Autodesk Sustainable Design Curriculum Lesson Five: Design of the Building Envelope Daylighting Glazing Use the Outdoor Environment Wall and Roof Assemblies Cool Roofs © 2009 Autodesk Why Spend So Much Effort on Daylighting? Data Source: 2005 Buildings Energy Data Book © 2009 Autodesk What is Wrong with This Picture? © 2009 Autodesk Myths from the Daylighting Collaborative Myth: Costs More Fact: Not if done using integrated design approach. Decreased internal load from electric lighting allows HVAC equipment downsizing. Myth: Complicated Fact: No need to reinvent the wheel. Many resources are available. Myth: Too Much Heat Gain Fact: Light-to-heat ratio is better than using efficient electric lights. Proper design can block most of the sun’s heat and provide 50 fc of light. *Source of myths & facts: Daylighting Collaborative. http://www.daylighting.org/why.htm © 2009 Autodesk More Myths Myth: Causes glare. Fact: Proper designs use shading and appropriate glass to avoid too much light. Myth: It is better to upgrade lighting and HVAC efficiency. Fact: It is better to reduce need for electric lighting and cooling in first place with the right building form and properly designed daylighting. Myth: Requires use of clear glass. Fact: Clear glass may work well for the “daylighting” glass, but depending on opening size may cause excessive glare to be used as vision glass. Source of myths & facts: Daylighting Collaborative. http://www.daylighting.org/why.htm © 2009 Autodesk More Myths Myth: Requires use of skylights. Fact: Building form is crucial. In many situations windows and clerestories can provide most of the light needed. Myth: Only works on sunny days. Fact: Even overcast skies provide 5,000 or more foot-candles—most spaces need no more than 50 fc. The amount of daylight through a window can be HIGHER on a day with a thin cloud layer than with clear skies. Myth: Requires an all-glass building. Fact: All-glass buildings have problems with heat gain and glare. Windows in daylit buildings typically are not different from non-daylit buildings—just better planned. Start with a band of glass, and see how much you need. Source of Myths & Facts: Daylighting Collaborative. http://www.daylighting.org/why.htm © 2009 Autodesk Daylighting Elec. Lighting Reduction for South Facing Office Space Percentage Reduction in Electric Lighting from Daylighting Assumes: Electric light comes on when daylight is below 50 fc. DEC NOV OCT SEP AUG JUL M o nt h JUN M AY APR Glare can be controlled by window shades. Glare is not a major issue for south-facing glazing with well-designed overhangs when the sun is high. M AR FEB JAN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Ho ur 0-20 © 2009 Autodesk 20-40 40-60 60-80 Even light distribution is very important and can be more important than the absolute illuminance level. Skylights: Big Box Retail Example Least energy use: 8% skylight to roof ratio (SRR) 4 – 5% SRR delivers significant savings, with reduced capital costs 8% SRR is not optimal from a capital cost/ROI perspective Incremental lighting savings decrease and cooling penalty increases past 5% SRR. Electric lighting must be controlled to realize savings © 2009 Autodesk Cooling energy increases faster than lighting savings Natural Light – How much is enough? For 100% daylighting from skylights – approx. 5% of roof area should be in skylights, (5% SRR). Depends on Building Type, Tvis., SHGC. The skylight specifications depend on climate. All climates should have high visible transmittance (Tvis). Hot climates should have low SHGC. Rectangular with Louvers Skylight Tubular skylights require a much lower SRR, approx. 