Once the framing is up in a basement, the next question is what goes inside those walls before the drywall covers them. Basement walls are harder to insulate than the walls upstairs. The concrete stays cool, moisture can move through it, and the wrong materials in the wrong order can leave a finished wall damp behind the paint.
Here’s what the energy code generally asks for in Colorado, the common ways to build an insulated basement wall, and what to check before the drywall goes up.
Which code applies at your address
In Colorado, building codes are adopted by cities and counties, and the energy code is part of that package. A 2022 state law, House Bill 22-1362, set a minimum. A city or county that has building codes and updated them between July 1, 2023, and July 1, 2026, had to adopt an energy code at least as strong as the 2021 International Energy Conservation Code, or IECC. Updates after July 1, 2026, have to meet a newer state model code aimed at low energy use and carbon emissions. Some small rural counties have a different path.
Each department adopts on its own schedule and can adopt a stronger code, so two neighboring cities may not be on the same edition. Ask your building department which energy code applies at your address before you settle on an insulation plan. The numbers below come from the 2021 IECC as a model code, which is a common starting point.
How much insulation the model code asks for
The IECC sets insulation levels by climate zone. Under the 2021 edition, Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, and Jefferson counties are all in Climate Zone 5B. Several mountain counties are in zones 6B and 7.
For a basement wall in Climate Zone 5, the 2021 IECC gives three ways to comply:
- R-15 continuous insulation on the inside or outside surface of the wall
- R-19 cavity insulation on the inside of the wall
- R-13 cavity insulation on the inside, plus R-5 continuous insulation on the inside or outside
Zones 6, 7, and 8 list the same three options for basement walls.
Continuous insulation is a layer that runs unbroken across the wall, like foam board. Cavity insulation fills the spaces between studs. The code also sets how far down basement wall insulation goes: from the top of the basement wall down to 10 feet below grade or to the basement floor, whichever is less.
Finishing an existing basement can be treated as an alteration or as a change in space conditioning, and your building department decides which. Under the IECC, an alteration has to meet the requirements for new construction, but only for the parts you change. If the basement wasn’t heated or cooled before, Section R502.2 can apply instead, and it brings the newly conditioned space into full compliance with the code. Either way, the new walls you build have to meet the table, and the rest of the house doesn’t have to be brought up to it.
Three common ways to build the wall
The U.S. Department of Energy’s Building America program has studied basement insulation for years. Its researchers found that the best interior systems share a few traits: the insulation touches the concrete with no air channel behind it, the assembly is air sealed so basement air can’t get into it, nothing moisture-sensitive touches the concrete walls or floor, and the system lets some water vapor pass through.
Most finished basements use one of these three approaches.
- Foam board on the concrete, then a stud wall. Rigid foam boards (XPS, EPS, or polyiso) are glued or fastened to the wall, with seams taped and edges sealed. A stud wall goes up in front, and the cavities can take batt insulation. According to DOE, XPS foam is about R-5 per inch. One inch of XPS plus R-13 batts in a 2×4 wall lines up with the code’s “R-13 plus R-5” option.
- Spray foam on the concrete, then a stud wall. Spray foam seals air well and works on uneven walls. DOE’s Building America profile says closed-cell spray foam gives the best moisture control of the interior options. DOE also notes that it usually costs more to install than foam boards.
- A stud wall with fiberglass batts only. A deep enough stud cavity can meet the R-19 option. DOE’s caution is that fiberglass or cellulose installed in contact with basement walls “can absorb large amounts of moisture and stay chronically damp.” If this is the plan, ask how the batts will be kept off the concrete and how the wall will be sealed.
Which one fits depends on the wall, the budget, and how much room you can give up. Each layer of foam and framing takes a few inches off the room, so settle the wall build along with the other choices in our guide on what to decide before the framers arrive.
About the plastic sheet
Many people picture a plastic vapor barrier stapled over the studs before the drywall. On basement walls, the model code doesn’t ask for it. The 2021 International Residential Code, or IRC, requires a vapor retarder on the inside of frame walls in Climate Zone 4 and colder, but it lists basement walls and the below-grade part of any wall as exceptions.
