Earth is one of the best insulating materials for a home. So, areas of the home like basements that are built below ground level are buffered from dramatic temperature spikes and dips. To improve on that natural insulation and to make the basement even more livable, you’ll want to install basement insulation. Installing effective, water-resistant basement insulation opens up an unused area of your home, increasing floor space and providing more opportunities for living, playing, and entertaining.
How Basement Insulation Works
Basement insulation differs from the insulation system used throughout the rest of the house on the above-grade floors. On those upper floors, wood stud walls enclose soft fiberglass insulation batts. The batts are faced with paper. The edges of the paper are stapled to the studs.
This system does not work for basement walls. Fiberglass insulation facing foundation walls readily absorbs moisture if it is exposed to humidity or if it becomes wet. After being water-logged, fiberglass loses some of its R-value. Worse, the highly porous material becomes a breeding ground for mold and other allergens.
Rigid foam insulation is the material of choice for insulating against foundation walls in basements. You have two options with rigid foam: rigid foam only or rigid foam with a stud wall system.
2 Methods for Insulating a Basement
-
Foam attaches to foundation wall with adhesive
-
Fiberglass insulation added
-
From R-18 to R-23
-
Drywall attaches to framed stud wall
-
Uses more space
-
More expensive
Rigid Foam and Furring Strips Basement Insulation
Large sheets of 1- or 1-1/2-inch-thick rigid foam are attached directly to the foundation wall with concrete screws. The screws are driven through one-by-three wood furring strips. Later, drywall is installed on the furring strips.
This method is straightforward and less expensive than the stud system method. Being thinner, the wall conserves room. But it has about half of the insulating value of the stud system method. Plus, driving multiple screws into concrete can be taxing.
This method works well for basements that need minimal insulation and where space is at a premium.
| R-Value | Total Wall Thickness | |
| 1-inch rigid foam | R-5 | 2-1/4-inch |
| 1-1/2-inch rigid foam | R-7.5 | 2-3/4-inch |
Rigid Foam and Stud System Basement Insulation
Large sheets of 1-inch to 2-inch-thick rigid foam are attached directly to the foundation wall with foam adhesive. A stud wall system, similar to those on upper levels, is built using wood or metal studs. The wall system touches the rigid foam but is held in place at the top and bottom. With the wall system fully watertight, soft R-13 or R-15 insulation can be added between the studs as an option. Finally, drywall is added.
This method gives the basement more insulating value, by allowing you to add fiberglass in watertight wall cavities. The wall system is structurally independent of the foundation wall. In some ways, construction is easier than the furring strips method because fewer fasteners need to be driven into concrete.
But this method requires more building materials, making it more expensive and overall more difficult to construct. Plus, with thicknesses up to 6-3/4 inches, this wall system will take up a lot of valuable basement space.
The full stud system is best for larger basements that need maximum insulating value.
| R-Value | Total Wall Thickness | |
| 1-inch rigid foam and fiberglass | R-18 (R-5 + R-13) | 5-3/4-inch |
| 1-1/2-inch rigid foam and fiberglass | R-20.5 (R-7.5 + R-13) | 6-1/4-inch |
| 2-inch rigid foam and fiberglass | R-23 (R-10 + R-13) | 6-3/4-inch |
Safety Considerations
Rigid foam insulation is made from extruded polystyrene, which is generally safe for handling without gloves. Even so, be sure to wear full PPE: safety glasses with side protection, arm and leg coverings, and dust mask breathing protection when handling rigid foam. With fiberglass insulation, full PPE is required, including a NIOSH-approved dust respirator (N95 or greater).
Warning
Extruded polystyrene foam on the interior must be covered with 1/2-inch drywall or another code-approved material for fire safety. It cannot be left uncovered.
Materials
Rigid Foam and Furring Strips Method
- 3 sheets extruded polystyrene foam insulation, 4-foot by 8-foot
- 6 one-by-sixes, 8-foot
- 3 sheets drywall, 1/2-inch
- 2 tubes foam adhesive
- 1 box concrete screws, 2-1/2-inch
- 1 box drywall screws
- 1 can spray foam sealant for cracks
Rigid Foam and Stud System Method
- 3 sheets extruded polystyrene foam insulation, 4-foot by 8-foot
- 13 two-by-fours, 8-foot
- 3 sheets drywall, 1/2-inch
- 2 tubes foam adhesive
- 3 rolls fiberglass insulation, 15-inch by 32 feet
- 1 box drywall screws
- 1 can spray foam sealant for cracks
How to Prepare the Basement for Insulation
-
Test for Moisture
Foundation walls must have no moisture before they are insulated. At least two weeks before insulating the basement, tape a 24-inch by 24-inch section of clear plastic against the foundation wall. Tightly cover all four sides. If condensation does not collect between the plastic and the wall, the wall is dry enough for insulation.
