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AIR CONDITIONING & HEAT PUMP SYSTEMS INSULATION & VENTILATION INSPECTION & IMPROVEMENT INTERIORS of BUILDINGS ACOUSTICAL SEALANTS AGE of a BUILDING - how to determine AIR BYPASS LEAKS AIR LEAK DETECTION TOOLS AIR LEAK MINIMIZATION AIR SEALING STRATEGIES ANIMAL ALLERGENS APPLIANCE EFFICIENCY RATINGS ASBESTOS IDENTIFICATION IN BUILDINGS ATTIC LEAKS, CONDENSATION & ATTIC MOLD ATTIC VENTILATION BASEMENT HEAT LOSS BASEMENT LEAKS Moisture or Mold BASEMENT WATERPROOFING BATHROOM VENTILATION BLOWER DOORS & AIR INFILTRATION BRICK LINED WALLS BUCKLED FOUNDATIONS due to INSULATION? CATHEDRAL CEILING INSULATION CHIMNEY INSPECTION DIAGNOSIS & REPAIR COMBUSTION AIR for TIGHT BUILDINGS CRAWL SPACES Crawl Space Dryout Procedures Crawl Space Safety Advice Crawlspace Mold Advice MOLD CLEANUP by MEDIA BLASTING Mold on Dirt Floors DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE DEFINITION of Heating & Cooling Terms Definition of BTUs, BTUH, & Calories Definition of K value K-coefficient heat transmission Definition of U value or U-coefficient heat loss resistance Definition of R-Values for Insulation or Buildings Definition of Design Temperature for Buildings Definition of Heating or Cooling "Degree Day" Definition of Tons of Cooling Capacity ELECTRICAL INSPECTION, DIAGNOSIS, REPAIR ENERGY SAVINGS in BUILDINGS ENERGY SAVINGS PRIORITIES ENERGY SAVINGS RETROFIT CASE STUDY ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE ENERGY SAVINGS RETROFIT OPTIONS ENVIRONMENTAL HAZARDS FIBERGLASS INSULATION FIBERGLASS HAZARDS FIBERGLASS MOLD FIREPLACE Damage & Unsafe Hearths - Settlement FLAT ROOF MOISTURE & CONDENSATION FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP FLOODS IN BUILDINGS-mold FLOORING CHOICES OVER CONCRETE SLABS FLOORING TYPES & DEFECTS FOUNDATION WATERPROOFING FRAMING DETAILS for BETTER INSULATION FRAMING DETAILS for DOUBLE WALL HOUSES FREEZE-PROOF A BUILDING HEAT LOSS in BUILDINGS How to measure heat movement through a wall How to measure building insulation How leaky is the building BASEMENT HEAT LOSS ENERGY AUDIT - How to Use a Free One ENERGY SAVINGS PRIORITIES HEAT LOSS CALCULATIONS HEAT LOSS DETECTION TOOLS HEAT LOSS INDICATORS HEAT LOSS PREVENTION PRIORITIES HEAT LOSS R U & K VALUE CALCULATION Heat Tapes: Use on Roofs for Ice HEATING SYSTEMS HEATING COST FUEL & BTU Cost Table HEATING COST SAVINGS METHODS HEATING LOSS DIAGNOSIS-BOILERS HEATING LOSS DIAGNOSIS-FURNACES HEATING SYSTEM INSPECTION PROCEDURE HOUSEWRAP AIR & VAPOR BARRIERS HOUSEWRAP - TYVEK INSTALLATION DETAILS HUMIDITY LEVEL TARGET ICE DAM PREVENTION Ice Dams: Comparing Two Houses INDOOR AIR QUALITY & HOUSE TIGHTNESS INDOOR AIR QUALITY IMPROVEMENT GUIDE INSULATION CHOICES INSULATION FACT SHEET- DOE INSULATION for GREENHOUSE or SOLARIUM INSULATION IDENTIFICATION GUIDE INSULATION LOCATION in BUILDINGS - WHERE TO INSULATE INSULATION LOCATION for BASEMENTS INSULATION LOCATION for BRICK VENEER WALLS INSULATION LOCATION for CAPES, CRAWLSPACES INSULATION LOCATION for CATHEDRAL CEILINGS INSULATION LOCATION for GREENHOUSE or SOLARIUM INSULATION LOCATION for PASSIVE SOLAR FLOOR SLAB INSULATION LOCATION for SOUND CONTROL in BUILDINGS INSULATION LOCATION for SWIMMING, INDOOR INSULATION MOLD INSULATION R-Values & Properties INSULATION & VENTILATION INSPECTION & IMPROVEMENT INSULATION R-Values & Properties Air Bypass Leaks, Thermal Tracking AIR LEAK DETECTION TOOLS AIR LEAK MINIMIZATION AIR SEAL STRATEGIES Asbestos Identification in Buildings ATTIC CONDENSATION CAUSE & CURE BASEMENT HEAT LOSS Blocked Soffit Intake Vents BLOWER DOORS & AIR INFILTRATION BRICK VENEER WALL INSULATION CATHEDRAL CEILING INSULATION ENERGY SAVINGS in BUILDINGS ENERGY SAVINGS PRIORITIES ENERGY SAVINGS RETROFIT CASE STUDY ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE ENERGY SAVINGS RETROFIT OPTIONS FIBERGLASS DUCT, RIGID CONSTRUCTION FRAMING DETAILS for BETTER INSULATION FRAMING DETAILS for DOUBLE WALL HOUSES Inspect Attics for Moisture or Mold Inspect Basements for Moisture or Mold Inspect Building Exterior Inspect the Ridge Vent System from the Attic Inspect the Soffit Vent System from the Attic Insulation Air & Heat Leaks INSULATION for GREENHOUSE or SOLARIUM INSULATION CHOICES INSULATION FACT SHEET- DOE INSULATION PLACEMENT in BUILDINGS INSULATION R-Values & Properties LOG HOME WALL INSULATION VALUES POLYISOCYANURATE FOAM INSULATION POLYSTYRENE FOAM INSULATION SUPER HI-R INSULATION RADIANT BARRIERS RIGID FOAM USE INDOORS ROOF VENTING ENERGY SAVING DETAILS ROOF VENTING NEEDED? PASSIVE SOLAR DESIGN METHOD PASSIVE SOLAR HEAT PERFORMANCE PASSIVE SOLAR HOME, LOW