HEAT ENERGY FLOWS IN
BUILDINGS
Building energy fundamentals
SENSIBLE VS. LATENT HEAT FLOW
Sensible Heat Flow – Results in a change in air
temp
 Latent Heat Flow – Results in a change in
moisture content. Release or storage of heat
associated with change in phase of a substance,
without a change in substance temp.
 Total Heat Flow – sum of latent and sensible
heat flows
 Heat Energy Flows in Buildings

SENSIBLE VS. LATENT HEAT FLOW

Sensible vs. latent heat: it takes over 5x as much
heat to turn water into steam at the same temp
than it does to heat liquid water from freezing to
boiling temps.
SENSIBLE VS. LATENT HEAT
Whenever an object is at a temp different from its
surrounds, heat flows from hot to cold
 In similar fashion moisture flows from areas of
greater concentration to areas of lower
concentration
 Buildings lose sensible heat to the environment
(or gain) in 3 principle ways

CONDUCTION, CONVECTION, RADIATION
Conduction: transfer of heat between substances which
are in direct contact with each other
 Convection: movement of gases/liquids caused be heat
transfer. As a gas or liquid is heated it warms,
expands and rises because it is less dense
 Radiation: electromagnetic waves travelling through
space. When these waves hit an object they transfer
their heat to it

Conduction takes placed through
envelope assemblies
 Convection is the result of wind or
pressure drive air movement
 Radiant heat is primarily from the
sun

THERMAL EFFECTS
Principals are the same, but heat flow under
changing conditions is more complex than under
static conditions
 Heat storage within materials is of greater
concern during dynamic conditions
 Under static conditions, heat flow is primarily a
function of temp difference and thermal
resistance

THERMAL EFFECTS
Under dynamic conditions, those two factors are
still important, but the heat storage in the
building envelope is a compounding issue
 Heat storage is a function of the density of the
material and specific heat; product of these two is
the thermal capacity (thermal mass)

THERMAL PROPERTIES

Every material used for the envelope has
properties that determine their energy
performance
THERMAL CONDUCTIVITY (K)
Material’s ability to conduct heat
 The faster heat flows through a material the
more conductive it is
 q=Resultant heat flow (watts)
 k=thermal conductivity (W/mK)
 A=surface area (m2)
 T=temp diff between warm and cold sides(K)
 L= thickness or length of material (m)

THERMAL CONDUCTANCE
Conductivity per unit area
 In basic building materials heat flow is generally
measured by conductance (C), not conductivity
 Is an object property which relies on the
materials and the size

U-FACTOR
In layered assemblies, conductance is combined into a
single number called the U-factor or U-value
 Lower U factor means worse conduction, which means
better insulation
 Does not include latent heat (moisture related)
 Used only to describe air flow from the outside of the
envelope to airflow on the inside of the envelope (ie not
for basement walls)

THERMAL RESISTANCE (R-VALUE =1/U)
How effective a material is as an insulator
 R is measured in the hours needed for 1BTU to flow
through 1ft2 of a given thickness of material when the
temp diff is 1f
 Object property, not material



A 2x6 pine stud has three times the R-value as a 2x2 pine
stud
Higher R value indicates better insulating properties
BUILDING ENERGY LOADS
How much energy your building needs
 Can be provided by electricity, fuel, or passive
means
 Lots of terms that can get confusing, next slide
has a chart to help with these terms

ENERGY LOADS

Thermal Loads – heating and cooling energy
needed to keep people comfortable
Heating loads – energy required to heat the building
when to cold
 Cooling loads – energy required to cool the building
when to hot


Not just about temp, include moisture control (latent heat)
LOADS

Heating and cooling loads are met by the HVAC
system

Uses energy to add/remove heat and condition the space
Equipment loads – HVAC etc, met by energy or fuel
 Plug loads – electricity for computers and appliances
 Lighting loads – electricity used for lighting

