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How to Measure or Calculate Building Heat Loss, Insulation, Heating Efficiency InteriorAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• How to measure or calculate heat loss (or gain) in a building
• 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?

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. Photograph at page top © 2007 Daniel Friedman. 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 (R) building supply center. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

What is a BTU or BTUH? A Definition of BTUs

When we are evaluating the quality and effectiveness of insulation in a building or the adequacy of a building heating or cooling system, we need to use measurements that permit us to describe the rate at which a building loses heat under various conditions (such as outdoor temperature, wind velocity, how leaky the building is, the area of its windows and perhaps doors, and the amount of insulation in the building walls, floors, and ceilings. A few of these critical definitions is given just below, followed by some simple formulas used to calculate the heat loss in a building.

Definitions of BTUs, BTUH, and Calories

Definition of BTUs and BTUH: a BTU is one "British Thermal Unit" which is defined as the quantity of heat that would be required to increase the temperature of one pound of water by one degree Fahrenheit.

A BTUH is defined as the number of BTU's lost (if we're talking about heat loss or air conditioning), or provided (if we're talking about providing heat for a building) in one hour. You'll often see BTUH as a number on data plates on air conditioners and on heating systems.

One BTU is also equal to 252 calories. So what's a calorie?

Definition of Calorie or Calories: a calorie is defined as the quantity of heat needed to raise the temperature of one gram of water by one degree Centigrade.

How do we measure heat transmission or movement through a building wall, insulation, or any other material?

How do we measure and express how well a building is insulated? or How much heat loss is ocurring at a specific building?

Many people have heard of using "R" values to describe "how good" a building's insulation is. Here we discuss three measures of the flow of heat out of or into a building: R-values, K-values, and U-values. Each of these is defined below. But before moving on to these basic concepts of building heat loss (or gain) theory, it is essential that this still more basic point be considered:

How leaky is the building with respect to heat loss (in a heating climate) (or gain in a cooling climate)?

It doesn't matter much how wonderful the building insulation is, how thick it is, or what the insulating material's "R" value is (see R defined below) if the building is leaky. If, for example, we're considering an older home with leaky windows or doors, or if we're considering a tall building with poorly controlled heat in winter, such that occupants of the upper floors are leaving windows open in winter then the heat flow out of these openings will be so terrific that the amount of insulation won't matter much.

What are the priorities when working to make a building energy efficient, warm, or cool?

Therefore when the object is to make a building more energy efficient, and before any more sophisticated analyses are performed using thermography, insulation evaluations, or even calculations of areas, "R" values, "K" values, or "U" values (defined below), remember this order of concerns when working for building efficiency. The order of magnitude of sources of un-wanted heat loss in a building are pretty much in this order:

1. Close open windows or doors when a building is being heated or cooled by other than "natural means" (like using fans, summer breezes or evaporative coolers in windows). Where older windows are leaking air but are otherwise in good condition, it may be most-economical to install a high quality, well-installed, storm window.
2. Investigate and cure leaky windows or doors that are producing drafts; also check for drafty wall or ceiling vent fan openings such as kitchen fans and whole house ceiling exhaust fans that have been left un-covered during the heating season.
3. Investigate and make sure that the top floor ceiling or attic floor (or cathedral ceilings) have been insulated, with no insulation voids or areas where insulation was removed or omitted
4. Investigate and consider installing or adding wall insulation.
5. Investigate and insulate any other un-insulated building perimeter areas such as the building rim joist or band joist accessed from a basement or crawl space.
6. Insulate under floors over uninsulated crawl spaces (we prefer to make the whole crawl space an enclosed and conditioned space).
7. Insulate building foundation walls below grade in basements or in conditioned-space crawlspaces.
8. Investigate the efficiency and state of tune of the building's heating or cooling equipment, including boiler or furnace and the condition of the heating or cooling delivery system (baseboards or ductwork, for example). (Warning: have heating systems cleaned and tuned by an expert before accepting a measurement of the system's efficiency.)

How to Really Foul Up a Radiant Heat Concrete Floor Installation - Mistakes to Avoid

This article has been relocated to Radiant Heat Floor Mistakes to Avoid where we describe installation specifications for radiant heat flooring in a poured concrete slab along with a detailed report of just how bad a radiant heat floor slab installation can be. The article's conclusions include this insulation advice:

• Insulate below the floor slab
• Insulate the slab perimeter, making sure that the insulation design does not rely on foam placed against the slab perimeter and extending above grade up to siding where it will invite termites or carpenter ants into the structure
• Place the radiant heat tubing at the industry-recommended depth down from the surface of the slab. Typically the maximum depth that tubing should be placed in a concrete floor slab is 2" down from the finished floor surface.
• If you cannot be present at the job site at critical stages in construction, find someone knowledgeable who can inspect for you before the work continues
• If your contractor is an opinionated bully, find someone else as soon as possible, even if his or her other work was good.

