HEAT FLOW TO OR FROM A SPACE No habitable space has an envelope that is made of different materials with a consistent value of heat transmission through.

HEAT FLOW TO OR FROM A SPACEHEAT FLOW TO OR FROM A SPACE

No habitable space has an envelope No habitable space has an envelope that is made of different materials with a that is made of different materials with a consistent value of heat transmissionconsistent value of heat transmission through the separating barrier. through the separating barrier.

Spaces are constructed of several Spaces are constructed of several layers of building materials, installed for a layers of building materials, installed for a specific purpose, and all of them likely have specific purpose, and all of them likely have a different heat flow / resistance a different heat flow / resistance characteristic.characteristic.

In addition, a space is likely to have In addition, a space is likely to have more than one type of separating barrier, more than one type of separating barrier, such as the composition for walls, roofs, such as the composition for walls, roofs, floors, windows, doors, skylights . . . Most floors, windows, doors, skylights . . . Most have multiple layers of construction make-have multiple layers of construction make-up and none are the same.up and none are the same.

Consider the wall of a space. Take, for Consider the wall of a space. Take, for example the exterior wall of a residence, example the exterior wall of a residence, made up of: made up of:

–gypsum board surface insidegypsum board surface inside–2”x4” wood frame2”x4” wood frame–3.5” thickness of wall insulation3.5” thickness of wall insulation–¾” polyfoam sheathing¾” polyfoam sheathing–¾” thickness of air space¾” thickness of air space–3 5/8” thickness brick veneer3 5/8” thickness brick veneer–Then consider there is a still air film Then consider there is a still air film insideinside

and a moving air film outside.and a moving air film outside.

All the individual layers have different All the individual layers have different heat flow characteristics.heat flow characteristics.

THE “U” FACTOR THE “U” FACTOR

The rate of heat flow through an The rate of heat flow through an assembly of materials that form the assembly of materials that form the thermal barrier of a building envelope is thermal barrier of a building envelope is called a “U” factor. It considers the C called a “U” factor. It considers the C value, k value, and / or R value of each of value, k value, and / or R value of each of the materials that make up the assembly.the materials that make up the assembly.

First, each material must have its C or First, each material must have its C or k value converted to an R value, then all R k value converted to an R value, then all R values added together. C and k and R values added together. C and k and R values cannot be added to obtain a viable values cannot be added to obtain a viable heat flow rate. It is rather like adding heat flow rate. It is rather like adding apples, oranges, and bananas.apples, oranges, and bananas.

With all the R values known; With all the R values known;

And since R is the And since R is the reciprocalreciprocal of C, of C,

The “U” value for an assembly of The “U” value for an assembly of materials is the materials is the reciprocal of the reciprocal of the sumsum of of the R valuesthe R values. .

Like C, U is a rate of heat flow, Like C, U is a rate of heat flow,

except that “U” is the rate of heat flow for except that “U” is the rate of heat flow for a a combination of materials combination of materials that make up that make up part or all of a space envelope, rather part or all of a space envelope, rather than one material.than one material.

Still Air

Moving Air

CONVERSTION TO AND TABULATION OF CONVERSTION TO AND TABULATION OF R R VALUESVALUES FOR A RESIDENTIAL FRAME WALL FOR A RESIDENTIAL FRAME WALL WITH BRICK EXTERIOR:WITH BRICK EXTERIOR: material material R R valuevalue

1 outside air1 outside air 0.170.17

2 brick veneer 2 brick veneer k = 9.0; k = 9.0; R = in. thk / k = 3.625 / 9 = R = in. thk / k = 3.625 / 9 = 0.410.41

3 still air space 3 still air space 1.181.18

4 ¾” polyfoam sheathing; 4 ¾” polyfoam sheathing; k = 0.20; R = in. thk / kk = 0.20; R = in. thk / k R = 0.75 in / 0.20 =R = 0.75 in / 0.20 =

3.753.75

5 3 ½” insulation5 3 ½” insulation 11.011.0

6 ½” gypsum board; C = 2.226 ½” gypsum board; C = 2.22 R = 1/C = 1 / 2.22 =R = 1/C = 1 / 2.22 = 0.450.45