COST PHOTOVOLTAIC POWER SYSTEMS SLAB INSULATION, PASSIVE SOLAR Urea Formaldehyde Foam Insulation UFFI VAPOR BARRIERS & AIR SEALING at BAND JOISTS VAPOR BARRIERS & CONDENSATION in BUILDINGS VAPOR BARRIERS & HOUSEWRAP VAPOR CONDENSATION & BUILDING SHEATHING URETHANE FOAM Deterioration, Outgassing Vermiculite Insulation LOG HOME GUIDE MOBILE HOME INSPECTIONS MOISTURE CONTROL in BUILDINGS Mold Growth Resistance of Foam Insulation MOLD INFORMATION CENTER ODORS & SMELLS DIAGNOSIS & CURE PAINT FALURE, DIAGNOSIS, CURE, PREVENTION PLASTER & BEAVERBOARD & DRYWALL RADIANT BARRIERS RADIANT HEAT RADIANT HEAT Floor Mistakes to Avoid RADIANT SLAB FLOORING CHOICES RADIANT SLAB TUBING & FLUID CHOICES RIGID FOAM USE INDOORS ROOF VENTILATION SPECIFICATIONS ROT, FUNGUS, TERMITES ROT, TIMBER FRAME SAFETY HAZARDS & INSPECTIONS SEARS KIT HOUSES SHEATHING, FOIL FACED - VENTS SOLAR ENERGY SYSTEMS PASSIVE SOLAR DESIGN METHOD PASSIVE SOLAR HEAT PERFORMANCE PASSIVE SOLAR HOME, LOW COST PHOTOVOLTAIC POWER SYSTEMS SLAB INSULATION, PASSIVE SOLAR SOLAR COLLECTOR AIR or GAS COLLECTION SOLAR COLLECTOR FILMS SOLAR COLLECTOR WOOD HOUSINGS SOLAR HEATING SYSTEMS SOLAR HOT WATER HEATERS SOLAR HOUSE EVALUATION SWIMMING POOL SOLAR HEAT, INDOOR SWIMMING POOL SOLAR HEAT, OUTDOOR DIAGNOSIS SOUND CONTROL in BUILDINGS STAIN DIAGNOSIS STAIRS, RAILINGS, LANDINGS, RAMPS STUCCO OVER FOAM INSULATION STRUCTURAL INSPECTIONS & DEFECTS SUMP PUMPS GUIDE THERMAL EXPANSION of MATERIALS THERMAL MASS in BUILDINGS THERMAL MASS in UPSTAIRS THERMAL TRACKING Indicates Heat Loss Ceiling Thermal Tracking Marks Wall Thermal Tracking Stains Floor Carpet Thermal Tracking Stains Air Bypass Leaks Marks on Insulation Thermal Tracking to Diagnose IAQ Stains HVAC Supply Registers Pet Stains on Floors Pet Stains on Walls Human Occupant Stains on Walls Stains from Candles, Woodstoves, Fireplaces Other Stains on Indoor Walls & Ceilings What to Do About Thermal Tracking THERMOSTATS VAPOR BARRIERS & AIR SEALING at BAND JOISTS VAPOR BARRIERS & CONDENSATION in BUILDINGS VAPOR BARRIERS & HOUSEWRAP VAPOR CONDENSATION & BUILDING SHEATHING VENTILATION in BUILDINGS WATER ENTRY in BUILDINGS WINDOWS & DOORS WINTERIZE A BUILDING WOOD Burning Heaters Fireplaces Stoves More Information  InspectAPedia ® Home & Site Map InspectAPedia Blog - News Updates Air Conditioning & Heat Pumps Bookstore Electrical Environment Exteriors Heating Home Inspection Insulate Ventilate Interiors Plumbing Water Septic Roofing Structure Accuracy & Privacy Policies Contact Us |
This article defines Heat Loss, R-value, U-value, & K-Value measures of heating loss rate or insulation effectiveness and provides basic building insulation and heat loss guidelines 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. Because no amount of insulation can keep a drafty building warm, also review ENERGY SAVINGS PRIORITIES. Also see HEAT LOSS INDICATORS (where is the building losing heat during the heating season, or gaining un-wanted heat during the cooling season), and see HEAT LOSS R U & K VALUE CALCULATION for a guide to calculating heat loss (or gain) rates for buildings and building insulation. 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. © Copyright 2010 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. 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 CaloriesDefinition 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 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 the K value or K-coefficient of heat transmissionA 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:
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 U value or U-coefficient of heat loss resistanceComputing "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". Definition of Insulation or Building R-Values: Rate of Heat Loss Per Hour for a BuildingHow to Calculate the R value U value & K value for a Building & How to Use These Numbers: luckily, after having already discussed "K" values, "U" values, and "R" values as measures of heat loss at ---, 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: 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: More considerations when measuring home energy use or heat lossBut 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 lossWe'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 gainThis 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 thermodynamicsYou 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. Definition of 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. Definition of 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. Share this Article
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