Warm Roof Air Gap

Between the insulation and the ceiling finish.

Warm roof air gap.

Flat roof ventilation allows for the circulation of air in the property particularly for warmer rooms in residential buildings. Heat flow through air gaps associated with floors ceilings or roof elements will have r values that are greater downwards than upwards. Generally vertical air gaps in walls have the same heat flow inwards and outwards. The map at right which is based on table r806 4 of the irc lists the minimum r values required to prevent condensation in unvented assemblies in various climate zones.

Hot air exhaust vents located at the peak of the roof allow hot air to escape. Unvented roof assemblies should be insulated either with air impermeable insulation rigid foam or spray polyurethane foam or with a combination of foam and air permeable insulation. A cold roof can be cheaper as ridged warm roof insulation tends to be more expensive a warm roof is sometimes the only option when it is difficult to get the required cross ventilation rigid warm roof insulation can allow for smaller timber depths as the 50mm air gap does not have to be maintained. If i had to choose between a 1 2 inch gap and a 1 inch gap i would be inclined to go with the 1 inch gap if i were building in climate zone 5 or anywhere colder.

The colder the climate the more essential the air gap. There are two types of flat roof systems warm and cold roofs which affect the positioning of the ventilation. The warm side of the insulation i e. The roof should also have ventilation openings of at least equal to a 25mm continuous gap running the full length of the eaves on two opposite sides.

Insulating along the sloping rafters forms a warm roof and is an option for both new builds and refurbishment projects. If you don t want to use sips or nailbase read this article tolearn the four basic approaches to insulating unvented roof assemblies. In the case of sips it s essential in my view to have an air channel between the roofing underlayment and the metal roofing. Taking advantage of this natural process referred to as passive ventilation is the most common way to vent an attic.

Therefore this element alone can account for as much as 25 of a buildings heat loss. An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it.