2 Thermal Insulation by yurtgc548


									    Chapter 3    Thermal Insulation
    Why do we talk about thermal insulation?

   Thermal insulation is the major factor in
    reducing the loss of heat from buildings.
Benefits of good thermal
insulation ?

(1) to maintain a constant temperature within a building, we
need to reduce the heat exchange with the surroundings
(2) to save energy needed to run the cooling plant
(3) to save energy needed to run the heating plant
(4) One of the benefits of good thermal insulation
  is that the risk of surface condensation is
  reduced because of the warmer internal
(5) Good insulation can also reduce the time taken
  for a room to heat up to a comfortable
 The relatively small cost of extra insulating
  materials is quickly paid for by the reduction in
  the size of the heating plant required and by the
  annual savings in the amount of fuel needed.
     A  well-insulated structure and a
     poorly-insulated structure, which one
     will stay cooler in summer?

   It is useful to remember that good thermal insulation
    will also reduce the flow of heat into a building, when
    the temperature outside is greater than the
    temperature inside.
   《黄帝宅经》
   治宅极宜壮实
four topics are discussed in this chapter

3.1 Insulating materials 绝热材料
3.2 U-values      U-值
3.3 Thermal Bridging 热桥
3.4 Structural temperature 结构温度
3.1 Insulating materials 绝热材料
 What is a thermal insulator (隔热体)?
    A thermal insulator is a material which
     opposes(反对) the transfer of heat
     between areas at different temperatures.
    what should a good insulator have?

   The best insulating materials tend to be porous(多
    孔的) and of low density.
   Gas, are the best insulators against conduction.
   Air, which is a mixture of gases, is the basis of
    insulators such as aerated lightweight concrete(轻
    型加气混凝土), expanded plastics(发泡塑料),
    and cavities(空腔).
   For air to act as an insulator it must be stationary
    (静止的), otherwise moving air if allowed to move
    will transfer heat by convection.
  3.1.1 Types of insulator
(1) Rigid preformed materials: aerated concrete blocks
  预制刚性材料 : 加气混凝土砖
(2) Flexible materials: fibreglass quilts
  柔性材料                  玻璃纤维布
(3) Loose fill materials: expanded polystyrene granules
 疏松填充材料                 膨胀聚苯乙烯颗粒
(4) Materials formed on site : foamed polyurethane
 现场加工材料                       泡沫聚亚安脂
(5) Reflective materials : aluminium foil
  反射材料                  铝箔
3.1.2 Properties of insulators

 When choosing materials for thermal insulation of buildings
  the physical properties of the material need to be
 Thermal insulation suitable for the purpose.

 Strength (强度)or rigidity(硬度) suitable for the
 Moisture resistance. (防潮性能)

 Fire resistance. (防火性能)

 Resistance to pests and fungi.(防虫和防霉)

 Compatibility(兼容性) with adjacent materials
 Harmless to humans and the environment.
3.1.3 Thermal conductivity , lamda-value
      导热系数                     lamda-值
 thermal conductivity
 Unit: W/m K ( W/m℃)
Referring to figure3.1 the general formula is: P39

           H kA(1   2 )
           t     d
                  Resistivity 热阻

   Resistivity (r)is an alternative index of conduction
    is the reciprocal of(成反比) thermal conductivity,
    so that r = 1/lamda
   The thermal conductivities of some building
    materials are given in table 3.1.
   The thermal conductivity of practical building
    materials Varies with moisture content as the
    presence of water increases conduction.
   Variations in density have significant effects on the
    k-values of brickwork, concrete and stone and more
    comprehensive tables of thermal conductivities
    should be used to obtain specific values.
    3.1.4     Emissivity and absorptivity
              发射率           吸收率

    the ability of a material to absorb or give off radiant
    heat is a property of the surface of the material.
   Rough black surfaces absorb most heat and emit most
   Conversely, shiny slivered surfaces absorb least heat
    and emit least heat.
black body

Emissivity is the fraction of energy radiated by a
body compared to that radiated by a black body at
the same temperature
Absorptivity is the fraction of (               ) by
a body compared to that (         ) by a black body
3.1.5 Clear sky radiation 晴空辐射
  At night time a building emits radiant heat to its
   surroundings and the rate of this heat loss from the roof
   will be increased if the night sky is clear and cloudless.
 This radiant “suck”(允吸)
occurs because a clear dark sky is
closer in form to a black body

The same radiant mechanism
causes dew(露)or ground frost (霜)
to occur during a clear
starry (布满星星的) night.
    Now the next topic
    3.2    U-values

    3.2.1 Thermal transmittance ,U-value

   Unit : W/m2K (W/m2℃)
   The rate of heat flow in watts through 1m2 of a structure when
    there is a temperature difference across the structure of 1 ℃
   Lower U-values provide better thermal insulation.
    3.2.2 Elemental U-value

   Standard U-values are calculated by making certain
    assumptions about moisture contents of materials and about
    rates of heat transfer at surfaces and in cavities.
   not always agree exactly with U-values measured on site.
   the values for (some common types of construction)are
    given in table 3.3
Figure 3.1 Maximum U-values for new dwellings

3.2.3 Thermal resistance , R-value
      热阻 R-值
   Unit : m2K/w
   Higher thermal resistance gives better thermal
   There are there general types of thermal resistance
    which need to be determined , either by calculation or
    by seeking published standard values.
(1)Material resistances 材料热阻
(2)Surface resistances 表面热阻
(3)Airspace resistances 空气间层热阻
    (1)Material resistances材料热阻

   Assuming that a material is homogeneous均匀的, this
    type of resistance can be calculated by the
    following formula.

