Roof Insulation, Ventilation, and Moisture Control
This domain tests the thermal and moisture side of roofing: choosing insulation, placing the vapour retarder, sloping the roof with tapered insulation, and calculating attic ventilation so the assembly dries rather than accumulates moisture.
Insulation
Common roof insulations are polyisocyanurate, EPS, XPS and mineral wool, each with its own R-value per inch and its own tolerance for moisture. Tapered insulation creates slope to drains on a flat structural deck. Install two layers with offset joints where specified, use a cover board to protect the insulation and spread point loads, and never compress insulation, because compressed thickness means lost R-value. Fastener length must account for the full insulation thickness plus penetration into the deck.
Vapour and air control
The vapour retarder goes on the warm side of the insulation and must be continuous, sealed at laps and penetrations. Two vapour retarders in one assembly trap moisture between them. Air leakage carries far more moisture than diffusion, so air sealing matters more than the retarder rating.
Ventilation
Steep-slope roofs are vented at a ratio of about 1/150 of the attic floor area, relaxed to 1/300 when intake and exhaust are balanced and a vapour retarder is present. Intake is low at the soffit, exhaust is high at the ridge, and baffles keep insulation out of the soffit path. Compact low-slope roofs are generally not vented; they rely on the vapour retarder and insulation placement.
Worked example
An attic floor is 150 m². At 1/150, the net free area is 150 ÷ 150 = 1.0 m², split 50/50 into 0.5 m² intake and 0.5 m² exhaust. If each soffit vent provides 0.02 m² net free area, intake needs 0.5 ÷ 0.02 = 25 vents. Note that vent area means net free area from the manufacturer's table, not the rough opening.
Traps that cost marks
Counting the rough opening as net free area. Blocking soffit vents with insulation batts, which turns the intake into zero. Compressing insulation into a tight cavity. Double vapour retarders. Venting a compact low-slope roof. Leaving a ridge vent with no matching intake.