How to Insulate Basement Walls

There are three ways to insulate a basement: from the outside, from the middle of the foundation itself and from the inside of the structure.
There are three ways to insulate a basement: from the outside, from the middle of the foundation itself and from the inside of the structure.
iStockphoto/George Peters

­A few generations ago, many houses didn't have basements. They had cellars. Damp and musty, their dirt floors provided cool, consistent temperatures that were ideal for storing perishables like apples, jellies, fruit preserves and pickled vegetables.

With the evolution of building construction and poured concrete foundations, however, basements are now considered integral to a home's living space. They typically house the furnace, water heater, laundry machines and essential utilities. But recent trends have expanded the unfinished basement's utilitarian role into a more feature-rich, family-friendly environment such as a game room, home theater, office, craft or exercise area.

Because basements are mostly located below ground level, they are by nature cool and damp. They can also account for significant heat loss from a home. Porous foundations can allow cold air and water to seep in; flooding or bad drainage can allow water to run in through cracks.

That's where insulation comes in. Installed properly, insulation allows us to make basements comfortable, limit escaping heat and help keep moisture at bay.

­But before launching your own basement transformation, consider some basics of basement insulation. There are three ways to insulate a basement: from the outside, from the middle of the foundation itself and from the inside of the structure. Both external and middle insulation are typically done during the construction of the house, although external insulation can be done in conjunction with a drainage project aimed at keeping water away from the foundation.

Unless the house is still in the planning phase, insulating a basement from the inside is usually the most practical way to go. That's what we'll focus on in this article: interior insulation. The good news -- it's the most cost-effective way to insulate a basement. The bad news -- if it's not done properly, it can result in moisture problems.

Read on to learn about the tools you need to insulate basement walls.

Tools Needed to Insulate Basement Walls

Local jurisdictions might have building requirements that cover interior insulation. Fire codes require that foam or batt insulation be covered with drywall or another type of finished wall. Check with your local building department to make sure your project is up to code. The last thing you want is to fail the ­inspection after you've put time and money into the project.

The first thing you'll need to figure out is the R-Value (thermal resistance) of the insulation you'll be installing -- you can find recommendations at the International Energy Conservation Code.­ In the United States, there are five zones. The farther north the zone, the higher the thermal resistance (R-value) your insulation should provide. Most of our country falls between the R-5 and R-10 ratings [source: U.S. Department of Energy].

You could already have some of the following tools you need for your basement insulation project. If not, look for the following at a local hardware store:

  • safety glasses
  • dust mask or respirator
  • protective clothing
  • stapler (electric, standard, hammer type)
  • three-eighths or half-inch staples
  • work light
  • extension cord
  • tape measure
  • utility knife and blades; straightedge for cutting insulation
  • stepladder [source: Home Improvement Web]

The following items may also be useful, depending upon the type of insulation being installed:

  • expanding foam sealant
  • caulk and caulking gun
  • chalk line
  • gypsum drywall (half-inch)
  • 2 x 2 lumber
  • tape: white vinyl, duct tape
  • extruded polystyrene foam
  • power-activated gun with fasteners
  • construction adhesive
  • wire insulation hangers
  • noise protection for ears
  • masonry screws, drill and carbide drill bit
  • asphalt roofing cement [source: Home Improvement Web]

Now that you have the tools you'll need, don't forget the insulation materials. Find out more on the next page.

Materials Needed to Insulate Basement Walls

Homeowners have access to a number of materials that can be used to insulate a basement from the inside. The ty­pe ­of insulation and the installation process should be driven by climate, the best way to control moisture in that particular climate and the basement configuration itself.

Here are a few options:

  • Blanket insulation -- This is the most common type of insulation; it is made of flexible fibers, most commonly fiberglass. It comes in batts, rolls or blankets. Some come with a facing attached that's made from kraft paper, foil or vinyl. Widths typically come in standard sizes of wall studs. Batts are typically 4 feet (1.22 meters) or 8 feet (2.44 meters) long. Rolls can be hand-cut and trimmed to fit. Blankets come in rolls as large as 64 feet (19.5 m). Standard fiberglass blanket insulation will usually have an R-value from 2.9 to 3.8 per inch (7.36 - 9.65 cm) of thickness. This type of insulation is typically cheaper than foam board, loose fill and spray-applied insulation.
  • Rigid foam board insulation -- This is made from polystyrene, polyisocyanurate (polyiso) and polyurethane. R-values run between 3.8 to 5 per inch of thickness for boards made from polystyrene. Foam boards made of polyiso and polyurethane offer higher R-values, from 5.6 to 8 per inch of thickness. The advantage of foam board is that it can keep water vapor in its gaseous state, keeping it from condensing on the wall. Foam board must be covered with a fire barrier, typically a half-inch (1.27 cm) gypsum wallboard.
  • Loose-fill insulation -- This method uses small particles of fiber or foam that can conform to any space. It can be installed by pouring it or blowing. Cellulose, fiberglass and mineral wool are common ingredients of loose-fill insulation. R-values range from 2.2 to 3.8 per inch, with fiberglass being on the low end and cellulose being the most efficient. The key to installing this type of insulation is achieving the proper density and getting complete coverage.

To learn about the benefits of insulating basement walls, read on.

Benefits of Insulating Basement Walls

The obvious benefit of basement wall insulation is the substantial energy cost savings it should provide. However, your savings will depend upon the following factors: local climate, type of heating system, fuel cost and lifestyle of the people who use the basement.

Ultimately, the thermal performance of any insulation or any combination of insulation depends upon proper installation, which is why it might be a good idea to hire a professional contractor who has a lot of experience with basement wall insulation. The most cost-effective way to insulate a basement of an existing structure while minimizing moisture problems involves a combination of rigid foam insulation and an insulated frame wall assembly.

It's all about airflow. Experts talk about the importance of "inward drying," meaning the foundation's ability to disperse moisture into the basement, allowing it to dry. Restricting airflow causes moisture to be trapped. A true vapor barrier, used to block moisture from coming into the basement, often succeeds in trapping the moisture between the insulation and the wall. If the insulation is installed on a wood wall frame and encased in a vapor barrier like polyethylene sheeting, the moisture will be trapped inside, which will damage both the insulation and the wood frame.

Now that you've learned the basics on kinds of insulation, tools, materials and benefits you should be ready to start your basement wall insulation project and watch those energy dollar savings add up each year. To learn more, visit some of the links on the following page.

Related HowStuffWorks Articles


  • Carter, Tim. "Basement Insulation." Ask the Builder. (Accessed 2/17/2009)
  • Building Science. "BSD-103: Understanding Basements." Building Science Corporation. Last Updated 10/20/2008. (Accessed 2/17/2009)
  • Darling, David. "Basement Insulation." The Encyclopedia of Alternative Energy and Sustainable Living. (Accessed 02/16/2009)
  • The Home Improvement Web. "Basement and Stud Wall Fiberglass Insulation Installation Instructions." Accessed 02/16/2009.
  • ICAA. "Contractor Locator." Insulation Contractors Association of America. (Accessed 02/18/09)
  • Minnesota Department of Commerce. "Basement Insulation." (Accessed 2/17/2009)
  • Silva, Tom. "Insulate a Basement?" This Old House. (Accessed 2/17/2009),,396510,00.html
  • U.S. Department of Energy. "Adding insulation to an existing home." A Consumer's Guide to Energy Efficiency and Renewable Energy. (Accessed 2/17/2009)
  • U.S. Department of Energy. "Basement Insulation." A Consumer's Guide to Energy Efficiency and Renewable Energy. (Accessed 2/17/2009)