I’m a big fan of exterior insulation. It’s rarely used in my area, mostly because the State of Minnesota has eliminated that code requirement. It has to to with our wide use of polyethylene sheeting as a vapor retarder on the warm in winter side of a wall assembly and then adding a low permeance plastic insulation product as exterior insulation. These plastic foams would be the choice for most contractors, lower cost and easy to source. Very slow vapor movement in either direction when a wall assembly becomes wet. This posting isn’t going to get into the foam insulations, but more into what exterior insulation can do for a home.

I see the photo above often when conducting energy audits and assessments on local homes. Thermal bridging happens when the lower insulating values of some building products, such as wood framing, have a direct pathway to the temperature differences between outside and inside the home. Wood framing will conduct heat easier than cavity insulation, metal and concrete are even better at conducting heat. Pretty easy concept.

In a Minnesota code built home, (and homes around the country built before 2012 that weren’t required to have exterior insulation), the wall assembly might be something like this: From the exterior, lap siding, wall sheathing, 2 x 6 wall framing, cavity insulation, and drywall. There’s probably some sort of house wrap and maybe an interior vapor retarder that could be included, for simplicity, I’m not including those products. With a typical 16″ on center framing assembly, the wood framing will account for around 22% of the wall. With this information, we can calculate the weighted average R-value of a wall assembly. A weighted average R-value is the average of the entire wall assembly, not just the cavity insulation R-value. I often hear something like “My walls are R-21”, when in reality, the actual R-value is something much less. As you might have guessed, here comes the math!

To calculate the weighted R-value of a wall, you’ll need the R-values of each of the components that make up the wall assembly along with the percentage of wall framing. You’ll also need to know how to convert an R-value to a U-value. What is a U-value? It’s the value used to calculate the energy needed for heating. It’s an easy calculation, U=1/R-value. The inverse is also true, if you need the R-value, R=1/U-value. If you look at the required stickers on new windows, they are always listed as a U-value. A code built home might have windows with U-.30, which means that window has a resistance to heat flow of R-3.33. We will talk about the effects of windows on the weighted wall assembly later.

Back to the assembly listed above, here are the R-values of the listed components:

Wood lap siding R-.80 7/16 OSB sheathing R-.50 Fibrous insulation-5.5 inch R-21 2 x 6 wood framing R-6.88 Drywall-1/2 inch R-.45

We need to separate the framing R-values from the cavity insulation R-values. The good news is we can simply add the R-values together. Our wall framing R=value is R-.80 (lap siding) + R-.50 (OSB sheathing) + R-6.88 (2 x 6 stud) + R-.45 (drywall) = R-8.63.

The cavity R-value where we have the fibrous insulation is R-.80 (lap siding) + R-.50 (OSB sheathing) + R-21 (cavity insulation) + R-.45 (drywall) = R-22.75

We now need to change both the R-values to U-values for the next part of the equation. U=1/R, the framing U-value is .116 and the cavity insulation area is U-.044. The formula for calculating the weighted wall assembly is:

U = (wall framing U-value x area percentage of the wall framing) + (cavity insulation U-value x area percentage of the cavity insulation)

We have calculated the percentage of wall framing at 22% and cavity insulation at 78%. (U-.116 x .22) + (U-.044 x .78) = .06. So the weighted U-value for the wall assembly is U-.06, 1/.06 = R-16.67. There’s our weighted R-value of the wall assembly without windows, R-16.67. A far cry from the R-21 cavity insulation.

Next, lets look at what the heat loss is for this wall assembly in three different areas of the US, Atlanta, GA, Kansas City, MO and Grand Rapids, MN. To do this, we will use the 99% heating load supplied by Manual J. What this temperature for the given city means is that 99% of the time, the temperature is warmer than the design temp. It’s the point where, if a heating system is designed correctly, the system should be operating continuously. Atlanta’s design temperature is 26°F, Kansas City is 9°F and Grand Rapids is -17°F. To calculate the heat loss in BTU’s per hour, we need the delta tee or difference between inside and outside. I’m going to use an indoor temperature of 68°, a typical indoor temperature during the heating season in my area. This gives Atlanta a difference between inside and outside of 42°F, KC gets 62°F and my town, Grand Rapids gets 85°F. We will also need the wall area, we will use a 30′ x 40′ home with 8′ sidewalls. This gives us a total area of 1120 square feet. We are just calculating the heat loss out the wall assembly, not the roof or floor systems nor are we looking at losses due to air leaks. The formula we will use is:

BTU’s/hour = U-value x area x delta tee

Our weighted average R-value is 16.67, U-.060 The equation for Atlanta is:

BTU’s/hour = .060 x 1120 x 42 = 2822 BTU’s per hour of heat loss in Atlanta. Kansas City will have 4166 BTU’s/hour and my town of Grand Rapids, MN will have 5712 BTU’s/hour. These heating loads are for identical homes without any exterior insulation in three different climates.

Now, lets add exterior insulation! I’m a big fan of Rockwool ComfortBoard 80 used as exterior insulation. We will use two different thicknesses to illustrate how adding insulation to the exterior of our wall assemblies will reduce heating loads. We will use two inch, R-8, and Rockwools newer thickness, five inch, R-20. Because the insulation is outboard the wall assembly, we can simply add the exterior wall R-values to the original weighted wall calculation without the windows. R-16.67 + R-8, two inches of exterior insulation gives us a total wall assembly R-value without windows of R-24.67. Five inches gets us to R-36.67. The U-values needed changes to U-.041 for the two inch and U-.027 for the 5 inch.

Atlanta’s heat loss reduces to 1929 BTU/hour with the two inch and 1270 BTU/hour for the five inch. Kansas City goes to 2847 BTU’s/hour and 1875 BTU’s/hour. Grand Rapids goes to 3903 BTU’s/hour and 2570 BTU’s/hour.

Let’s concentrate on the very cold climate location of Grand Rapids. The code minimum assembly heat loss out the walls without windows and without loss due to air leaks was 5712 BTU’s per hour. If heating using natural gas at $2 per therm, not taking into account efficiency loss of the heating system, the heat loss is costing $.115 per hour at the design temperature of -17°F. That costs drops to $.05 per hour with five inches of exterior insulation. Does exterior insulation make sense? I don’t recommend looking at the energy costs savings alone, though reducing your wall heat loss by half is a big incentive, there are other benefits gained. Building durability by eliminating condensing surfaces inside walls, a major increase in comfort and a decrease in outside noise entering the home. All should be taken into account.

Now, lets move to the scary part by add 15 percent windows to this assembly. We will use code minimum windows with U-.30 and our weighted wall assembly with five inches of insulation in my very cold climate.

U = (.30 x .15) + (.027 x .85) = .067 1/.067 = R-14.9. Wow, our weighted average wall assembly is now down to R-14.9 from R-36.67. All by adding 15% windows with an R-3.33 and U-.30, a typical code minimum window assembly. Another important consideration when adding exterior insulation, install better windows. This is why we see window u-values nearing .12 or R-8.3 in high performance homes, the windows become the weak spot.

If you are interested in learning more about Rockwool, I’ve been involved in a webinar with them for nearly a year now. The webinar happens the first Thursday of every month. Here’s a link to sign up.

Affordable Efficiency: Continuous Insulation Unwrapped (rockwool.com)