                         R d /k
    (2)Surface resistances表面热阻
   The thermal resistance of an open surface depends upon
    the conduction, convection and radiation at that surface.
   Some useful values are given in table 3.5**
Factors which affect surface resistance are given below
   Direction of heat flow: upward or downward
   Climatic effects: sheltered or exposed
   Surface properties: normal building materials with high
    emissivity or polished metal with low emissivity.
(3)Airspace resistances 空气间层热阻

 The thermal resistance of an airspace
 or empty cavity depends on the nature
 of any conduction, convection and
radiation within the cavity.
 useful airspace resistances are given in table 3.5 P47

 Factors affecting airspace resistance are given below

 Thickness of the airspace

 Flow of air in airspace ; ventilated or unventilated

 Lining(衬里, 内层)of airspace; normal surfaces or

  reflective surfaces of low emissivity.
    Total thermal resistances
   Thus the total thermal resistance is the sum of the thermal
    resistance of all components in a structural element.
3.2.4   Calculation of U-values

The thermal transmittance , or U-values, is the
reciprocal of the total thermal resistance and can
be calculated using the following formula

   1                        1
U       U
   RT       Rsi  R 1  R 2  ...  R a  R so
    Worked example 3.1
   Calculate the U-value of a cavity wall(空心墙体)
    with a 105mm(millimetre ) thick brick out leaf(外砖
    墙), a 75mm unventilated cavity containing 50mm of
    fibreglass quilt, then a 100mm lightweight concrete
    block inner leaf (内砖墙)with a 15mm layer of
    lightweight plaster(轻质石膏板). The thermal
    conductivities in W/m K are: brickwork 0.84,
    lightweight concrete block 0.19, lightweight plaster
    0.16, fibreglass 0.04. standard thermal resistances in
    m2K / W are: internal surface 0.123, external surface
    0.055, cavity 0.18
Step 1: sketch a figure indicating all parts of
the construction with surface layers
Step 2: tabulate all information and ,where
necessary ,calculate thermal resistance using
 R d /k

Step 3: using      U
                      Rsi  R 1  R 2  ...  R a  R so

 注意作图,分母是热阻, 不要带入导热率
 Work to a final accuracy of 2 decimal places
 Lay the calculation out in a table
     3.2.5     U-values of floors

    The calculation of the actual U-values for ground floors is
    complex and it is useful to use table or graphs to find the
    minimum thickness of insulation needed to meet a particular
    average standard required by regulations.
   For some large buildings the standard may be achieved without
    the use of insulation because the ratio of exposed floors edges
    to total area is proportionally small.
    3.2.6    Adjustments to U-values
1)calculate the effect that additional insulating material
 has upon a U-value,
2) calculate the thickness of material that is required to
 produce a specified U-value.
   Use the following guidelines to make adjustments
   U-values can not be added together or subtracted
    from one another
   Thermal resistances can be added and subtracted.
    Worked example 3.2 illustrates the

   A certain uninsulated cavity wall has a U-value of
    0.91W/m2K. If expanded polyurethane(聚亚安酯)board
    is included in the construction what minimum
    thickness of this materials is required to reduce the U-
    value to 0.45 W/m2K? Given that the thermal
    conductivity of the expanded polyurethane =
Target U-value                   U2=0.45
Target total resistance(1/U)     R2=1/0.45=2.222
Existing U-value                  U1=0.91
Existing total resistance(1/U)     R1=1/0.91=1.099
Extra resistance required (所需的额外热阻)
the k-value of proposed insulating material k=0.025
So using formula R=d/k
Thickness of material
So minimum thickness of insulating board needed to
  give 0.45U-value=28mm
实例:墙角结露卧室像澡堂 设计方施工方各有说法

日期:2005-6-10 8:48:27 来源:
3.3 Thermal bridges, cold bridges
   A thermal bridge is a portion of a structure whose
    high thermal conductivity lowers the overall thermal
    insulation of the structure.
   There is increased heat flow across the thermal bridge
    and the surfaces on the interior side of the bridge
    therefore become cooler, giving rise to the informal
    term of “cold bridge”.
   There is an increased risk of condensation and mould
    growth (发霉)on these internal surfaces around
    cold bridges.
   热桥以往又称冷桥,现统一定名为热桥。热桥是指
   由于热桥部位内表面温度较低,寒冬期间,该处温
   通风\加热\保温\赔偿
    Pattern staining 暗斑

