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How to Insulate the Flat Roofs

Updated on November 9, 2012

If you are having a flat-roofed addition built, then insulating the roof is easily done at the building stage. Existing flat roofs pose more of a problem since it is not easy to get at the voids above the ceiling.

It is possible to add secondary roofing above the existing one and sandwich insulation be­tween the two. But this is a very expensive solution and not likely to be cost-effective for many years - if ever. Should the existing roof be in need of replacement, however, then this method might be a possibility.

Other methods should be tackled from inside the house. Taking down an existing ceiling is a fairly clean exercise if it is a modern sheetrock (drywall) type, but quite a dirty job if you have to knock down a lath and plaster construction.

Polystyrene slabs or insulation blanket are fitted between the roof joists and a layer of polythene stapled to the joists. The polythene prevents condensation forming in the roof void and damaging the insulation. It also serves as a vapor barrier to moisture created in the room below which, in the case of a kitchen or bathroom, could be considerable. You can then fix a new sheetrock ceiling to the joists.

You must ensure that the roof space remains well ventilated, otherwise the roof structure could become moist and start to rot. Allow for a 2in wide gap between the top of the insulation and the underside of the roof and drill ventila­tion holes in the soffits on both sides of the roof so that there is a continuous cross-flow of air.

An alternative method involves adding a sheetrock ceiling to the interior surface, while sandwiching insulation material in between the old and new ceilings.

First you need to nail a framework of spacers about 4in below the existing ceiling. Then add insulation blanket or polystyrene slabs between the two surfaces, followed by a polythene vapor barrier and a layer of sheetrock. Alternatively, you can nail sheets of insulating board directly to the ceiling. This board incorporates a vapor barrier and a layer of rigid foam insulation.

Another option is to fix a new tongued and grooved board ceiling and fill the cavity above with polystyrene slabs or insulation blanket. Since wood itself has excellent insulation prop­erties, this is a particularly effective method.

Finally, you can use a proprietary suspended ceiling comprising a framework of metal chan­nels supporting insulating tile panels.

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