Chapter 4: Introduction to thermal control

1. Chapter 4: Introduction to thermal control • As we take on issues of thermal control it is important to keep in mind the following issues: • Human comfort is the ultimate objective; numbers aren’t everything. • There are psychological / cultural predispositions • temperature, humidity, air motion • personal control • If you do calculations, don’t forget to look at the larger picture. What design strategies do they suggest. • 3. There are 2 objectives to doing calculations: • a) how big should the heating/cooling system be? • (extreme design conditions) • b) how much energy will the building use in a typical season?

2. Chapter 4: Introduction to thermal control cont’d • Buildings, like our bodies, exchange heat with the outside • environment in 2 ways: • Through the envelope (skin) - materials, areas, rates of heat flow. • With incoming fresh air (respiration) – volume of space plus rates of fresh air. • Both can be manipulated through design to improve thermal • performance. • Buildings usually experience in thermal heat production. • It is usually best to minimize these gains - reduce electric lights • (an argument for daylight design) • Thermally, an occupied building can be very different from • unoccupied . • 8 hours, 5 days a week = approximately ¼ year. • One hour’s surplus heat can be another hour’s needed warmth.

3. Chapter 4: Heat Flow 4.1 The building envelope “is not merely a set of two-dimensional exterior surfaces, it is a transition space, a theater where the interaction between outdoor forces and indoor conditions can be experienced.” 4.2 Connectors, Filters and Barriers Fig. 4.3 open frame and closed shell 4.3 Switches and User’s choice Fig. 4.4 vernacular thermal switches 4.4 Heat flow, seasons and the building convection, conduction and radiation Fig. 4.7 Nature of heat flow through materials

4. 4.5 Heat flow through solids (Table 4.1, Figure 4.8) a) conductivity – rate at which heat (Watts) will flow through an area of material (1m²) 1 meter thick when the temperature drop through that material is 1˚K Unit: Watts/mK Passive heating depends heavily on the rate at which heat is conducted into a material. When we touch a surface we sense its conductivity rather than its temperature b) conductance – Used for materials that come in standard thicknesses. The rate of heat flow through one square meter of a given thickness of material when the temperature difference is 1˚K. Units: Watts/m²K

5. 4.5 Heat flow through solids c) resistance How effective a material is as an insulator. (ie the reciprocal of conductivity or conductance.) Useful for comparing insulating materials. It is sometimes listed as “per inch” of thickness. The higher the number, the better the insulative value. In the imperial system, it is the time in hours it takes for 1 Btu to flow through 1ft² of a given thickness when the temperature drop through that material is 1˚F. So a typical R-value for 5.5” of fiberglass batt insulation is 20. (SI Unit: m²K/W and since a watt is 1 joule/sec = m²K sec/joule)

6. 4.5 Heat flow through solids • c) Insulators • Types: • inorganic fibrous or cellular • (glass, rock wool, slag wool, perlite, vermiculite) • organic fibrous or cellular • (cotton, synthetic fibers, cork, foamed rubber or polystyrene) • metallic or metalized organice reflective membrane • (must face an airspace to be effective) • Forms: • loose fill (above a ceiling) • insulating cement (troweled on) • formed in place (expanded pellets or liquid fiber) • flexible batts • semirigid blanket • rigid

7. 4.6 Heat flow through air a) Resistance insulating layer of air (no value in wind conditions) b) Conductance c) Emittance Radiation is influenced by surface characteristics Shiny materials are less able to radiate than rough building materials This characteristic is called emittance: the ratio of the radiation emitted by a given material to that emitted by a blackbody at the same temperature. aluminum foil e=0.05 wood, paper, masonry e= 0.9 The lower the emittance, the lower the radiative heat exchange A low reflectance (high absorptive) material will tend to have a high emittance. d) Insulation dead air spaces are effective insulators.

8. 4.6 Heat flow through the opaque building envelope • Heat flow calculations must be undertaken to establish equipment size requirements and approximate energy consumption. • To do this, you need to know: • the rate at which heat flows through the various envelope assemblies • the area of each of these assemblies • the temperature difference between inside and outside for the time of the calculation. • a) U-value • thermal transmittance of an element or construction (W/m²K) • it is the reciprocal of the sum of the resistances. Figure 4.9

9. b) Trends in U and R values SIP’s - structural insulated panels gas filled panels - sealed honeycomb baffles filled with inert gas. silica aerogel (R20/in) can be foamed into cavities (no CFC’s) windows – dramatic changes in recent years. tinted, reflective, photochromic, low emissivity coatings (low e) low conductivity gases between glazings, intermediate films between glazings, operable blinds between glazings, lower conductivity glazing spacers and frames, more air-tight weather-stripping. spectrally selective glazing (Vt p. 138) smart windows – electrochromic, thermochromic

10. c) Walls Fig. 4.10 Consider the framing area as thermal bridge e) Basements h) Overall envelope heat flow: thermal gradient Table 4.2 Thermal properties of building materials Table 4.3 Properties of surface air films and air spaces Table 4.4 Thermal resistance of air spaces Table 4.13 Window characteristics 4.8 (d) Solar Heat Gain Coefficient (SHGC) The portion of the solar radiation that passes through a window Scale 0-1 Table 4.16 Solar optical properties of several glazings 4.8 (k) Shading Table 4.19

11. 4.9 Moisture and Infiltration a) cold climate moisture control b) hot humid climate moisture control c) heat flow by infiltration e) forced ventilation Table 4.22 Recommended outdoor air requirements for ventilation