AA DAYLIGHT AND HEAT DAYLIGHT AND HEAT GAINGAIN

1) 1) HEAT GAINHEAT GAIN is the amount of heat is the amount of heat that gets into a space FROM the that gets into a space FROM the outside in summer conditions when air outside in summer conditions when air conditioning is desired. Air conditioning is desired. Air conditioning is simply removal of heat conditioning is simply removal of heat from a space.from a space.

2) 2) HEAT LOSSHEAT LOSS is the opposite. The is the opposite. The amount of heat that gets away from a amount of heat that gets away from a space TO the outside when comfort space TO the outside when comfort heating is desired. Comfort heating is heating is desired. Comfort heating is the process of adding heat to a space.the process of adding heat to a space.

3) Two kinds of heat gain:3) Two kinds of heat gain:BY CONDUCTION through the variousBY CONDUCTION through the variousmaterials of the building envelope whenmaterials of the building envelope whenheat is transferred from molecule toheat is transferred from molecule tomolecule. The number of BTUs thatmolecule. The number of BTUs thattravel through one square foot in onetravel through one square foot in onehour per degree Fahrenheit temperaturehour per degree Fahrenheit temperaturedifference is called a “U” factor.difference is called a “U” factor.

BY RADIATION of electromagnetic energyBY RADIATION of electromagnetic energydirectly through glass – which does notdirectly through glass – which does notbecome heat energy until sufficient massbecome heat energy until sufficient masschanges EM energy to heat.changes EM energy to heat.

DIRECT SUNLIGHTDIRECT SUNLIGHTa) Sunlight is the source of a) Sunlight is the source of electromagnetic energyelectromagnetic energy which in turn which in turn becomes a source of heat, but not becomes a source of heat, but not untiluntil

b) The rays strike a surface of b) The rays strike a surface of sufficient densitysufficient density to convert to heat. to convert to heat.

c) Sunlight c) Sunlight increases the temperature increases the temperature of a concreteof a concrete walkway much higher walkway much higher than ambient temperature, because of than ambient temperature, because of its density, but does not for surfaces its density, but does not for surfaces such as grass and soft earth.such as grass and soft earth.

d) An adjacent grassy lawn surface d) An adjacent grassy lawn surface may seem cool to the touch, because may seem cool to the touch, because of the difference in the density. of the difference in the density.

e) The softer surface e) The softer surface acts to absorb acts to absorb the sun’s rays with too little densitythe sun’s rays with too little density to to produce heat. produce heat.

f) Rays of the sun may penetrate f) Rays of the sun may penetrate glass glass but not become heat until they but not become heat until they contact a dense surface insidecontact a dense surface inside, such , such as a floor, wall, furniture, people, etc. as a floor, wall, furniture, people, etc. Glass transforms some EME to heat, Glass transforms some EME to heat, but not much, because of its thinness.but not much, because of its thinness.

g) Then, electromagnetic energy g) Then, electromagnetic energy produces heat that becomes a cooling produces heat that becomes a cooling load during air conditioning seasons.load during air conditioning seasons.

h) If glass has a mirrored surface such h) If glass has a mirrored surface such as as a coating sufficient to reflecta coating sufficient to reflect direct direct sunlight, then the source of heat is sunlight, then the source of heat is diminished. diminished.

i) But there is a tradeoff. A surface i) But there is a tradeoff. A surface that can reflect light that can reflect light also may also may diminish light transmissibilitydiminish light transmissibility. .

SHADING DEVICESSHADING DEVICES

a) A shading device may be a) A shading device may be anything anything that keeps direct sunlight from striking that keeps direct sunlight from striking a surface – a surface – or from penetrating a or from penetrating a surface to produce heat on the other surface to produce heat on the other side.side.

b) Shading may be a b) Shading may be a tree, an overhang, tree, an overhang, a directional louvera directional louver, a window , a window drape or drape or blindblind..

c) Glass itself can be a shading device c) Glass itself can be a shading device if if it is tintedit is tinted to reduce transmissibility of to reduce transmissibility of sunlight, or sunlight, or if coated with a reflective if coated with a reflective surfacesurface to deflect heat-producing rays. to deflect heat-producing rays.

d) Not many shading methods allow d) Not many shading methods allow free use of daylight, except for free use of daylight, except for strategically placed mechanical strategically placed mechanical shades that protects only in areas shades that protects only in areas needed.needed.

