{"id":34,"date":"2022-08-02T18:40:01","date_gmt":"2022-08-02T22:40:01","guid":{"rendered":"https:\/\/opentextbc.ca\/plumbing3f\/chapter\/heat-loss-and-heating-requirements\/"},"modified":"2022-08-10T16:31:21","modified_gmt":"2022-08-10T20:31:21","slug":"heat-loss-and-heating-requirements","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/plumbing3f\/chapter\/heat-loss-and-heating-requirements\/","title":{"raw":"Learning Task 3","rendered":"Learning Task 3"},"content":{"raw":"

Transmission Loss<\/h1>\nIn the heating season, we want to prevent the heat that we\u2019ve put into the building from passing through building materials to the great outdoors. Heat that is lost via conduction through the molecules of the building structure\u2019s materials is known as transmission loss.<\/em>\n\n[caption id=\"attachment_33\" align=\"aligncenter\" width=\"368\"]\"\" Figure 1 Transmission losses through the materials of the building envelope via conduction[\/caption]\n\nThrough many decades now, manufacturers and industry associations have tested and certified the materials commonly used in construction as to their ability to resist the flow of heat through them, and have applied a recognized identifier to them. This identifier is known as an \u201cR\u201d value, with \u201cR\u201d signifying \u201cthermal resistance\u201d. Here\u2019s a common example of a material with its \u201cR\u201d value clearly shown on its packaging.\n\n[caption id=\"attachment_33\" align=\"aligncenter\" width=\"400\"]\"\" Figure 2 R14 Batt Insulation[\/caption]\n\n\u201cR\u201d values are indicative of the number of hours it would take for 1 BTU to pass through 1 ft\u00b2 of this material when the \u0394T between the two sides of the material is 1\u00b0F. This is sometimes written as:\n

R = 12 hrs\/btu\/ft\u00b2\/\u00b0F<\/p>\nR12 fiberglass insulation is commonly used in 2 \u00d7 4 outer wall construction typical of the 1960\u2019s to 1980\u2019s. R14 fiberglass batts can also be found in 2 \u00d7 4 construction, but the material is slightly more dense than the R12 and therefore has a better insulating quality to it. Today\u2019s construction, typically using 2 \u00d7 6 exterior wall studs, uses R20 to R28 batt insulation or, better yet, spray foam between the studs. In essence, the higher the \u201cR\u201d value, the better the insulator.\n\nAn \u201cR\u201d value is an expression of how long, in hours, it would take for 1 BTU to pass through 1 ft\u00b2 of material when there is a 1\u00b0\u0394T between the two sides. In the heating industry, however, this expression is exactly backwards to what we need to know. Our industry is based on information expressed in BTUs per hour, not in hours per BTU. To be able to do any of the calculations necessary in building, we must turn an \u201cR\u201d value into what is known as a \u201cK\u201d value. This is done by simply dividing the single material\u2019s \u201cR\u201d value into 1. As an example, if we want to calculate how many BTUs per hour will pass through R14 insulation, we would divide 1 by 14 to get a \u201cK\u201d value of 0.07. This means that 0.07 BTUH will pass through 1 ft\u00b2 of this material when the \u0394T between the two faces is 1\u00b0F. If we double the thickness of the material, it will take twice as long for heat to pass through it. And, if we double the \u0394T between sides, it will take \u00bd the time for the heat to pass through.\n\nSo, just like our previous calculations regarding BTU calculations and water, there is a formula to use when trying to estimate the transmission losses through a material. The formula is:\n

Transmission loss = Area (ft\u00b2) \u00d7 \u201cK\u201d value \u00d7 \u0394T (design temperature difference)<\/p>\nLet\u2019s use the R12 material in an example. Suppose we have a 2 \u00d7 4 wall that is 15 feet long by 8 feet high, and we\u2019re trying to maintain a temperature on one side of 72\u00b0F while the temperature on the other side could be as cold as \u201310\u00b0F. We would have 120 ft\u00b2 of material, the \u201cK\u201d value would be [latex]\\dfrac{1}{12}[\/latex] = 0.08 and the \u0394T would be 82\u00b0F. The calculation would then be:\n

120 \u00d7 0.08 \u00d7 82 = 787.2 BTUH<\/p>\nWhat this means is that, every hour, we could expect 787 BTUs to pass through this 120 ft\u00b2 of material as a \u201ctransmission loss\u201d. We can adopt the widespread rationale within the industry that we don\u2019t use decimals of btus; instead we usually round them to the nearest whole number.\n

