建筑围护结构的热透射系数的计算方式
How to Calculate the Thermal Transmittance (U-Value) in the Envelope of a Building

由专筑网李韧，杨帆编译

在设计项目的围护部件时，建筑师们需要特别考虑其内部组成部分，因为每一层都有着特定的功能，这对建筑的热传递将起到决定性作用。

在不同的表面中，如果我们通过温差来区分1平方米的立面区域，就会得到其中的热传递数值，也就是U值。U值说明的是建筑的隔热层与其通过的能量的百分比，如果U值较低，那么建筑的隔热性能就较好，相反，如果U值较高，那么其隔热性能就相对欠缺。

U值通过公式W/m2•K来计算，同时它与隔热层每个组成元素的热阻密切相关，热阻是建筑构件与热量传递的比率，另外U值也与隔热层的厚度和导热性能相关，这也就涉及到各个部件的所用材料。让我们来回顾一下计算立面热传递性能的公式。

建筑热维护结构

建筑热维护结构也就是我们常常说到的表皮系统，它能够有效地保护建筑室内的热舒适性和声学系统，并且通常由不透明的墙体（墙体、地面、天花板），还有诸如门窗等活动元素，以及热桥而组成，这些要素的存在都让热量更加容易地进行传递。

When designing the envelope of projects, we must pay special attention to each of the elements that comprise it, since each of these layers has specific qualities that will be decisive in the thermal behavior of our building as a whole.
If we divide 1 m2 of our envelope by the temperature difference between its faces, we will obtain a value that corresponds to the thermal transmittance, also called U-Value. This value tells us a building's level of thermal insulation in relation to the percentage of energy that passes through it; if the resulting number is low we will have a well-isolated surface and, on the contrary, a high number alerts us of a thermally deficient surface.
Expressed in W/m2•K, the U-Value depends on the thermal resistance of each of the elements that make up the surface (the percentage in which a building element is opposed to the passage of heat), and this, in particular, obeys the thickness of each layer and its thermal conductivity (ability to conduct the heat of each material). Let's review the formulas necessary to calculate the thermal transmittance of our envelope.

Thermal Envelope
The thermal envelope is defined as the "skin" of the building, which protects the thermal and acoustic comfort of its interiors. This is made up of its opaque walls (walls, floors, ceilings), its operable elements (doors and windows), and its thermal bridges, which are all those points that allow heat to pass more easily (points with geometric variations or changes of materials).

而就立面而言，其延展状态也各不相同，例如金属与木材，就需要在不同的材质区域进行不同的计算，这样才能获得更高的准确度。然后通过每个要素的大致百分比计算出总数值，有些系数则在项目的当地规范中可以查询。

热传递计算

U值的计算公式为：

U = 1/Rt

在其中：
U=热透射系数(W/m2•K)*
Rt =每个组成部分的总热阻(m2•K/W)，其数值来源于公式：

Rt = Rsi + R1 + R2 + R3 + ... + Rn + Rse

在其中：
Rsi =室内表面的热阻（按气候分区的规范划分）
Rse =外部表面的热阻（按气候分区的规范划分）
R1, R2, R3, Rn =每一层的热阻，其数值来源于公式：

R = D / λ

在其中：
D =材料厚度（m）
λ =每种材料的导热系数(W/K•m)

热透射系数与热阻成反比关系，也就是说，立面材料的热阻越大，热透射率便越小

U = 1/R
R = 1/U

In the case of envelopes that are not completely homogeneous in their extension, for example in metal or wooden structures, it's possible to perform differentiated calculations for different areas and obtain a greater accuracy in the results. The total is then calculated on the approximate percentages to each of them, which can be found in the local standards and regulations corresponding to the location of the project.

Thermal Transmittance Calculation

The general formula for calculating the U-Value is:

U = 1/Rt

Where:
U = Thermal Transmittance (W/m2•K)*
Rt = Total Thermal Resistance of the element composed of layers (m2•K/W), obtained according to:
Rt = Rsi + R1 + R2 + R3 + ... + Rn + Rse

Where:
Rsi = Interior Surface Thermal Resistance (according to the norm by climatic zone)
Rse = Exterior Surface Thermal Resistance (according to the norm by climatic zone)
R1, R2, R3, Rn = Thermal Resistance of each layer, which is obtained according to:
R = D / λ

Where:
D = Material Thickness (m)
λ = Thermal Conductivity of the Material (W/K•m) (according to each material)
The Thermal Transmittance is inversely proportional to the Thermal Resistance: the greater the resistance of the materials that make up an envelope, the lower the amount of heat that is lost through it.

U = 1/R
R = 1/U

气候分区

当计算出U值时，我们需要将项目所在地冬季与夏季的最大或极限热透射系数进行比较，这个数字由当地规范决定，因此在计算过程中必须结合考虑。

* W =功率（瓦）-K=温差（Kelvin）

Climatic Zones
When obtaining our U-Value, we must compare it with the value of the maximum (or limit) thermal transmittance specified for the climatic zone in which our project is located, in winter and summer. This number has been determined by official local regulations, which you must review carefully to ensure proper functioning.

* W = Power (Watts) - K = Temperature difference (Kelvin)