Walls: finding and filling the gaps in older houses

How heat escapes through walls

Heat moves through a wall by conduction through solid materials, by convection as air circulates in open spaces, and by radiation. In an uninsulated cavity, air picks up heat on the warm side and gives it up on the cold side (NRCan ↗). Insulation stops that by trapping still air, but only if it fills the space completely and evenly (NRCan ↗).

Framing leaks heat too. A stud is a thermal bridge, a solid path from the warm side to the cold side. NRCan's example: R-20 (RSI 3.52) batts in a 2×6 wall perform like R-17 (RSI 2.99) once the studs and plates are counted, and foam board over the framing acts as a roadblock to that heat flow (NRCan ↗).

Know your wall

• Frame walls have a cavity to fill. In older houses it is often only 89 to 102 mm (3½ to 4 in.) deep, which limits how much can be added (NRCan ↗).
• Balloon framing was common up to the early part of the 20th century. Its studs run past the floors to the roof framing, so wall cavities connect floor to floor unless fire blocks were added (CMHC ↗). CMHC lists balloon-frame walls among the hidden air paths into the attic (CMHC ↗), and NRCan notes these open cavities can be the easiest to fill from the attic or basement (NRCan ↗).
• Brick veneer has a 25 to 50 mm (1 to 2 in.) air space behind the brick for drainage. Do not fill it (NRCan ↗).
• Solid walls of brick, stone, concrete block, log or plank have no cavity, so insulation must go inside or outside. Many double-brick walls have a gap under 25 mm that drains water; never insulate it or plug its drain holes (NRCan ↗).
• Concrete block cores can be filled, but the webs and mortar keep conducting heat, so sealing air paths and capping the top course matter more (NRCan ↗).

How much is enough?

For retrofits, NRCan recommends walls of RSI 3.9 (R-22) where a location has fewer than 3,000 heating degree-days, RSI 4.8 (R-27) from 3,000 to 4,999, and RSI 7.1 (R-40) at 5,000 and above (NRCan ↗). Heating degree-days measure how cold a climate is: Toronto has about 3,520 and Winnipeg 5,670 (National Building Code ↗). New homes without a heat recovery ventilator need effective wall values of RSI 2.78 to 3.85 (about R-16 to R-22) (National Building Code ↗).

Signs your walls are under-insulated

• Drafts at outlets and switches on exterior walls, and even on interior walls, since air can travel through partitions and along floor joists (NRCan ↗).
• Staining or mould on exterior walls and ceilings, often in closets and bathrooms (NRCan ↗).
• Staining, rot, cracked interior or exterior finishes, or windows and doors that stick because they are out of square. NRCan says to fix these moisture or structural problems before insulating (NRCan ↗).
• Little or nothing in the cavity. NRCan suggests looking behind an outlet on an exterior wall, with the power off, to see what is in the wall (NRCan ↗).

What an energy advisor checks

An EnerGuide evaluation records the insulation levels of walls, ceilings and basement and measures airtightness (NRCan ↗). For walls, the most useful checks, by an advisor or an insulation contractor, are:

• Thermal imaging with the house under blower door pressure, which NRCan calls “a powerful tool to locate air leaks and missing insulation” (NRCan ↗).
• What is already in the cavities, and what blocks them: fire stops, bracing, wiring, plumbing, windows and doors (NRCan ↗).
• The wall type (frame, balloon, veneer or solid), which decides whether drill-and-fill is possible at all (NRCan ↗).

An advisor is not a home inspector (NRCan ↗). If the house still has knob-and-tube wiring, have an electrician assess it before any insulation goes in: NRCan warns that insulation must never contact old knob-and-tube wiring because of the fire hazard (NRCan ↗).

Fixes, from simplest to biggest

Dense-packing suits empty or partly filled frame cavities. Because older stud spaces are shallow, the gains are limited, but NRCan says good results can still be had (NRCan ↗).

Mistakes to avoid

• Filling the wrong space. The air gap behind brick veneer and the narrow drainage gap in double-brick walls must stay open, along with their weep holes (NRCan ↗).
• Blowing insulation over knob-and-tube wiring, a fire hazard (NRCan ↗).
• Wrong density. Too little leaves voids and settling gaps; too much can make walls bulge. Agree with the installer on the manufacturer's density and the number of bags (NRCan ↗).
• Leaving holes open. Have exterior holes patched section by section so a storm cannot soak the wall (NRCan ↗).
• Counting on insulated siding. It adds only RSI 0.35 to 0.70 (R-2 to R-4) and still needs a drainage plane (NRCan ↗).
• Putting the vapour barrier in the wrong place. At least two-thirds of the wall's insulating value should sit on the cold side of the vapour barrier (NRCan ↗).
• Believing miracle claims. NRCan says long-term claims above RSI 1.14 per 25 mm (R-6.5 per inch) may not be substantiated, and reflective bubble foils range from RSI 0 to 0.62 (R-0 to R-3.5) (NRCan ↗).
• Retrofitting solid masonry without advice. NRCan suggests consulting an architect, building conservator or engineer for older and heritage houses (NRCan ↗), and flags freeze–thaw risk when old stone and rubble foundation walls are heavily insulated from inside (NRCan ↗). We found no official Canadian guidance that sets a safe limit for insulating above-grade solid masonry from inside, so get that advice before you start.

Rebates and next steps

NRCan notes walls can be insulated as part of a major repair or renovation, and that exterior insulation can be combined with re-siding (NRCan ↗).

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.