The R-Value and U-Value are two different types of insulation ratings that measure how well a material resists heat transfer. The higher a building’s R-value, the better it insulates against heat loss.

If you are in the market for replacement windows, there are some key terms and requirements you’ll want to know. These terms may be used inaccurately in some literature you may find, so we’re shedding light on the proper information related to R-Value, U-Value, Heat Gain, and Title 24 requirements. Also, understanding this information will help you choose the right energy-efficient windows for your home.

What is R-Value?

R-Value is the measure of the resistance of the material to heat flow. The higher the R-value, the better insulation that material provides. To calculate the total R-value for a wall or roof you must add up all of the individual values for each inch in thickness of insulation used.

What is U-Value?

U-Values are thermal transmittance rates through building materials such as windows and walls. In addition, A lower U value means less heat can escape from your home.

The Window U value

The U-Factor measures how well the window insulates. While a range of values is possible, in general, it ranges from 0.20 to 1.20 and lower is better for windows. In addition, the SHGC measure how much heat comes through the window which can vary but generally falls between zero and one with higher numbers being worse (better).

What is the best U-factor for a window?

In cold climates, it’s recommended to have windows with an efficiency between 0.17 and 0.39 (R6 – R2). The low end of this range only achievable through triple glazing or other high-quality materials that are more expensive than double-paned glass.

R-Value, U-Value, and Heat Gain

Both U-value and R-value measure energy efficiency. However, they are not the same nor are they interchangeable.

U-value is a measure of heat transfer, which is the heat gain or loss through the glass. R-value is the measure of heat resistance. The U-value is not rating a material. Rather, it’s the calculation of the conduction properties of various materials used to build windows, doors, and skylights.

On the other hand, R-value measures the performance of a specific material, such as insulation, based on the material’s ability to reduce the transfer of heat. It measures a material’s resistance to heat flow, which is important in the rating insulation. It measures the rate at which heat is transferred or lost and is based on conduction and resistance. A high R-value indicates better insulation and resistance to heat transference.

U-value is a scientific calculation measuring the rate at which heat flows through one square foot of material. Simply put, it measures how well the product keeps heat from escaping a space. In other words, the lower the U-value, the better it is at reducing heat transfer and relates to the entire energy efficiency of a window. Also, this value is especially important during the winter heating season.

Here is a breakdown of how to interpret these two different values:

	R-Value	U-Value
Indicates Energy Efficiency	Yes	Yes
Measures insulating ability	Yes	Yes
Measures a specific material value	Yes	No
Based on different components of a product and calculations	No	Yes
Indicator of heat transfer	No	Yes
Indicator of heat resistance	Yes	No
Measures airflow	No	Yes
Measures radiated or reflected heat	No	Yes

Why do R-Value and U-Value matter in the rating of energy-efficient windows?

R-value is relevant in terms of insulation used around the window during installation. This includes the use of caulking and insulation around the window where it meets the walls of the structure. The higher the R-value of any material, the greater the insulating value. In relation to windows, U-value takes more than the conduction of heat into consideration. It also considers airflow or convection around the window and the amount of radiated or reflected heat off the glass.

Title 24 Requirements

Another important aspect of choosing replacement windows is the Title 24 requirements. Also, Title 24 in California relates to windows, glazed doors, and skylights and how these elements impact energy use in a home. Title 24 has mandatory requirements regarding the U-value and solar heat gain coefficient (SHGC) ratings of windows.

We’ve already addressed U-value, so here’s a quick review of SHGC—it’s the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed then released inward. It’s a number between 0 and 1, with the lower the number indicating less solar heat transmission.

Title 24 mandatory measures require that both the U-factor and the SHGC of manufactured fenestration products be determined from the National Fenestration Rating Council’s (NFRC) Certified Product Directory or from the California Energy Commission (CEC) approved default tables.

At the time of inspection, the actual fenestration U-factor and SHGC values as shown on NFRC labels or in the default tables must result in equal or lower overall energy consumption than the values indicated on the compliance documents.

CEC default values are on the poor side of the performance range for windows. For performance glass such as low-e coatings and thermal break frames, the manufacturer must test the window per NFRC procedures.

Improved Window Performance

With the 2016 Title 24 energy standards update, the prescriptive maximum u-factor for all windows, glass doors, and skylights has been maintained at 0.32 in all climate zones. The mandatory maximum u-factor has been maintained at 0.58. Up to 10 feet of fenestration or 0.5% of the conditioned floor area (whichever is greater) is exempt from the maximum U-value requirement.

Performance Features Impacting U-Value and SHGC

There are many features that can impact these measurements in the world of windows, including:

• Frame material, design, and configuration
• Thermal breaks
• Number of panes of glazing, coatings, and fill gases
• Gap width (the distance between panes)
• Window type
• Spacer material (the type of material separating multiple panes of glass)

In conclusion, when choosing your replacement windows for your California property, compliance with Title 24 is critical, so it’s important to know what it is and how other elements impact it. At The Window & Door Shop, we are well-versed on the subject and ready to answer any questions you may have. Contact us today to learn more.