What’s the Difference Between R-Value and U-Factor?

This post first appeared on the Andersen Windows and Door Website.

We were taught in junior high that hot air rises, the key to that phrase is air.  Heat itself moves from someplace warm to someplace cool.  A good example of this happens often in cold climates.  When standing in front of an old window on a cold night, you feel a chill.  This chill is the result of heat leaving your body and moving towards the colder surface of the window glass, heat moving from hot to cold.  To slow this movement of heat, we use insulation, by putting on another layer of clothes, we reduce or resist the movement of heat, we have added R-value.

R-value is the resistance to heat flow, conductive heat flow to be specific.  With R-value, the higher the number, the more resistance to the movement of heat.  Another term that is commonly used in construction is U-factor.  U-factor is the rate or speed of heat loss (or gain), with U-factor, the smaller the number (most U-factors will be less than 1), the slower the rate of heat loss.

U-factor is the amount of heat that transmits through 1 square foot of building assembly or product (roof, floor, wall, window, etc…) in one hour with a 1°F temperature difference between the two sides.

Window manufacturers are required to indicate the rate of window heat loss through their window assembly by using the U-factor metric.  A typical window in my very cold climate might have a U-factor of .30.  For some, U-.30 might not be relatable.  We can convert U-factor to R-value with the simple formula: 1/U-factor=R-value.  That U-.30 window will have an R-value of 3.33.  We can also convert R-value to U-factor by the formula 1/R-value=U-factor.  The reason U-Factor is the metric for windows is because a window is an assembly of different materials.  The glass will have a different heat flow than the frame.  We cannot simply add the R-values of these different components together.  We have to calculate both series and parallel paths through the different materials by using a weighted average of the U-factors of the various pieces that create the window.  These calculations get us into the weeds, so we won’t go there.  Just be aware, calculating the insulation values of a window (or any assembly of products) isn’t as easy as simply adding R-values together.

What is a good R-value and what is a good U-factor?  The answer is a phrase that I unfortunately have to say a lot, it depends.  If you have a wall assembly that is insulated to R-30, I would say that’s awesome!  But that same R-30 makes for a below average roof resistance to heat flow and R-30 would be very difficult to achieve in a window.  The code minimums in the 2021 IRC have improved and moved very close to the 5-10-15-30-60 and 1 rule of what I like to see.  R-5 windows (U-.20), R-10 under slab insulation, R-15 slab edge/foundation insulation, R-30 wall insulation and R-60 in the roof.  The “and 1” is the recommended air tightness level in air changes per hour at 50 Pascals (ACH50).

Let’s dive a little deeper into the wall assembly and how the building materials, insulation and windows affect performance.  Most common walls in the residential market are constructed using wood framing lumber, typically 2 x 4 or 2 x 6.  The framed cavities are filled with insulation.  The outside is usually covered by some sort of sheathing, such as plywood or oriented strand board (OSB).  There may or may not be a layer of continuous exterior insulation.  The exterior will have some sort of cladding to protect the moisture sensitive surfaces of the inner wall.  The interior side of the wall will have some sort of finish, drywall is most common.  All these surfaces and products have an insulation value.  Cavity insulation will have a higher resistance to heat flow than wood framing.  Because of this, the actual R-value of the entire wall is different than the listing for the cavity insulation.  The code may require that the insulation value of a wall, for instance, be R-20, but the actual R-value will usually be something less.  When we calculate the lesser R-values of wood framing (and windows and doors) compared to the higher insulation value of the insulation, we produce what is called the effective R-value.

The effective R-value, (sometimes called area weighted average) is the whole wall resistance to heat flow which takes all the building components into account.

The calculation for the effective R-value is beyond the scope of this article but be aware it requires knowing the area of framing, the area of insulation and area of windows and doors of the entire wall assembly.  You also need to know the U-factor of each of these components.  What starts off as an R-20 wall might end up being R-12 or less by the time all the component insulation values and ratios are calculated.  The effective R-value is needed when calculating the heating and/or cooling needs of the structure.

How do you achieve the best possible effective R-value?  There are a few suggestions. Some of the more successful strategies are to move as much of the insulation to the outside of the structure as possible (as shown in the illustration).  When insulation is outboard of the framing, the effect of thermal bridging can be reduced.  Moving to an advanced framing technique (where framing members are spaced on 24” centers rather than the traditional 16” center) is an option.  Windows with lower U-factor ratings and lowering the window to wall ratio can also help.

R-value and U-factor are two important subjects that builders should know.  Knowing that a wall’s performance with regards to heat flow is much lower than the listed R-value of the wall’s insulation is a topic that is often misunderstood.  The choice of insulation levels and the location of that insulation in the wall assembly, framing type, size and spacing along with the selection of a quality window or door will all affect comfort, cost and even the durability of the structure.  For more information about the performance of Andersen Windows and Doors can be found at: Performance Documents | Andersen Windows

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