How To Insulate A Flat Roof Externally

When a homeowner really needs to cut down on costs some contractors may suggest the following.

How to insulate a flat roof externally.

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

The typical u value of an existing uninsulated flat roof is 1 5w m2k. 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. Ventilation is necessary to allow moisture accumulated inside to escape. The target should be to achieve a u value of 0 25w m2k or better.

Like other roofs insulating a flat one requires ample ventilation a vapor barrier and sealed bypasses. Then install r 30 fiberglass batts between the rafters. It should be installed on tapered 2x furring strips in order to slope the surface of the roof to drains. The material that works best depends largely upon preference in a given situation.

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. To achieve this add 100 to 160mm of insulation above the structural deck dependant upon insulant conductivity. Insulation material is placed on top of the roof rather than in between the ceiling and the decking.

The correct method for a flat roof is under the current regulations to have a warm roof. You would only do this however if replacing the roof covering on top and you have the height above the roof to do this.