Pitched Roof Insulation: Insulating a pitched roof at the ceiling


Mineral wool insulation

Ceiling joists

Mannok Therm Roof / MR PIR insulation boards

Timber flooring

Insulating a pitched roof at ceiling level is a cost-effective way of installing roof insulation where the attic space will not be used as a living area. Excellent thermal performance can be achieved by using Mannok Therm Roof / MR PIR insulation boards at ceiling level. The thickness of insulation is largely unrestricted by construction so the layer of insulation can be deeper to achieve better performance as required.

Airtightness and ventilation in the roof space are important considerations when installing insulation at ceiling level to avoid condensation forming on top of the insulation and other surfaces in the cold space. To avoid moisture getting into the space, the ceiling design should be airtight.

Ensuring any warm air which does pass into the cold loft space does not form condensation can be achieved by incorporating ventilation in the design. A low vapour resistance or breathable sarking can be used in the roof structure to allow the moisture to pass through. Where a high resistance sarking is used, the space will require cross ventilation.

How to install pitched roof insulation at ceiling level

1. