What the roof gains
In winter, a well-insulated, well-sealed attic stays cold. Snow melts less from underneath, and less ice forms at the overhang. The deck stays drier. It is less likely to blacken and keeps its stiffness better.
Conversely, badly installed insulation blocks the soffits and holds moisture under the roof. Insulation and attic ventilation are planned together.
First, know what is already there
Some older houses have a granular, golden grey insulation in the attic: vermiculite. Vermiculite can contain asbestos. If you see some from the hatch, do not touch it and do not take a sample yourself. Have a professional take the sample and analyze it before anyone works there.
This glance only applies if the hatch opens without your having to climb, and you stay at the opening. If it is in the ceiling, out of reach without a stepladder, do not climb up: tell the contractor you do not know what the attic contains, and they will check before starting. Do not enter the attic.
Step one: an airtight ceiling
Loose-fill insulation slows heat. It does not block air. Warm air from the rooms passes through loose-fill insulation almost effortlessly, and its water vapour follows it up under the deck.
The contractor starts by sealing the openings in the ceiling.
- around wires and pipes
- electrical boxes
- the top of partition walls
- the hatch frame
This painstaking work is done before blowing: once the new insulation is in place, these openings become practically inaccessible.
Step two: loose-fill insulation
In an open attic, insulation is often blown in by machine. Two materials are common: cellulose, a treated recycled paper, and fibreglass. The first is denser and slows air a little better. The second weighs less and handles moisture better. Neither replaces a sealed ceiling.
Compared with batts, loose fill settles into corners and goes over the joists. Thermal bridges are reduced as a result.
Step three: keeping the bottom of the roof open
At the bottom of the slopes, there is little height between the ceiling and the deck. Yet the air from the soffits has to rise through that spot.
Blown in carelessly, the insulation flows into the soffits and blocks them. The attic then has no air intake, and the airflow stops, no matter how many vents the roof carries.
The solution is the baffle, a cardboard or plastic channel shaped like a trough. One is fastened under the deck in each space between two rafters. It holds the insulation below and keeps an air channel against the deck.
Coordinating insulation and re-roofing
Let us know if the insulation is to be redone: we can adapt the air intakes and exhausts. And when we open the deck at the bottom of the slopes during a re-roofing, the baffles are easier to install from above.
On the budget side, programs from the Quebec government and Hydro-Québec target insulation. Their conditions can be read on their official websites.
Frequently asked questions
Insulation or roofing: which comes first?
Both orders are possible, as long as the insulation contractor and the roofer talk to each other. When the re-roofing is near, it allows the baffles to be installed and the ventilation corrected. The blowing can come after, with no risk to the soffits.
Can an attic be over-insulated?
The thickness of the insulation is not a problem. The problem comes from insulation laid over a leaking ceiling, or insulation that closes the air intakes. In the first case, the attic becomes colder and the frost under the deck can increase.
Does better insulation make the ice at the roof edge disappear?
Not on its own. Insulation helps a lot when the ceiling is airtight and the attic well ventilated. The heat the roof lets escape melts the snow: the less escapes, the less ice forms at the overhang.