e) Well designed e) Well designed landscape is useful landscape is useful on east and west exposureson east and west exposures when sun when sun angles are low.angles are low.

f) Landscape may be ineffective on f) Landscape may be ineffective on south exposures since the south is south exposures since the south is subject to direct light most of the day, subject to direct light most of the day, and sun angles vary.and sun angles vary.

g) Landscape plants of deciduous g) Landscape plants of deciduous variety are particularly useful, since in variety are particularly useful, since in summer they shade, but in winter with summer they shade, but in winter with no leaves they allow heat producing no leaves they allow heat producing sunlight.sunlight.

h) Shading devices on h) Shading devices on high rise high rise buildings are impractical, which may buildings are impractical, which may be limited to the use of reflective be limited to the use of reflective coatings on transparent surfacescoatings on transparent surfaces..

SHADING COEFFICIENTSSHADING COEFFICIENTSa) Refer to supplementary material for a) Refer to supplementary material for charts that contain shading data.charts that contain shading data.

b) A shading coefficient is b) A shading coefficient is a number a number between zero and onebetween zero and one, that represents , that represents the value of a shading device in the value of a shading device in diminishing the effect of direct rays of diminishing the effect of direct rays of sunlight.sunlight.

c) It is c) It is a percentage of direct suna percentage of direct sun, hence , hence the smaller the number, the better the the smaller the number, the better the material as a shading device.material as a shading device.

d) Shading coefficients are used to d) Shading coefficients are used to modify the calculation of the amount of modify the calculation of the amount of heat that gets through glass into a space heat that gets through glass into a space from measured affects of direct sunlight from measured affects of direct sunlight at specific times of the year called at specific times of the year called “solar “solar gain.” gain.”

B HEAT GAIN THROUGH B HEAT GAIN THROUGH WINDOWSWINDOWS

1) HEAT FLOW THROUGH GLASS1) HEAT FLOW THROUGH GLASS

a) a) Heat caused by radiationHeat caused by radiation is is technically not ‘heat flow’, since the technically not ‘heat flow’, since the heat is not transformed until it is heat is not transformed until it is inside the space – by connecting with a inside the space – by connecting with a dense surface.dense surface.

b) Direct heat travels by conduction, b) Direct heat travels by conduction, from warm to coolfrom warm to cool. Conduction tends . Conduction tends to balance both sides of a barrier by to balance both sides of a barrier by heating the material and flowing heating the material and flowing through.through.

c) Conduction of c) Conduction of heat is transferred heat is transferred from molecule to moleculefrom molecule to molecule of a of a material, and if unabated will continue material, and if unabated will continue until the entire material is the same until the entire material is the same temperature as the ambient air. temperature as the ambient air.

d) The d) The difference in temperaturedifference in temperature between outside and inside is the between outside and inside is the force that causes heat flow through a force that causes heat flow through a building envelope. building envelope.

e) Regardless of shading devices, e) Regardless of shading devices, coated glass, coated glass, or landscaping, or landscaping, heat will heat will travel through glass travel through glass simply because of simply because of the difference in the difference in temperaturetemperature between between the inside and outside the inside and outside surfaces. surfaces.

f) Tinted Glass, also called “heat f) Tinted Glass, also called “heat absorbing glass” absorbing glass” was developed was developed because dark colors absorb heatbecause dark colors absorb heat. The . The premise was that glass in sunlight premise was that glass in sunlight would absorb heat and the would absorb heat and the surrounding air would dissipate the surrounding air would dissipate the heat before it penetrated inside. heat before it penetrated inside. The The process is effective slightlyprocess is effective slightly..

