{"id":39,"date":"2022-04-27T12:33:43","date_gmt":"2022-04-27T16:33:43","guid":{"rendered":"https:\/\/opentextbc.ca\/plumbing3f\/chapter\/calculating-transmission-loss\/"},"modified":"2022-08-10T16:31:22","modified_gmt":"2022-08-10T20:31:22","slug":"calculating-transmission-loss","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/plumbing3f\/chapter\/calculating-transmission-loss\/","title":{"raw":"Learning Task 4","rendered":"Learning Task 4"},"content":{"raw":"Rooms have ceilings, floors, and walls with doors and windows. Transmission losses must be calculated for each surface that has unconditioned space on the other side of it. This means that a floor with a heated room below it would be considered to not have any downward heat loss. The same is said for a ceiling with a heated room above it or a wall that abuts a heated room. In short, only surfaces that separate a heated space from the outdoors need to have heat losses calculated for them.\n

Transmission Losses through Windows and Doors (Openings)<\/h1>\nThe \u201cU\u201d values used for doors are the same as for windows. A \u201cU\u201d value, based on parameters such as single, double or triple glazed with or without special coatings and fillers, is selected from a table, and multiplied by area in ft\u00b2 and by the DTD. As an example, 48 ft\u00b2 of doors and windows with a \u201cU\u201d value of 0.59 in Penticton would calculate to be 48 \u00d7 0.59 \u00d7 67 = 1,897 BTUH.\n

Walls and Openings<\/h1>\nWalls that separate heated (conditioned) spaces from the outdoors are called \u201cexposed walls\u201d, and they will have an area that includes windows and doors. The losses through windows and doors (\u201copenings\u201d) must be calculated separately, as they will have different \u201cU\u201d factors than will the walls themselves (see above). The easiest way to perform an exposed wall calculation is to do a takeoff of the area of the openings first. If a room has a total of 108 ft\u00b2 of openings in its exposed walls, that area would be subtracted from the total area (length \u00d7 height) of the exposed wall. So, an exposed wall that measures 28 feet long by 8 feet high would have a gross area of 224 ft\u00b2. When the openings are subtracted from that gross area, the net area of exposed wall becomes 224 \u2013 108 = 116 ft\u00b2. The heat loss formulas can now be applied to the net wall area. Table 2 lists the \u201cU\u201d values for framed walls whose exterior finishes are lumped together into one category. In other words, it doesn\u2019t matter what the exterior wall finish is \u2013 the \u201cU\u201d value depends upon the \u201cR\u201d value of the insulation in the wall cavities. So, for our exterior wall in Penticton, with a net area of 116 ft\u00b2, using R20 insulation in the walls, the calculation would be 116 \u00d7 0.05 \u00d7 67 = 389 BTUH\n

Ceiling and Floor Losses<\/h1>\nCeiling losses will only be counted for rooms on a top floor, and are listed as either \u201croof with attic space\u201d (normal residential construction using trusses) or \u201cbuilt up roof, no attic space\u201d (typically flat or vaulted roofs) with an insulation value attached to them. Floor losses (other than basements) will only be counted if they are located over unheated space such as an open carport (\u201cwood over exposed space\u201d) or a garage (\u201cwood over enclosed space\u201d). The insulation values will determine the \u201cU\u201d value chosen from the table, and their transmission losses are calculated in the same manner as with openings and exposed walls.\n

Thermal Bridging<\/h1>\nOur previous example did not make mention at all of a thermal resistance for the 2 \u00d7 6 studs themselves. This is because they are spaced at either 16\u2033 or 24\u2033 centres along the wall and are difficult to allow for. Some heat load designers will include a small allowance for the studs when coming up with the \u201cU\u201d value for an assembly but this is a matter of designer preference.\n\nThe studs that form the support system for walls will have a heat loss through them which is higher than that of the insulation between them. This occurrence is known as \u201cthermal bridging\u201d. By definition, thermal bridging, also known as cold bridging or thermal bypass, is an area or component of an object which has a higher thermal conductivity than the surrounding materials, creating a path of least resistance for heat transfer. Overlooked for a great period of time, thermal bridging has recently come to the forefront of heat load calculations as designers strive to be as accurate and comprehensive as possible in their estimates. Thermal bridging not only causes a loss of heat within the space, it can also cause the warm air inside the space to cool down. When heat attempts to escape a room, it follows the path of least resistance. Likewise, the same process occurs during the summer, only in reverse, allowing heat to enter your otherwise cool building, called heat gain. Thermal bridging happens when a more conductive material allows an easy pathway for heat flow, usually where there is a break in (or penetration of) the insulation. Some common locations include:\n