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99 Cards in this Set
- Front
- Back
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What is the job of envelope thermal analysis?
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"The job of envelope thermal analysis is to ensure that a proposed envelope will meet the design intent and criteria (including building codes)." (171)
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What is the envelope of a building from a functional perspective?
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"From a functional perspective, the envelope of a building is "a three-dimensional transition space...where the interactions between outdoor forces and indoor conditions occur under the command of material and geometries." (171)
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What are the basic components of the building envelope?
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"...the basic components of a building envelope include windows, doors, floors, wall, and roofs." (172)
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What are the four ways that envelope components exchange energy?
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"...design intent relative to the exchange of energies...
filter connector barrier switch" (172) |
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What is a connector?
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"In general we define a connector as a means to establish direct connection..." (172)
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How is a filter defined?
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"In general we define... a filter as a means to make the connection indirect (controlled)..." (172)
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How is a switch defined?
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"In general we define... a switch as a regulating connector..." (172)
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How do we define a barrier?
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"In general we define...a barrier as a seperating element... (172)
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What is a transformer?
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"A transformer is intended to convert an environmental force (such as solar radiation) directly into a different and desirable energy form (such as electricity)." (172)
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What is the closed shell approach to a building envelope and where is it used?
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"In harsh climates, (or where unwanted external influences such as noise or visual clutter abound)...conceives of the building envelope as a closed shell and proceeds to selectively punch holes in it to make limited and special contacts with the outdoors. " (172)
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Between the closed shell and open frame approaches, which one is barrier dominated and which one is connector dominated?
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Closed shell "might be called barrier-dominated."
Open frame "might be called connector-dominated." (172) |
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What is the essence of an open frame building envelope and when would it be used?
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"When external conditions are very close to the desired internal ones, the envelope often begins as an open structural frame, with pieces of building skin selectively added to modify only a few outdoor forces." (172)
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What is a designer's use of a switch?
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"Switches are a designer's way of having an envelope respond in a variable manner and /or giving building occupants some control of their own environment." (174)
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What is thermal sailing?
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"Passive heating and cooling systems are especially reliant upon switches, hence the knowledge and cooperation of their users. These users often must base their actions in manipulating thermal switches at some point in time on what effect will be needed at a later time. This practice, called thermal sailing..." (176)
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How does heat flow through a building envelope vary with the seasons?
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"The flow of heat through a building envelope varies both by season (heat generally flows FROM a building in winter and TO a building in summer)." (176)
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What is sensible heat?
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"Sensible heat is a form of energy that flow s whenever there is a temperature difference and that manifests itself as an internal energy of atomic vibration within all materials... essentially the'density' of heat within a material." (178)
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How is sensible heat measured?
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temperature (178)
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What is latent heat?
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"Latent heat is sensible heat use to change the state of (evaporate or condense) water." (179)
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What is power?
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"Power refers to the instantaneous flow of energy (at a given time)." (179)
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What is energy?
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"In buildings, energy refers to power usage over time." (179)
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What is the key determinant of steady-state heat flow?
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"The key determinant of steady-state heat flow is thermal resistance." (179)
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What are the two forms of heat flows and what are their results?
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"Sensible heat flow results in a change in temperature. Latent heat flow results in a change in moisture content (often humidity in the air)." (179)
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When an object is at a temperature different from its surroundings, in what direction does heat flow?
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"When an object is at a temperature different from its surroundings, heat flows from the hotter to the colder." (179)
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What are the 3 principal ways in which buildings experience heat transfer to and from the environment?
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"Buildings...experience sensible heat loss to, and gain from the environment in three principal ways.
convection conduction radiation (179) |
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How does moisture flow?
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"...moisture flows from areas of greater concentration to areas of lesser concentration." (179)
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What are the key thermal properties of materials used in envelope assembly?
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The key thermal properties of building envelope components are:
Conductivity Conductance Resistance Emittance (179-182) |
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What is CONDUCTIVITY and how is it designated?
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"Each material has a characteristic rate at which heat will flow through it. For homogeneous solids this is call CONDUCTIVITY."
