Montgomery County, MD has adopted the 2021 International Energy Efficiency Code, effective as of December 10, 2024, with a grace period ending March 31st, 2025. To see more about what changed and how to get out of it if you want to (not my preference), see my other post about this recent Energy Efficiency Code change.
That other post has the what and when and a little bit of the how…to deal with it. This post will be more technical, as I'll show you a few ways to construct walls to meet the new code standards.
The legal aspect of this is specific to Montgomery County, Maryland, but the technical aspect should be useful to anywhere else in Climate Zone 4, which includes Montgomery County, IL, Montgomery County, KS, Montgomery County, MO, Montgomery County, PA and Montgomery County, TN.
If you're in Montgomery County, TX you're climate zone 2A. Montgomery County, AL, Montgomery County, AR, Montgomery County, GA, Montgomery County, MS and Montgomery County, NC you're in 3A
Montgomery County, IN, Montgomery County, IA and Montgomery County, ND, you guys are 5A. Montgomery County, NY is in climate zone 6A.
RESIDENTIAL INSULATION REQUIREMENTS FOR WALLS
The 2021 International Residential Code (IRC) and International Energy Efficiency Code (IECC) set the following minimum standards for wall insulation:
- R30 -or-
- R20+5ci -or-
- R13+10ci -or-
- 20ci
The R value represents insulation within the wall, often called cavity insulation. It is placed between the studs. The ci value represents continuous insulation. This is a an uninterrupted layer of insulation (running past the face of the studs, not between them) that is often placed outside of the studs, but can be done inside as well. So to meet the standards, you could build walls that have
- R30 insulation between the studs -or-
- R20 insulation between the studs AND R5 continuous -or-
- R13 insulation between the studs AND R10 continuous -or-
- No insulation between the studs and R20 continuous
There are probably a million different wall assemblies you could choose from to meet these standards. It's pretty fun thinking of different ways to construct an energy efficient wall assembly. I'll start with the three most basic and might add more variations later. This is 101 level stuff. If you want to get more educated start browsing Green Building Advisor or Building Science Fight Club.
R30 Wall Assembly
The most simple way to meet the new standard is to provide R30 insulation within the wall cavity. Two basic ways this can be done:
2X6 Wall with 5.5 Inches of Closed-Cell Spray Foam (CCSPF)
Pros: Easy, keeps walls at 2X6, which has been the default for a while now. See other post. A 2X6 wall is 5.5″ thick, which should get you R33 or better with closed-cell foam. Spray foam of all types aT that thickness provide additional air-tightness benefits and help strengthen the wall.
Cons: CCSPF is expensive. And it is (or was) ironically not great for the environment. I'm admittedly not up to date on the latest discussions, but the gist of it is that the blowing agent used to install CCSPF was hundred of times worse than carbon when measuring global warming potential, and there was a debate as to whether or not the lifetime carbon savings of using CCSPF (by reduced energy use) would ever “pay off” the initial damage done when it was installed. A more clear downside to this method is that you still have a thermal bridge every 16″ to 24″, which reduces the effective R-value of the wall.
2X8 Wall with 7.25 inches of Batt Insulation
Pros: Easy to install. Nothing really changes for builders as they can still use the same details and processes they are used to.
Cons: You need 2X8 studs, which can be close to twice as expensive as the 2X6 studs commonly used today. They also might require more labor cost to install as well, because pre-cut 2X8 studs are not that common yet, so every single stud would have to be cut to length for standard 8, 9 and 10 foot ceiling heights…not the case when using pre-cut 2X6 or 2X4 studs. You also lose 1.75″ of interior space at every wall, which might not sound like a lot, but I can tell you it adds up and can get really annoying when you need those inches.
R20+5 Wall Assembly
I think this is going to be the most common wall now that the new code is in effect. I did a slightly better version of this in my own energy efficient home. Download the project book for fun insulation pictures!
2X6 Wall with 5.5 Inches of BATT INSULATION and a ‘zip' r6 sheathing panel
Pros: You can still use 2X6 studs, see CONS of larger studs above. You have a wide range of insulation options to choose from to use within the wall. R-21 Fiberglass batts are the most common, but you can also use R-23 Rockwool batts (my preference) or get about R-20 with open-cell spray foam, which is what I ended up using at my house. The Zip System Insulated R-Sheathing has an integrated layer of R6 foam so there should not be additional labor costs to install the continuous insulation. The biggest benefit of this assembly is that the layer of insulation in the Zip panel is on the inside of the panel. Still continuous across the face of the studs, but the exterior face of the sheathing/water barrier remains the same, so most window install details won't change. I include an air gap between sheathing and siding in my typical details, as shown here. This is technically not required, but it's a good thing to do for the health of the wall.
Cons: It's going to be more expensive to build, but that is (sort of) unavoidable with the code change. If you use Zip R panels greater than the 1.5″ wide panel make sure you are clearing that with your structural engineer, as there is now more space between the face of sheathing and studs, and this could be an issue for wind bracing.
R13+10 Wall Assembly
This one will be less common than the other two. This might be useful on Accessory Dwelling Units or other small structures when you need every single inch of interior space.
2X4 Wall with 3.5 Inches of BATT INSULATION, a REGULAR ‘zip' sheathing panel AND THEN 2″ OF XPS FOAM
Pros: You can use 2X4 studs, which are totally fine for most residential use up to 9 feet ceilings. You save the most interior space of any of these wall assemblies and you have lots of options for the wall cavity insulation (basically the same as the 2X6 wall options above). The more insulation you can get continuously outside of the wall the better, so this wall will probably perform better than the previous two. The 2″ of exterior foam being outside the face of sheathing can provide some benefit in keeping the face of sheathing warm and above the dew point.
Cons: The 2″ of exterior foam outside of the face of sheathing significantly changes way windows and doors are installed, which is a bigger change than that one sentence might indicate. See this short post about Innie vs Outie Windows from Building Science Legend Martin Holladay. This assembly is not standard practice for most builders around here. There are plenty of building-science-forward builders around here, but they are not the inexpensive option. They are usually the premium price for premium quality option. If you want to build an energy efficient new home, ADU, whatever, and you think you might want to start explore Passive House wall assemblies, you are not likely to have success without a builder and an architect who already understands.