Homeowners expect a basement that’s warm and dry, whether they’re using the space as an extension of the living space, a hobby room, or just a controlled-environment storage area. It’s tempting to simply consider finishing the basement walls, flooring, and ceiling as an after-move-in project; however, for a truly warm, dry basement, it helps to take a whole house approach.
Water management and insulation systems are best installed during foundation construction and landscaping. Building a dry, energy-efficient basement involves applying a combination of materials and techniques on the exterior and interior.
Foundation wall exterior
To protect the exterior of the foundation walls, follow this checklist:
Apply a capillary break to the top of footings. Use an elastomeric asphalt coating or polyethylene sheet to prevent water from wicking up through the footings and wetting the basement slab and walls.
Install a perimeter drain alongside the footings. This will collect any rising groundwater and carry it away from the foundation.
Seal all penetrations in the foundation walls. Use an approved mortar or low-shrink sealant.
Apply a waterproofing system to the exterior of the foundation walls. This two-part system includes an elastomeric asphalt coating to seal the wall’s porous concrete surface and 1” to 3” polystyrene panels that extend from the footings to the top of the foundation wall for insulation and drainage.
Install a capillary break under the slab. Place a 4” to 6” layer of clean gravel in the foundation footprint to control groundwater infiltration via capillary action.
Install a vapor barrier under the slab. Position a 10-mil polyethylene vapor barrier to fully cover the foundation footprint, and extend it 2” to 4” up the foundation wall, fixing it to the wall with construction tape or adhesive to prevent water vapor and bulk water intrusion.
Backfill the foundation properly. Remove any debris, and use backfill material that allows for good drainage. Take care not to puncture the waterproofing system.
Be sure to also slope the rough and final grades away from the home, establish swales, and install flatwork and landscaping that allows for good drainage. All of these practices will help minimize water against the foundation wall and the amount of water that penetrates the wall into the basement space.
Basement wall interior
To finish the interior of the basement walls, a common mistake is to simply add 2x4 stud framing, batt insulation, and drywall over the concrete foundation. Sometimes, a polyethylene vapor barrier is also added in the wall assembly, or the concrete wall is sealed with paint or epoxy. These practices are problematic for the same reason: they trap moisture in the wall assembly. This causes many problems, including reducing the insulation’s R-value, promoting mold growth on insulation and drywall, and eventually rotting the framing members.
The best practices for insulating and finishing the basement walls are to construct the wall assembly so it can dry when it gets wet and to physically separate all framing, insulation, and drywall from the concrete foundation walls. It's best to install extruded polystyrene sheathing panels to function as both a moisture barrier and an insulator. Extruded polystyrene sheathing panels have a very low perm rating, so they won’t absorb moisture, support mold growth, or deteriorate if they get wet. They stop moisture from moving through the wall, and instead, allow the wall to dry to the exterior. They’re also excellent insulators. A 1” panel is rated R-5.
Before installing polystyrene panels, be sure that all wall penetrations are sealed with a low-shrink mortar or approved sealant and that the basement has no pre-existing moisture problems. Next, attach the polystyrene panels directly to the interior of the foundation walls using an approved adhesive. Seal all seams with an approved mastic and mesh tape. Be sure the tape is centered over each joint; intersecting or jointed tapes should overlap 2”.
To further increase the R-value of the wall assembly, construct 2x4 framing using pressure-treated lumber, fill the stud cavities with insulation, and finish with drywall. If no additional insulation is needed, simply attach furring strips to the polystyrene panels, and then attach drywall to the furring strips.
Protecting the basement from water intrusion and insulating it well yields a basement that's comfortable and energy-efficient inside and out. For more information about insulating finished basements, visit the Basement Insulation section of A Consumer's Guide at the U.S. Department of Energy's Efficiency and Renewable Energy website.
Other Best Practices® for foundation construction can be found in the following online training courses from BuildIQ University:
Foundations: Basements & Crawlspaces
Foundations: Slab on Grade
Water Management: Basements & Crawlspaces
Water Management: Slab-on-Grade Foundations