Conductivity = k Number of British thermal units per hour flowing through 1 square ft of 1" thick material when the temperature difference across the material is 1 degree F. Btu in./h ft2 degrees F. (179) |
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What is CONDUCTANCE and how is it designated?
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"CONDUCTANCE.
The rate of heat flow through a specific non-homogeneous object (such as as concrete masonry unit) or a defined thickness of a homogeneous material. Conductance = C Btu in/hr ft2 degrees F the number of Btus per hour that flow through 1 ft squared of a given thickness of material when the temperature difference is 1 degree F. also..the rate of heat flow through defined sizes of modular units of nonhomogeneous materials." (180, 182) |
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What is RESISTANCE and how is it designated?
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"...resistance indicates how effective any material is as an insulator.
Resistance = R the reciprocal of conductivity (or conductance) hours needed for 1 Btu to flow through 1 ft. squared of a given thickness of material when the temperature difference is 1 degree F. h ft squared degrees F/Btu |
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What is EMITTANCE?
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"emittance, the ratio of radiation emitted by a given material to that emitted by a blackbody at the same temperature." (183)
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How is emittance related to radiative heat exchange and absorptance?
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"the lower the emittance, the lower the radiative heat exchange. For most materials, emittance is related to absorptance: a highly absorptive (low reflectance) material will usually have a high emittance as well." (183)
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What are the three broad categories of insulation materials?
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"Materials used for insulation fall into three broad categories:
(1) inorganic fibrous or cellular products (such as glass, rock wool, slag wool, perlite or vermiculite) (2) organic fibrous or cellular products (such as cotton, synthetic fibers, cork, formed rubber or polystyrene), (3) metallic or metalized organic reflective membranes (which must face an air space to be effective)." (183) |
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Name several form fitting insulating materials.
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"Form-fitting materials include
loose fill (as above a ceiling on the floor of an attic); insulating cement, a loose material mixed with a binder and troweled onto a surface; and formed-in-place materials such as expanded pellets or liquid fiber mixtures that are poured, frothed, sprayed, or blown in place." (183) |
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What are the basic forms of insulating materials?
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Basic forms of insulation are:
loose fill formed-in-place flexible, semirigid rigid (183) |
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What is EIFS?
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"Exterior insulation and finish systems (EIFS)...expanded polystyrene boards applied to exterior...substrates...covered with fabric reinforced acrylic..." (183)
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Why does a combination of dead air spaces and reflective surfaces create some of the most effective insulating products?
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"...all enclose vast numbers of dead-air spaces per unit volume. When they are bonded to reflective films and properly installed (the shiny film facing a dead air space), high resistance to heat flow is achieved." (183)
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What is the inverse of conductivity or conductance
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The inverse of conductivity (k) or conductance (C) is resistance (R). (182)
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What is the inverse of resistance?
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The inverse of resistance (R) is conductivity (k) or conductance (C). (182)
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How are conductance (C), conductivity (k) and resistance (R) mathematically related?
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C = k/x
R = x/k |
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How is U-factor defined and how is it expressed?
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"...there is one overall property that expresses the steady-state rate at which heat flows through architectural envelope assemblies. This property is the U-factor. U is the overall coefficient of thermal transmittance, expressed in terms of Btu/h ft squared degrees F..." (184)
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How is U-factor calculated?
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"U-factors are calculated for a particular element (roof, wall, etc.) by finding the resistance of each constituent part, including air films and air spaces, then adding these resistances to obtain a total resistance. The U-factor is the reciprocal of this sum of resistances: U = 1/sum of R." (184)
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What is thermal bridging?
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"...thermal bridging: where framing interrupts insulation, there are actually two different wall constructions and an averaged (an insulated portion with an uninsulated portion) U-factor must be found." (185)
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Why is thermal performance considerably improved with SIPs (structural insulated panels)?
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"Thermal performance is considerably improved (with SIPs) because no framing members penetrate the insulated core." (187)
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How is the effect of thermal bridging mitigated in roof structures?
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"Roof insulation... is often placed entirely above (or entirely below) the supporting structure, greatly reducing the effects of thermal bridging." (187)
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What is the limitation on using U-factor calculations?