THERMAL LOADS
Understanding the heating and cooling loads
helps to provide the right sized HVAC system for
a space
 Reduce the loads as much as possible, and meet
them as efficiently as possible

EXTERNAL
Heat transfer through the building envelope from
the sun and outside environment
 Building envelope includes the roof, walls, floors,
windows. Anything that separates inside from
outside

EXTERNAL

Common ways heat flows into or out of the
building
Heat conduction from the envelope to outside air or
ground
 Sunlight shining through windows to heat int.
 Sunlight warming up ext. of building
 Losing inside air to outside, or vice versa, through
leaks


How much energy from the sun’s radiation,
outside air temp, latent heat in the airs moisture
that reaches the inside to affect the comfort
depends a lot on the envelope


Materials, design and how well it is sealed
Understanding where heat energy is gained/lost
is important for successful passive design
strategies
INTERNAL THERMAL LOADS
Come from heat generated from people, lighting, and
equipment (core loads, internal gains)
 Thermal loads from lighting and equipment is
generally equal to their use

When a light fixture converts a watt-hour of electricity into
photons, those photons bounce around until they are
absorbed which turns light energy into heat energy
 All electrical energy not turned into photons is turned
directly into heat energy due to inefficiency

INTERNAL THERMAL LOADS
Similarly, electrical energy used to move mechanical
parts is transformed into heat via friction
 Energy used to power this equipment is turned into
heat energy via electrical resistance
 Thermal loads from people depend on the number of
and what activity they are doing

Office buildings are generally dominated by internal loads
 Single family residences are typically dominated by
external loads

INTERNAL THERMAL LOADS
HEATING AND COOLING LOADS
How much energy you need to heat and cool the
building and control moisture within
 Gains that are more than envelope and
ventilation losses would cause a net cooling load
(the building is too hot)
 Losses that are more than the internal gains
would cause a net heating loads (the building is
too cold).
 The heating setpoint is often different than the
cooling setpoint, so the distribution of heating
and cooling loads is climate dependent

EQUIPMENT AND LIGHTING LOADS
Lighting loads – energy used to power electric
lights, make up nearly 1/3 of commercial building
energy use (10-15% in residential)
 Look for more efficient lighting

Reduce lighting loads
 Reduce cooling loads for the same visible brightness

PLUG LOADS

Electricity used for other equipment
20-30% of energy in commercial
 15-20% in residential (these numbers are growing)

Equipment Rated Power (watts)
Desktop computer 120
Notebook computer 45
17” LCD Display 75
Desktop laser printer 120
Office laser printer 250
Office copier 750
Refrigerator 750
Dishwasher 1,200
Television 100
Commercial refrigerator 1,000
Commercial fryer 10,000
Clothes washer 350
Clothes dryer 2,000
MEASURING ENERGY USE

Energy Use Intensity - Energy intensiveness is
simply energy demand per unit area of the
building's floorplan, usually in square meters or
square feet
EUI BASED ON BUILDING TYPE
EUI BASED ON FLOORSPACE
SITE ENERGY VS. SOURCE ENERGY
Energy intensiveness only considers the amount
of electricity and heat that is used on-site
("secondary" or "site" energy)
 Does not consider the fuel consumed to generate
that heat or electricity. This "primary" or
"source" energy can be generated on-site or at a
power plant far away.

SOURCE ENERGY
When measure energy used to provide thermal or
visual comfort, site energy is the most useful
measurement
 When measuring total energy usage, source energy is
more accurate
 Low on-site energy can cause more use upstream

Ex, 2kW of natural gas burned on site is better than 1 kW
of electricity used on site. 1kW of site electricity from the
average US electrical grid is equal to 3.3kW of source
energy due to inefficiencies
 Energy Efficiency

SOURCE-SITE RATIOS
ENERGY END USE

Commercial and residential use energy
differently
Commercial are dominated by internal thermal loads
(more people and equipment)
 Residential are dominated by external loads, larger
percentage of energy use is for heating and cooling to
meet those

SANKEY DIAGRAMS