Insulating concrete foundations from inside the basement

Why insulate foundations from the interior rather than the exterior?

Interior wall insulation of foundations is often easier to carry out. It is also less expensive than exterior insulation. You avoid excavation work around exterior walls while improving long-term energy efficiency.

What checks should be done before insulating a foundation wall?

Foundations are put under significant stress. Before insulating them, a few foundation inspection steps are necessary.

Inside the basement

Before installing insulation panels or sprayed foam insulation on your walls, check that your foundation is in good condition:

• Structural inspection: Check for cracks, concrete crumbling, or deformations.
• Water and radon tightness: Inspect the walls. Ensure there are no cracks, water leaks, or efflorescence. If any issues are detected, repair the foundation before installing insulation.

Outside the basement

Your foundations must be protected against excessive water pressure. Check that:

• The soil around your home is properly graded, with a slope of at least ¾ in per foot over 5–6 ft, to direct runoff away from the foundation.
• Your eavestroughs discharge water at least 5–6 ft from the house.
• Your French drains and sump pumps are functional and not blocked.

What thermal resistance should you aim for when insulating your foundation walls?

Here are the recommended thermal resistance (insulation R-value) thresholds to target for reducing your electricity bills:

*The rim joist is the wooden board that surrounds the main floor structure. It rests on top of the foundation wall.

Best practices for insulating foundation walls

Concrete foundations can cause condensation, especially when in contact with a crawl space or freezing soil. Installing continuous insulation on interior walls allows you to:

• Reduce thermal bridges, which transfer heat from the framing to the foundation.
• Protect moisture-sensitive materials, such as wood or batt insulation.

Do not forget rim joist insulation, a critical area often overlooked but prone to significant air leaks.

Finally, stone or concrete block foundations should not be insulated to full R-value from the inside, unless a waterproof membrane has been installed on the exterior.

Which materials to use for insulating foundations?

The most effective insulation products for limiting water infiltration and ensuring long-lasting insulation are:

• Extruded or expanded polystyrene panels (rigid insulation panels).
• Spray polyurethane foam, which perfectly conforms to concrete walls and seals gaps.

Avoid placing gypsum directly against foundation walls. Also, do not use tarboard panels, as they emit toxic vapours indoors.

Key features of insulation materials

Should you install a vapour barrier for basement insulation?

Yes. A vapour barrier or moisture barrier is essential, except for stone walls, which must breathe. It prevents indoor moisture from penetrating walls, causing mould and damaging the foundation. Polyethylene is typically used between the insulation and the finishing material.

How to insulate a foundation wall: Two effective methods

Method 1 – Continuous insulation + Wood framing + Batt insulation

• Install continuous insulation: Place a 1-in (25mm) polystyrene panel over the entire concrete surface (R-5). If the wall is irregular, apply ¾ in (2cm) of spray polyurethane foam.
• Attach framing: Install 2x4 studs (38 x 89mm) at 24 in (600mm) spacing against the continuous insulation.
• Insert batt insulation: Fill the framing with 3½ in (89mm) batt insulation (R-12).
• Install vapour barrier: Place a vapour barrier on the warm side (interior) and seal the joints.
• Finish with fire-resistant material: Attach ½ in (13mm) gypsum board according to safety standards.
• Ensure tight sealing between insulation panels, framing, and rim joist to reduce heat loss and air infiltration.

Method 2 – Rigid panels + Wood furring

• Install two layers of rigid panels: Apply 1 ½ in (38mm) extruded polystyrene panels in staggered joints (total R-15).
• Seal joints: Fill and seal all edges; at this thickness, the panel can act as a vapour barrier if well sealed.
• Attach furring strips: Install 1x3 in (19 x 64mm) furring strips to create anchoring space.
• Finish: Install ½ in (13mm) gypsum board and seal joints with the vapour barrier if needed.
• Ensure continuous insulation and eliminate thermal bridges around the rim joist and floor junctions.

Electrical and plumbing precautions

• Install airtight electrical boxes or seal them with polyethylene on the vapour barrier.
• Always run water pipes on the interior (warm) side of insulation to prevent freezing.

DIY or professional work?

Some repairs can be done yourself, but others require specialized tools or expertise.

Steps you can do yourself

• Prepare and install insulation: Cut and fit panels on straight, sound walls.
• Attach furring strips according to the framing plan.
• Clean and prepare surfaces: Walls must be clean and dry before installation.

Steps to leave to a professional

• Major foundation repairs: Structurally damaging cracks, shifting, or French drain issues.
• Spray polyurethane foam application: Ensures perfect adhesion and airtight sealing.
• Exterior waterproofing and French drain repairs.
• Crawl space work: Technical tasks requiring experience and safety.

Document your renovation projects: keep estimates, before/after photos, and intervention certificates for warranty and future renovations.

Precautions before insulating a basement

• Do not insulate a block foundation to full R-value from the interior without an exterior waterproof membrane.
• Seal all joints between panels, rim joist, and floor to reduce heat loss and water infiltration.
• Cover all combustible insulation (panels, foam) with fire-resistant material according to standards.
• Follow local code updates and building code rules for renovation projects, adjusting R-values and vapour barriers as needed.

Always check for radon before insulating a basement, especially in risk areas.

FAQ on interior foundation insulation

Should a stone basement be insulated from the inside?
Yes, but stone walls must breathe. Avoid an impermeable vapour barrier that traps moisture. Opt for breathable insulation or consider exterior insulation.

What is the best insulation for a damp basement?
Spray polyurethane foam is the most effective. It seals cracks and prevents air and water infiltration. If your walls show signs of moisture or if you notice a musty smell, first address the cause (French drain, exterior waterproofing) and dry the foundation before insulating.

Does interior insulation increase home value?
Yes. An insulated basement improves comfort, reduces heating costs, and provides extra living space, increasing your property’s appeal when reselling.