1-2%. U-value is more important in cold climates and has energy and condensation implications. Dampers (to darken space) are available with most skylights. Tubular © 2009 Autodesk Skylights/Windows – What Size? SKYLIGHTS Size of skylight Estimate = (Floor to Ceiling Height x 1.5)2 x SRR = area of one skylight. Choose a size that is appropriate. Start with 5% SRR and modify depending on climate and building use. More windows => fewer skylights. In a high rise…skylights may not be possible/useful. For example with a 12' ceiling and 5% skylight-to-roof ratio the right size skylight would be approximately: (12 x 1.5)^2 x 5% = 16.2sf => should use a 4x4 or 8x2' skylight for good light distribution. WINDOWS Great deal of variation in window area depending on climate and building type. In a sunny climate, a 2.5 – 3 ft. perfectly clear opening (does not exist) will provide 40 – 50 fc in perimeter offices. If the glass has a visible transmittance of 50%, you need twice as much glass to achieve the same amount of light. Glazing should be divided into “daylighting glass” and vision glass. A light shelf of approximately 2 – 2.5 ft. shades the lower vision glass and distributes light from the daylighting glass above. © 2009 Autodesk Common Question – Should I use PVs or skylights on the roof? Skylights and PVs in a School Turning Off vs. Powering a Lighting System Cover a 5x6' roof area with PV or Skylight 1,200 1,000 $1,115 w rebates $900 w/o rebates 800 kWh/yr Energy OR 600 $ Installed Cost 400 The Small Print •1500 hrs/yr daylight/sunlight that can be used. •10% PV System Efficiency •Automatic controls turn off electric when daylight is available. •PV cost - $8/Watt without incentives, $4/Watt with incentives •No rebates assumed for skylight •Northern California climate and rebate assumptions 200 0 Skylight/Lighting Control Avoided/Produced kWh per YEAR PV System First Cost including Incentives Cost per kWh over 10 yrs Photovoltaic - $0.267/kWh Daylighting - $0.12/kWh © 2009 Autodesk Site, Shading, and Daylighting – Autodesk Ecotect Environmental Analysis Desktop Application Schematic •Lighting •Daylighting/Shading •Overhangs •Also used as a visualization interface for analyses that take place outside of Ecotect •Very visual, presentation graphics © 2009 Autodesk Daylighting © 2009 Autodesk Site, Shading and Daylighting - A Few Graphics © 2009 Autodesk © 2009 Autodesk The Building Envelope – Modes of Heat Transfer Conduction Convection Radiation Which is most important? Modes of Heat Transfer in Residential Buildings © 2009 Autodesk Heat Transfer Conduction Through Solids Convection Through Fluids (gases and liquids) Dependent on Velocity Radiant Heat Transfer Proportional to T4 Difference Does not Require Matter for Transmission © 2009 Autodesk Alternative Envelopes Structural Insulated Panels (SIPS) - rigid panels of foam insulation joined to OSB. Typically used as floors, walls, and roofs on smaller buildings. Insulated Concrete Forms (ICFs) are forms/molds or hollow blocks with built-in insulation. These are stacked and filled with reinforcing bar and concrete. Both SIPS and ICFs approximately R5 per inch, ICFs have thermal mass benefit. Both also reduce infiltration/ noise, mostly residential/light commercial. Many variants – Benefit is dependent on climate. Do not obsess on a single component (i.e., opaque envelope) – The right question is: Do I need low conduction (High Rval) for high performance? Depends on building type, climate, and occupancy pattern. © 2009 Autodesk Alternative Envelopes – Metal Framing R-19 in metal framing => Utotal = .183 = R 5.5 with metal framing R-19 in wood framing => Utotal = .074 = R 13.5 with wood framing Do I need low conduction (High Rval) for high performance? Depends on building type, climate, and occupancy schedule. House in Maine – High R-value – Good, winter dominated climate. Retail store in Los Angeles – High R-value may increase energy use. Develop a theory and test your assumptions by analyzing the impact of improving the thermal envelope © 2009 Autodesk Case Study - Community Center Question: Will this project benefit from high R-value, but expensive wall/roof systems, or should the resources be spent on other efficiency measures? Project Description Community Center – 21,000 square feet Location – Northern Ca Building Form – As shown below © 2009 Autodesk Wall Insulation Sensitivity Wall Insulation Sensitivity--R19 Roof Total Building Energy Use (MMBtu/year) 1,600 1,400 1,200 1,000 San Jose, CA 800 W. Palm FL Anchorage, AK 600 400 200 0 R7 Wall © 2009 Autodesk R11 Wall R15 