How To Insulate A Flat Roof Externally

Like other roofs insulating a flat one requires ample ventilation a vapor barrier and sealed bypasses.

How to insulate a flat roof externally.

The correct method for a flat roof is under the current regulations to have a warm roof. Add an additional layer of osb over the sheathing. If 180mm is to be installed then 100mm board might be introduced between the rafters there has to be a minimum 25mm air gap between the insulation and the underside of the tiles or slates with an 80mm board running across the rafters. The typical u value of an existing uninsulated flat roof is 1 5w m2k.

Proper ventilation in flat roofs. With a warm roof the usual process is to split the insulation into two layers. It also lets hot air escape in the summer. Insulation is placed in between the joists underneath the roof surface above.

In a nutshell you would sandwich 130mm of solid insulation between two decks allowing to place a vapour barrier on top of the first deck. 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. Ventilation is necessary to allow moisture accumulated inside to escape.

The material that works best depends largely upon preference in a given situation. You would only do this however if replacing the roof covering on top and you have the height above the roof to do this. Flat concrete roofs are generally not ventilated since the ceiling internally is generally a plaster and skim coat directly to the underside of the cold concrete. Creating a void between the old ceiling and new is better but ideally you need to eventually consider insulating the top of the concrete roof with solid insulation and a better covering grp or epdm.

It should be of the maximum available thickness. When a homeowner really needs to cut down on costs some contractors may suggest the following.