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"The term U-factor should be used only where heat flow is from air to air through an envelope assembly." (185)
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What is more conductive, earth or air?
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"...earth is more conductive than air." (189)
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What is a thermal gradient?
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"The variance in temperature through a cross section of a construction assembly is called a thermal gradient." (191)
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How can a thermal gradient be predicted?
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"For any construction, the thermal gradient can be predicted by proportioning the collective thermal resistance at any point in the assembly to the overall difference in temperature across the assembly." (191)
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Why is it important to understand envelope thermal temperatures?
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"Understanding envelope surface temperature is fundamental to predicting thermal comfort." (192)
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Why is it important to understand temperature patterns within materials?
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"Understanding temperature patterns within materials is critical to establishing where water vapor might condense within or on a wall, roof, or floor assembly." (192)
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Under static conditions, what primarily influences heat flow?
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"Under static conditions, heat flow is primarily a function of temperature difference (the driving force) and thermal resistance (the resisting force)." (192)
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What is thermal capacity?
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"Heat storage is a function of the density of a material and its specific heat; the product of these two properties is known as thermal capacity." (192)
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What is density?
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"Density is the weight of a material per unit volume." (192)
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What is specific heat?
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"Specific heat is the measure of the amount of heat required to raise the temperature of a given mass of a material by 1 degree...expressed as Btu/lb degree F." (192)
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What is time lag?
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"Time lag is a measure of the delay in the flow of a pulse of heat through a material that results from thermal capacity." (192)
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What is the difference between static and dynamic heat flow conditions?
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Static (steady state) conditions:
"usually presumed for analysis of winter heat loss (assumed to occur under darkness of night)." "...heat flow is primarily a function of temperature difference (the driving force) and thermal resistance (the resisting force)." Dynamic (rapidly changing) thermal conditions: (such as solar radiation impingment on building envelope) usually presumed for analysis of summer heat gain (assumed to occur during daylight hours)" In addition to temperature difference and thermal resistance, heat storage (thermal capacity) in the building envelope becomes a compounding issue." (192) |
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What drives moisture flow through a building envelope assembly?
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"A difference in vapor pressure is the driving force behind moisture flow through components of an intact building envelope." (193)
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What is permeance?
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"The permeance of materials of construction is the latent equivalent of sensible conductance..." (193)
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What is the differnce between permeance and permeability?
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"As with conductance, permeance refers to the bulk properties of a material and permeability refers to unit thickness properties." (193)
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What is a vapor retarder?
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"Materials with low permeance are termed vapor retarders, and are incorporated in envelope construction as a means of reducing the flow of water vapor and subsequently the risk of condensation of the vapor within the envelope assembly." (193)
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From a design perspective, how would you reduce water vapor flow through an envelope assembly?
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"From an architectural design perspective, reducing water vapor flow is accomplished using very thin materials (membranes) that must be carefully located to ensure that they work as intended." (193)
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What is the fundamental principle of properly positioning a vapor retarder within an envelope assembly?
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"The fundamental principle... is that the retarder stop the flow of water vapor before the vapor can come in contact with it dew point temperature within the assembly." (193)
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In cold climates, what is the usual positioning of a vapor retarder within the building envelope assembly?
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In cold climates, install vapor retarder as close the warm (interior side) of the envelope as possible to prevent the warm air flowing to a layer where it can reach its dew point and condense. This is usually just inside the finish surface (gypsum board, wood flooring, etc.). (194)
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In cold climates, if you have higher insulation R values, where could you position your vapor retarder and what would be the advantage?
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"...with higher insulation R values, it is becoming common to install the vapor retarder within the insulation at a point about one-third of the distance from the interior to the exterior. This allows the inner one-third of the wall to be used as a chase for wiring or plumbing without penetrating the retarder, yet with enough insulation beyond... to prevent condensation by maintaining the temperature above the dew point." (194)
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What are the ups and downs of aluminum foil-faced insulation?
