HEAT FLOW TO OR FROM A SPACE

2. HEAT FLOW TO OR FROM A SPACE No habitable space has an envelope that is made of different materials with a consistent value of heat transmission through the separating barrier. Spaces are constructed of several layers of building materials, installed for a specific purpose, and all of them likely have a different heat flow / resistance characteristic. In addition, a space is likely to have more than one type of separating barrier, such as the composition for walls, roofs, floors, windows, doors, skylights . . . Most have multiple layers of construction make-up and none are the same.

3. Consider the wall of a space. Take, for example the exterior wall of a residence, made up of: • gypsum board surface inside • 2”x4” wood frame • 3.5” thickness of wall insulation • ¾” polyfoam sheathing • ¾” thickness of air space • 3 5/8” thickness brick veneer • Then consider there is a still air film inside and a moving air film outside. All the individual layers have different heat flow characteristics.

4. THE “U” FACTOR The rate of heat flow through an assembly of materials that form the thermal barrier of a building envelope is called a “U” factor. It considers the C value, k value, and / or R value of each of the materials that make up the assembly. First, each material must have its C or k value converted to an R value, then all R values added together. C and k and R values cannot be added to obtain a viable heat flow rate. It is rather like adding apples, oranges, and bananas.

5. With all the R values known; And since R is the reciprocal of C, The “U” value for an assembly of materials is the reciprocal of the sum of the R values. Like C, U is a rate of heat flow, except that “U” is the rate of heat flow for a combination of materials that make up part or all of a space envelope, rather than one material.

7. Still Air Moving Air

13. CONVERSTION TO AND TABULATION OF R VALUES FOR A RESIDENTIAL FRAME WALL WITH BRICK EXTERIOR: • material R value • 1 outside air 0.17 • 2 brick veneer k = 9.0; R = in. thk / k = 3.625 / 9 = 0.41 • 3 still air space 1.18 • 4 ¾” polyfoam sheathing; k = 0.20; R = in. thk / k R = 0.75 in / 0.20 = 3.75

14. 5 3 ½” insulation 11.0 • 6 ½” gypsum board; C = 2.22 R = 1/C = 1 / 2.22 = 0.45 • 7 Inside air film R = 0.68 Total R value = 17.64 With a total R value of 17.64 for all the materials that make up the wall, the U value equals: U = 1 / summation of individual R values U = 1 / 17.64 = 0.0567 Which means that for each square foot of wall, per hour, per degree Fahrenheit, 0.0567 BTU will move through the wall - - - because of heat flow caused by temperature difference.

15. Condensation of moisture must be given consideration to the composition of exterior walls and how they are insulated. Condensation occurs because water vapor in the air reaches a certain ambient temperature called the “dew point.” Dew point is a temperature at which moisture in the air reaches a saturation point and cannot remain as vapor, but condenses, changing state from vapor to liquid form.

16. Two things the designer also wants to occur in a building envelope. One: A vapor barrier placed on the warm side of insulation – generally on the interior of the space because that is where moist air is most likely to remain. Moist air occurs on the outside, but exterior conditions change – and we don’t care if condensation occurs on the outside. Besides, the exterior surface of the envelope is made to resist moisture. A vapor barrier can be any surface that expels water.

17. Two: The designer would desire that the dew point temperature occur within the insulating barrier where there is no water vapor present. Illustration: On a hot summer day, say the outside surface temperature is around 100+ degrees, and air conditioning inside at a cool, comfortable seventy two degrees. There is enough water vapor in the air inside, such that the dew point temperature is between 72 and 100+ degrees. The tendency for heat flow is from outside to inside - - -

18. At this point, say there is no insulation and no vapor barrier, and there is little to keep heat from penetrating, and at some point between the outside and inside surfaces, the dew point temperature is reached and water happens because there is moisture present. Condensation occurs (wet water) within the wall – resulting in potential damage to the enclosure. But suppose there is insulation and a vapor barrier. The inside surface finish remains at 72 degrees, and within the insulation material, heat flow is slowed to the point that dew point occurs in a dry area, protected from the intrusion of moisture by the vapor barrier.

19. AFFECT OF SUN RADIATION ON A BUILDING ENVELOPE Previously, calculations of heat flow have been directed based on BTU/h x area x temperature differential. During cooling season the surface of a building envelope, even though it may be opaque, is influenced by radiant energy from the sun. Consider the mass of a building material and its ability to retain heat. After having been heated by the sun, dense materials such as brick and concrete will stay warm for a longer period of time than lighter materials such as wood, or materials with low mass because they are thin, as is glass or sheet metal.

20. When a building is subjected to radiant energy during the day, the walls and roof are heated because of the change of electromagnetic energy to heat energy, AND conduction because of ambient temperature. As time progresses, part of the heat will dissipate into the atmosphere, but most will penetrate the surfaces because of the greater temperature differential. So the MASS of a building envelope has an affect on heat flow in summer because of time lag. Glass is considered separately regarding conduction and radiation, because radiant energy can penetrate a translucent/transparent barrier.