How to Calculate the R value U value & K value for a Building & How to Use These Numbers

Definition of & How to Calculate the R value or R-coefficient of Resistance to Heat Loss in a Building or its Insulation

The "R" value of a material is its resistance to heat flow through the material. When buying various insulation materials you will almost always see an "R" value quoted for the material. In general, higher "R" means more resistance to heat loss and therefore lower heating or cooling bills for the building.

Mathematically, "R" is simply the reciprocal of the two measures of resistance to heat flow "K" (R = 1/K) or "U" (R_{(whole building)} = 1/U) defined below. As you'll read next, "K" measures the heat flow through an individual substance and "U" measures the overall building heat loss by adding all of the various areas and substances together.

Definition of & How to Calculate the K value or K-coefficient of heat transmission

A building's K value or K-coefficient of heat transmission is one way to express the heat loss in a building. "K" is defined as the number of BTU's of heat moving through any material with these details:

• Per square foot of area of the material
• Per degree Fahrenheit of temperature difference
• Per inch of thickness of the material

So "K" takes a lot of variables into consideration and gives us the rate of heat loss per square foot of building surface, per inch of thickness of material in that building surface, per degree of temperature difference in Fahrenheit, in BTUs per hour.

By "degree of temperature difference in Fahrenheit" we mean that we are taking into consideration the difference in temperature on the two sides of our building surface. For example, if the indoor temperature in a building is 68 deg. F. and the outdoor temperature is 48 deg. F., then we have a 20 degree temperature difference on the two sides of the building (wall or roof for example).

This temperature difference on the two sides of a surface, say an insulated building wall, for example, is very important in understanding how a building loses heat (in the heating season) or gains heat (in the cooling season). That's because the rate of heat transfer through a material increases exponentially as a function of the temperature difference. This is intuitively obvious and is confirmed by physicists. For example, if the temperatures on either side of a building wall were the same, there would be no heat loss or gain through the building wall. As the temperature difference on either side of that same wall increases, say from one degree of difference to 20 degrees of difference the rate of heat transfer increases.

An interesting version of this heat transfer theory was shared with the author in a class on how to minimize building heating costs when the instructor told us that "the thermal conductivity of finned copper heating baseboard is exponentially greater at higher temperatures". He was saying that if we ran heating water from our heating boiler through the baseboards at 200 deg.F. we would see much more efficient heat transfer from the heating baseboards into the building. There are other factors involved in heating system efficiency such as the length of boiler "on" cycle (longer is more efficient), so there was more to think about, but the instructor was applying classic heat transfer theory that is reflected in the "K" values of building insulation as we've discussed here.

Definition of & 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".

How 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.) / (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.

How to make use of a home energy audit or free home energy use survey

A less precise and less computerized method for calculating building heat loss (or gain) is used by people who perform an "energy survey" or energy audit for a building. Home energy audit services may be free from your local utility company. The energy survey technician uses a pre-printed form whereon s/he records the areas of the building's walls, top floor ceilings, foundation walls, floors, and the number and type of windows and doors. An "R" value is assigned to these and the sheet is used to manually calculate the building's rate of heat loss. We had one of these "free" surveys performed on a home built in 1900 when we were renovating it years ago. Regrettably the surveyor was not very observant. He rated our walls at a very high rate of heat loss by assuming that they were not insulated whatsoever (and then proceeded to try to sell us an insulation service).

What that particular home energy audit surveyor failed to notice was that the building walls had been insulated (with blown-in foam) - a condition that was quite easy to see since we had removed the building's exterior siding and wall sheathing. He just didn't look. So while home energy audits are a great idea, make sure your auditor is awake before you believe the results of the home energy survey. And remember that some "home energy auditors" are really trying to sell you replacement windows (very long payback time) or building insulation. (Remember the urban legend about the home energy auditor who was using a camera light meter as an "energy loss" indicator to convince home owners that they needed new windows?)