Wall R19 Wall Roof Insulation Sensitivity Roof Insulation Sensitivity--R11 Walls For this building form, adding insulation beyond code minimum has only minimal benefit in San Jose, CA and no benefit in Miami. There is considerable benefit in Fairbanks, AK. Total Building Energy Use (MMBtu/year) 1,600 1,400 1,200 1,000 San Jose, CA 800 W. Palm FL Anchorage, AK 600 400 200 0 R11 Roof © 2009 Autodesk R19 Roof R30 Roof R38 Roof © 2009 Autodesk National Fenestration Rating Council Windows require NFRC label for factory built windows Label must contain: U-value Solar Heat Gain Coefficient (SHGC) Visible Light Transmittance Air Leakage Value © 2009 Autodesk Windows: U-Factor (Conduction) The U-factor defines how well a window prevents conductive heat flow. The rate of heat conductivity is defined by the U-value (inverse of the Rvalue) of the window assembly. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value. (source: Efficient Windows website) The definition of a “good” U-factor depends on the building type and location. In Los Angeles – U-factor does not matter as much. In a cold climate, a low U-factor is important – 0.25 - 0.40 Btu/hr-ft2-degF. Low U-factors are important for skylights in cold climates for both energy and water condensation reasons. © 2009 Autodesk Windows: Solar Heat Gain Coefficient (SHGC) Radiation The SHGC measures the portion of incident solar energy that passes through a window. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits. The definition of “good” depends on building location and the amount of glass. In Tucson, Miami, and Dallas a low is beneficial – 0.20 - 0.35 would be good. In Fairbanks, AK a high SHGCis Good – 0.5-0.7 would be good. SHGC is very important for windows/skylights in hot climates and for buildings with a large amount of unshaded glass. Some building designs vary the SHGC of the glass by orientation. © 2009 Autodesk Windows: Visible Light Transmittance (VLT or VT) Visible light transmittance measures the portion of visible light that passes through a window. The NFRC's tabulation of VT is a whole window rating and includes the impact of the frame, which does not transmit any visible light. Most VT values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is typically desirable to maximize daylight. Definition of Good Depends on Building Type/Amount of Glass. Too high and => glare if large glass area. Too low and => spent money on glass and did not capture enough daylight. Modern selective coatings combine high VLT and low SHGC – Do not need mirrored glass to get low heat gain. High performance glass can have 65% VLT, 30% solar heat gain and 0.30 Uvalue. © 2009 Autodesk I Specify Low-E Windows…Good Enough? All of the glass listed above is Low-E. -Some of the glass is suitable for Fairbanks, Alaska -Some is suitable for Fresno, California You would not need such a low “U-value” in Fresno, and could use an even lower one in Fairbanks, AK. © 2009 Autodesk Radiant Barriers Reduce heat transfer from the underside of the roof deck—primarily used in residential and small commercial. Radiant barriers/foil faced insulation are either metallic foils or plasticized foils that reduce the amount of radiant heat transfer between the roof and the plenum or conditioned space. Radiant barriers should be installed with the shiny side facing the interior of the building. Radiant barriers require air spaces to be effective. If a radiant barrier is in contact with sheathing or facing the wrong direction, they are not effective. © 2009 Autodesk Shading and Solar Studies Use BIM tools for shading studies. The combination of tools enable you to visually control direct solar (such as Autodesk® Ecotect ®) and quantify Energy/cooling load impacts (such as Autodesk® Green Building Studio®). © 2009 Autodesk Cool Roofs •Rolled, BUR, composition roofs absorb between 70-90% of incident radiation. •Cool roofs absorb 25-40% of incident radiation (aged performance.) •Trade-off with roof insulation. •In-house tests of coating on pitched new composition roof—disappointing. •Better luck on flat roofs. •San Francisco vs. Tucson—where is a