University of Florida Building Envelope
What is a Building Envelope? Answer: Everything that separates the interior of a building from the outside environment • Foundation or building slab • Walls and ceilings • Roof • Doors • Windows • Insulation
Foundation or Building Slab • Insulating foundations or bldg slabs is important for energy efficiency • For new construction, pre-insulated and pre-cast foundation panels or insulating concrete forms: • Minimize heat loss through the foundation • Protects the foundation from the effects of the freeze-thaw cycle in extreme climates • Reduces the potential for condensation on surfaces in the basement Interior basement insulation Exterior basement insulation
Wall/Ceiling Construction Considerations • Advanced framing techniques help to achieve E efficiency • Framing (of ceilings & walls) can also be avoided entirely with Structural Insulating Panels (SIPs): • Prefab panels sandwich a foam core between two sheets of plywood • Made to precise design specifications • Insulated concrete forms, are now also being used to form insulated concrete walls Ski Lodge in Canada Constructed w/ SIPs
Wall/Ceiling Construction: Alternative Building Materials A wide variety of alternative materials is now being used to construct buildings. Many have energy efficiency as well as environmental benefits. These materials include: • Adobe (clay and straw) • Straw Bale • Rammed earth • Tires and other recycled materials Mixing mud and straw in brick frames
Roof Considerations • White or reflective roofing help reflect heat and keep buildings cool • Ventilation should be considered to avoid moisture build-up • Studs, sills, and other building components can act as thermal bridges, conducting heat past a building's insulation White acrylic elastomeric roof coating protects the roof of a chemical manufacturing plant
Heat Loss Through Doors • Exterior doors generally comprise a small area of the building envelope • Even though most door types may not be very well insulated, they usually do not contribute substantially to the overall heat transfer of the envelope • The primary source of heat loss related to doors: • is through air leakage due to poor fitting doors and weatherstripping • and through the door being physically opened for building access
Overhead Doors • Overhead doors used for loading and unloading material or vehicle access are often left open for convenience • If used frequently, overhead doors can cause excessive air leakage and result in substantial heat loss or gain • This can lead to unnecessary cycling of heating and cooling systems as well as reduce comfort in surrounding areas
Overhead Doors • Evaluate loading schedules for frequency of overhead door use and identify problem areas and retrofit potential • Loading dock curtains made of plastic strips can be installed to reduce mixing of outside and conditioned air while permitting access to the loading dock • Other alternatives include reducing the door size or installing air curtains, radiant heating systems, conveyor belts, and controls to lock out HVAC equipment when the doors are open • Overhead doors in conditioned areas should also be insulated and weatherstripped to prevent heat loss when closed
Industrial Door Options • Roll-up Doors can effectively block air movement while not slowing down production. • Rapid open / close options are available. http://www.youtube.com/watch?v=XsLkoJIPym0 • Vinyl strip doors and air curtains
Windows • Labels from: • ENERGY STAR® • National Fenestration Rating Council • indicate: • Solar heat gain coefficient (SHGC) - roughly equivalent to the solar shading coefficient • U-value - which indicates how well the window insulates • Visible transmittance - which indicates how well light passes through the window • High-tech efficiency options include windows with: • Argon between the window panes • Low-emissivity (low-e) coatings
Energy-Efficient Windows • A deposit of microscopically thin, virtually invisible, metal or metallic oxide layers reduces the U-factor by suppressing radiative heat flow • Heat transfer in multilayer glazing is thermal radiation from a warm pane of glass to a cooler pane • Low-E coatings are transparent to visible light • Different types of Low-E coatings have been designed to allow for: • high solar gain (for cool climates) • moderate solar gain (for temperate climates) • or low solar gain (for cooling dominant climates) Pyrolitic window: high solar gain, low-e, double glaze / argon fill
Effect of Building Variables and Window-Oriented Heating Costs • By using energy efficient window technologies, the effect of: • shading, • window orientation • window area is minimized Top (red): clear, single glaze through bottom (purple): low-e, triple glaze
Insulation • Need to insulate indoor thermal sources: • Process heating equipment • HVAC Ductwork/ piping • Steam lines • Separate areas with AC from those without with air curtains or strip doors • Weatherstripping and caulking
Envelope Heat Loss The ability to hold indoor air temperature at the desired level is affected by all three methods of heat transfer: • Conduction • Convection • Radiation
Conduction • Requires that surfaces touch for solid-solid heat transfer. • Because the different materials in an insulated assembly touch each other, conduction heat loss occurs through solid components of the building envelope. • For example, heat flows by conduction from warm areas to the cooler areas of concrete slabs, window glass, walls, ceilings, and other solid materials.
