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 is great! But that same R-30 makes for poor roof resistance to heat flow, and R-30 would be very difficult to achieve in a window. The code minimums in the 2021 IRC are good achievements, but if you are looking to build above code, the 5-10-15-30-60 rule is what I suggest. 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.
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 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 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 be R-20, for example, 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.