INSULATING GLASSINSULATING GLASS

aa) Consider ) Consider ttwo panes of glasswo panes of glass held held apart with edges sealed together, and apart with edges sealed together, and dry air in the space between panes. dry air in the space between panes.

b) Dry air prevents condensation from b) Dry air prevents condensation from forming inside the space between the forming inside the space between the panes because no moisture is present panes because no moisture is present there.there.

c) c) Resistance to heat flow is doubledResistance to heat flow is doubled with two panes of glass and the with two panes of glass and the air air space adds an insulating valuespace adds an insulating value..

d) Insulating glass with two panes d) Insulating glass with two panes and one air space is a little more than and one air space is a little more than twice as good at resisting heat flow by twice as good at resisting heat flow by conductionconduction as a single pane of glass. as a single pane of glass.

e) Insulating glass made with clear e) Insulating glass made with clear panes panes does nothing to prevent heat does nothing to prevent heat caused by radiationcaused by radiation from entering a from entering a space, because radiation passes space, because radiation passes through and is not heat until through and is not heat until transformed by striking a dense surface transformed by striking a dense surface on the other side.on the other side.

f) Formerly, insulating glass was very f) Formerly, insulating glass was very expensive, had to be ordered in exact expensive, had to be ordered in exact dimensions, and took a long time to get dimensions, and took a long time to get – because the panes had to be made by – because the panes had to be made by fusing the edges of glass togetherfusing the edges of glass together..

g) Today, insulating glass is made by g) Today, insulating glass is made by putting two or more panes together putting two or more panes together with a metal strip at the perimeter with a metal strip at the perimeter edges between the panes and fastening edges between the panes and fastening with epoxywith epoxy glue – exhausting the air in glue – exhausting the air in the space and replacing it with dry air.the space and replacing it with dry air.

REFLECTIVE GLASSREFLECTIVE GLASS

a) Since insulating glass does not a) Since insulating glass does not stop radiation of direct sunlight, stop radiation of direct sunlight, a a reflective coating is placed on one reflective coating is placed on one sideside of an insulating glass panel. of an insulating glass panel.

b) The process is b) The process is much the same as much the same as coating a glass to make a mirrorcoating a glass to make a mirror, , except it is not opaque.except it is not opaque.

c) Coatings on glass are subject to c) Coatings on glass are subject to deterioration due to exposure to deterioration due to exposure to weather and cleaning, so weather and cleaning, so the coating the coating must be placed inside the insulating must be placed inside the insulating air spaceair space..

d) Coatings range in appearance from d) Coatings range in appearance from light to dark in a variety of colors, and light to dark in a variety of colors, and in a range of effectiveness to REFLECT in a range of effectiveness to REFLECT radiation penetrationradiation penetration by 70% to 20%. by 70% to 20%.

1” INSULATING GLASS1” INSULATING GLASS

C SOLAR GAIN AND DIRECTIONC SOLAR GAIN AND DIRECTION

ORIENTATION OF BUILDINGSORIENTATION OF BUILDINGS

a) a) Orientation is related to sun Orientation is related to sun directiondirection – not only east – west, but – not only east – west, but sun angle height at worst conditions.sun angle height at worst conditions.

b) East or west sides are good at b) East or west sides are good at selective times for horizontal daylight selective times for horizontal daylight – – effective if shading is not effective if shading is not prohibitive.prohibitive.

c) c) South exposure is worst for South exposure is worst for buildingsbuildings above 23 degrees latitude above 23 degrees latitude since that direction is vulnerable to since that direction is vulnerable to radiant energy the whole day.radiant energy the whole day.

d) d) North exposure is best for buildingsNorth exposure is best for buildings above 21 degrees, since direct sun above 21 degrees, since direct sun exposure is minimum. exposure is minimum.

e) e) North exposure is best for North exposure is best for daylighting and best for minimal heat daylighting and best for minimal heat gain due to sun radiationgain due to sun radiation..

f) North exposure is f) North exposure is NOT the worst NOT the worst condition for winter heating conditionscondition for winter heating conditions, , because HEAT LOSS from a space to because HEAT LOSS from a space to the outside is mostly by conduction.the outside is mostly by conduction.

g) g) Ambient temperature differenceAmbient temperature difference is is the only consideration for temperature the only consideration for temperature for winter heating conditions – for winter heating conditions – radiation is not a factor. radiation is not a factor.

SOLAR GAINSOLAR GAIN

a) Solar gain is the amount of heat that is a) Solar gain is the amount of heat that is produced as the result of direct radiation produced as the result of direct radiation from the sun. It is measured in terms of the from the sun. It is measured in terms of the number of BTU per hour per square foot of number of BTU per hour per square foot of glassglass that is subject to direct radiation from that is subject to direct radiation from the sun – the sun – diminished only by a shading diminished only by a shading coefficientcoefficient

b) Solar gain varies in its intensity b) Solar gain varies in its intensity at at different times of the daydifferent times of the day. Obviously when . Obviously when the sun’s rays extend through a maximum of the sun’s rays extend through a maximum of atmosphere, the affects of radiation are atmosphere, the affects of radiation are filtered.filtered.

c) c) Solar gain also varies at different times of Solar gain also varies at different times of the yearthe year, because of the tilt of the earth , because of the tilt of the earth relative to the sun.relative to the sun.

