Principles of Thermal Comfort
Heating and cooling of spaces is typically done passively
as we tend to lose or gain heat from surrounding objects more than directly from heating and cooling systems.
Two materials must be in contact with each other
for conduction to work
Electromagnetic waves cover all spectrum's of light
from Cosmic Rays to visible light to radio waves.
Buildings lose heat and gain heat
from numerous sources.
Materials with high thermal capacity
have low thermal resistance.
Specify the correct materials to
inhibit hidden moisture developing.
Condensation in
interior spaces can cause great damage.
Evaporation
is a method of temperature change through phase changes in material state (liquid to gas).
RH over 80% will cause mold and mildew to form. High humidity can be
managed with mechanical, refrigerant dehumidifiers, which chills the air causing the humidity to condense into a drain pan or line.
Thermal bridges are
material pathways allowing heat to conduct more easily (wood or metal studs).
Heat given off by our bodies is called
metabolic heat.
The rate at which our bodies generate heat is called our
metabolic rate
Comfort is determined by
more than air temperature.
Infrared (IR) light is electromagnetic energy that is not visible to the
naked eye, but can be felt as heat
Provide proper mechanical ventilation
of high humidity air.
Humidity is
water vapor in the air.
If the air temperature drops further,
water will condense as dew, and liquid water droplets will form.
Insulation is
the primary defense against heat loss through a building envelope.
Thermal mass refers to
the quantity of material that exhibits good retention of heat.
Relative humidity is
the ratio of the amount of water vapor in the air to the maximum amount of water vapor the air could hold. Expressed as a percentage.
If the amount of water vapor remains the same (in lbs.) and the temperature drops,
the relative humidity will increase.
The complex physical and chemical processes involved in the maintenance of human life is called
Metabolism
Heat is gained internally from:
People Equipment Lighting Heating systems
Thermal resistance of materials can be measured in
R-Value.
Heat energy is transferred in three ways:
Radiation Conduction Convection
Buildings interact with thermal radiation in three ways:
Reflectance absorptance emittance
Consider how warmed surfaces may effect the perception of warmth.
Stone, or solid surfacing vs. wood or upholstery.
Heat is gained from the exterior by:
The Sun Outside air temperature Radiant heat from surrounding surfaces.
Evaporation is
a process that results from the three types of heat transfer (radiation, conduction, and convection).
Conduction takes place within
a single material when one part is warmer than the other.
Architects increase thermal resistance to heat loss by
adding more insulation, reflective surfaces, or more layers of air.
The way in which heat is lost or gained is an
advantage or disadvantage when specifying interior finishes.
Warm large surfaces have
advantages and disadvantages.
Radiation travels through
air or vacuums such as space.
All objects give off IR radiation,
and all objects receive IR radiation.
Be aware of existing mechanical system grilles and diffusers
and where they may blow conditioned air. Do not interrupt flow without considering alternatives.
Super-insulated buildings (Passive House) .
are designed to eliminate the need for heating systems
Convection causes heat flow
as a fluid stream of air or liquid passes a material of a different temperature.
The moist air cools and cannot hold
as much water vapor. Some of it condenses into water droplets.
Higher temperature air
can hold more water vapor than cooler temperature air.
Air should be
circulated near cold windows. RH should be controlled to between 20% to 50% for comfort.
A layer of air helps slow
down the transfer of heat.
The internal energy that sets molecules in motion
emits electromagnetic waves.
IR behaves
exactly like visible light.
The R-Value is the number hours required
for 1 BTU to pass through 1 square foot of material when the temperature difference is 1-degree F.
A building's heating system can
heat very large surfaces, which then warm us.
As long as there is a temperature difference from one area to another,
heat will always move from regions of higher temperature to regions of low temperature.
IR radiation flows from
hot to cold in a direct line of sight
Thermal resistance indicates
how effectively a material serves as an insulator.
The construction of a building influences
how much heat is lost or gained from the outside environment.
Evaporation removes heat
in order for a liquid (sweat) to change to a vapor. This has a cooling effect as it draws heat away from the skin.
Use humidity to your advantage in
making occupants feel warmer or cooler with having to raised or lower air temperature
The cooling load
is the amount of cooling needed to offset the net gain of heat.
The Mean Radiant Temperature (MRT)
is the area weighted mean temperature of all the objects surrounding the body.
Good insulators have
low thermal capacity because they cannot retain heat. They prevent heat from passing through them.
Every substance has a
property called specific heat. This is the heat required to change the temperature a specific amount.
Reflectance
refers to the amount of incoming radiation that bounces off of a material.
When liquid evaporates it
removes a large amount of heat from the surface it is leaving.
Each layer of material in the envelope helps
resist the flow of heat.
Convection is
similar to conduction in that heat is lost through contact with another material.
Low humidity cause
static electricity problems.
The warmer the air,
the more water vapor it can hold.
A relative humidity of 50% means
that the air contains half of the maximum it can at that temperature.
If the air temperature drops
the RH will increase until it reaches 100%.
An RH of 100% means
the air is completely saturated with water vapor (gas) and can hold no more.
Emittance
the amount of energy radiated outward from an object.
Thermal capacity is
the amount of heat a material can hold.
The heating load is
the amount of heat needed in the winter time to offset heat lost through the envelope.
Absorptance
the amount of thermal energy that enters a material thereby raising its temperature.
The operative temperature is
the average of the dry-bulb temperature and the MRT.
A good insulator resists
the conduction of heat.
Effective temperature is
the effect of feeling cooler due to evaporation, even though the air temperature has not changed.
The larger the temperature difference between two sides of an insulator,
the faster the heat flow.
The larger the temperature difference between the interior and exterior of a building envelope,
the faster the heat gain or loss.
Conduction is
the flow of heat from two materials in direct contact with each other.
Sensible heat is
the heat required to change the temperature of a substance.
The latent heat of vaporization is
the sensible heat required to change a liquid to a gas.
The latent heat of fusion is
the sensible heat required to change a solid into a liquid.
Latent heat is
the sensible heat required to change the state of a substance (e.g. solid to liquid).
The higher the thermal resistance,
the slower the heat loss.
Heat transfer occurs faster when
the temperature difference between the air and body is greater, and when the air flow is greater.
When no temperature difference exists,
there is thermal equilibrium.
Condensation forms when
warm, humid air makes contact with cold surfaces.
Heat flows from the
warmer material to the cooler material.
When we go from gas to solid
we acquire heat (it is given off).
Provide adequate space around
windows for proper air circulation to prevent condensation.
