Increasing thermal performance is always a high priority for homeowners when looking to upgrade the insulation of their pitched roof.

As well as adding comfort to your home, a well-insulated roof increases property value (when seeking to create an additional room), significantly reduces heating bills and most importantly provides a warm, airtight space without the need for additional pipework.

To ensure you get specification right and achieve the desired results from your roof refurbishment, here are our top 7 considerations:

1. Choose the right type of insulation board

When it comes to choosing an insulation board for your project, you need to consider a product that meets and exceeds performance requirements. A range of insulation boards are available and consist of rigid, semi-rigid or flexible materials.

PIR insulation boards are highly recommended for roof refurbishments; these boards are made by blending a number of materials together to form a rigid foam product. The heat generated during the reaction allows the gases to evaporate and become trapped within cells, delivering premium thermal performance.

Due to the irreversible chemical reaction that takes place within its formation, PIR insulation boards cannot melt; this means that the board achieves high fire performance ratings. PIR is also designed to suppress the potential for condensation, whilst its’ structural strength ensures that it can cope with light foot traffic and loading.

As the rigid board structure cannot absorb moisture, change shape or be affected by air permeation, you can be confident of achieving consistent performance that will typically last the full lifetime of the building.

2. Consider the positioning of your insulation

There are different options to consider for the positioning of your pitched roof insulation: typically, insulation is placed either between and under the rafters, or between and over.

Whilst both are acceptable options for roof refurbishment, placing insulation between and under the rafters may be more suited for pitched roof refurbishment if you’re unable to remove/replace the roof covering; choosing this option will also have little impact on headroom space as no alterations to the roofline will be required.

However if the only work intended within the refurbishment is the re-roofing of the roof above an existing room in the roof, then situating the insulation between and over the rafters may be the best-suited option.

3. Prevent condensation risk

Whilst this is an area that rightfully concerns homeowners and building designers, it is also a requirement of the building regulations that pitched roofs are designed and constructed to minimise risks of harmful condensation.

Condensation can be controlled by ventilating under any non-breathable roof covering or membrane, whilst adding a vapour control layer to the warm side of the insulation material.

A vapour control layer (VCL) helps to reduce water vapour transfer through the pitched roof and as a result prevents condensation risk.

4. Meet the requirements of Approved Document L

So what do the building regulations require in terms of insulation when refurbishing a pitched roof?

Part L states that where more than 50% of the roof is being renovated, the insulation value should be upgraded to achieve a U-value of 0.18 W/m2K.

In most cases, this U-Value could only be achieved by either raising the roof height or by stripping all the ceilings, which is difficult when working on the refurbishment of an existing building.

However Part L goes on to state that building control accept a lesser U-value as long as you make reasonable provision to achieve the best standard possible.

As a basic rule of thumb, when complying to the requirements of Approved Document L, always look to achieve a U-value of no less than 0.35 W/m2K.

You can access the Approved Document L document here.

5. Choose the right thickness

The thickness of your pitched roof insulation will always depend upon the required thermal performance, the details of which will be outlined in Approved Document L.

However for homeowners and building designers alike the objective is to keep the insulation thickness to a minimum, in order for it to fit snugly within the existing rafters. The thicker the insulation the more space is required, and in turn this can lead to an unacceptable reduction in headroom. Thicker insulation is also difficult and expensive to fit into your pitched roof refurbishment.

Note: Building regulations requires you to take into account the effect of thermal bridging when calculating the thermal performance (and required thickness) of your pitched roof refurbishment. Refer to BS 5250 Section 9.6 and BRE document 262: Thermal insulation: avoiding risks to find out more.

6. Ensure your roof is airtight

Ensuring your roof is airtight is crucial to getting the most out of your efforts and achieving long-term satisfaction. This will require a roof system that prevents the free movement of air through any section of the pitched roof. When carrying out your pitched roof refurbishment, ensure that your building designer is able to identify any gaps or breaks in the insulation envelope, as well as avoiding the unnecessary cuttings of boards. Your chosen insulation board should always be installed to fit tightly around penetrations and ridges.

7. Minimise work to existing roof

Very often in roof refurbishment, implementing insulation that meets Approved Document L of the building regulations would mean raising the height of the roof; this significantly increases internal work and in most cases is simply not an option.

Celotex has worked with fellow insulation manufacturer TLX to create a new product that overcomes the challenges of improving thermal performance, whilst minimising internal works.

Celotex Rafter-Gold is designed for re-roofing existing properties using a combination of Celotex GA4000 high performance PIR insulation board and TLX gold – the unique intelligent 2 in 1 insulating breather membrane. Most importantly, Rafter-Gold does not require a vapour control layer, which can require the replacement of all the ceilings in the property whilst being both expensive and disruptive to the project.

The system enables the upgrade of pitched roof insulation to provide excellent thermal comfort by allowing work to be carried out from the outside of the building during re-roofing works.

To find out more about specifying the correct insulation for your pitched roof refurbishment, contact us on 01473 822093, or for more information on Celotex Rafter-Gold visit