7 Inside air film R = 7 Inside air film R = 0.680.68 Total R value = 17.64Total R value = 17.64

With a total R value of 17.64 for all the With a total R value of 17.64 for all the materials that make up the wall, the U value materials that make up the wall, the U value equals:equals:

U = 1 / summation of individual R valuesU = 1 / summation of individual R valuesU = 1 / 17.64 = 0.0567U = 1 / 17.64 = 0.0567

Which means that for each Which means that for each square square foot of wallfoot of wall, , per hourper hour, per , per degree degree FahrenheitFahrenheit, 0.0567 BTU will move through , 0.0567 BTU will move through the wall - - - because of heat flow caused the wall - - - because of heat flow caused by temperature difference. by temperature difference.

Condensation of moisture must be Condensation of moisture must be given consideration to the composition of given consideration to the composition of exterior walls and how they are insulated. exterior walls and how they are insulated.

Condensation occurs because water Condensation occurs because water vapor in the air reaches a certain ambient vapor in the air reaches a certain ambient temperature called the “dew point.”temperature called the “dew point.”

Dew point is a temperature at which Dew point is a temperature at which moisture in the air reaches a saturation moisture in the air reaches a saturation point and cannot remain as vapor, but point and cannot remain as vapor, but condenses, changing state from vapor to condenses, changing state from vapor to liquid form. liquid form.

Two things the designer also wants to Two things the designer also wants to occur in a building envelope.occur in a building envelope.

One: A vapor barrier placed on the One: A vapor barrier placed on the warm side of insulation – generally on the warm side of insulation – generally on the interior of the space because that is interior of the space because that is where moist air is most likely to remain. where moist air is most likely to remain. Moist air occurs on the outside, but Moist air occurs on the outside, but exterior conditions change – and we don’t exterior conditions change – and we don’t care if condensation occurs on the care if condensation occurs on the outside. Besides, the exterior surface of outside. Besides, the exterior surface of the envelope is made to resist moisture. the envelope is made to resist moisture. A vapor barrier can be any surface that A vapor barrier can be any surface that expels water. expels water.

Two: The designer would desire that the Two: The designer would desire that the dew point temperature occur within the dew point temperature occur within the insulating barrier where there is no water insulating barrier where there is no water vapor present. vapor present.

Illustration: Illustration:

On a hot summer day, say the outside On a hot summer day, say the outside surface temperature is around 100+ degrees, surface temperature is around 100+ degrees, and air conditioning inside at a cool, and air conditioning inside at a cool, comfortable seventy two degrees. There is comfortable seventy two degrees. There is enough water vapor in the air inside, such enough water vapor in the air inside, such that the dew point temperature is between that the dew point temperature is between 72 and 100+ degrees. 72 and 100+ degrees.

The tendency for heat flow is from The tendency for heat flow is from outside to inside - - -outside to inside - - -

At this point, say there is no insulation At this point, say there is no insulation and no vapor barrier, and there is little to and no vapor barrier, and there is little to keep heat from penetrating, and at some keep heat from penetrating, and at some point between the outside and inside point between the outside and inside surfaces, the dew point temperature is surfaces, the dew point temperature is reached and water happens because there is reached and water happens because there is moisture present. Condensation occurs (wet moisture present. Condensation occurs (wet water) within the wall – resulting in potential water) within the wall – resulting in potential damage to the enclosure.damage to the enclosure.

But suppose there is insulation and a But suppose there is insulation and a vapor barrier. The inside surface finish vapor barrier. The inside surface finish remains at 72 degrees, and within the remains at 72 degrees, and within the insulation material, heat flow is slowed to the insulation material, heat flow is slowed to the point that dew point occurs in a dry area, point that dew point occurs in a dry area, protected from the intrusion of moisture by protected from the intrusion of moisture by the vapor barrier. the vapor barrier.

