How To Insulate A Flat Roof Externally

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.

How to insulate a flat roof externally.

Like other roofs insulating a flat one requires ample ventilation a vapor barrier and sealed bypasses. The typical u value of an existing uninsulated flat roof is 1 5w m2k. It should be of the maximum available thickness. Then install r 30 fiberglass batts between the rafters.

With a warm roof the usual process is to split the insulation into two layers. It also lets hot air escape in the summer. 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. Proper ventilation in flat roofs.

You would only do this however if replacing the roof covering on top and you have the height above the roof to do this. 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. 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. Insulation material is placed on top of the roof rather than in between the ceiling and the decking.

When a homeowner really needs to cut down on costs some contractors may suggest the following. To achieve this add 100 to 160mm of insulation above the structural deck dependant upon insulant conductivity. Insulation is placed in between the joists underneath the roof surface above. Ventilation is necessary to allow moisture accumulated inside to escape.

The material that works best depends largely upon preference in a given situation. Add an additional layer of osb over the sheathing.