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aluminum foil-faced insulation: "it is very effective thermally but less effective as a vapor retarder... must also face an air space...to be effective as a radiant barrier" (194)
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In hot, humid climates, what is the approach to vapor barriers?
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"In hot, humid climates, the object is to keep the moisture in the warmer outside air from penetrating to the cooler (and usually less humid) interior." (7.4, 194)
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Why do windows and skylights merit special attention with regards to heat flow?
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Windows and skylights merit special attention because:
they generally have the lowest R (highest U) of all building components. They are major contributors to infiltration of outdoor air. They admit solar heat (for better or worse). Radiation passes easily through them complicating thermal analysis. They often admit daylight and desired ventilation. All in all, they play important roles in a building construction. (195) |
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What is SHGC?
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Solar Heat Gain Coefficient (SHGC) "represents the percentage of solar radiation (across the spectrum) incident upon a given window or skylight assembly tht ends up in a building as heat. It is the ability of a window to resist heat gain from solar radiation. SHGC can theoretically range from 0 to 1, with 1 representing no resistance and 0 representing total resistance. SHGC is dimensionless." (196)
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Where would you use a window with a high SHGC?
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"A high SHGC (meaning poor resistance to radiant gain) is desirable for solar heating applications..." (196)
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Where would you use a window with a low SHGC?
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"...a lower SHGC (good resistance) is better for windows where cooling is the dominant thermal issue." (196)
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What does SHGC depend on?
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"The SHGC depends upon the type of glass and the number of panes, as well as tinting, reflective coatings, and shading by the window or skylight frame." (196)
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What is VT?
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Visible Transmittance (VT)
"...represents the percentage of incident light (only the visible spectrum) at a normal angle of incidence that passes through a particular glazing. VT is dimensionless. The higher the visible transmittance, the greater the daylight transmission." (196) |
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What influences VT?
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"VT is influenced by the color of the glass (clear glass has the highest VT) as well as by coatings and the number of glazings." (196)
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What are three common types of low-e coatings and their uses?
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Three common types of low-e coatings are:
High-transmission low-e: "...for passive solar heating applications...". Selective transmission low-e: "...where winter heating and summer cooling are both important, requiring low U factor and low SHGC but hight VT for daylighting..." Low-transmission low-e: "...where the sun is the enemy, low U factor, low SHGC and even low VT..." (197) |
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What does a low-e coating do?
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"A low-e coating blocks a great deal of the radiant transfer between the glazing panes, reducing the overall flow of heat through the window and thus improving the U-factor." (197)
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What is infiltration?
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"Infiltration is an unintended influx of outdoor air due to air leakage through the building skin." (7.7 202)
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What is ventilation?
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"Ventilation is a deliberate, designed introduction of outdoor air." (7.7 202)
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How are both infiltration and ventilation expressed quantitatively?
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"Both infiltration and ventilation airflow rates are expressed in cubic feet per minute (cfm);" (7.7 202)
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What are the 2 primary means of calculating the infiltration airflow rate during design?
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"...2 primary means of calculating the infiltration airflow rate during design:
the airchange method and the crack method." (7.7a 202) |
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What is the air-change method of estimating infiltration through a given building in a certain climate?
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"Air-change is a correlational method wherein observed performance of a set of existing buildings is matched to key characteristics of a proposed building...(ie) construction type and climate." (7.7a 202)
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What is ACH?
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"ACH is indicative of the 'turnover' of air within a building or space. The greater the ACH, the greater the rate of outdoor airflow." (7.7a 202)
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What data is the crack method based on?
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"The crack method... assumes that data on window and door construction and wind velocities are known... also assumes that all infiltration under design conditions is due to cracks around doors and windows." (7.7a 202)
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How do you calculate the amount of ventilation needed based on population of a building?
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"...when a building's required outdoor air is based on population:
V = (cfm of outdoor air per person) x (number of people)" (7.7b 203) |
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How do you calculate the amount of ventilation needed based on floor area of a building?
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"when the required outdoor airflow rate is based upon floor area:
V = (cfm of outdoor air per unit floor area) x (total floor area)" (7.7b 203) |
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How do you figure out the required minimum ACH per person or square footage in a given location?
