The insulation of a roof, whether flat or pitched, is crucial on many levels: not only does it keep the house cool in summer and warm in winter, but it also extends the life of a house, for example by preventing condensation issues and/or water infiltration.

Condensation issues can look like infiltration. Some buildings were built during a period when technical changes did not all evolve at the same pace, which caused some problems. It should be noted that when an attic is poorly ventilated or poorly insulated, moisture accumulates in the insulation and interferes with its insulating properties. Moisture freezes during very cold weather and thaws like in those old freezers we used to have, giving the impression that the roof is leaking.

In addition, if the insulation is deficient, the building structure itself can be damaged and costly work may be required, not to mention the health risks if mold develops as a result of water infiltration.

Specialists will be able to give you an accurate picture of the quality of your insulation and the cost of any work that may be required.

Insulating a flat roof

External roof insulation, and more specifically the insulation of a flat roof from the exterior—also known as ‘warm flat roof’ because the insulation is on the outside—is quite simple to achieve.

However, certain steps must be followed, and several factors must be taken into account: the structure of the building—wood or concrete—the thickness of insulation required, the type of material used, etc.

How do you insulate a flat roof from the outside?

Here are the steps to insulate an unvented flat roof that is insulated on the deck after tearing off the roof sheathing:

1. Strip the existing roof down to the wood deck and dispose of it. 
2. Inspect and repair the deck, if necessary. 
3. Provide and install a vapor barrier. 
4. Provide and install slope insulation rows if the roof has a bad slope. 
5. Install rigid insulation, either polystyrene or polyisocyanurate. 

For flat roofs with a ventilated attic, we must at least remove a good part of the wood deck in order to allow the insulation of blown wool or sprayed urethane, here are the steps in this case:

1. Strip the existing roof down to the wood deck and dispose of it. 
2. Remove the boards from the entire wood deck. 
3. Clean the attic and prepare for insulation. 
4. Lay 2 rows of 2x4s crossed on the entire deck with a slope of 1 to 2% towards the drain.

    

   If there is not enough air space or no air space at all, we raise the deck to create an air space and take this opportunity to optimize the slope towards the drain(s).

5. Install new 5/8” plywood over the entire area.