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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

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Building envelope l.jpg

University of Florida

Building Envelope

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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

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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

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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

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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

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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

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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

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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

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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

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Industrial Door Options

  • Roll-up Doors can effectively block air movement while not slowing down production.

  • Rapid open / close options are available.

  • Vinyl strip doors and air curtains

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  • Labels from:


    • 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

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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

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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

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Insulation Costs

  • 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

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Envelope Heat Loss Costs

The ability to hold indoor air temperature at the desired level is affected by all three methods of heat transfer:

  • Conduction

  • Convection

  • Radiation

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Conduction Costs

  • 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.

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Conductance Costs

  • 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.

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Convection Costs

  • 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

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Radiation Costs

  • 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.

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Thermally Light and Thermally Heavy Buildings Costs

  • 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.

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Thermal Weight Costs

  • 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

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Thermal Mass Costs

  • 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.

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Thermal Mass Costs

  • 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

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Thermal Mass Costs

  • 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.

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Thermal Mass Costs

  • 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

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Solar Heat Gain Costs

  • 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

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Solar Heat Gain Costs

  • 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.

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Building Pressure Costs

  • 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