How do I insulate a flat roof?

You need to insulate a flat roof. But where? And how? The answer is especially important if the room below generates high levels of humidity like a bathroom.


Why insulate a flat roof?

warm deck flat roof insulation is now the standard way to insulate a flat roof

Warm deck flat roof insulation is now the standard way to insulate a flat roof – click to enlarge

Flat roofs, for example on extensions, bathrooms and dormer windows, are not the best of ideas. They lose loads of heat, and are prone to leaking, causing the materials to deteriorate. Unfortunately, too many dwellings suffer from this problem.

The best way to deal with this is to add insulation. But where? And how?

Firstly, it is always more economical to insulate a flat roof when you are replacing the roof covering anyway. Secondly, it’s important to understand how to keep it airtight and prevent condensation.


Cold flat roof insulation

Lots of traditional flat roofs are of the ‘cold roof’ construction type. This means there is a vapour control layer in the ceiling and cross ventilation at the eaves or the top of the walls. This may help with condensation, but causes such a loss of heat that it has been banned in Scotland.


Warm flat roof insulation

Inverted warm deck flat roof insulation puts the insulation above the waterproof membrane

Inverted warm deck flat roof insulation – click to enlarge

Nowadays, flat roofs have the ‘warm roof’ type.

Insulation can either be put below the weather proof membrane, which is called a ‘warm roof deck construction’, or above the waterproof membrane, which is called an ‘inverted warm deck’.

The only point of putting the insulation on top of the waterproof membrane is when you can’t take it off, as when, for example, it is bitumen.


Vapour control layer

This goes on first, above the deck. This vapour check and airtight membrane should always be laid on the warm side of the insulation. It takes the form of reinforced bitumen sheeting or polyethylene laminated with metal foil to reflect the heat back in.

It is appropriate for ventilated or unventilated roofs (all flat roofs are unventilated). A product like pro clima INTELLOplus features humidity-variable diffusion resistance that is effective in most if not all climates. It protects thermal insulation where there is a possibility of moisture entry, e.g. through leaks, damp building materials or diffusion through adjoining structural surfaces.

This means that it is possible to use it in combination with all fibrous, organic insulating materials. You can choose this if you don’t want to use mineral wool, polyurethane (PUR), expanded polystyrene (EPS) or rigid polyisocyanurate (PIR). It can be used with roofs covered in bitumen sheeting or metal sheet, or green roofs with diffusion resistant exterior cladding.

It should be lapped up around the insulation and affixed securely to prevent any air infiltration.


Flat roof insulation thickness

The chosen insulation laid on top of this should be able to take the weight of anything put above it on the roof without compression, or protected from compression by further sheeting. It should be of the maximum available thickness.

The typical U-value of an existing, uninsulated flat roof is 1.5W/m2K. The target should be to achieve a U-value of 0.25W/m2K or better. To achieve this, add 100 to 160mm of insulation above the structural deck (dependant upon insulant conductivity).

A typical installation would find 80-100mm of PUR insulation laid over a deck made of plywood (conforming to BS5268 Part 2 and to BS EN 636 Part 3, and of quality grades C-D and C-C) or Orientated Strand Board (manufactured to BS EN300 and be certified by BBA or WIMLAS), with a further 38mm of PIR insulation fitted between the roof joists, immediately below the deck. This would give a U-value of around 0.21W/m2K.

Insulation thickness (mm) U-value W/m2K
Wood fibre batts (adhered / mechanically fixed) 300 0.22
400 0.17
500 0.15
Mineral wool slab (adhered / mechanically fixed) 150 0.22
200 0.17
250 0.15
Polyurethane (PUR) fully bonded 100 0.22
130 0.18
150 0.16
Polyisocyanurate (PIR) fully bonded 100 0.25
150 0.18
200 0.14
Extruded polystyrene (XPS) fully bonded 110 0.23
150 0.19
200 0.14

These figures are approximate and vary according to the construction type and material used. Assumptions include:

  • timber roof: 12.5mm plaster board, 150mm timber roof space with no insulation, 20mm timber decking, insulation, 6mm felt weather cover;
  • concrete roof: 12.5mm plasterboard, 22mm batters, 150mm concrete deck, insulation, 6mm felt weather cover

In practice, the depth of insulation that can be added is often determined by features such as flashings, kerbs and upstands.

Contact the manufacturer of the selected insulation to discuss the technical requirements and assess the potential for interstitial condensation.


Roof covering

On top of this goes the roof covering. Close attention must be paid to drainage patterns, to ensure no pools of water can gather.

A single ply EPDM or TPO membrane may be used, which is easy to lay down.

Reinforced bitumen membranes can also be used but must conform to BS EN 13707, and be secured in accordance with the guidance in BS 8747.

Mastic asphalt is a further option, to be applied according to the guidance in BS 8218 and BS 8000: Part 4.


With an inverted warm roof construction, put ballast, usually graded pebbles or concrete paving slabs, on top of the insulation in order to keep the insulation down in the event of wind.

Green roofs

A green (sedum or grass) roof can also be put onto a flat roof, provided it can take the weight.

Whether ballast or green roof is chosen, a filter membrane is a good idea just beneath it, to prevent dirt clogging up the drainage channels.

Protexia Geofabric is one heavy duty all-in-one solution I have found to both the roof covering and filter membrane.

Watch the edges

When you insulate a flat roof it is important that moisture or air are kept well out, all around. Careful detailing at the edge and parapet areas of a flat roof is vital for reliability and durability.

Why not put the insulation inside?

Now you might say, why not put the insulation under the ceiling? After all, it’s easier to get at. But this is more likely to cause problems with damp, which be a risk to the electrics. Putting it on top of the roof helps to prevent thermal bridging, and keeps the thermal mass of the structure on the warm side.

Why not forget the flat roof altogether?

If you are replacing a flat roof, then it’s worth seriously considering a pitched roof. This is easier to insulate and to maintain.

For more detailed guidance on roof insulation please google for the following Energy Saving Trust literature:

  • Practical refurbishment of solid-walled houses (CE184)
  • Energy efficient loft conversions (CE120)
  • Energy efficient refurbishment of existing housing (CE83)

Thanks to the Energy Saving Trust for the illustrations

Also See:
Sedum roof covering
Best insulation material
insulating a solid wall
Draught-proofing- a good thing?


Visit a house with flat roof insulation in Sep 2016
You can see flat roof insulation at four open home events around the UK this month. See our events listing for details and simply filter for ‘flat roof insulation’. SuperHome tours and Q&A sessions will let you quiz the owners, so you can discover what worked and get frank feedback on anything that didn’t. Entry is free.  SuperHomes are older homes refurbished by their owners for much greater comfort, lower bills and far lower carbon emissions – at least 60% lower! This makes them a great source of ideas to help you green your own home.

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© David Thorpe Jan 2013. David is Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency