Flat roof insulation

Description
Layer of insulating material above, or built into, a flat roof achieving a U-value of 0.25 W/m²K (Building Regulations).

Unit cost of installation
Data not available.

Bulk cost of installation
Data not available.

Indicative cost and carbon saving

The figures quoted are for a whole house. However, it is unusual for dwellings (other than flats and high rise blocks) to have a flat roof. More usually part of the roof (for example an extension) will be flat with the majority being pitched. In these circumstances the savings attributed to the area of flat roof will be proportionately lower.

Maintenance
None.

Lifetime
Lifetime of the waterproof membrane (for warm roof construction - see below).

Technical suitability
A number of constructions are possible.

Warm deck flat roof: Insulation is installed on a deck above the joists and covered by the waterproof membrane.

Inverted flat roof: Insulation is placed above the waterproof membrane and retained in place by heavy ballast or paving slabs.

Cold deck flat roof: Insulation is placed between the joists that support the deck and membrane. A ventilated void is required between the insulation and the deck that supports the waterproof membrane.

Insulation to the ceiling internally: Insulation-backed plasterboard is fixed internally in a similar way to internal wall insulation.

Warm deck and inverted flat roof constructions can be used in most cases where increased thickness is not a problem, but checks need to be done to ensure that the roof joists are able to take the extra weight of any ballast materials. Upstands, together with any stepped DPC or cavity trays, also need to be checked and raised where necessary.

Inverted construction is a less robust solution compared to warm deck, but has the advantage of being applicable without removing the existing waterproof membrane, which may have a number of year's life remaining. However, there is a risk of water percolating through the insulation, cooling the deck sufficiently to cause condensation to occur beneath it. BS5250 should be consulted.

Warm deck should be considered where the waterproof membrane is being replaced. If the existing roof is a cold vented roof, the existing insulation between the joist should be removed, and all perimeter walls built up to the underside of the new insulated deck to avoid cold bridging. Where the roof does not have a significant fall the addition of firring pieces or the use of tapered insulation should be considered to avoid ponding of rainwater. (Ponding of water can have a significant impact on roof loading, and will reduce the life expectancy of the membrane). Warm deck construction (including a new waterproof membrane) can also be applied over the existing waterproof membrane of an uninsulated roof, which may be preferable to creating an inverted roof.

Cold deck construction is not generally recommended because of the difficulty of ensuring effective ventilation of the void between the top of the insulation and the underside of the deck. In Scotland Section 3 - Environment (clause 3.10.7) of the building standards specifically warns against the use of this form of construction (although the regulations there do not prevent its use).

Non-technical constraints

Potential concerns with fuel poor
No specific issues.

Ancillary benefits

Trigger points for work
Repairs to roof, replacement of waterproof membrane. (N.B. Building Regulations Approval is required in England & Wales whenever any thermal element is renovated.)

Performance Limitations

Technical Guidance Available
» CE 101/GPG 171 'Domestic energy efficiency primer'
» CE 83/GPG 155 'Energy-efficient refurbishment of existing housing'
» The Flat Roofing Alliance: www.fra.org.uk
» Single Ply Roofing Association www.spra.co.uk
» BS5250:2002 Code of Practice for control of condensation in buildings.
» BS8217: 1994 Code of Practice for Built-up felt roofing
» BS6229:2003 Flat roofs with continuously supported coverings - Code of practice