is a rate of air flow, but rather than a measured rate, it is relative to the volume of the house. You can convert one to the other using the volume of the house:
  ACH = (CFM*60)/Volume.

Pascal - a unit of pressure (in metric units).  It is equal to 1 Newton per square meter. It is a relatively small unit of pressure.  One PSI, the unit often used for tire pressure, is equivalent to 6,894 Pascals.