AFFECT OF SUN RADIATION ON A BUILDING AFFECT OF SUN RADIATION ON A BUILDING ENVELOPEENVELOPE

Previously, calculations of heat flow have Previously, calculations of heat flow have been directed based on BTU/h x area x been directed based on BTU/h x area x temperature differentialtemperature differential. . During cooling During cooling season the surface of a building envelope, season the surface of a building envelope, even though it may be opaque, is influenced by even though it may be opaque, is influenced by radiant energy from the sun.radiant energy from the sun.

Consider the mass of a building material Consider the mass of a building material and its ability to retain heat. After having been and its ability to retain heat. After having been heated by the sun, dense materials such as heated by the sun, dense materials such as brick and concrete will stay warm for a longer brick and concrete will stay warm for a longer period of time than lighter materials such as period of time than lighter materials such as wood, or materials with low mass because they wood, or materials with low mass because they are thin, as is glass or sheet metal.are thin, as is glass or sheet metal.

When a building is subjected to radiant When a building is subjected to radiant energy during the day, the walls and roof are energy during the day, the walls and roof are heated because of heated because of the change of the change of electromagnetic energy to heat energyelectromagnetic energy to heat energy, AND , AND conduction because of ambient temperatureconduction because of ambient temperature. . As time progresses, part of the heat will As time progresses, part of the heat will dissipate into the atmosphere, but most will dissipate into the atmosphere, but most will penetrate the surfaces because of the greater penetrate the surfaces because of the greater temperature differential. So the MASS of a temperature differential. So the MASS of a building envelope has an affect on heat flow building envelope has an affect on heat flow in summer because of time lag.in summer because of time lag.

Glass is considered separately regarding Glass is considered separately regarding conduction and radiation, because radiant conduction and radiation, because radiant energy can penetrate a energy can penetrate a translucent/transparent barrier. translucent/transparent barrier.

The amount of heat retained by a The amount of heat retained by a building envelope is also affected by HOW building envelope is also affected by HOW MUCH radiant energy is converted to heat MUCH radiant energy is converted to heat energy because of its color hue. Recall energy because of its color hue. Recall that dark hues absorb heat and light hues that dark hues absorb heat and light hues reflect heat. So a variation of reflective reflect heat. So a variation of reflective quality exists from white (the equal quality exists from white (the equal combination of all colors) to black (the combination of all colors) to black (the absence of light)absence of light)

The following lists approximate The following lists approximate general reflective value of colors:general reflective value of colors:

graygray 25%25%dark reddark red 26%26%light greenlight green 50%50%creamcream 65%65%whitewhite 75 – 95%75 – 95%

A factor called A factor called “EQUIVALENT “EQUIVALENT TEMPERATURE DIFFERENTIAL”TEMPERATURE DIFFERENTIAL” ( ETD ) ( ETD ) approximate the construction assembly’s approximate the construction assembly’s interrelationship between conductance, interrelationship between conductance, thermal time-lag, and color.thermal time-lag, and color.

ETD is defined as ETD is defined as the outdoor-indoor the outdoor-indoor TEMPERATURE DIFFERENCE that will be TEMPERATURE DIFFERENCE that will be equal to the solar, conduction, and radiation equal to the solar, conduction, and radiation heat flow into a space with allowance for heat flow into a space with allowance for time lagtime lag. .

In calculating HEAT GAIN caused by In calculating HEAT GAIN caused by radiation onto a building component of radiation onto a building component of mass, the ETD value is used instead of the mass, the ETD value is used instead of the difference in outdoor/indoor temperature. difference in outdoor/indoor temperature. The chart in the packet labeled “ETD” The chart in the packet labeled “ETD” indicates values for types of construction indicates values for types of construction and time of day. and time of day.

EQUIVALENTEQUIVALENTTEMPERATURETEMPERATUREDIFFERENCEDIFFERENCE

Use this chart forUse this chart fortemp difference temp difference

InIncalculating heat calculating heat

GAINGAIN for walls and for walls and

roofroof

This chart is This chart is based onbased on

Location of 40 Location of 40 degreesdegrees

N latitudeN latitude

ASHRAE ASHRAE handbook ofhandbook of

FundamentalsFundamentals

TEMPERATURE ZONES WITHIN BUILDINGSTEMPERATURE ZONES WITHIN BUILDINGS

All areas within buildings exposed to All areas within buildings exposed to exterior walls are subject to change in exterior walls are subject to change in temperature simply because of the temperature simply because of the changing position of the sun.changing position of the sun.

