Insulating Existing Walls in Ontario: Options, Costs and Moisture Risks

Insulating existing walls in Ontario usually means one of three approaches: blowing dense-pack insulation into closed wall cavities, insulating from the inside during a renovation, or adding continuous insulation on the outside when you replace siding. In homes with empty or partly filled walls, which is common in houses built before about 1960, wall insulation can reduce heating energy by roughly 10 to 20% (estimate), but it is rarely the first or cheapest upgrade.

This guide explains how to tell what is in your walls, compares the main methods and their typical costs, and covers the moisture and safety issues that matter in Ontario's climate.

Should you insulate walls before the attic or basement?

Usually not. For most Ontario homes the order of priority is:

Air sealing, especially the attic floor and basement rim joists
Attic insulation to R-50 to R-60
Basement walls and rim joists to about R-20 or more
Walls, particularly if they are empty

Attics and basements are cheaper per unit of heat saved because access is easier. Walls make sense earlier when they are completely empty, when you are already opening them up, or when you are re-siding. Our guides to attic insulation and air sealing cover those first steps.

How do I know what is in my walls?

Remove an outlet cover on an exterior wall (turn off the breaker first) and look beside the box with a flashlight. You may see fibreglass, loose fill, shavings or nothing.
Look in the attic at the top of the wall plates, where you can sometimes see down into cavities.
Use an infrared camera on a cold day. Uninsulated cavities show up as warm stripes from outside or cold stripes from inside.
Get a home energy assessment. A Registered Energy Advisor will check walls, measure air leakage and model savings. See our guide to home energy assessments.

Caution: If you find knob-and-tube wiring, do not insulate around it until a licensed electrician has assessed it. Many insurers and the Electrical Safety Authority treat insulation around knob-and-tube as a concern because the wiring was designed to release heat into open air. Learn more at esasafe.com (opens in a new tab).

What are the options for insulating existing walls?

Method	How it works	Typical added R-value	Rough cost (estimate)	Best for
Dense-pack cellulose	Holes drilled from outside or inside; cellulose blown in at high density	R-12 to R-14 in a 2x4 wall	$3 to $6 per ft² of wall	Empty wood-frame cavities
Injection foam	Low-expansion foam injected through small holes	R-12 to R-16 in a 2x4 wall	$5 to $9 per ft² of wall	Closed cavities where cellulose is hard to install
Interior retrofit	Remove drywall or plaster, insulate cavities, add vapour control and new drywall	R-14 to R-24 depending on framing	$8 to $15+ per ft² including finishing	Gut renovations
Exterior insulation	Rigid boards or mineral wool added over sheathing before new siding	R-5 to R-15 continuous	$10 to $25+ per ft² including new siding	Re-siding projects

Costs vary by region, access, number of storeys and finishing. Get at least two written quotes that state the material, installed density or thickness and the patching method.

Dense-pack cellulose

This is the most common approach for empty frame walls. Installers drill holes of about 2 to 3 cm (1 in) in each stud bay, usually from outside after removing a row of siding, and fill each cavity at high density so it does not settle. Done well, it also reduces air leakage noticeably.

Ask how the installer will handle fire blocks, horizontal blocking and window headers, since each creates a separate cavity that needs its own hole.

Interior retrofits

If you are renovating a room down to the studs, insulate the walls properly at the same time. In Ontario this generally means filling the cavity, installing a vapour retarder on the warm side and sealing it carefully at outlets and joints, then new drywall. Building permits may be required for this work; check with your municipality.

Exterior insulation

When siding is being replaced, adding rigid mineral wool, EPS or polyiso boards over the sheathing creates a continuous layer that covers the studs, which are a weak point in any frame wall. This is the best-performing retrofit but also the most expensive. Window and door openings need extension jambs and careful flashing, and the wall needs a drainage gap behind the new cladding.

What about brick and masonry walls?

Many older Ontario homes, especially in Toronto, Hamilton, Kingston and Ottawa, have solid masonry walls: two or three layers of brick with no cavity. Others have brick veneer over a wood frame.

Brick veneer over wood frame: The frame cavity can usually be insulated with dense-pack from inside or through mortar joints, by an experienced installer.
Solid masonry: Insulating from inside makes the brick colder in winter, so it dries more slowly. That can increase freeze-thaw damage (spalling) and moisture problems at embedded wood such as joist ends. This does not mean it can never be done, but it needs an assessment of brick condition, drainage, gutters and exposure, and usually a vapour-open approach.

If you are unsure what kind of wall you have, look at the brick pattern. Rows of bricks laid end-out every few courses usually indicate solid masonry.

How much will wall insulation save?

Savings depend heavily on what is there now and how you heat. Here is an illustrative example for a 1950s two-storey home near Toronto with empty walls, about 140 m² (1,500 ft²) of exterior wall, and gas heat.

Item	Estimate
Annual gas use before	2,400 m³
Heating share	About 2,000 m³
Saving from dense-packing empty walls (10 to 20%)	200 to 400 m³ per year
Value at 45¢/m³	About $90 to $180 per year
Installed cost (dense-pack, 1,500 ft² of wall)	About $4,500 to $9,000

With natural gas at current prices, the simple payback is long, often 30 years or more. The numbers improve considerably with electric baseboard, oil or propane heat, where each unit of heat costs more, and in colder places like Sudbury or Thunder Bay. The comfort gain from warmer wall surfaces and fewer drafts is real and often the main reason people do it.

Insulation may qualify under the assessment stream of the Home Renovation Savings Program, which requires pre- and post-retrofit assessments and at least two upgrades. Amounts and deadlines change; confirm on saveonenergy.ca (opens in a new tab) and see our rebate finder.

Moisture and safety checklist

Fix roof leaks, gutters and downspouts before insulating
Check for knob-and-tube wiring and have an electrician assess it
Look for rot or water staining at window sills and wall bottoms
Seal penetrations in the attic first so warm, moist air does not flow up through newly filled walls
Make sure bathroom and kitchen fans vent outside
For masonry walls, get an assessment of the brick and mortar first
Ask how holes will be patched and painted to match

Frequently asked questions

Can I insulate walls without removing drywall or siding?

Yes. Dense-pack cellulose and injection foam can be installed through small holes drilled from either side. Exterior drilling usually means removing and reinstalling one course of siding, which leaves no visible patches if done carefully.

Does wall insulation need a vapour barrier?

In new interior retrofits in Ontario, a warm-side vapour retarder is generally required. In dense-pack retrofits of closed walls, the existing paint and plaster usually act as the vapour retarder, and air sealing matters more. Follow the installer's and building department's guidance.

Will insulating walls make my house too airtight?

Dense-pack reduces air leakage, but most older homes remain leaky enough for natural ventilation. If your home becomes much tighter after several upgrades, a balanced ventilation system may be recommended.

Is spray foam better than cellulose for walls?

Spray foam is effective in open walls during renovations, but it costs more and cannot be sprayed into closed cavities. Low-expansion injection foam is the foam option for closed walls.