\u201cU\u201d Value<\/h2>\n\u201cK\u201d values are not normally involved in calculating heat losses in building construction due to the fact that they are representative of a single material only. Walls are an assembly in the form of drywall, studs, fiberglass batts, wood sheathing, vapour barriers, air films and outside wall coverings such as wood siding or stucco.\n\nWhen we need to calculate the estimated heat lost through an assembly that is made up of many different materials, we have to come up with an aggregate or summary \u201cR\u201d value. As mentioned previously, every material used in construction will have an \u201cR\u201d value attached to it. In making a list of exterior wall construction from one face to the other, it might look like this, as an example:\n\nInside air film on \u00bd\u2033 gypsum wall board on vapour barrier on 2 \u00d7 6 wood studs with fiberglass batts on [latex]\\dfrac{3}{8}[\/latex]\u2033 plywood sheathing on lapped, bevelled wood siding on outside air film.<\/em>\n\nEach of these components has an identified \u201cR\u201d value that can be found in numerous tables in industry publications as well as on the internet. If any values differ between tables, the difference will be very minimal. Here are examples of \u201cR\u201d values that include those in the wall construction listed above.\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Material<\/th>\nR per inch thickness<\/th>\nR for given thickness<\/th>\n<\/tr>\n
Interior air film<\/td>\n<\/td>\n0.68<\/td>\n<\/tr>\n
Exterior air film<\/td>\n<\/td>\n0.17<\/td>\n<\/tr>\n
Gypsum wallboard \u00bd\u2033<\/td>\n<\/td>\n0.45<\/td>\n<\/tr>\n
Vapour barrier<\/td>\n<\/td>\nNegligible<\/td>\n<\/tr>\n
Fiberglass batt<\/td>\n4.0<\/td>\n<\/td>\n<\/tr>\n
Polyurethane (spray-in foam)<\/td>\n5.9<\/td>\n<\/td>\n<\/tr>\n
Concrete block 8\u2033<\/td>\n<\/td>\n1.11<\/td>\n<\/tr>\n
Brick, common<\/td>\n0.20<\/td>\n<\/td>\n<\/tr>\n
Plywood<\/td>\n1.25<\/td>\n<\/td>\n<\/tr>\n
Polystyrene<\/td>\n5.00<\/td>\n<\/td>\n<\/tr>\n
Bevelled, lapped siding<\/td>\n<\/td>\n1.05<\/td>\n<\/tr>\n
Single pane glass<\/td>\n<\/td>\n1.13<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nAs you can see, depending on the type of material, the \u201cR\u201d value can be expressed either by the inch of thickness and then adjusted, or by a given thickness. Many materials such as plywood and polystyrene come in many thicknesses so a per-inch value is usually listed once. Other materials such as concrete block and brick are found as certain standard sizes and are listed accordingly.\n\nTo assign an aggregate \u201cR\u201d value to the wall listed above, simply identify the \u201cR\u201d value for each material and add them together.\n\nThe air that clings to a material\u2019s surface by adhesion has actually been assigned an \u201cR\u201d value. For the wall\u2019s exterior surface, it\u2019s listed as 0.17 and for the interior surface it\u2019s 0.68. Working from inside-to-outside through the wall, the numbers to add together would be:\n

0.68 (inside air film) + 0.45 (\u00bd\u2033 gypsum wallboard) + 0 (vapour barrier) + 22.00 (fiberglass batts) + 0.47 ([latex]\\dfrac{3}{8}[\/latex]\u2033 plywood sheathing) + 1.05 (bevelled lapped siding) + 0.17 (outside air film) = \u201cR\u201d 24.82<\/p>\nThe inside air film, outside air film, \u00bd\u2033 drywall and lapped siding are all unit values, whereas the values for the fiberglass batts and plywood sheathing must be calculated according to thickness. For the fiberglass batts, the \u201cR\u201d value is 4.0 per inch of thickness. Thus, \u201cR\u201d of 4.0 per inch thickness \u00d7 5.5\u2033 = \u201cR\u201d of 22.00.\n\nThe process is the same for the plywood sheathing; \u201cR\u201d of 1.25 per inch thickness \u00d7 [latex]\\dfrac{3}{8}[\/latex]\u2033 = \u201cR\u201d of 0.47\n\nFor reasons that are unclear, the plastic vapour barrier that is sandwiched between the drywall and studs is considered to have a \u201cnegligible\u201d resistance to the flow of heat and is therefore omitted from heat loss calculations.\n\nNow that we\u2019ve established an aggregate \u201cR\u201d value for a wall assembly, we can come up with a \u201cU\u201d value just as we did for establishing the \u201cK\u201d value for a single material. In our case, we would divide 24.82 into 1 to get \u201cU\u201d of 0.04 (round \u201cR\u201d. \u201cK\u201d and \u201cU\u201d values to two decimals). Remember that a \u201cU\u201d or \u201cK\u201d value is an expression of a material or assembly\u2019s ability to conduct<\/em> heat through it in BTU\/HR, whereas an \u201cR\u201d value is an expression of a material or assembly\u2019s resistance<\/em> to the flow of heat through it in HOURS\/BTU. It should be clear, then, that we are always interested in coming up with the \u201cU\u201d value so it can be used in a multiplication formula to give us a BTUH heat loss. The lower the \u201cU\u201d value, the better the insulator.\n\nIt\u2019s important to note that there isn\u2019t much difference between the thermal conductivity of a 2 \u00d7 4 wall assembly with wood siding and one with stucco or brick siding. Where the greatest difference lies is in the insulation within the stud space. Exterior walls have been mandated to be at least 2 \u00d7 6 construction for many decades now due to the fact that, with an extra 2 inches of insulation in the walls, they are much more energy efficient. The move to 2 \u00d7 6 exterior walls versus 2 \u00d7 4 was mainly energy-based and not particularly based on strength of the wall.\n\nTECA as well as other associations involved in the heating industry have come up with abbreviated lists of \u201cU\u201d factors. The TECA \u201cU\u201d values are shown in Figure 3 below.\n\n[caption id=\"attachment_33\" align=\"aligncenter\" width=\"1429\"]\"\" Figure 3 Heat Loss \"U-Factors\" table[\/caption]\n\nRather than having to go through the process of identifying all the components of a wall assembly, adding together their \u201cR\u201d values and dividing that into 1 to get a \u201cU\u201d value, one simply has to identify a wall assembly by its general makeup. There is not a lot of variation in wall assemblies used in residential construction, so these \u201cU\u201d values listed are particularly helpful in more quickly obtaining a heat loss while still applying a reasonable degree of accuracy.\n

Media Attributions<\/h3>\n