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"Many building codes require a minimum outdoor airflow rate based upon either building population or floor area." (7.7b 203)
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Which three types of heat flow are of primary interest in design and construction of a building?
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"Three types of heat flow are usually of primary interest:
(1) a design heat loss... (2) a design heat gain... (3) an annualized heat flow..." (7.8 203) |
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What is design heat loss based on and what is it used for?
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"...a design heat loss based upon 'worst-hour' conditions, which is used to size heating systems;" (7.8 203)
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What is design heat gain based on and what is it used for?
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"...a design heat gain (or cooling load)...based upon 'worst-hour conditions, which is used to size cooling systems;" (7.8 203)
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What is annualized heat flow based on what are it uses?
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"...an annualized heat flow, based upon year-long climate conditions, which is used to predict annual energy usage and costs and /or demonstrate compliance with energy standards." (7.8 203)
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What is calculation of design heat loss and how does this affect heating system capacity?
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"Calculation of design heat loss is an estimation of the worst likely hourly heat flow from a building to the surrounding environment... the greater the design heat loss, the larger the required heating system capacity." (7.8a 203)
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Is design heat loss the highest heat loss that can ever occur in a building? Why or why not?
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"The design heat loss is not the highest heat loss that can (or will) ever occur; rather it is a statistically reasonable maximum heat loss based upon a chosen out-side air temperature." (7.8a 203)
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By convention, what are the assumptions made in calculating design heat loss?
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"By convention, design heat loss is assumed to occur at night (no sun), during the winter (coldest temperatures), with no occupants, lights or equipment to offset heat lost through the envelope." (7.8a 203)
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What information do you need in order to calculate design (statistically-worst-hour) heat loss through a building's envelope?
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"To calculate design (statistically-worst-hour) heat loss through a building's envelope, the following information is necessary:
1. The RATE at which heat flows through each of the elements that make up the building envelope - the assembly U-FACTORS 2. The AREA of each of these assemblies 3. The design TEMPERATURE DIFFERENCE beween inside and outside." (7.8a 204) |
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What is SOL-AIR TEMPERATURE?
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"SOL-AIR TEMPERATURE is the apparent outdoor air temperature that would produce the same heat flow experienced under the combined effects of temperature difference (based upon actual outdoor air temperature) and radiation." (7.8b 206)
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In addition to the design variables that affect design heat loss, what factors affect design cooling load?
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"In addition to the design variables that affect design heat loss... the following variables will affect design cooling load:
- The orientation of an assembly (north, south, etc.) - The tilt of an asssembly (vertical, horizontal, inclined) - The surface reflectance of an assembly - the thermal capacity of an assembly - The solar heat gain coefficient of a transparent/translucent assembly - Shading for any envelope component - Heat gain from occupants - Heat gain from lights - Heat gain from equipment" (7.8b 206) |
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How does design HEAT GAIN differ from design COOLING LOAD?
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HEAT GAIN is the total of instantaneous heat flows at a given hour that entered or originated inside of a building envelope.
COOLING LOAD is "equal to instantaneous heat gain minus any part of that heat gain stored within the building plus any previously stored gains that are now affecting air temperature." (7.8b 206) |
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Energy codes give what two paths to compliance?
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"It is typical for energy codes to provide two paths to compliance:
(1) a PRESCRIPTIVE PATH, which provides precise statements of minimal acceptable component characteristics... (2) a PERFORMANCE PATH which sets a minimum level of overall energy performance that may be met using a wide range of solutions." (7.9 207) |
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Can you gain any points toward green building rating by meeting minimum energy code requirements?
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NO.
"To gain any points toward a green building rating, a design must exceed these minimum requirements (by 15% to 60%, depending upon the points sought)." (7.9 207) |
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Why is shading so important in passively cooled buildings?
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"If a building is arranged to intercept the intense rays of the sun BEFORE they pass through it transparent envelope elements, instead of afterwards, the cooling load can often be cut in half. In approximate terms, effective external shading rejects about 80% of the fierce attacks of solar energy, whereas internal shading absorbs and reradiates 80% of it." (7.5i 198)
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