Between Deck Roof Insulation, What is between deck roof insulation

The between deck roof insulation speaks to what is between deck roof insulation and why between deck roof insulation.

It is the traditional location for low slope roof insulation, that is, it is between the deck and the roof membrane. Insulation in this position must be in the form of low density rigid boards or lightweight concrete in order to support the membrane. The insulation protects the deck from temperature extremes and is itself protected from the weather by the membrane.

However the roof membrane in this type of insulation remains exposed to extreme temperature variations. As well, any moisture that may accumulate in the insulation is trapped beneath the membrane, which can lead to decay of the insulation and roof deck, as well as blistering and eventual rupture of the membrane from vapor pressure.

When installing between the deck and the membrane in cold climates, two precautions may be considered, a vapour retarder installed below the insulation and ventilation within the insulation to allow the escape of any moisture that reaches there.

Ventilation is accomplished through the installation of topside vents, one per 1000 square feet that allow water vapour to escape upward through the membrane. Topside vents are most effective with a loose laid membrane. That is a membrane that is not adhered to the underlying surface which allows trapped moisture to easily work its way toward the vents from any part of the insulating later


With this type of roof the insulation is placed above the roof deck but below the weatherproofing. There should be no insulation below the deck and no ventilation is required.

Some forms of roof decking may also have insulation bonded to them.

The external wall insulation needs to be extended up to the underside of the roof insulation to prevent cold bridging.

All roofs should be insulated. The thickness of insulation will depend on the type of insulant used and whether you are constructing a cold or warm roof.

Insulation levels have increased dramatically.

When constructing flat roofs please consider the following, materials used must be suitable for use in a flat roof, roof needs to achieve an external fire rating, vapour barriers should be used, flat roofs should be anchored to walls to prevent uplift, flat roofs should be laid to falls and flat roofs require regular maintenance.

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