Insulating basement walls the right way
Basement insulation works best when moisture is dealt with first, the rim joist is sealed and insulated, and the walls get a material that tolerates damp concrete, such as rigid foam or spray foam, followed by a finished wall. Insulating from the inside is the usual retrofit, while outside insulation is mostly done when the foundation is already excavated.
Why insulate a basement at all?
Below-grade walls look protected by the soil, but in a Canadian winter the ground near the surface gets cold and bare concrete sheds a lot of heat. The top of the wall and the rim joist are the worst spots, because they sit above or near the frost line. An uninsulated basement also feels cold underfoot, raises heating bills and invites condensation on the walls. In many older houses it ranks right after the attic and air leaks in the upgrade priorities guide.
What must be fixed before you insulate?
Insulation on a wet wall traps moisture, so check these first.
- Exterior water. Grading should slope away from the foundation and downspouts should discharge well clear of it.
- Leaks and efflorescence. White powder, damp patches or active seepage need a proper repair, which may involve a foundation specialist.
- Cracks and gaps. Seal what can be sealed so that outdoor air and groundwater stay out.
- Humidity. A dehumidifier or better ventilation may be needed in summer.
Radon is also worth testing for before sealing a basement more tightly. Ask your provincial health or housing authority how to get a test.
Interior or exterior insulation: which one?
Exterior insulation keeps the concrete warm and dry and protects the foundation from freeze-thaw cycles, but it requires excavating around the house, so it is usually done during new construction or when waterproofing is replaced anyway. Interior insulation is cheaper and far more common in retrofits. The wall is covered with rigid foam board, spray foam, or a framed wall with an insulating layer, and then a finish such as drywall.
Whichever you choose, the choice of material near the concrete matters. Materials that soak up water, such as unprotected fibreglass batts pressed against bare concrete, can grow mould if the wall is damp. Foam products cope better, and the pros and cons of foam are covered in spray foam insulation. The handling of vapour barriers differs by assembly and code, so confirm the layers you plan with your building department or a qualified contractor.
How do you handle the rim joist and the floor?
The rim joist, the wood band that sits on top of the foundation wall, leaks both heat and air. Cut-and-fit rigid foam sealed at the edges, or spray foam, is often used to insulate and air-seal it in one step. Details of sealing gaps are in the air sealing guide. Slab floors can be insulated under a new floor system with foam and a subfloor, but this adds height and cost, so many owners start with the walls and a warm floor covering.
How much insulation to aim for varies with climate and local code. Look at R-values by region for the way recommendations are set.
What does it cost, and is it DIY work?
Costs depend on wall area, material, finish, the amount of moisture repair and your region, and quotes decide. Simple rigid foam on dry walls can be a DIY job for a handy owner. Wet walls, spray foam and any work that changes electrical, plumbing or fire separation are better handled by licensed trades, and a permit may be needed, particularly when the basement becomes living space. Ask contractors to price the same scope and to explain how moisture, the rim joist and fire protection for foam will be handled. If the basement also houses the furnace or a hot water tank, see how combustion air and clearances are kept intact. A home energy audit will show how much heat the basement is losing before you spend.
Quick answers
Should I insulate basement walls inside or outside?
Outside insulation performs very well but needs excavation, so it is usually chosen when digging is already planned. Inside is the practical retrofit for most homes.
Can I use fibreglass batts in a basement?
They are used in framed walls with a foam layer or air gap against the concrete, but placed against bare, damp concrete they risk mould. Foam products are safer where moisture is possible.
Does basement insulation need a permit?
Insulation alone often does not, but finishing the space or changing walls and wiring can. Check with the municipal building department.