    Pattern staining on a ceiling is the formation of a
    pattern , in dirt or dust, which outlines the hidden
    structure of the ceiling.
    It is a particular result of thermal bridging and
    also depends upon the frequency of

   Pattern 模范, 式样,图案
   Staining 着色
Average U-values

If a wall, or other element , is composed of
different constructions with different U-values then
the overall insulation of the wall depends upon the
relative areas of the different constructions.
The general formula is as follows

             A1U1  A 2 U 2  .....
U(average) 
               A1  A 2  ...
Worked example

   A wall has a total area of 8 m2 of which 2m2 are
    windows. The U-values are 0.35 W/m2K for the
    cladding(覆层)and 2.8W/m2K for the glazing. Calculate
    the average U-value for the wall.
                 A1U1  A 2 U 2  .....
    U(average) 
                   A1  A 2  ...
       6  0.35  2  2.8
    U                     0.96
3.4 Structural temperature 结构温度

The thermal insulation installed in a building affects
the rate at which the building loses heat energy
which is measured by the U-value.
The thermal performance of the building also
depends upon the thermal capacity of the insulating
material, which affects the times taken to heat or
cool the structure,
 and the position of the insulation, which affects the
temperatures in the structural elements
     3.4.1 Response times            响应时间

    .
                                          Figure 3.6
                                           thermal response

In general, lightweight structures respond more quickly to
surrounding temperature changes than do heavyweight structure .
This is because heavyweight materials have a higher thermal
capacity and require more heat energy to produce given
temperature changes
   A structure made up of different materials,

   will have varying temperature gradients between the
    inside and outside
                                                     R
                                                   T R T

   The boundary temperatures between layers in a structural
    element can be determined from the thermal resistances which
    make up the U-value of that element
   The ratio of the temperature changes inside a structure is
    proportional to the ratio of the thermal resistance.
Worked example 3.5

  A room has an external wall with a U-value of 1.5 W/m2K
   and contains air at 20 ℃ when the outside temperature is
   5 ℃. The internal surface resistance is 0.123 ㎡ K/W.
   calculate the boundary temperature on the internal surface
   of the wall.
               inside temperature= 20 ℃
                outside temperature= 5 ℃
       resistance of that layer R= 0.123㎡K/W
       total resistance of the structure RT=1/U=1/1.5=0.667
Using                 R
 
   ⊿=2.77R        T R T
 So temperature on the inside surface=20-2.77=17.23 ℃
    T    RT
Chapter 3           Thermal Insulation
3.1 Insulating materials 绝热材料
3.1.1    Types of insulator
3.1.2     Properties of insulators
3.1.3     Thermal conductivity , k-value
3.1.4     Emissivity and absorptivity
3.1.5    Clear sky radiation 晴空辐射
3.2     U-values
3.2.1   Thermal transmittance ,U-value
3.2.2   Elemental U-values
3.2.3   Thermal resistances
3.2.4   Calculation of U-values
3.2.5   U-values for floors
3.2.6   Adjustments to U-values
3.3     Thermal Bridging
3.3.1   Thermal bridge
3.3.2   cold bridges
3.3.3   pattern staining
3.3.4   average U-values
3.4 Structural Temperature
3.4.1 response time
3.4.2 temperature gradients
   That’s all for chapter 3
   Since there’s time, please read the text
    carefully and try to find the main points in
    chapter 3.
   If you have questions, you can ask me.
    some useful sentence patterns
Factors which affect xxx are given below
   影响表面热阻的因素如下
   影响空气层热阻的因素如下
Some useful xxx are given in table 2.5
   表3.5给出了一些有用的空气热阻值
   表3.5给出了一些常用结构的U值
   xxx是yyy的倒数
xxx is the reciprocal of yyy
   传热系数是总热阻的倒数
   xxx可用下式计算
   xxx can be calculated using the following
   传热系数可用下式计算
   最终结果精确到小数点后xx位
   Work to a final accuracy of xx decimal places
   最终结果精确到小数点后2位
   最终结果精确到小数点后3位
   列表计算
   Lay the calculation out in a table
 Exercises in class
1 Benefits of good thermal insulation ?(     )
A to maintain a constant temperature within a building
B to reduce the heat exchange with the surroundings
C to save energy needed to run the cooling plant
D to save energy needed to run the heating plant
E to reduce the risk of surface condensation
F to reduce the time taken for a room to heat up to a
  comfortable temperature
G energy conversation
2 Rough black surfaces(   )
A absorb most heat
B emit most heat.
C absorb least heat
D emit least heat
3 Shiny silvered surfaces (   )
A absorb most heat
B emit most heat.
C absorb least heat
D emit least heat
4(       ) is a measure of the overall rate of heat
 transfer ,by all mechanisms under standard
 conditions, through a particular section of
A a U-value
B a r-value
C a R-value
D a lamda-value
5(       ) gives better thermal insulation
A    higher thermal resistance
B    higher U-value
C    lower thermal resistance
D    lower U-value
6(     ) be added together or subtracted
 from one another
A U-values
B R-values
C r-values
D lamda-values

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