Insulating a Flat Roof - DIY Tips for Energy Efficiency

When it comes to improving your home's energy efficiency, the flat roof is one area you shouldn’t overlook. Without proper insulation, flat roofs can experience significant heat loss in winter - and let heat in during the summer. This adds up to making your space uncomfortable and your energy bills higher than they need to be.

If you're a DIYer tackling a roof upgrade or even planning a new build, this guide will walk you through how to insulate your flat roof the smart way and introduce you to PIR, designed to meet modern efficiency needs.

Why insulating flat roofs matters

Flat roofs have less natural airflow than pitched ones, which means they can become heat traps in summer and cold sinks in winter - unless they're well insulated. Here's why insulating is a must:

• Reduces energy loss
• Lowers heating and cooling bills
• Improves year-round comfort
• Prevents condensation and damp
• Increases property value and performance

Choosing the right insulation

PIR (Polyisocyanurate) is one of the most effective insulation materials available today. It offers excellent thermal performance in a thin profile, developed for flat roofs where space is at a premium.

Resista MG PIR insulation

Resista MG is a premium PIR board that’s ideal for warm flat roof systems. Here’s what makes it a standout:

• Super low thermal conductivity (λ 0.022 W/mK) for excellent insulation
• Foil-faced for moisture and damp resistance
• Rigid and lightweight for easy handling during DIY installs
• Strong compressive strength, ideal for roofs with foot traffic (e.g., balconies or terraces)
• Compatible with most waterproofing systems including EPDM and Liquid PU

DIY install tips for flat roof insulation

There are two main ways to insulate a flat roof - from above (warm roof) or below (cold roof). For most DIYers and modern builds, a warm roof system is easier, safer and more effective.

Warm roof (external insulation)

This method places the insulation on top of the roof deck, under the waterproofing membrane.

Step-by-step:

• Prepare the roof deck: Ensure it’s in good condition, clean, level, and dry.
• Lay a vapour control layer (VCL) to prevent condensation (where appropriate).
• Fix Resista MG PIR boards across the roof, staggering the joints.
• Tape or seal joints to minimise thermal bridging.
• Install a suitable waterproofing membrane (e.g., EPDM or Liquid PU) over the insulation.

Tools You’ll Need:

• Utility knife or insulation saw
• Vapour barrier tape
• Fixings and washers for PIR boards
• Spirit level and measuring tape

Pro Tip: Warm roof systems don’t interfere with your indoor space, making them perfect for garages, extensions, and garden rooms.

Cold roof (internal insulation)

This method installs insulation between or below joists, inside the roof structure.

• Typically used in retrofits where external work isn't possible
• More prone to condensation issues—needs excellent ventilation
• Less thermally efficient than a warm roof

While doable, we generally recommend warm roof systems for long-term performance and simplicity.

How much could you save?

Upgrading your flat roof insulation can reduce heat loss by up to 25%, depending on the structure and climate. That means warmer winters and cooler summers, with potential annual savings of £150 - £250 or even more on energy bills.

Ready to get started?

If you're looking to boost your home's energy efficiency and tackle a satisfying DIY project, insulating your flat roof is a great move. With high-performance materials like Resista MG PIR, you can do the job right the first time - and enjoy the comfort and savings for years to come.

If you need help choosing the right thickness, quantifying or installing, please give the expert roofing team a call, we’re here to help. View our full range of PIR insulation boards, all in stock and ready-to-go.

Download a RESISTA Technical Information Sheet >