cool roof more important? California Requirement as an example Applies to Low Slope Roofs (2:12 slope or less) Minimum solar reflectance of 0.70 Minimum Thermal Emittance of 0.75 Certified by Cool Roof Rating Council © 2009 Autodesk More information www.coolroofs.org www.coolroofs.info www.energystar.gov http://eetd.lbl.gov/coolroofs/ Time Erodes the Cool Factor 17°F Fresh application of cool roof coating © 2009 Autodesk 8°F 10-month old cool roof coating Uncoated “light” asphalt shingles A Cool Roof Coating on a Composition Roof - Results 151.9F 160 143.6F 150 140 130 120 Temperature F 110 100 90 80 70 60 50 40 roof standard aged roof cool coating outdoor ambient 30 © 2009 Autodesk Cool Roof Conclusions Asphalt shingles are cheap and prevalent. All roofers know how to install them. BUT, they are a very poor choice for reflecting incoming solar radiation. The solar reflectance of all commercial asphalt shingles is low (premium white shingles are only about 30% reflective, and other colors reflect less.) The low solar reflectance can be attributed to several factors. Limited amount of pigment in the granule coating. Roughness of the shingle contributes to multiple scattering of light, and thus to increased absorption. The black asphalt substrate is not 100% covered, and reflects only about 5% of the light that strikes it. © 2009 Autodesk Why do I need AC when it is 60 degrees outside? 20 80.0 18 16 75.0 65.0 14 12 60.0 10 55.0 8 6 70.0 50.0 40.0 4 2 35.0 0 Tim e Supply air typically at approximately 50 - 55 degrees © 2009 Autodesk 8/6 19:12 8/6 16:48 8/6 14:24 8/6 12:00 8/6 9:36 8/6 7:12 45.0 Unit Power (kW) Outside air – 60 – 70 deg F this day. 85.0 8/6 4:48 Air Tem perature (°F) Oakland AC-4 Air Temperatures and Unit Power Supply Return Outside Mixed Unit Pow er Are we addicted to air-conditioning? AC is assumed for new buildings. Buildings are uninhabitable without AC. Thermal comfort is #1 occupant complaint. AC systems can have comfort and IAQ problems. Need to think in terms of comfort and not in terms of tons of cooling. Source: Erik Ring, PE, Glumac, April 2007 Presentation at the PEC © 2009 Autodesk Most of the World’s Roofs Packaged airconditioning units. This is the fresh air intake © 2009 Autodesk The More Difficult But More Efficient Cooling Strategy: Passive/Low-Energy Cooling Natural Ventilation Thermal mass Evaporative Cooling Backup with compressors where needed This type of scheme is HARDER than traditional HVAC, even though it may require fewer tons of cooling and will save energy. © 2009 Autodesk Bioclimatic Charts Examines four passive cooling strategies: Natural Ventilation High Thermal Mass High Thermal Mass with Night Ventilation Evaporative Cooling Limitations Charts typically applicable for residences and projects with small internal heat gain. For more information: G.Z. Brown. Sun, Wind, and Light. John Wiley & Sons, 1985. Pg 50 © 2009 Autodesk Natural Ventilation Design issues: Shade Must have inlet and outlet for air Building should have thin cross-section Challenges Not effective above 90ºF More difficult at higher humidity © 2009 Autodesk High Thermal Mass Design issues: Shade Need wide daily temperature variation straddling comfort zone Isolate from exterior temperatures Mass should be distributed Challenges Not effective above 95ºF More difficult at higher humidity © 2009 Autodesk Evaporative Cooling Design issues: Shade Requires air movement Provides greatest savings at highest temperatures Challenges Not effective at high humidity Evaporative media and pump requires maintenance Requires water © 2009 Autodesk Autodesk, Ecotect, and Green Building Studio are registered trademarks or trademarks of Autodesk, Inc. and/or its subsidiaries and/or affiliates, in the USA and/or other countries. All other brand names, product names, or trademarks belong to their respective holders. Autodesk reserves the right to alter product offerings and specifications at any time without notice, and is not responsible for typographical or graphical errors that may appear in this document. © 2009 Autodesk, Inc. All rights reserved © 2009 Autodesk .