Space on the east side during mornings Space on the east side during mornings are exposed to radiant heat from the sun are exposed to radiant heat from the sun until mid-day, while space on the west side until mid-day, while space on the west side are in the shade of the building. are in the shade of the building.

The situation reverses itself during the The situation reverses itself during the afternoon, giving the west side to exposure afternoon, giving the west side to exposure of direct radiation Spaces on the south have of direct radiation Spaces on the south have sun all day while the north side has virtually sun all day while the north side has virtually none.none.

As in the diagram, a central mechanical As in the diagram, a central mechanical unit with only one control, the dilemma unit with only one control, the dilemma remains as to where to place the thermostat.remains as to where to place the thermostat. Not such a problem for a residence, in Not such a problem for a residence, in that the use is limited to a small number of that the use is limited to a small number of people and minimal activity.people and minimal activity.

But for an office area where each space is But for an office area where each space is in use by personnel all through the day, the in use by personnel all through the day, the problem remains except as mitigated by problem remains except as mitigated by architectural planning. architectural planning. Consideration must be given to areas Consideration must be given to areas within a space that are subject to extreme within a space that are subject to extreme heat gain due to sun position. heat gain due to sun position. A space may A space may be large enough laterally that some of the be large enough laterally that some of the areas within are not affected by the areas within are not affected by the condition of the exterior walls. condition of the exterior walls.

Such spaces might be in very large Such spaces might be in very large single story buildings and those with single story buildings and those with multiple floors where the space is very large.multiple floors where the space is very large.

The limit of distance from exterior walls The limit of distance from exterior walls where the space is affected by the where the space is affected by the condition of the exterior is much the same condition of the exterior is much the same as considerations given to daylighting of as considerations given to daylighting of interior spaces as limited by the distance interior spaces as limited by the distance from windows. from windows. There is a limit to the There is a limit to the distance from the exterior wall to spaces distance from the exterior wall to spaces that will be affected by heat loss / gain that will be affected by heat loss / gain through the exterior wall.through the exterior wall.

The following diagram represents the The following diagram represents the floor of a multi-story building where all floor of a multi-story building where all exterior walls are subject to exterior exterior walls are subject to exterior conditions, whether it be cold, heat, or conditions, whether it be cold, heat, or radiation from the sun. radiation from the sun.

In order to maintain a thermal comfort In order to maintain a thermal comfort level in all areas, each zone must be level in all areas, each zone must be treated and controlled as a separate treated and controlled as a separate entity. entity.

A mechanical system must necessarily A mechanical system must necessarily be flexible enough to provide for varying be flexible enough to provide for varying conditions during the day.conditions during the day.

Realize that varying zones are created Realize that varying zones are created within a space as the result of the varying within a space as the result of the varying position of the sun during the day. Of the position of the sun during the day. Of the zones created, 2 through 9 are influenced zones created, 2 through 9 are influenced by HEAT GAIN variance by HEAT by HEAT GAIN variance by HEAT RADIATION from the sun, and affects RADIATION from the sun, and affects COOLING COMFORT ONLY during air COOLING COMFORT ONLY during air conditioning season.conditioning season.

Only TWO ZONES exist during winter Only TWO ZONES exist during winter conditions, when heat is lost in areas 2 conditions, when heat is lost in areas 2 through 9 through the exterior walls and through 9 through the exterior walls and must be replaced – since HEAT LOSS is a must be replaced – since HEAT LOSS is a factor of heat flow by CONDUCTION. Zone 1 factor of heat flow by CONDUCTION. Zone 1 all year long is air conditioning only. all year long is air conditioning only.

So much for day to day requirements. So much for day to day requirements. But what about requirements that aren’t But what about requirements that aren’t the same one or two days of the week – and the same one or two days of the week – and on holidays.on holidays.