Using infra-red or thermography to screen buildings for un-wanted heat loss, leaks, or heat gain points

Home energy loss surveys using thermography or simple infra-red thermometers are a great way to pinpoint individual points of heat loss (or unwanted heat gain) in a building. In the hands of a properly-trained expert (not a window salesman) this equipment can help find unexpected building air leaks or heat loss points even when you think that the building has already been insulated. Having a "high-R" insulated wall or ceiling is not going to be enough to make a building energy efficient if there are many unidentified air leaks or insulation voids in the building's walls, ceilings, or floors.

What is the Typical Design Temperature for Buildings and Building Insulation?

The "indoor design temperature" for a building refers to the assumed target indoor temperature that the building owner or occupants want. Typically 70 deg.F. is used unless the owner specifies something different.

The "outdoor design temperature" for a building is (for heating purposes) assumed to be the average lowest recorded temperature for each month between October and March (the heating season in most climates). If we are specifying a "design temperature" for cooling climates we'd use the average outdoor highest recorded temperature during the heating season, perhaps April through September.

What is a heating or cooling "degree day"?

Some building insulation designers and architects look at the number of "degree days" as an easy way to get at the average outdoor temperatures for an area and a season. A "degree day" is the daily average number of degrees Fahrenheit that the outdoor temperature is below 65 deg.F. The number of "degree days" during a heating season is easy to obtain: call your local oil delivery company or utility company. These energy providers keep close tabs on degree days for their area since this number is used in planning for the automatic delivery of energy. It's the number of "degree days" that have occurred in a given period, combined with a building's historic rate of heating oil use, for example, that tells an oil company when to schedule that building for an automatic delivery of heating oil.

Definition of Tons of cooling capacity

"One ton" of cooling capacity, historically, referred to the cooling capacity of a ton of ice. One ton of cooling capacity is the same as 12,000 BTU's/hour of cooling capacity. Tons of ice does not, however, explain an important factor in the comfort produced by air conditioning systems, reduction of indoor humidity - that is, removing water from indoor air. Cool air holds less water (in the form of water molecules or gaseous form of H2O) than warm air. Think of the warmer air as having more space between the gas molecules for the water molecules to remain suspended. When we cool the air, we in effect are squeezing the water molecules out of the air. When an air conditioner blows warm humid building air across an evaporator coil in the air handler unit, it is not only cooling the air, it is removing water from that air. Both of these effects, cooler air and drier air, increase the comfort for building occupants. One ton of cooling capacity equals 12,000 BTU's/hour of cooling capacity.

Table of Characteristics of Various Insulating Materials: fiberglass, mineral wool, cellulose, foam insulating board, UFFI, vermiculite, others

More Reading About Building Insulation

• Asbestos pipe insulation in buildings
• Brick "Insulation" in Building Walls
• Heat Loss Calculations, Insulation Properties, Definitions of R, K, U values, Insulation Design
• How to Choose an Air Conditioner - BTU Chart
• How to Detect and Correct Attic Condensation & Prevent Ice Dam Leaks in Buildings
• How to Inspect Building Interiors and Building Insulation/Ventilation list of articles about building insulation inspection, defects, design, and ventilation requirements
• Insulation Materials as Indicators of Building Age
• Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
• Insulation Identification Photographs - Cellulose insulation photos, Mineral wool insulation photos, rock wool insulation photos, cotton insulation photos, balsam wool insulation photos
• Insulation Identification Photographs - Vermiculite insulation photos
• LP or Natural Gas Pressures & BTUH per Cubic Foot
• Insulation Properties, Table of R-Values, density, moisture permeability, fire safety, aging effects on various insulation materials
• Mold in Fiberglass in Insulation
• Radiant Heat Floor Mistakes to Avoid
• Rated Cooling Capacity - How to Determine Air Conditioning Equipment Rated Cooling Capacity
• Un-Vented Roof Solutions - How to Prevent Attic Condensation, Ice Dam Leaks, Roof Mold, & Roof Structural Damage in Buildings with Un-vented Roof Cavities
• Vermiculite Building Insulation & Asbestos

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Use the links at the left of each page or these links just below to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

How to Calculate Heat Loss in a Building
Definition of BTU BTUH Calorie
How to measure heat movement through a wall
How to measure building insulation
How leaky is the building
Priorities for building energy efficiency
Radiant Heat Floor Mistakes to Avoid
Calculate the R U & K values
Calculate Heat Loss Per Hour
How to use a free home energy audit
Design Temperature for Buildings
Definition of "degree day"
Definition of Tons of cooling capacity
Table of Insulating Materials

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