Why is actual R-Value different from advertised R-Value?
The photo above is from a newly built (2024) home in my neighborhood. The builder used very typical home framing techniques; nearly all homes built in Colorado are framed this way. Notice all the wood (studs, header, blocking), window and framing, and electrical boxes – all these elements take up space in the wall assembly. That means that there will be less insulation in those areas.
Simplified wall assembly
The wall section below represents a very common construction method here in Colorado. Typically, the builder will space the 2×6 studs 16” apart, center to center. Which means that we have a 1-1/2” stud, then a 14-1/2” insulated cavity followed by another 1-1/2” stud. The builder will repeat this pattern around the entire outside perimeter of the house. Notice that while the fiberglass insulation is R-19, the wood 2×6 studs are only R-5.5. To calculate the actual R-Value of the wall section, we average the wood R-Value and the insulation R-Value. When we calculate the average R-Value for this small section of wall we get R-15.4. Not too bad but wait – there’s more.
Actual Wall Assembly
Let’s a quick deep dive into the typical home framing techniques reduced the insulating characteristics of the wall. Around the window (R-2.2), we have eight 2×6 studs, plus four more cripple studs above the window (there will be more cripples below the window; not shown). Above the window, we see the header which means we have an area of 30”x10” with zero insulation. On either side of the window, we see a narrow gap between the studs, not the normal 16” spacing. Insulation installers usually try to insulate in these areas, but it can be difficult and is very often poorly or not at all insulated. We also see seven pieces of 2×6 blocking, which reduces the amount of insulation in those areas. Finally, there are three electrical boxes, which are very difficult to insulate and often ignored by insulation installers. The presence of electrical wiring in the wall cavities also makes proper insulating difficult.
So, what’s the R-Value of this wall? Calculating the exact R-Value would be a significant chore, but from my experience it’s probably not better than R-8.
The wall area around doors will have similar issues as the window area above. The wall areas near corners will likely have many studs in the way of insulation. I’ve seen house with as many as ten studs all sitting next to each other; creating a large and very noticeable cold spot on the wall.
These are all common issues in most contemporary homes. However, with proper design and building techniques these issues can be eliminated or significantly reduced. We will address these design and building techniques in future blogs.
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