Since heat gain in cooling Since heat gain in cooling season can beseason can be

diminished by preventing radiant diminished by preventing radiant energy fromenergy from

the sun, the orientation, color, and the sun, the orientation, color, and reflectance reflectance

of the building envelope must address of the building envelope must address the the

issue.issue.

Because of the difference in Because of the difference in ambient ambient temperature from inside to temperature from inside to outside, the outside, the composition of the building composition of the building envelope must envelope must maintain an integrity to maintain an integrity to reasonably reduce the reasonably reduce the gain or loss of gain or loss of heat due to conduction.heat due to conduction.

HEAT FLOW VARIATION BY DAY AND BY SEASONHEAT FLOW VARIATION BY DAY AND BY SEASON

AA Heat flow in winter is caused by the Heat flow in winter is caused by the difference in temperature on both sides of the difference in temperature on both sides of the barrier. With the exception of ventilation and barrier. With the exception of ventilation and infiltration, heat flow is by conduction.infiltration, heat flow is by conduction.

BB Accumulation of heat within a space in Accumulation of heat within a space in summer is caused by a combination of things; summer is caused by a combination of things;

Conduction through opaque surfacesConduction through opaque surfacesConduction through transparent surfacesConduction through transparent surfacesRadiation through transparent and opaque Radiation through transparent and opaque surfaces *surfaces *Heat from infiltrationHeat from infiltrationHeat from outside make-up air due to Heat from outside make-up air due to ventilationventilation

CC Since the sun seems to trek across the sky Since the sun seems to trek across the sky during the day from east to west, the intrusion of during the day from east to west, the intrusion of radiant energy varies at opposite times on the east radiant energy varies at opposite times on the east and west. and west.

DD With sites above 23 degrees north latitude, With sites above 23 degrees north latitude, radiant energy is not a major factor in solar gain radiant energy is not a major factor in solar gain on the north side.on the north side.

EE For any site above 23 degrees north latitude, For any site above 23 degrees north latitude, the south exposure is subject to direct sun the south exposure is subject to direct sun radiation during most of the day. radiation during most of the day.

FF The earth rotates (day / night), and also tilts The earth rotates (day / night), and also tilts on its axis relative to the sun by an amount of 46 on its axis relative to the sun by an amount of 46 degrees and 42 minutes. (summer / winter)degrees and 42 minutes. (summer / winter)

FF Because of the tilt, any point on the earth’s Because of the tilt, any point on the earth’s surface is at a varying distance from the sun, and surface is at a varying distance from the sun, and at a varying angle from the horizon.at a varying angle from the horizon.

GG Consider in Lubbock, Texas at 34 degrees N, Consider in Lubbock, Texas at 34 degrees N, when the sun is at its highest peak in the summer, when the sun is at its highest peak in the summer, which means the sun’s position would be at 23 which means the sun’s position would be at 23 degrees and 21 minutes north of the equator. The degrees and 21 minutes north of the equator. The sun would be at its closest point to Lubbock. sun would be at its closest point to Lubbock. Summer solstice, the most severe radiant energy.Summer solstice, the most severe radiant energy.

HH But then the sun (so it seems) moves toward But then the sun (so it seems) moves toward the south, passing the equator at the autumnal the south, passing the equator at the autumnal equinox, then on south of the equator an angle of equinox, then on south of the equator an angle of 23 degrees 21 minutes south.23 degrees 21 minutes south.

II At that point the sun is at its farthest At that point the sun is at its farthest distance from Lubbock, (winter time) and at its distance from Lubbock, (winter time) and at its lowest angle with the horizon from Lubbock. lowest angle with the horizon from Lubbock. Winter solstice, the least amount of radiant Winter solstice, the least amount of radiant energy.energy.

The calculation for the maximum amount of heat The calculation for the maximum amount of heat gain for a space happens during the period of the gain for a space happens during the period of the summer solstice.summer solstice.

