Rigid insulation under a concrete slab can impact a home’s energy efficiency, moisture control, and overall comfort. This comprehensive guide explains the types of insulation available, building codes, costs, and proper installation techniques to help homeowners make informed decisions about their concrete slab insulation.
Facts About Rigid Insulation and Concrete Slabs
Before we can really dive into explaining insulation under concrete slabs, it’s important to review some important factors and terms. Here are some things you need to know:
R-Value
R-value refers to the insulation level per inch of thickness. It measures the insulation’s ability to stop heat from traveling through it. The higher the R-value, the better the insulation is and the better it can prevent heat from entering or leaving the home. However, higher R-values also mean higher costs.
Climate Zone
Homeowners don’t necessarily have to go with the highest R-value. Different insulation levels are needed throughout the United States to meet the demands of various climates. The U.S. Department of Energy Building America Program created eight climate zone designations to help homeowners and builders improve a home’s energy efficiency. The zones take temperatures and precipitation into account.
For example, homes in South Florida (Climate Zone 1) require a much lower R-value than homes found in North Wisconsin (Climate Zone 7).
Building Codes
Building codes for rigid insulation under concrete slabs can vary depending on location. The International Energy Conservation Code (IECC) sets minimum energy-efficiency requirements for buildings.
According to these standards, rigid foam insulation is required if the surface is less than 12 inches below grade. In Climate Zones 1 to 3, rigid foam insulation is optional but recommended for improved energy efficiency. Heated slabs in these zones also require insulation to a depth of the footing or 2 feet.
In Climate Zones 4 to 8, the IECC mandates a minimum of R-10 insulation under concrete slabs. Therefore, homes in these climate zones may need several inches of insulation in order to meet the requirements.
Vapor Barriers
Vapor barriers protect concrete slabs from moisture and should be used with rigid insulation for the best protection. The American Concrete Institute (ACI) states that vapor barriers shouldn’t be less than 10 one-thousandth of an inch thick (mils). It also shouldn’t exceed a permanence rate (perms) of 0.3. For heated structures, the ACI recommends a minimum 6-mil vapor barrier.
What Are the 3 Main Types of Rigid Foam Insulation?
Rigid foam insulation is the most common type of insulation used under concrete slabs. There are three main options:
Expanded Polystyrene
Expanded polystyrene (EPS), also known as beadboard, is a lightweight, moisture-resistant, and cost-effective option for under-slab insulation. It’s composed of 98% trapped air and 2% plastic and has an R-value of 3.6 to 4.0. Extremely durable, EPS maintains its R-value over time.
Extruded Polystyrene
Extruded polystyrene (XPS) is a popular choice for under-slab insulation due to its higher R-value (4.5 to 5.0) and excellent moisture resistance. Typically available in pink, blue, or green, XPS is also very compression-resistant. However, it tends to be more expensive compared to EPS and other options. However, it may lose some R-value over time.
Polyisocyanurate
Polyisocyanurate, also known as polyiso or PIR, offers the highest R-value per inch of the three rigid foam insulation types, coming in at 6.5 to 6.8. Moisture-resistant and fire-resistant, it also maintains its R-value longer. As a result, it’s the most expensive in most cases, but XPS can cost more in certain situations.
Cost Considerations for Rigid Insulation
The cost of rigid insulation can vary widely based on the home’s location, the size of the home, code requirements, and the type of insulation selected. Here’s a general overview of what homeowners can expect to pay, according to Home Advisor:
- EPS: $0.40 – $0.75 per square foot
- XPS: $0.60 to $1.50 per square foot
- Polyiso: $0.65 to $1.50 per square foot
How to Insulate a Concrete Floor Before Pouring
For homeowners tackling a new construction project or major renovation, here’s a step-by-step guide to insulating a concrete floor before pouring:
- Measure the slab area and calculate the amount of insulation needed based on your area’s regulations and your energy efficiency goals.
- Excavate the area 12 inches below the planned slab level.
- Add a 4-inch layer of gravel or crushed stone and rake it evenly.
- Lay down the vapor barrier, fully covering the area and using construction tape to seal seams.
- Install the chosen rigid foam insulation, including insulation around the slab’s edges.
- Pour the concrete and level it using a rake or shovel.
Homeowners may also choose to add a 2-inch layer of sand over the insulation.
Insulating Basement Floors
Basement floors present unique challenges for insulation due to potential moisture and temperature variations. Homeowners can choose between EPS, XPS, or polyiso, or they may opt for closed-cell polyurethane spray foam, which comes with additional air-sealing capabilities.
Before getting started, address any structural issues or air leaks before insulating to prevent future complications. Consider upgrading plumbing and wiring during the insulation process to avoid the need for future access beneath the insulated floor.
Insulating Garage Floors
Garages, while often considered less critical for insulation compared to living spaces, can still benefit from proper insulation, especially if the homeowner plans to regularly use the garage for more than parking the car.
Common garage insulation options include rigid foam, fiberglass, radiant barriers, reflective insulation, cellulose, and spray foam. Before adding insulation, fill air gaps to prevent cold or hot air from bypassing the insulation layer.
Final Thoughts About Rigid Insulation Under Concrete Slabs
Rigid insulation under concrete slabs is necessary when constructing or renovating an energy-efficient home. By selecting the right type of insulation, adhering to building codes, and following proper installation techniques, homeowners can significantly improve their home’s energy performance and overall comfort.
