Although the concept of U-value (or U-factor) is universal, U-values can be expressed in different units. In most countries, U-value is expressed in SI units, as watts per square metre-kelvin:
:W/(m⋅K)
In the United States, U-value is expressed as British thermal units (Btu) per hour-square feet-degrees Fahrenheit:
:Btu/(h⋅ft⋅°F)
Within this article, U-values are expressed in SI unless otherwise noted. To convert from SI to US customary values, divide by 5.678.
Well-insulated parts of a  have a low thermal transmittance whereas poorly insulated parts of a building have a high thermal transmittance. Losses due to thermal radiation, thermal convection and thermal conduction are taken into account in the U-value. Although it has the same units as heat transfer coefficient, thermal transmittance is different in that the heat transfer coefficient is used to solely describe heat transfer in fluids while thermal transmittance is used to simplify an equation that has several different forms of thermal resistances.
It is described by the equation:
:Φ = A × U × (T - T)
where Φ is the heat transfer in watts, U is the thermal transmittance, T is the temperature on one side of the structure, T is the temperature on the other side of the structure and A is the area in square metres.
Thermal transmittances  most walls and roofs can be calculated using ISO 6946, unless there is metal bridging the insulation in which case it can be calculated using ISO 10211. For most ground floors it can be calculated using ISO 13370. For most windows the thermal transmittance can be calculated using ISO 10077 or ISO 15099. ISO 9869 describes how to measure the thermal transmittance of a structure experimentally. 
Choice of materials and quality of installation has a critical impact on the window insulation results. The frame and double sealing of the window system are the actual weak points in the window insulation. 
Typical thermal transmittance values for common building structures are as follows: 
*Single glazing: 5.7 W/(m⋅K)
*Single glazed windows, allowing for frames: 4.5 W/(m⋅K)
*Double glazed windows, allowing for frames: 3.3 W/(m⋅K)
*Double glazed windows with advanced coatings: 2.2 W/(m⋅K)
*Double glazed windows with advanced coatings and frames: 1.2 W/(m⋅K)
*Triple glazed windows, allowing for frames: 1.8 W/(m⋅K)
*Triple glazed windows, with advanced coatings and frames: 0.8 W/(m⋅K)
*Well-insulated roofs: 0.10 W/(m⋅K)
*Poorly insulated roofs: 1.0 W/(m⋅K)
*Well-insulated walls: 0.15 W/(m⋅K)
*Poorly insulated walls: 2 W/(m⋅K)
*Well-insulated floors: 0.2 W/(m⋅K)
*Poorly insulated floors: 1.0 W/(m⋅K)
In practice the thermal transmittance is strongly affected by the quality of workmanship and if insulation is fitted poorly, the thermal transmittance can be considerably higher than if insulation is fitted well