How to keep a finished basement dry and warm

Why basements need special care

Basements sit against cold, damp soil. Natural Resources Canada's Keeping the Heat In guide says basements can account for about 25 percent of a home's total heat loss, and that earth is a poor insulator. It also points to air leaks at windows, cracks and the top of the foundation wall.

Adding insulation inside makes the concrete behind it colder. NRCan explains that humid indoor air touching those cold surfaces can condense, and that finished walls can hide moisture problems until they become mould problems.

Step one: fix water first

NRCan's advice is clear: do not insulate a basement with moisture problems from the inside. Signs include leaks, dampness, efflorescence (white mineral deposits) and blistering paint. Correct the moisture first, or new walls can rot.

Grading, eavestroughs and downspouts

The cheapest fixes are often outside. NRCan says minor leaks can sometimes be corrected by sloping the grade, aiming downspouts away from the house and patching the foundation. Its guide describes directing surface water away from the foundation by at least 2 m (6 ft.) with eavestroughs and downspouts, and suggests a slope of about 10 percent for the first 2 m. CMHC's mould guidance also advises sloping grading away from the foundation to divert water from the walls.

• Clean eavestroughs so they do not overflow at the foundation
• Extend downspouts well away from the house
• Rebuild soil that has settled against the foundation so it slopes away
• Keep window wells clear and draining

Foundation cracks

Not every crack is a problem, but active ones matter, and it is much easier to deal with them before the walls are finished. NRCan says that if a crack is active, meaning it gets bigger or smaller, seek professional help to find out whether structural repairs are needed. Leaking cracks should be repaired before framing.

For serious or repeated leaks, NRCan describes exterior waterproofing from grade down to the footings, sometimes with a drainage membrane. That is a larger project, often done at the same time as exterior insulation.

A simple dampness test

A basement can look dry and still let moisture through the concrete. NRCan suggests taping a sheet of plastic about 1 m (40 in.) square to the wall and floor and checking after a day or two.

Condensation on top of the plastic points to high indoor humidity. Condensation or darker concrete under the plastic points to moisture coming through from the ground. NRCan notes this can also indicate a possibility of radon entry.

Insulating basement walls: common approaches

NRCan describes several ways to insulate from the inside once the basement is dry. The right choice depends on moisture risk, how you use the space and budget.

• Framed wall with batt insulation: NRCan suggests covering the concrete with house wrap from grade down, not plastic, so small amounts of moisture can dry
• Rigid foam board against the concrete, then a framed wall: NRCan warns not to add a polyethylene vapour barrier here, to avoid a double vapour barrier
• Closed-cell spray foam applied by a certified installer, followed by a framed wall: again, NRCan does not recommend polyethylene
• Exterior insulation: often the best choice for foundations with moisture problems, since repairs and waterproofing can happen at the same time

Vapour and air control

NRCan says a polyethylene vapour barrier is suitable in basements that have proven to be dry. If there is any doubt, it suggests two alternatives: a "smart" vapour retarder that lets the wall dry inward when humidity rises, or the airtight drywall approach, where drywall is carefully sealed to act as the air and vapour barrier.

NRCan also warns against using asphalt building paper (tar paper) inside the house because it can release toxic vapours. Any exposed foam may need fire protection under the Code.

Don't forget the rim joist

The rim joist area, where the floor framing sits on the foundation, is prone to air leakage and is seldom properly insulated, according to NRCan. That leads to drafts, dust and pests. Sealing and insulating it is an important part of any basement finish.

Floors and subfloor options

NRCan notes that most basement heat is lost through the upper walls, so slabs are seldom insulated. For comfort, moisture control and radon reduction, it suggests applying a moisture barrier to the floor or sealing it so moisture does not collect between the insulation and the slab. A floor drainage system under the new floor is another option.

In practice, homeowners often choose between flooring laid directly on a sealed slab, a dimpled or insulated subfloor panel, or a framed floor. Each suits different ceiling heights, budgets and moisture levels. If you are replacing a concrete slab anyway, NRCan says it is a good time to consider in-floor heating or making the floor radiant-ready.

Permits and insulation work

Insulation changes can need a permit. The City of Kawartha Lakes lists alterations to building systems, such as insulation changes and additions and foundation drainage, as permit work on its building page. Other municipalities include insulation in basement finishing permits. Check locally before you start.

Start with a dry basement and a clear plan

At a free site meeting, Oakridge looks at visible conditions in your basement and talks through what needs attention before finishing. We explain options in plain language and put the agreed approach in your written scope. Call 289-731-2086 or request a free site meeting online.