If the space is a multi - use area, If the space is a multi - use area, occupied by a number of occupants that occupied by a number of occupants that have varying work times, the mechanical have varying work times, the mechanical system must be flexible from that system must be flexible from that standpoint. That is, if efficiency is to standpoint. That is, if efficiency is to remain a priority.remain a priority. Say maybe two or three of the tenants Say maybe two or three of the tenants work Monday through Friday - closed on work Monday through Friday - closed on weekends.weekends.

But maybe another group of tenants But maybe another group of tenants has a six-day work week – and maybe has a six-day work week – and maybe another tenant works seven days per another tenant works seven days per week.week.

The equipment must be staged in an The equipment must be staged in an efficient way in order to meet the efficient way in order to meet the requirements of differing temperature requirements of differing temperature zones, and differing daily needs. zones, and differing daily needs.

When refrigerated comfort cooling When refrigerated comfort cooling first became prominent, a unit used to first became prominent, a unit used to describe an amount of cooling capacity describe an amount of cooling capacity was called a “ton” of air conditioning. was called a “ton” of air conditioning. The term originated when air conditioning The term originated when air conditioning consisted of blowing warm air over ice to consisted of blowing warm air over ice to allow it to absorb the heat. allow it to absorb the heat.

The amount of heat required to The amount of heat required to change one pound of ice at 32 degrees to change one pound of ice at 32 degrees to one pound of water at 32 degrees is 144 one pound of water at 32 degrees is 144 btu – a change of state of a substance – btu – a change of state of a substance – solid ice to liquid water. solid ice to liquid water.

The melting of a ton of ice (2000 lb) The melting of a ton of ice (2000 lb) over 24 hours will cool ( 2000 lb x 144 over 24 hours will cool ( 2000 lb x 144 btu ) / 24 hours = 12,000 btu/hour. btu ) / 24 hours = 12,000 btu/hour. Hence, a ‘ton’ of air conditioning capacity Hence, a ‘ton’ of air conditioning capacity is 12,000 btu/h.is 12,000 btu/h.

So an exchange of heat is required to So an exchange of heat is required to change the state of a substance - - - at the change the state of a substance - - - at the same temperature; water to ice, water same temperature; water to ice, water vapor to water, etc. vapor to water, etc.

It takes one btu to change one pound It takes one btu to change one pound of water from 211 degrees to 212 of water from 211 degrees to 212 degrees, F. But it takes 1061 btu to degrees, F. But it takes 1061 btu to change one pound of water at 212 change one pound of water at 212 degrees to steam at 212 degrees. Steam degrees to steam at 212 degrees. Steam is water vapor.is water vapor.

NOW TAKE A CLEAN, 8 ½” X 11” SHEET OF NOW TAKE A CLEAN, 8 ½” X 11” SHEET OF PAPER, PAPER,

SMOOTH EDGES,SMOOTH EDGES,

RULED OR UNRULED, ANDRULED OR UNRULED, AND

PRINT YOUR NAME AT THE TOP, THENPRINT YOUR NAME AT THE TOP, THEN

ANSWER THE FOLLOWING QUESTIONANSWER THE FOLLOWING QUESTION

When someone makes home-made ice When someone makes home-made ice cream, using a mechanical ice cream cream, using a mechanical ice cream freezer (either hand-crank or electric freezer (either hand-crank or electric motor)motor)

All the ingredients are in the container, All the ingredients are in the container, then Ice is added to the unit, then salt is then Ice is added to the unit, then salt is poured over the ice. You begin to turn the poured over the ice. You begin to turn the crank until ice cream happens . . . crank until ice cream happens . . .

Explain why the salt is poured over the ice, Explain why the salt is poured over the ice, and what happens in the process to make and what happens in the process to make the contents freeze.the contents freeze.

WHEN YOU ARE FINISHED, WHEN YOU ARE FINISHED,

FOLD YOUR PAPER ONCE, THEN TURN IT IN FOLD YOUR PAPER ONCE, THEN TURN IT IN TO METO ME