How To Insulate A Flat Roof Cold Deck

Refurbishing an existing timber joisted flat roof which has no insulation.

How to insulate a flat roof cold deck.

In warm climates zones 1 through 4 low density open cell spray foam insulation is the typical choice since it is vapor permeable and will allow drying of the exterior sheathing and roof framing if they happen to get wet via diffusion into the interior of the building. Many properties use this option as there are no additional costs for scaffolding or organising how to gain safe access to the roof exterior. The insulation material is placed between the supporting joists leaving a gap of 50 60mm between the roof and insulation. The insulation is below the deck but above the plasterboard.

Quinn therm qrfr ffr insulation is a foil faced insulation board which helps further improve the thermal performance to help achieve the desired u value. There are a number of quinn therm roof insulation boards which will suit a warm deck flat roof construction built with a concrete deck each with different facings. I explain problems with insulating a roof using the warm and cold roof construction methods. The typical u value of an existing uninsulated flat roof is 1 5w m2k.

Whatever new flat roof covering you choose epdm rubber kemper. On a warm flat roof the layer is installed underneath the insulation to stop moisture interacting with it through rising thermal pressure lower down in the building. For cold flat roofs the vapour layer needs to be positioned underneath the insulation layer on top of the room below to create an effective barrier. Where insulation depth is restricted by the flat roof construction it may be necessary to install the roof insulation between and below the joists to create a cold deck structure.

The target should be to achieve a u value of 0 25w m2k or better. To achieve this add 100 to 160mm of insulation above the structural deck dependant upon insulant conductivity.