The calculation of heat gain is affected by two The calculation of heat gain is affected by two major considerations, both caused by the varying major considerations, both caused by the varying position of the sun.position of the sun.

Equivalent Temperature Difference (ETD), which Equivalent Temperature Difference (ETD), which takes into account the density of mass of a takes into account the density of mass of a building envelope (to retain or expel heat), the building envelope (to retain or expel heat), the outside color of the envelope (to reflect or to outside color of the envelope (to reflect or to absorb heat), and the time lag during the day absorb heat), and the time lag during the day when part of the heat absorbed by the envelope when part of the heat absorbed by the envelope escapes into the space.escapes into the space.Solar Gain, which is the amount of heat that gets Solar Gain, which is the amount of heat that gets into a space by the inclusion of electromagnetic into a space by the inclusion of electromagnetic energy through translucent and transparent energy through translucent and transparent surfaces.surfaces.

HEAT FLOW BY CONDUCTION AND RESISTANCE HEAT FLOW BY CONDUCTION AND RESISTANCE TO HEAT FLOWTO HEAT FLOW

AA Obviously, heat flow must have a scale of Obviously, heat flow must have a scale of units. Heat is measured in terms of BTUs per units. Heat is measured in terms of BTUs per hour, and measured in quantity through a material hour, and measured in quantity through a material or assembly of materials in BTUs per hour, per or assembly of materials in BTUs per hour, per square foot of surface area, per degree Fahrenheit square foot of surface area, per degree Fahrenheit difference in temperature between the inside and difference in temperature between the inside and outside of the space.outside of the space.

BB BTU is an acronym for British Thermal Unit, BTU is an acronym for British Thermal Unit, which is the amount of heat required to raise one which is the amount of heat required to raise one pound of water one degree Fahrenheit. pound of water one degree Fahrenheit.

CC All building materials are laboratory tested to All building materials are laboratory tested to determine their rate of heat flow by conduction, determine their rate of heat flow by conduction, which is their measure to conduct heat.which is their measure to conduct heat.

DD A material has a A material has a Conductance factorConductance factor, , designated designated CC, which is the amount of heat that will , which is the amount of heat that will flow through flow through a given standard thickness, a given standard thickness, such as such as a 1” thick board, or a 3 5/8” thick bricka 1” thick board, or a 3 5/8” thick brick..

EE A material has A material has a a resistance to heat flowresistance to heat flow, , designated as designated as RR, which is its measure of its value , which is its measure of its value as an insulator.as an insulator.

FF A material has by the Inch-pound system of A material has by the Inch-pound system of measure a rate of measure a rate of conductivityconductivity, designated as , designated as kk, , which is the amount of heat flow through the which is the amount of heat flow through the material material per inch of thicknessper inch of thickness..

GG Since conductance and conductivity is heat Since conductance and conductivity is heat flow through a material by conduction, and flow through a material by conduction, and resistance opposes heat flow, it follows that resistance opposes heat flow, it follows that conduction and resistance are reciprocals, so conduction and resistance are reciprocals, so there is a mathematical relationship.there is a mathematical relationship.

C = 1 / R, and R = 1 / CC = 1 / R, and R = 1 / C

C = k / inches thickness, and k = inches C = k / inches thickness, and k = inches thickness x Cthickness x C

R = inches thickness / k, and k = inches R = inches thickness / k, and k = inches thickness / R thickness / R

WhereWhere C is conductanceC is conductance k is conductivityk is conductivity

R is resistanceR is resistance

CONVECTIONCONVECTIONConvection is simply the Convection is simply the

movement of warm air within a space movement of warm air within a space

For the simple reason that warm air is less For the simple reason that warm air is less dense than cold air (molecules are further apart), dense than cold air (molecules are further apart), warm air rises in an uneven temperature and cold warm air rises in an uneven temperature and cold air falls. That is the reason why hot-air balloons air falls. That is the reason why hot-air balloons rise in the atmosphere. The air in the balloon is rise in the atmosphere. The air in the balloon is warmer than the air outside – hence the mass of warmer than the air outside – hence the mass of the balloon is less dense because the air is the balloon is less dense because the air is warmer.warmer.

Considering only density, that is why a stick Considering only density, that is why a stick of wood held under water will rise to the surface of wood held under water will rise to the surface when released; the wood is less dense than water when released; the wood is less dense than water so the denser medium forces it upward.so the denser medium forces it upward.