DOE’s Building America profile goes a step further. It warns that plastic on the interior of a basement wall “will trap moisture in the wall.” DOE’s Basement Insulation Basics guide adds that, in Building America studies, insulation systems stayed drier when water vapor could move between the basement and the concrete wall. If plastic is in someone’s plan, ask why, and check what your building department expects.
Water comes before insulation
Insulation can’t dry out a wet wall. DOE’s Basement Insulation Basics guide, written for new and existing homes, says moisture management must be addressed before a basement is insulated. Its checklist for existing homes asks whether groundwater and storm water can be kept out of the basement, and puts those measures ahead of choosing an insulation system. The IRC also says interior drywall can’t be installed where it is directly exposed to water.
Insulation can’t dry out a wet wall.
If you’ve seen water on the floor or damp patches on the walls after a storm, have that looked at before anyone frames or insulates.
Where the drywall comes in
Three drywall details matter in a basement.
- It covers the foam. Under the IRC, foam insulation has to be separated from the room by a thermal barrier, and the most common one is 1/2-inch drywall. Foam board and spray foam both count, so where foam is used, the drywall is part of the fire protection.
- It stays off the floor. DOE’s Building America guidance says to leave at least a 1/2-inch gap between the drywall and the floor. Baseboard covers the gap.
- It respects the floating walls. Many Colorado basements have floating walls that let the slab move. Drywall on those walls is fastened to the wall itself, never to the fixed plate or the ceiling framing above.
The inspection before the drywall
Under the 2021 IECC, the framing and rough-in inspection happens before any interior finish goes on. Part of its job is to check the insulation: the type, the R-value, where it is, and how it was installed, along with air sealing.
How each building department schedules that check varies, so ask whether insulation is looked at during its own inspection or along with another one. Our post on the permit and inspection process covers the order of inspections in more detail.
Five questions to ask before the walls get covered
- Which energy code applies at my address? Ask the building department for the edition and any local changes.
- Which code option does this wall use? Get the R-value of each layer and where it goes.
- Is anything moisture-sensitive touching the concrete or the slab? Ask about the batts, the drywall, and any untreated wood.
- Is plastic sheeting part of the plan, and why? The model code doesn’t require it on basement walls.
- When is the insulation inspected? Ask if you can walk the walls yourself before the drywall goes up.
The short version
Basement wall insulation in the Denver metro usually falls under Climate Zone 5, where the 2021 IECC model code calls for R-15 continuous insulation, R-19 in the stud cavity, or R-13 plus R-5. Your city or county may use a different edition, so confirm it first. Keep moisture-sensitive materials off the concrete, think twice before adding plastic, cover foam with drywall, and fix any water problem before the walls get built.
If you’re planning a basement and want to talk through drywall and insulation, start with the five questions above.
Sources
- Colorado General Assembly, House Bill 22-1362, Building Greenhouse Gas Emissions (amending C.R.S. 30-28-211 for counties and 31-15-602 for municipalities)
- International Code Council, 2021 International Energy Conservation Code, Table R301.1, Climate zones by state and county (Colorado)
- International Code Council, 2021 International Energy Conservation Code, Table R402.1.3, Insulation minimum R-values by component (basement wall, with footnote on “15ci or 19 or 13&5ci”)
- International Code Council, 2021 International Energy Conservation Code, Section R402.2.8.1, Basement wall insulation installation
- International Code Council, 2021 International Energy Conservation Code, Section R502.2, Change in space conditioning
- International Code Council, 2021 International Energy Conservation Code, Section R503.1, Alterations
- International Code Council, 2021 International Energy Conservation Code, Section R105.2.2, Framing and rough-in inspection
- International Code Council, 2021 International Residential Code, Section R702.7, Vapor retarders (exceptions for basement walls and below-grade walls)
- International Code Council, 2021 International Residential Code, Section R316.4, Thermal barrier
- International Code Council, 2021 International Residential Code, Section R702.3.5, Application (gypsum board not exposed to water)
- U.S. Department of Energy, Building America Top Innovations Hall of Fame Profile, Basement Insulation Systems
- U.S. Department of Energy, Building America, Measure Guideline: Basement Insulation Basics (Aldrich, Mantha, and Puttagunta, 2012)
