Why and how to insulate a roof from the outside?

There is nothing like insulating the roof to improve the energy efficiency of a home! Conventionally, the insulation of a pitched roof is installed from the inside. But insulation from the outside or ‘sarking’ has its advantages.

External roof insulation: the principle

With interior insulation, the entire space between the rafters is filled with insulating material and then covered with a plasterboard or wood panel finish that is then painted or wallpapered.

As its name suggests, external insulation is placed outside the house:

1. The roof trusses and purlins are exposed (by removing tiles or slates, battens, and counter-battens).
2. A kind of sloping base is formed by attaching wooden panels or boards to them.
3. A damp-proof membrane is attached to this base.
4. The insulation, composed of rigid panels (e.g. extruded polystyrene, polyurethane, wood fibres, etc.) is applied.
5. A new set of battens and counter-battens are nailed to a rainscreen membrane on which the tiles or slates are laid.

Why should I choose insulation from the outside?

Even though it is more expensive, there are several reasons why insulation from the outside may be preferred to insulation from the inside:

For practical reasons

• the work is fast, with little dust and damage, and the house remains habitable.
• a maximum amount of living space is maintained inside the dwelling

For energy efficiency reasons

• the thickness of insulation can be at the maximum, not limited by the thickness available between the rafters as is the case in the interior technique.
• the existing roof trusses and purlins are now entirely included in the protected volume and therefore protected from strong variations in temperature and humidity.
• thermal bridges at the rafters are non-existent, contrary to the case of insulation from the inside.

For aesthetic reasons

• to highlight beautiful old roof timbers if you have a house with character.
• the interior finish, if it existed, is intact; and if it did not exist, it can easily be done by painting the inside of the wooden roof panels.

What are the disadvantages of external insulation?

• the price of the insulation material used for sarking is more expensive than traditional materials, but this difference can be compensated for by ease of installation. On second thought…
• the work can only be carried out in dry weather and, in any case, never when it freezes, in order to protect the workers’ safety,
• rigid insulating panels are less efficient in terms of sound insulation than glass wool or rock wool materials,
• you need to hire an experienced company. Achieving an air seal and removing the thermal bridge at the bottom of the slope, in particular, is complicated. Ask for references from the companies from which you are requesting a quote and consult the CSEC’s advice to fully understand the proposals you will receive.