How to Calculate Building Heat Loss, Insulation Values & Heating Efficiency
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How to calculate the rate of heat loss (or gain) in a building through insulation, walls, etc.
How to measure heat transmission in materials: definition of R-values, U-values, K-values, BTU, calorie, and rates of heat loss or gain
Building design temperatures & how to use a home energy audit or heat loss analysis
What insulation "R" values should be used in a building insulation?
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This article explains how to insulate a building and how much insulation is needed including how to measure or calculate heat loss in a building, defines thermal terms like BTU and calorie, provides measures of heat transmission in materials, gives desired building insulation design data, and shows how
to calculate the heat loss in a building with R values or U values. Be sure to review the definitions of R, U, & K Values at Definition of Heating & Cooling Terms. Because no amount of insulation can keep a drafty building warm, also review ENERGY SAVINGS PRIORITIES. Also see our detailed table of Insulation R-Values & Properties and our list of building HEAT LOSS INDICATORS (where is the building losing heat during the heating season, or gaining un-wanted heat during the cooling season).
Formula-R™ and Owens Corning™ which may be visible in this photograph of pink Styrofoam™ insulation boards are registered trademarks of Owens Corning® and were photographed at a Home Depot® building supply center.
How to Calculate the U value or U-coefficient of heat loss resistance
Computing "K" values tells us the heat loss rate for a specific material, thickness, area, and temperature difference but while
we need to be able to calculate "K" values, those alone don't tell us what's going on in an actual building. We need to be able to combine all of the rates of heat loss (or gain) across all of the types of surfaces, insulation, and building material for the
whole building - at least for all of its external or perimeter surfaces including roofs, walls, and floors as well as windows and doors.
That's where the "U" value makes its appearance.
A building's "U" value or U-coefficient of resistance of heat loss is a related measure of resistance
to thermal energy or heat flow out of a building (if it's warmer inside than outside) or conversely
the same concept works in a warm climate where air conditioning is in use, except that we expect outside heat to be flowing into the building.
A building's "U" value is much more complete, and therefore useful than "K" values alone because
a building's "U" value combines the "K" factors for all of the building's surfaces and
materials. In other words, we add the effects of heat loss (or gain), still expressed in the
number of BTU's per hour per square foot of area, and still expressed per degree of Fahrenheit of temperature
difference and still expressed per inch of thickness of material (just as with "K" values), for
all of the substantial areas and surfaces of the exterior of a building's floors, walls, windows, doors,
ceilings, or roofs (if cathedral ceilings are present).
To calculate the "U" value, or overall heat loss (or gain if we're air conditioning) for a building, we need to
add the "R" values for each material in the structure, and to factor in the total area of each material in the structure.
We discuss this procedure in more detail below at "Calculating Heat Loss for a Building".
Formulas Used to Calculate the Rate of Heat Loss Per Hour for a Building Using it's "R" Values or "U" Values
Luckily, after having already discussed "K" values, "U" values, and "R" values as measures of heat loss just above,
calculating a building's actual rate of heat loss is pretty simple - it's a "cookbook" process that uses the
following formula:
Heat Loss using "R" values:
(Building Heat Loss in BTU's per hour) = [(Building Total Surface Area in sq.ft.) / (Surface Area "R" value)] x (Temperature Difference)
Temperature Difference = the difference in temperature in deg F. on the two sides of the building surface, typically indoors and outdoors
Surface Area "R" value = the "R" value of the surface area being evaluated (say an insulated wall).
Heat Loss using "U" values:
(Building Heat Loss in BTU's per hour) U = 1/R, - or in other words -
(Building Total Surface Area in sq.ft.) x (Surface Area "U" value) x (Temperature Difference)
More considerations when measuring home energy use or heat loss
But there's more work to do for a complete answer to building heat loss. We need to make up a simple table which will contain
the total surface area of each type of material (since each will have it's own "R" value) and then plug in the area's "R" value
and the temperature difference. Usually we assume the same temperature difference for all of the areas of the building though this might
be a simplification since that may not be exactly true.
How to include the effect of wind on home energy use or heat loss
We're also missing, from this simple calculation, the effects of wind on a building's heat loss, though a more sophisticated version of this approach might simply adjust the temperature difference to include
the wind factor. For example, you could use a wind/temperature chart to derive the effective outdoor temperature when it's also windy.
In cold conditions, adding a wind velocity will lower the effective outdoor temperature and thus it will increase the temperature difference across the building wall. Use any "wind chill factor" chart for this data. Still more sophistication
of measures of heat loss are possible by adding the effects of moisture on heat loss from a surface, but while this is important
for a (sweaty) human in cold conditions it is generally ignored when considering building heat loss.
Using a spreadsheet to accurately calculate building heat loss or heat gain
This is a perfect application for an Excel or similar spread sheet, listing each building surface type (wall, window, door),
it's R, K, or U value, and its total area. Adding temperature difference across these surfaces permits a calculation of the
heat loss (or gain) through each surface type. These are simply added together to represent the entire building's heat loss or gain.
Heat loss vs. heat gain in buildings: applying the simple laws of thermodynamics
You may have noticed we keep talking about heat loss and then we add "or heat gain" in the same sentences or headings.
That's because heat loss analysis works just fine for both building heating and building cooling. The only differences
between looking at heat loss and heat gain for a building are the direction of heat flow and the fact that we may
be using different equipment with different equipment efficiencies (a heating furnace or boiler versus an air conditioner).
If we're in a heating climate and are in the heating season, heat will flow from the building interior to the outdoors.
If we're in a cooling climate and are in the cooling season, heat will flow from the outdoors to the building interior. Just remember that (according to the
laws of thermodynamics), heat (or energy) always flows from the warmer (or more exited state) into the cooler (or less excited state) area of a building.
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