Cold Roof

A cold roof is a roof construction in which the thermal insulation is placed at ceiling level, typically between and over the ceiling joists, so the roof space above stays at roughly outdoor temperature. The term is widely used in the UK and Europe; in North America the same arrangement is usually described simply as a vented attic.

How it handles moisture

Warm, moist air from the house can leak or diffuse into the cold roof space. If it meets cold surfaces such as the underside of the roof covering, underlay or sheathing, it can condense, wetting timber and insulation. Cold roofs control this mainly by:

• ventilating the loft or attic with outside air, through openings at the eaves and often at the ridge;
• reducing air leakage from the house through the ceiling;
• in some designs, using a vapour control layer at ceiling level.

In the UK, BS 5250 is the code of practice for managing moisture in buildings and sets out ventilation provisions for cold roofs, which depend on whether the underlay is high-resistance (HR) or low-resistance (LR). In the US, IRC section R806 sets minimum net free vent area for vented attics, generally 1/150 of the attic floor area, reducible to 1/300 under conditions such as balanced high and low ventilation.

Insulation at the eaves

A clear path must be kept for air to flow from eaves vents into the roof space. Baffles or eaves trays prevent insulation from blocking the vents; the IRC requires at least 1 in (25 mm) of space between insulation and the roof sheathing at the vent.

Cold flat roofs

The term is also used for flat roofs with insulation between joists and a ventilated void above. Many authorities now discourage cold flat roofs because the void is hard to ventilate reliably, preferring warm roof designs.