FLAT ROOF INSULATION

Flat roof insulation is a layer of insulation built into the roof structure either below the roof deck or above. Flat roof is understood as a roof that has a pitch of lower than 10 degrees. Insulating a flat roof can result in less energy use, a reduction in condensation, and an overall decrease in related household bills. Flat roof insulation boards are often considered with the Celotex brand being widely popular in the UK. Its TD4000 PIR product is known as a 3 in 1 material designed to provide decking, vapour control layer and insulation for new and refurbishment flat roof applications. Other popular products are: Recticel Powerdeck® F, Kingspan Thermaroof TR31, FR/TP Xtratherm Thin-R Thermal Ply, FR/BGM Xtratherm Thin-R and EcoTherm Eco Deck.

TYPES OF FLAT ROOF INSULATION

There are two methods of installing insulation in a flat roof: warm roof and cold roof insulation. Selecting which one is right for you will largely depend on what type of building you are working with.

WARM ROOF INSULATION (INSULATION ON TOP OF THE EXISTING ROOF)

Warm roof insulation works by putting the insulation material on top of the roof. It has been proven by German specialists that in order to improve the insulating properties of a flat roof, insulation must be fitted on the exterior site. By doing so the dew point is simply moved away from the roof. This insulation method stops the development of damp and decay by allowing moisture to escape, and as a result, it is thought to be the best option for the UK climate. It turns out that PIR (polyisocyanurate) boards are a most popular choice when it comes to warm roof insulation. They are highly durable and can handle being walked on. It is even more important if you consider the fact that roofs need to be able to withstand human weight. In fact, warm roof insulation is the most commonly used in domestic properties being cost effective. A warm deck roof can be very simple to install and offers higher thermal performance at the cost of a higher roof profile. Warm roof insulation ensures the entire roof structure is insulated, meaning a warm deck roof is more energy efficient than a cold roof. The insulation for a warm roof will usually be boards of extruded polystyrene or rigid compressed boards of glass fibre or rock fibre. These can be supplied with rebated edges which interlock reducing the risk of uplift.

COLD ROOF INSULATION (INSULATION BETWEEN JOISTS BELOW ROOF)

Cold roof insulation is an opposite method to warm roof insulation. In this technique, insulation is laid between the rafters below the roof level and the roof rafters are uninsulated from top, meaning on a winter's day the cold will conduct through the rafters and into the room below. Insulating an existing cold flat roof requires ripping up any existing roof surface to enable you to lay the insulation in between.

FLAT ROOF INSULATION GRANTS

You can qualify for flat roof insulation grants through the government’s ECO4 scheme. It is the government’s fourth and likely final phase of the scheme to help qualifying households receive funding for the installation of more energy efficient home improvements such as insulation. It is due to come into effect later on in 2022, replacing the recently ended ECO3. One of the main aims of the ECO scheme is to help the government lower carbon emissions in domestic homes across the UK in order to achieve its legally binding goal of reaching net-zero emissions by 2050. ECO4 is expected to run from late summer 2022 to 2026. If a member of a household is claiming an income-related benefit and the combined annual income of the household is less than £31,000, they could be eligible for free insulation grants through ECO4. In homes that have an existing EPC band of F or G, the grants will need to improve the rating to at least band D. Whereas if a home has an existing band of D or E, the funding grants are required to improve the rating to a minimum of band C.

FLAT ROOF INSULATION COST

The cost of insulating a roof will depend on 2 factors. First is the size of your property roof, second is the thickness. As of 2022 the average cost of installing a cold roof with 170mm fibreglass insulation is around £4.64 per m2, while with a warm roof it is £73.27 (174mm Xtratherm Plydeck PIR with OSB). Please note that this is an average cost and not the actual cost. Once you have determined what kind of roof you have, you can choose what type of roof insulation material you would like to use. Here is the handy guide based on 100mm insulation:

WARM ROOF

Xtratherm Plydeck PIR with OSB - £48.02 per m2
PLY FR/TP Xtratherm Thin-R - £42.25 per m2
FR/BGM Xtratherm Thin-R PIR  - £32.58 per m2
FR/MG Thin-R Xtratherm - £31.96 per m2
Kingspan Thermaroof TR31 - £58.60 per m2
Celotex Insulation TD4000 PIR Plywood - £45.25 per m2
Recticel Plylok PIR -  £52.89 per m2

COLD ROOF

Knauf Earthwool Loft Insulation Roll 44 Combi Cut - £2.30 per m2
URSA 10 Diverso Loft Insulation Roll - £2.76 per m2
Foil Faced RS45 Rocksilk Knauf - £21.48 per m2
Rocksilk RS45 Knauf Insulation Building Slab - £11.46 per m2
RWA45 Rockwool Acoustic Insulation Slab - £10.51 per m2
EPS70 Floor Polystyrene Insulation Jablite Jabfloor - £10.25 per m2
Celotex GA4000 PIR Insulation Board - £18.55 per m2

FLAT ROOF INSULATION RETROFIT

It is important to design the roof in collaboration with the manufacturers of the construction components to ensure compatibility, correct sizing and fixing to avoid premature failure of the roofing system. Adding insulation to an existing flat roof generally requires the removal and replacement of some of the existing structure. Depending on the condition of the existing roof, various retrofit options are available, including:

RENEWAL
It is appropriate where the existing deck and roofing system has failed. The deck and waterproof membrane is stripped and replaced new decking and roofing system.

OVERLAY
It is appropriate where the existing deck is sound and where the waterproof membrane is aged. Either a new membrane is placed over the existing complete with insulation and ballast (inverted warm) or the existing membrane is prepared as a vapour check and insulation is added complete with a waterproof membrane over (warm). Your roof can be insulated by installing several layers of rigid foam on top of the roof sheathing.

FLAT ROOF INSULATION BUILDING REGULATIONS

Whether your new flat roof is to be made of timber, concrete or felt you need to comply with flat roof building regulations. You will have to obtain a planning permission if you are making any structural changes to the roof. If the new roof covering is going to be different than the old one and this will affect the roof in the event of a fire and if you are replacing more than 25% of the roofing area, then you will also need to replace the thermal insulation. According to current Building Regulations to comply with Part L of the Building Regulations 2010, all roofs to rooms should be thermally insulated to a maximum U-value of 0.18 W/m²K. A lower U-value indicates better insulation properties, hence U-value requirements usually specify a maximum value. 

CONSTRUCTION ROOFING MATERIALS

You need to make sure that the roofing materials are the correct size so that the roof can support an average man’s weight. The insulation material for a warm flat roof should be able to cope with extremes of temperature and UV light without deterioration in thermal performance, be able to resist pressure from any anticipated foot traffic, be durable and suitable for external use and be capable of withstanding loads imposed on it during construction and future maintenance. An inverted warm roof must also be capable of receiving the extra load of the ballast.

GUTTERING

Under Part H of the Building Regulations, drainpipes and corresponding guttering must be fitted on at least two sides of the flat roof to ensure the flowing water doesn’t accumulate at the base of yours and neighbouring buildings. Water should drain to one or two edges. Waterproofing should extend up the adjacent walls at least 150mm from the roof surface. Depending on where you live in the UK, snowfall must also sometimes be considered in determining the gradient of the fall.

JOISTS

Flat roofs must be designed and built to be strong, durable and wind resistant.

WATERPROOFING
 
Warm roofs must have a VCL bonded to the deck. This can be made of polyethylene membrane, bitumen sheet, polymeric foil. Waterproofing should extend up the adjacent walls at least 150mm from the roof surface. Cold roofs must have ventilation.

If you decided to replace and insulate your roof, you will need to contact your Local Authority to issue firstly an Open Building Notice, that allows you to get started with your roofing projec and secondly a Certificate of Compliance on completion of the project, to prove to potential buyers that your flat roof meets all construction and energy efficiency guidelines.

Related articles:

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How to choose the right Kingspan insulation board for construction?

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2 Insulated plydeck compared

Cheaper alternative/equivalent to Kingspan insulation

Video credit:https://www.youtube.com/watch?v=RKssKIHs-io&t=14s