Flat roof insulation is used to maximise a building’s energy efficiency and minimise heat loss. A roof is typically classed as ‘flat’ if it has a pitch below 10°. The way flat roofs are insulated differs from standard pitched roofs. Insulating a flat roof can lead to lower energy use, a reduction in condensation and a decrease in associated household bills.

This guide explores the different types of flat roof insulation, their benefits and the installation options available. We’ll also look at the latest flat roof insulation regulations.

What is flat roof insulation?

Flat roof insulation is used to enable a room to feel warmer in the winter and cooler in the summer months. Wood fibre, cellular glass and polystyrene (EPS or XPS) all have good insulation properties, but foam works particularly well and is often found in modern constructions.

There are two approaches to installing flat roof insulation; warm flat roofing works by placing the insulation material on top of the roof, while cold flat roofing involves placing insulation between the joists.

If you’re seeking a reduction in energy usage (and your energy bills), warm flat roof insulation will work best for you. It’ll also improve your property’s Energy Performance Certificate and help to reduce condensation. Cold flat roofing has a much lower installation cost and is well suited to outbuildings, which don’t necessarily require all the benefits of warm flat roof insulation.

Warm roof vs cold roof insulation

Choosing which form of insulation is best for your project largely depends on the type of building you’re working with. Warm flat roof insulation works by ensuring the insulating layer sits above the timber rafts. This results in the roof and loft area being around the same temperature as the one inside the building. It’s by far the best choice if you’re insulating a room in your house; its simple design is both thermally efficient and cost-effective. However, be prepared to have scaffolding erected and a team of professionals on hand to install the insulation, as this is not a DIY job.

Alternatively, cold flat roof insulation works by placing the insulation layer between or under the timber rafters. This leaves the roof cold and is generally regarded as an outdated method of flat roof insulation. It’s not recommended in Scotland due to multiple condensation issues, but if you’re working on an outbuilding you’ll find it’s cheap to install and is an easy option if you’re creating a flat roof (as opposed to working with an existing flat roof).

Warm roof insulation

There are many benefits of installing warm flat roof insulation. It allows heat to be conserved without the need for roof void ventilation, it also prevents damp and decay by enabling moisture to escape, and is considered to be the best roof option for the UK climate. While there are many materials to consider for the job, PIR (polyisocyanurate) boards are a popular choice because they are extremely durable and can withstand being walked on.

This is an important factor to consider as, despite the addition of warm roof insulation, you’ll still need to access the roof to check for any damage over time.

Installing warm flat roof insulation requires specialist equipment, so be prepared to arrange installation with a local expert.

Cold roof insulation

Cold roof insulation is a cheap way of insulating outbuildings, although it typically requires the addition of a roof void ventilation system to prevent the accumulation of damp. It might not be the best solution for flat roof insulation, but it does still have its uses in certain projects. While PIR boards are one of the best materials to use in warm flat roof insulation, we recommend using loft insulation roll for cold roof insulation. This is because the insulation is most commonly placed between the rafters, and loft roll is lightweight and easy to manoeuvre, making it the perfect candidate for the job.

Are there building regulations for flat roof insulation?

The simple answer is yes, there are building regulations that must be adhered to. The latest flat roof insulation amendment came in 2010 in the form of Approved Document L. U-values are measured in Watts/metres squared Kelvin and the document stipulates a U-value target of 0.18W/m2K for new or replacement flat roofs. The idea behind this is that more insulation should provide better heat retention, which will lower bills and help meet the CO2 emission reduction EU directive.

Understanding U-value and regulations

A U-value is a unit used in the UK to tell builders, architects and engineers how good a material is at insulating. The lower the U-value the better the insulation properties of the material. To comply with Part L of the Building Regulations 2010, all roofs should be thermally insulated to a maximum U-value of 0.18W/m2K.

But how do you calculate the U-value of your project? There’s no denying a U-value is a difficult measurement to calculate, but there are some handy U-value calculators available online. With different materials being awarded a variety of U-values, it’s best to have computer software on hand to help you out.

For more information on how to calculate the U-value of a flat roof insulation project read our specialist guide.

How long does flat roof insulation last?

Insulation materials tend to be extremely robust. You can expect flat roof insulation to last anywhere between 25 and 40 years. It’s worth noting that warm roof insulation tends to degrade faster than cold roof insulation due to it being exposed to the elements. However, this shouldn’t cause too much concern as the insulation will last at least as long as the durable waterproofing material.

How to insulate a flat roof

While insulating a flat roof is a relatively straightforward process in terms of the materials and steps required, much of the job relies on specialist equipment. We advise contacting a professional to complete the project unless you have been trained or are specifically skilled in this area. You can find a comprehensive guide on how to install flat roof insulation here.

An expert will typically install it using the following steps:

  1. Begin by laying 12mm of plywood or OSB board across the timber joists. This acts as a base on which to build the flat roof up.
  2. Now it’s time to install the vapour control layer. This layer reduces the risk of condensation and moisture, which would compromise the flat roof structure if it developed.
  3. Make marks on the vapour control layer to indicate where fixings will pierce the membrane. Remember to mark rafter centres and the correct axial spacing.
  4. Place butyl tape over the marks indicating where the vapour control layer will be pierced. This will form a seal after penetration.
  5. Lay rigid insulation board to the desired thickness. The thickness will often be dictated by the height available due to guttering and upstands.
  6. Install an additional layer of plywood or OSB board.
  7. The deck now needs to be fixed together. This can be done with helical fixings that pierce each layer of the buildup, including the flat roof insulation.
  8. To complete the job, install a waterproof membrane. This finishes off the deck and protects it from the elements.

For a more in-depth step-by-step guide, read our project guide to insulating a flat roof.

Call our team banner

Was this guide useful?

Thanks for rating this article.

Mentioned in this project guide: