What is U Value

The U-value, also known as the thermal transmittance, is a measure of how effective a building material (such as windows, doors, or walls) is as an insulator. It indicates the rate at which heat is transferred through a material. The lower the U-value, the better the material is at insulating, meaning it is more effective at preventing heat from passing through.

1, Definition

U-Value: It measures the rate of heat transfer (thermal transmittance) through a building component over a given area, under standard conditions, per degree of temperature difference between the inside and outside. It is typically measured in watts per square meter per degree Kelvin (W/m²K).

2, Importance

Energy Efficiency: Lower U-values mean better insulation, leading to reduced heating and cooling costs and improved energy efficiency of buildings.

Comfort: Properly insulated buildings maintain a more consistent indoor temperature, contributing to occupant comfort.

Regulations: Many building codes and standards specify maximum allowable U-values to ensure energy efficiency and environmental sustainability.

1, Calculating U-Value

The U-value is calculated based on the materials' thermal conductivities, thicknesses, and the arrangement of layers in the building component. Here's a simplified way to understand it:

Identify the Materials: List all the materials that make up the building component (e.g., glass, air gaps, frames).

2, Thermal Conductivity

Obtain the thermal conductivity (k-value) of each material, which indicates how easily heat passes through the material.

Thickness: Measure the thickness (d) of each material layer.

Thermal Resistance (R-value): Calculate the thermal resistance for each layer using the formula

3, Sum of R-values

Add up the thermal resistances of all layers.

Calculate U-Value: The U-value is the inverse of the total thermal resistance

1, Factors Affecting U-Value

Glazing: Double or triple glazing with air or gas-filled spaces between panes significantly reduces U-values.

Frame Material: Materials like uPVC and thermally broken aluminum have lower U-values compared to standard aluminum frames.

Coatings: Low-emissivity (Low-E) coatings on glass surfaces reflect infrared energy and further reduce heat transfer.

2, Typical U-Values

Single Glazing: Typically has a higher U-value (5.0 W/m²K), indicating poor insulation.

Double Glazing: Lower U-values (around 1.2-3.0 W/m²K), offering better insulation.

Triple Glazing: Even lower U-values (as low as 0.8 W/m²K), providing excellent insulation.

The Window Energy Rating Scheme (WERS) allows for the rating and labelling of windows for their annual energy impact on a household anywhere in Australia.

Window makers who want to participate in the WERS scheme need to get energy ratings for their products. Rating organisations accredited by the Australian Fenestration Rating Council (AFRC) are authorised to give these ratings.

This simple and accurate energy rating system can help home and building owners choose products that satisfy their energy efficiency requirements. 

Different needs have specific requirements. For example, in cold rooms with no direct sunlight, reducing heat loss is more important than capturing solar gain. On the reverse, rooms that have direct sunlight in summer, need to reduce the solar gain to stop overheating.

Why we think U-values are important for windows:

A window with a low U value will lose less heat from a room and have the most direct impact on room comfort.

A house built with low U-value building components will use less energy and saves money on energy bills.

Good U-value building components increase the surface temperature on the inside which is critical in preventing surface mould growth.

Good U-values also improve the indoor thermal climate and create healthy buildings for their residents.

Choosing energy-efficient windows can help lower your energy bills and reduce your carbon footprint.

1. To improve the U-value of windows and doors:

Upgrade to Double or Triple Glazing: More layers of glass with insulating gaps can significantly reduce U-values.

2. Use Low-E Coatings:

These coatings on the glass surface reduce heat transfer.

3. Select Better Frame Materials:

Frames with thermal breaks and lower conductivity materials, like uPVC or thermally improved aluminum.

4. Consider Gas Fills:

Filling the gaps between glass panes with inert gases (e.g., argon, krypton) improves insulation.

As an expert in the windows and doors industry, where energy-efficiency is the talk of the town, ROPO is among the very few which actually fulfill their duties for the environment as well as the society. For years, ROPO has been a proud member in Australian Glass & Windows Association, WERS with AS certificates for the factory and all products. Energy-efficiency is our language and we shall never ever stop speaking it.