Convection is not a factor of heat flow.Convection is not a factor of heat flow.

Air FilmAir Film

At the exposed surfaces of solids, heat At the exposed surfaces of solids, heat transfer takes place by conduction and by transfer takes place by conduction and by radiation.radiation.

When air motion along surfaces is minimal, When air motion along surfaces is minimal, an insulating layer of air is createdan insulating layer of air is created – and – and heat heat transfer through the layer is by conductiontransfer through the layer is by conduction

When air motion is increased, such as When air motion is increased, such as outside with wind, the insulation value decreases.outside with wind, the insulation value decreases.

EMITTANCEEMITTANCE – the energy radiated by the surface – the energy radiated by the surface of a body per second, per unit area. Materials with of a body per second, per unit area. Materials with low emittance radiate less heat. Reflective low emittance radiate less heat. Reflective materials have low emittance.materials have low emittance.

Emittance is a factor in choosing the conductance Emittance is a factor in choosing the conductance and resistance value of air film, inside (still air) and resistance value of air film, inside (still air) and outside (moving air).and outside (moving air).

Table 4.3, of the text shows resistance values of Table 4.3, of the text shows resistance values of still air and moving air films, adjacent to a building still air and moving air films, adjacent to a building envelope. Column, Inch-Pound units, non envelope. Column, Inch-Pound units, non reflectivereflective

For still air (inside), vertical surface, For still air (inside), vertical surface, R = 0.68R = 0.68 for emittance of .90 for emittance of .90 For moving air (outside), 15 mph wind, For moving air (outside), 15 mph wind, R = R = 0.170.17

Which means air film has Which means air film has insulating value - insulating value - but not muchbut not much. .

INSULATING MATERIALSINSULATING MATERIALS

Most all building materials have some Most all building materials have some insulating value, but remain of secondary insulating value, but remain of secondary importance. Wood siding and brick is selected for importance. Wood siding and brick is selected for appearance, finish, durability – not for insulating appearance, finish, durability – not for insulating value. value.

In comparison: In comparison: material “R” valuematerial “R” value

3 5/8” thickness brick3 5/8” thickness brick .56.56 ¾” wood board¾” wood board .75.75 ¾” polyfoam board 4.68¾” polyfoam board 4.68 3 ½” fiberglass insulation 11.003 ½” fiberglass insulation 11.00

Examples of Conversion of Resistance and Flow:Examples of Conversion of Resistance and Flow:

1 If a material has a C value of 0.12, what is its R 1 If a material has a C value of 0.12, what is its R value?value?

Since R = 1/C, then Since R = 1/C, then R = 1 / .12 = 8.33R = 1 / .12 = 8.33

2 If a material has a k value (heat flow per inch of 2 If a material has a k value (heat flow per inch of thickness) 0.15 , what is the R value for a material that thickness) 0.15 , what is the R value for a material that is 4 ½” thick? Since R = inches thickness / k, thenis 4 ½” thick? Since R = inches thickness / k, then

R = 4.5 / .15 = 30R = 4.5 / .15 = 30

3 If a material 2 ½” thick has an R value of 16, what is 3 If a material 2 ½” thick has an R value of 16, what is its k value ? Since R = inches thickness / k, thenits k value ? Since R = inches thickness / k, then

k = inches thickness / R, and k = inches thickness / R, and k = 2.5 / 16 = .156k = 2.5 / 16 = .156..

4 If a material 6” thick has a k value of 0,156, what is 4 If a material 6” thick has a k value of 0,156, what is the C value?the C value?

Since C = k / inches thickness, then Since C = k / inches thickness, then C = .156 / 6 = C = .156 / 6 = 0.0260.026

Remember that insulation does not stop heat Remember that insulation does not stop heat flow . . . It only slows it over time.flow . . . It only slows it over time.

A building in a cold climate left vacant for A building in a cold climate left vacant for long periods of time that has water pipes inside long periods of time that has water pipes inside must be left with a source of heat. If not, the water must be left with a source of heat. If not, the water will eventually freeze and the pipes will burst.will eventually freeze and the pipes will burst.

The only thing that stops heat flow (by The only thing that stops heat flow (by conduction) is no medium, which is to say, a conduction) is no medium, which is to say, a vacuum.vacuum.

ENDEND