Foundations and basements: keeping heat in below grade

How heat and moisture escape through a basement

A basement loses heat through a large, often uninsulated surface both above and below grade, and the soil around it is a poor insulator. NRCan's Keeping the Heat In guide puts the basement at about 25% of a home's total heat loss (NRCan ↗). Most of that leaves through the upper part of the foundation wall, which is why basement floor slabs are seldom insulated (NRCan ↗).

Air leaks add to the loss, through basement windows, cracks, service penetrations and the sill plate and rim joist (the header), where the house framing sits on the concrete. NRCan says that band is prone to air leakage and seldom properly insulated (NRCan ↗), and in winter the stack effect draws cold air in through it (NRCan ↗).

Moisture moves in too. Groundwater can wick through concrete and evaporate on the wall, and in humid summer weather, warm air condenses on cool foundation walls (NRCan ↗). CMHC calls moisture “the most common problem in basements” (CMHC ↗).

Crawl spaces and slabs

NRCan treats a heated crawl space as a short basement: insulate its walls rather than the floor above, so pipes and ducts stay warm, and cover the earth floor with a moisture barrier. Slab-on-grade houses are insulated like the outside of a basement wall, with a sloped layer of foam in the ground on frost-susceptible soil (NRCan ↗).

How much insulation is enough?

For retrofits, NRCan recommends basement walls of RSI 3.3 (R-19) where a location has fewer than 3,000 heating degree-days (a measure of how cold the climate is), RSI 4.2 (R-24) from 3,000 to 4,999, and RSI 5.3 (R-30) at 5,000 and above (NRCan ↗). RSI is the metric R-value; multiply by 5.678 to get R. Toronto has about 3,520 degree-days, Ottawa 4,440 and Edmonton 5,120 (National Building Code ↗).

New homes without a heat recovery ventilator must meet minimum effective values, which count the framing, of RSI 1.99 to 3.97 (about R-11 to R-23) on foundation walls, and rim joists must match the above-ground wall requirement (National Building Code ↗).

Signs of trouble below grade

• Staining or mould, blistering or peeling paint, efflorescence (a white, chalky mineral deposit), crumbling concrete or a musty smell (NRCan ↗).
• Condensation on foundation walls in summer (NRCan ↗), or on windows and pipes (CMHC ↗).
• Lifting floor tiles, damp carpet, rust at the base of steel posts, or decaying wood where it touches concrete (CMHC ↗).
• Drafts, dust or pests getting in along the top of the foundation wall, at the rim joist (NRCan ↗).
• Cracks that grow or shrink; NRCan advises professional help to check whether structural repairs are needed (NRCan ↗).

NRCan's simple moisture test: tape a 1 m (40 in.) square of plastic to the wall and floor for a day or two. Moisture on top of the plastic points to high indoor humidity. Moisture underneath means water is moving in from the ground, and possibly radon with it (NRCan ↗).

What an energy advisor checks

In an EnerGuide evaluation, an NRCan-registered energy advisor records the insulation levels of your walls, ceilings and basement and measures airtightness with a blower door (NRCan ↗). In a basement, the most useful checks are:

• A blower door test, which depressurizes the house so leaks at the rim joist, windows and penetrations can be traced (NRCan ↗).
• Thermal imaging under blower door pressure, which NRCan calls “a powerful tool to locate air leaks and missing insulation” (NRCan ↗).
• Access to crawl spaces and mechanical equipment; NRCan asks homeowners to clear a path to both before the visit (NRCan ↗).
• The foundation type (poured concrete, block, brick, rubble or stone, or preserved wood), which shapes whether insulation can go inside, outside or both (NRCan ↗).

An energy evaluation is not a home inspection (NRCan ↗). Water, cracks and soil gas may also call for a basement specialist and a long-term radon test.

Fixes, from simplest to biggest

Inside or outside?

NRCan says outside is best, but inside is often the practical choice. Outside work lets you repair cracks and leaks, keeps the foundation warm and suits rubble, brick and leaky foundations, but it cannot be done in winter, and steps, driveways and landscaping can get in the way. Inside work can be done in stages, often by the homeowner, but only on a dry wall: it makes the concrete colder and can hide moisture problems (NRCan ↗).

Mistakes to avoid

• Insulating a damp basement from inside. NRCan says to correct leaks, dampness and efflorescence first “or your new walls will rot” (NRCan ↗). If future flooding seems unavoidable, CMHC advises against interior insulation or finishes that water will damage (CMHC ↗).
• Plastic against the concrete, or two vapour barriers. Polyethylene on basement walls has caused condensation and mould inside walls. NRCan recommends house wrap behind a framed wall, and no polyethylene over a wall that already has foam board; use a “smart” vapour retarder or the airtight drywall approach instead (NRCan ↗).
• Stuffing fibreglass into gaps. Glass and mineral fibre do not stop air; NRCan says stuffing them into header gaps “is not effective” (NRCan ↗).
• Too much foam on cast-in joists. Where joists are embedded in the concrete, NRCan caps foam at 25 mm (1 in.) so the floor above stays warm (NRCan ↗).
• Heavy interior insulation on stone or rubble. NRCan limits these walls to RSI 2.1 (R-12) inside because of freeze–thaw risk (NRCan ↗).
• Rushing new concrete. Allow about a year of drying before insulating a new foundation (NRCan ↗).
• Exposed foam, insulation around flue pipes, a dryer venting into the crawl space, or tar paper indoors. NRCan flags each as a fire, safety or moisture hazard (NRCan ↗).

Radon and soil gas: test before you finish

Radon is a radioactive gas from the ground with no colour, odour or taste. NRCan notes it may be present in any home, and the only way to know is to test (NRCan ↗; NRCan ↗). Health Canada recommends a long-term test of at least 3 months (Health Canada ↗) and a certified radon mitigation professional if the result is above the Canadian guideline of 200 Bq/m³ (Health Canada ↗).

The most common fix uses a pipe and small fan to draw soil gas from under the floor to the outdoors, and mitigation cuts radon by more than 80% in most homes. Sealing entry routes alone has limited effect (Health Canada ↗).

New homes built to the 2020 code need an air barrier against the ground and a rough-in for a future radon system (National Building Code ↗). In an older basement, NRCan notes insulation can be tied in with waterproofing, radon remediation or finishing, so plan them together (NRCan ↗).

Rebates and next steps

Programs open and close often, so check our rebates by province for what is available now. In Ontario, start with Home Renovation Savings assessments. Some programs include a post-upgrade EnerGuide evaluation that needs proof of the work, so keep paid invoices and receipts (NRCan ↗).

To compare this work with other upgrades at your local fuel prices, try the upgrade payback calculator, then put the order of work into your home plan. You can also find a registered energy advisor near you.