Insulating existing walls from the outside
An exterior wall insulation retrofit adds a continuous layer of insulation outside the existing wall, under new cladding. It reduces heat loss and thermal bridging without taking space from the rooms, but it is expensive and disruptive, so it makes the most sense when the siding is being replaced anyway.
What is an exterior wall insulation retrofit?
In a typical retrofit the old siding is removed, the wall is air sealed and made weather-tight, and one or more layers of rigid insulation, such as foam board or mineral wool, are fixed over the sheathing. A drainage layer and new cladding go on top. Windows may be moved outward so that they sit in line with the new wall. Because the insulation is continuous, it covers the studs that otherwise let heat through, a problem explained in thermal bridging.
Why choose outside rather than inside or in the cavity?
There are three ways to add insulation to a built wall, and each has a place.
- In the cavity. Dense-packed fibre or foam fills the existing stud space from outside or inside. It is cheaper and less disruptive, but the studs still bridge the insulation.
- On the inside. It takes floor area, disturbs finishes and moves the dew point, so it needs careful detailing.
- On the outside. It keeps the structure warmer and drier, improves airtightness when done well and leaves the interior alone. It costs the most.
Fibre options are compared in cellulose versus fibreglass, and foam in spray foam insulation.
When does it make sense?
The strongest case is a house that needs new siding or has damaged cladding, because the labour for removal and re-cladding is already in the budget. It also fits deep retrofits aimed at a net zero ready level, such as those described in the net zero home guide, where a very low heat load lets a small heat pump or solar system do the work. For a typical house with an uninsulated attic and a leaky envelope, it is not the first step. The insulation upgrade priorities guide explains why attics and air sealing come earlier.
What are the risks and design details?
Walls must keep drying. The design needs a weather-resistive barrier, correct flashing at windows and doors and enough exterior insulation so that the sheathing stays warm in winter. Too little exterior insulation with a vapour-closed interior can leave the sheathing cold and damp. Thick layers also change the details around eaves, roof overhangs, window sills, meters and vents, and the cladding needs fasteners that carry its weight through the foam to the framing.
Fire rules for foam, rules about property lines and the need for a permit vary by municipality, and some heritage districts restrict changes to the facade. Ask the building department early. A designer or building scientist familiar with retrofits should specify the layers for your climate zone, which the table in R-values by region helps you discuss.
What does it involve, and what will it cost?
Expect scaffolding, new window flashings, extensions to the roof edge and repair of whatever the old siding was hiding. The job takes weeks, not days. Costs depend on wall area, thickness, cladding, window work and region, and quotes decide, so ask contractors for line items and a scope that includes air sealing and flashing details. Ask who is responsible if moisture problems appear later. The overall benefit shows up in comfort and lower peak heat demand, and you can track it with an EnerGuide rating done before and after. Program support for deep retrofits changes over time, so check the Government of Canada energy-efficiency pages, your province and your utility.
Quick answers
Is exterior insulation better than interior insulation?
It keeps the wall structure warmer and avoids using floor space, so it is often the better technical choice. It also costs more and is best timed with re-cladding.
Can I do this one wall at a time?
In principle yes, but the details at corners, roof edges and windows get harder. Planning the whole house first avoids inconsistent results.
Does exterior wall insulation need a permit?
It often does, as it changes cladding, fire performance and sometimes the building footprint. Ask the municipal building department.