Technical Glazing Terms

What is a U Factor? A Guide to Thermal Insulation for Windows in the US

The thermal insulation factors possible from your windows and other fenestration are a key factor in specifying glazing in the US building market. With Stretch codes becoming more widely adopted, and the increased requirements of the IECC and ASHREA in regards to envelope insulation, glazing, and windows need to be carefully assessed in specification.  

Maximum U factors are now enforced in Climate Zones 2-8 so it is important for architects and other specifiers to understand what a U factor is, and which aspects of a window affect the overall thermal performance of the glazing installation.  

Read on below to learn more or get in touch with the team at IQ Glass with your U factor requirements and allow us to advise you on the best window or door system for your design and performance goals.

What is a U Factor?  

A U factor is the denotation of thermal performance for windows, most widely used and recognised in America and the United States.  

U factors are a measurement of how much energy can travel through a material, which showcases the material’s thermal insulation. The lower the U factor, the less energy that can travel through the material, and the better insulating it is.  

The unit of measurement for U factors is BTU/hsqftF. This is the amount of British Thermal Units that can travel through the material per hour, for each square foot of material.  

(British Thermal Units are a measurement of energy based on the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit, and is typically only used in the USA).  

What is the Difference Between a U Factor and U Value?  

A U value is the measurement of insulation that is typically used in the UK and Europe. It is measured in W/(m2K) (watts per meter squared, Kelvin). This measures the number of watts of energy that can travel through a m2 of a material.  

Both U factors and U values are recognised methods of calculating the insulation of a window or glass door.  

U values can be a little more complex as the overall thermal insulation for a window (known as the Uw value) is a combination of the thermal performance of the glass (the Ug value), the thermal performance of the frame (the Uf value) and the thermal performance of the spacer bar (the Psi value). All these factors come together to create the overall Uw value for the window or glass door.  

You can use various free online conversion tools to calculate the Uw value equivalent of a U factor, and vice versa.  

The quick table below shows the relationship between some of the most common insulation values for windows and doors, showing both the Uw value and the U factor.  

How Can I Improve the U Factor of a Window? (or How Can I Improve the Thermal Performance of a Window?)

Windows and glass doors are made up of the glass, frame, and spacer bar of the glass unit. The thermal insulation of all these individual materials has an impact on the overall thermal insulation of the window and your U factor.  

If you look at any window or glass door design, the glass is the largest surface area material of the installation. It is therefore logical to understand that the best improvements in thermal insulation of a window come from improving the performance of the glass.  

Improving the Thermal Performance of Glass  

By moving from single glazing to double glazing, you can half the thermal performance of a window. Further gains in thermal insulation of the window can be achieved by improving the gas insulation between the glass panes and including things like low e coatings within the glass specification.  

Further improvements in the U factor of a window can be seen by increasing the glass specification to triple glazing.  

A triple-glazed unit within a well-engineered aluminum system (thermally broken) will likely give you a U factor of around 0.14. If you are looking for thermal insulation levels below that for your window, we can then start looking at the specification and insulation of the window frame for gains.  

Improving the Thermal Performance of a Window Frame  

The thermal insulation of a window frame will depend on the material of the frame and any thermal breaks included within the frame design (if it is a metal frame).  

Timber is a natural insulator and is therefore often specified for high performance or sustainable builds however, this is not your only frame option.

Steel frames (including bronze, stainless steel, and Corten) have a slim metal exterior and are well suited to industrial designed buildings or designs of high luxury and opulence. If using a steel frame within the exterior of a building, it must have a thermal break to ensure heat cannot travel freely through the metal. A double-glazed steel door (like the Mondrian CWS 75 Slim) would be likely to have a U factor of 0.28, whereas using a triple glazed unit would achieve a U factor of 0.19.  

Have you considered using an aluminum steel look system? The Sieger Legacy TH+ system offers the same design as steel in an aluminum frame that provides much better insulation levels.  

Aluminum frames have a wide variety of performance possibilities thanks to the wide array of frame designs and thermal break designs available with this material. As a metal framing material, it must be thermally broken for exterior glazing. Most well-made, European aluminum systems can achieve a U factor of 0.2 using double glazing, down to 0.14 using triple glazing. Even further reductions in U factors can be seen with the use of multi-chamber thermal break systems, such as the Keller minimal windows NGS, which can offer U factors down to 0.11 with a triple glazed unit.  

Aluminum will give you the slimmest sightlines of any framing material, with frames of under 1 inch common across the European market.  

Timber frames are natural insulators and require no additional thermal break within them to provide insulation. The Avino Vero is a solid timber frame, and its lift and slide variation can provide a U factor of 0.1 using triple glazing.  

Timber frames are a great option for interiors that want a more natural design, using timber and other natural materials as part of the interior design.  

What Makes a Good Thermal Break in a Window?  

If you are using a metal framing material, it must have a thermal break. This is a layer of thermally non-conductive material that thermally separates the interior and exterior frame, making it harder for heat to move through the profile.  

There are various ways in which a metal framed system can be ‘thermally broken’, but below are some key things to look out for:  

The Thermal Breaks Must Be In Line  

A good thermal break will be in line with the gas cavity of the glass unit. This creates a continuous thermal break through the window. The thermal break of the fixed frame should line up with the opening frame and the gas cavity for a continuous insulation line within the window.  

Air Chambers in the Thermal Break Improve Performance  

Air is a natural insulator and therefore most good thermal breaks will have air gaps or cavities within them. The more air chambers within a thermal break, the better the insulation of the frame.  

What Can Affect the Thermal Performance of a Window?  

As well as your glass specification and frame material/design, there are other aspects of a window or glass door that can change the thermal performance.  

Size 

The size of your window or door will change the glass to frame ratio and therefore, change the overall thermal insulation factor of the window. As the glass is the best performing element, the larger the installation, the better the overall U factor.  

Configuration  

Similar to the above, the configuration of your opening window or door will change the amount of glass vs frame and therefore the U factor. Bifolding doors, sliding doors, casement doors and windows must always be calculated for the exact configuration of the installation.  

Glazing bars 

Sometimes, you may wish to incorporate glazing bars into your architectural glazing design as part of the building aesthetic. True glazing bars (that dissect the glass and run through the system) will have an effect on the thermal insulation of the whole window or door.  

If you are looking to maintain a high level of thermal insulation from your window, we recommend any glazing bars be ‘applied’ glazing bars (these are decorative and applied to the surface of the glass and therefore do not affect the thermal performance).  

What Thermal Performance Can I Achieve for My Design?  

The U factor and thermal performance achievable for your project will depend on many aspects of the design. You should always contact IQ with your performance requirements for your project, along with your intended designs, for us to provide accurate U factor calculations for your project.  

The chart below is intended as a quick overview of the possibilities.  

How can I specify glazing that will hit my performance requirements?  

Just speak to the team at IQ Glass.  

We are experts in architectural glazing and understand the performance requirements needed within all climate zones of the US and beyond. As experienced architectural glaziers across the Caribbean and mainland US, we can help you to select the right glazing solution for your design AND the performance you need.  

Click here to contact the team at IQ Glass to start your conversation with us today.