Building Envelope

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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.
• http//www.youtube.com/watch?vXsLkoJIPym0
• Vinyl strip doors and air curtains

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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

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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

• 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

• 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

• 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

• 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 Btus 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

• 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

• 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

• 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

• A "rule of thumb" for determining the thermal
  weight of a bldg
• look at heating and cooling needs at an outdoor
  temperature of 60F.
• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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