-
Mitigate Basement Moisture
Groundwater over the top of the wall and water through the wall usually need to be corrected from the exterior. Common methods of mitigating moisture include redirecting downspouts, creating a yard drainage system, grading the soil away from the wall, building French drains, and adding perimeter drain systems at the foot of the wall.
-
Seal the Wall and Fix Damage
Fix cracks and holes by using a trowel to fill them with watertight hydraulic cement. If necessary, apply basement waterproofing sealer to the wall.
-
Prepare the Wall
Because the insulation is directly applied to the wall and the strips or studs rest against the insulation, the underlying foundation wall surface must be flat and free of obstructions. The wall must be clean. Kill and encapsulate mold and mildew.
-
Insulate and Seal the Band Joist
The band joist is the outer box that forms the perimeter of the floor assembly above the basement. Each section between floor joists needs to be insulated. Cut small sections of rigid foam to fit in these areas. Adhere the foam to the band joist with foam adhesive. Fill the gaps with low-expansion spray foam adhesive.
How to Cut Rigid Foam Insulation
-
Cut the Foam Board Lengthwise
Use full-size foam sheets as often as possible. When the rigid foam needs to be cut lengthwise, first look for the pre-cut scores on the board. If the intended cut point happens to fall on a pre-scored line, you can break off the foam there. If not, score the board lengthwise with the utility knife down to the depth of the blade. The blade does not need to fully penetrate the board. Then snap the board on the score line.
-
Cut the Foam Board Widthwise
Foam boards will need to be cut widthwise to accommodate the basement’s height. Cut widthwise with a straight edge and a utility knife, scoring and snapping off the board rather than cutting all the way through.
-
Cut the Foam Board for Penetrations
When you need to cut rigid foam board all the way through for penetrations, use a 4- to 6-inch serrated knife, hand saw, or a drywall jab knife.
How to Insulate Basement With Foam and Furring Strips
-
Place Foam Boards Vertically
Cut the first foam board to the height of the basement. Start in a corner. Apply a continuous ribbon of foam adhesive to the back of the board. Press the board into place. Lean a section of scrap two-by-four against the board.
-
Cut Furring Strips
Measure and cut each furring strip individually to adapt to slight changes in basement ceiling height. Cut the boards on the electric miter saw or with a hand saw.
-
Install the Furring Strips
Fit the foam boards side-to-side. Butt the edges against each other. Install the furring strips horizontally on the face of the insulation. The furring strips will rest on top of the foam, with a 3/4-inch gap between the drywall and the foam insulation.
Use the drill and concrete screws to attach the furring strips. The screws must penetrate the concrete by 1-inch.
-
Install and Finish the Drywall
Install the drywall to the furring strips with drywall screws. Tape, mud, and sand the drywall.
-
Prime and Paint the Drywall
Apply drywall primer to the finished drywall. Paint with interior acrylic-latex paint.
How to Insulate Basement With Foam and Stud Wall
-
Apply Adhesive to Rigid Foam
Apply a continuous bead of foam adhesive to the back of the first insulation board.
-
Attach Boards to the Wall
Apply the foam boards vertically from the top of the basement floor to the sill plate with the edges tightly butted against each other.
-
Frame the Wall
Frame out the wall with studs. The frame should be built with a horizontal plate at the bottom, a horizontal plate at the top, and vertical studs spaced every 16 to 24 inches on-center.
The back of the framing can touch the front of the foam insulation. The top plate should be attached to the joists above and the bottom plate should be nailed or screwed into the concrete floor.
-
Add Fiberglass Insulation
Fiberglass insulation can be used in the framing. While this step is optional, it will double the R-value of the basement insulation. Staple the paper edges of the insulation against the studs.
-
Install and Finish Drywall
Attach the drywall to the stud framing. Finish the drywall by taping, mudding, and sanding it.
-
Prime and Paint the Drywall
Finish by priming the drywall with drywall primer, then painting the wall with acrylic-latex interior house paint.
When to Call a Professional
Water mitigation in basements and foundation walls—especially tracking down the source of the water—is often beyond the scope of a do-it-yourselfer and may require help from a water mitigation company. Similarly, minor mold growths can be managed by a homeowner, but large and pervasive swaths can be difficult to manage. A mold remediation company can help kill and encapsulate the mold.
Read the full article here