Conductance • The unit used for thermal transmittance (heat transfer) or conductance of a single building material or building is often called the U-value. • U-values are expressed in Btu’s per hour per square foot of area per degree temperature difference. • Windows are commonly described by their U-values. • Descriptions of building walls, floors, or ceilings, often use R-values instead of U-values. The two terms are reciprocal. • The U-value or conductance flows through a material and the R-value measures the resistance, or how slowly heat flows.
Convection • Transferring heat from one place to another by molecular movement through fluids such as water or air. • Heat loss by convection commonly results from exfiltration or air leakage. • Convective heat loss occurs when warm air is forced out, usually from the building (exfiltration), by cold incoming air, usually in the lower part (infiltration). • The rate of transfer is increased when the wind blows against the building or when the temperature difference between the inside and outside increases
Radiation • Radiation is the heat transfer by electromagnetic waves from a warmer to a cooler surface. • The transfer of the sun's heat to a roof or the warmth of a standing near a glass furnace are examples of radiant heat transfer.
Thermally Light and Thermally Heavy Buildings • Thermally light – A building whose heating and cooling requirements are proportional to the weather driven outside temperatures, e.g., most homes and commercial office buildings. • Thermally heavy – A building whose indoor temperature remains fairly constant in the face of significant changes in the outdoor temperature, e.g., a plastic injection molding facility, or a building with a high heat generating device or area in it.
Thermal Weight • A "rule of thumb" for determining the thermal weight of a bldg: • look at heating and cooling needs at an outdoor temperature of 60°F. • If the building requires heat at this temperature, it can, too, considered thermally light, • If cooling is needed, it is thermally heavy • Some areas within a building can be both thermally light and thermally heavy depending on their use. • A meeting room, for example, can have significant heat gains from people, equipment, and lights when the room is occupied and not require any heating from the HVAC system on a cold day. • The same meeting room, however, may require heat at the same outdoor temperature when the room is vacant
Thermal Mass • Thermal mass saves energy by storing and releasing heat • For a building to take advantage of thermal mass, there must be a source of free or less expensive energy to charge the mass. • The existence of thermal mass, such as concrete walls and floors, can have a substantial impact on the operation of HVAC system's but is difficult to analyze. • It can affect the HVAC systems ability to quickly respond to rapid changes in load caused by increased occupancy, equipment, or solar gains through windows.
Thermal Mass • The effect of thermal mass on the building systems will vary • by climate and type of building • by the location of the mass within the structure • Thermal mass in exterior walls, for example, will slow down heat flow through the wall allowing a reduction in insulation requirements while maintaining performance levels similar to standard frame construction. • High levels of mass located within the building tend to reduce the effectiveness of mass in the outside walls
Thermal Mass • Buildings that most benefit from thermal mass are typically those with substantial cooling loads • In this case, the thermal mass can be precooled at night using outside air for free cooling or less expensive offpeak electricity for mechanical cooling. • This allows the mass to absorb heat the following day, reducing the need for operation of cooling systems during peak utility demand hours.
Thermal Mass • Generally, thermal mass is part of the integral construction of the building and is not added for conservation reasons • Unfortunately, there are no easy rules to determine how thermal mass will affect different buildings • It is important to note its existence because it may help you understand behavior of the mechanical systems or reasons for some comfort complaints
Solar Heat Gain • Windows are subject to solar heat gains which can have significant impacts on HVAC operation and occupant comfort • The amount of heat gain is a function of orientation, season, time of day, glazing type, and shading by window coverings, overhangs, other buildings and vegetation • Solar gains through south facing glass will add heat to the building in the winter • East and West surfaces will gain the greatest amount of heat in the early morning and late afternoon hours during summer months
Solar Heat Gain • Winter heat gains may be desirable in thermally light buildings while any solar heat gains in a thermally heavy building will only contribute to the cooling load • East and west facing glass are primarily a problem during the summer. Low sun angles in the morning and late afternoon can result in substantial solar heat gains as well as unwanted glare • The problem of excess solar heat gains during the summer can be compounded by the build up of internal heat most buildings experience late in the day. • The combination of solar and internal heat gains can greatly increase the energy required for cooling.
Building Pressure • HVAC system balance can influence the amount of air leakage • Buildings can be slightly pressurized by bringing in more intake air than is exhausted to reduce infiltration • An easy method of determining if a building is under positive or negative pressure is to hold an exterior door open about 1 inch on a calm cool day and observe which way the air is flowing • If air is flowing into the building, that part of the building is under negative pressure and may have problems with infiltration