What is the U-value of a window or door?

In the context of energy-efficient buildings, "U-value" usually refers to the thermal conductivity of a material or component, also known as the U-factor or U-value, which is a measure of the ability of a material to transfer heat per unit of temperature difference per unit of time through a unit of surface area, and is usually expressed in units of W/m²-K (watts per square meter per kelvin).

The lower the U-value, the better the insulation of the material and the more effective it is at stopping the transfer of heat, thus improving the energy efficiency of the building. In energy-efficient building design, choosing materials with low U-values is critical to reducing energy consumption for heating and cooling.

Although U-value (or U-factor) is universal, U-values can be expressed in different units. In most countries, the U-value is expressed in SI units.

In most countries, the U-value is expressed as SI units,

W/(m2⋅K):

In the United States, the U-value is expressed as British thermal units (Btu) per hour-square feet-degrees Fahrenheit.

Btu/(h⋅ft2⋅°F)

Throughout this document, U-values are expressed as SI unless otherwise stated. To convert SI to U.S. customary values, divide by 5.678.

Thermal conductivity is lower in well-insulated portions of a building and higher in poorly insulated portions. u-values take into account losses due to heat radiation, heat convection, and heat conduction.

Although the units of thermal conductivity are the same as heat transfer coefficients, thermal conductivity differs in that heat transfer coefficients are only used to describe heat transfer in fluids, whereas thermal conductivity is used to simplify equations with several different forms of thermal resistance.

It is described by the following equation:

φ = A × U × (T1 - T2 ) where Φ is the heat transfer in watts, U is the thermal transmittance, T 1 is the temperature on one side of the structure, T 2 is the temperature on the other side of the structure, and A is the area in square meters.

The thermal transmittance of most walls and roofs can be calculated using ISO 6946 unless there is metal bridging insulation, in which case ISO 10211 can be used. For most ground floors, ISO 13370 can be used. For most windows, the thermal transmittance can be calculated using ISO 10077 or ISO 15099.

ISO 9869 describes how to experimentally measure the thermal transmittance of a structure. The choice of materials and the quality of the installation have a crucial influence on the effectiveness of window insulation. The framing and double sealing of the window system are the actual weak points of window insulation.