21. The amount of heat retained by a building envelope is also affected by HOW MUCH radiant energy is converted to heat energy because of its color hue. Recall that dark hues absorb heat and light hues reflect heat. So a variation of reflective quality exists from white (the equal combination of all colors) to black (the absence of light) The following lists approximate general reflective value of colors: gray 25% dark red 26% light green 50% cream 65% white 75 – 95%

22. A factor called “EQUIVALENT TEMPERATURE DIFFERENTIAL” ( ETD ) approximate the construction assembly’s interrelationship between conductance, thermal time-lag, and color. ETD is defined as the outdoor-indoor TEMPERATURE DIFFERENCE that will be equal to the solar, conduction, and radiation heat flow into a space with allowance for time lag. In calculating HEAT GAIN caused by radiation onto a building component of mass, the ETD value is used instead of the difference in outdoor/indoor temperature. The chart in the packet labeled “ETD” indicates values for types of construction and time of day.

23. EQUIVALENT TEMPERATURE DIFFERENCE Use this chart for temp difference In calculating heat GAIN for walls and roof This chart is based on Location of 40 degrees N latitude ASHRAE handbook of Fundamentals

24. TEMPERATURE ZONES WITHIN BUILDINGS All areas within buildings exposed to exterior walls are subject to change in temperature simply because of the changing position of the sun. Space on the east side during mornings are exposed to radiant heat from the sun until mid-day, while space on the west side are in the shade of the building. The situation reverses itself during the afternoon, giving the west side to exposure of direct radiation Spaces on the south have sun all day while the north side has virtually none.

26. As in the diagram, a central mechanical unit with only one control, the dilemma remains as to where to place the thermostat. Not such a problem for a residence, in that the use is limited to a small number of people and minimal activity. But for an office area where each space is in use by personnel all through the day, the problem remains except as mitigated by architectural planning. Consideration must be given to areas within a space that are subject to extreme heat gain due to sun position. A space may be large enough laterally that some of the areas within are not affected by the condition of the exterior walls. Such spaces might be in very large single story buildings and those with multiple floors where the space is very large.

27. The limit of distance from exterior walls where the space is affected by the condition of the exterior is much the same as considerations given to daylighting of interior spaces as limited by the distance from windows. There is a limit to the distance from the exterior wall to spaces that will be affected by heat loss / gain through the exterior wall. The following diagram represents the floor of a multi-story building where all exterior walls are subject to exterior conditions, whether it be cold, heat, or radiation from the sun.

29. In order to maintain a thermal comfort level in all areas, each zone must be treated and controlled as a separate entity. A mechanical system must necessarily be flexible enough to provide for varying conditions during the day. Realize that varying zones are created within a space as the result of the varying position of the sun during the day. Of the zones created, 2 through 9 are influenced by HEAT GAIN variance by HEAT RADIATION from the sun, and affects COOLING COMFORT ONLY during air conditioning season.

30. Only TWO ZONES exist during winter conditions, when heat is lost in areas 2 through 9 through the exterior walls and must be replaced – since HEAT LOSS is a factor of heat flow by CONDUCTION. Zone 1 all year long is air conditioning only. So much for day to day requirements. But what about requirements that aren’t the same one or two days of the week – and on holidays. If the space is a multi - use area, occupied by a number of occupants that have varying work times, the mechanical system must be flexible from that standpoint. That is, if efficiency is to remain a priority. Say maybe two or three of the tenants work Monday through Friday - closed on weekends.

31. But maybe another group of tenants has a six-day work week – and maybe another tenant works seven days per week. The equipment must be staged in an efficient way in order to meet the requirements of differing temperature zones, and differing daily needs.

32. When refrigerated comfort cooling first became prominent, a unit used to describe an amount of cooling capacity was called a “ton” of air conditioning. The term originated when air conditioning consisted of blowing warm air over ice to allow it to absorb the heat. • The amount of heat required to change one pound of ice at 32 degrees to one pound of water at 32 degrees is 144 btu – a change of state of a substance – solid ice to liquid water. • The melting of a ton of ice (2000 lb) over 24 hours will cool ( 2000 lb x 144 btu ) / 24 hours = 12,000 btu/hour. Hence, a ‘ton’ of air conditioning capacity is 12,000 btu/h.

33. So an exchange of heat is required to change the state of a substance - - - at the same temperature; water to ice, water vapor to water, etc. It takes one btu to change one pound of water from 211 degrees to 212 degrees, F. But it takes 1061 btu to change one pound of water at 212 degrees to steam at 212 degrees. Steam is water vapor.

34. NOW TAKE A CLEAN, 8 ½” X 11” SHEET OF PAPER, SMOOTH EDGES, RULED OR UNRULED, AND PRINT YOUR NAME AT THE TOP, THEN ANSWER THE FOLLOWING QUESTION

35. When someone makes home-made ice cream, using a mechanical ice cream freezer (either hand-crank or electric motor) • All the ingredients are in the container, then Ice is added to the unit, then salt is poured over the ice. You begin to turn the crank until ice cream happens . . . • Explain why the salt is poured over the ice, and what happens in the process to make the contents freeze. WHEN YOU ARE FINISHED, FOLD YOUR PAPER ONCE, THEN TURN IT IN TO ME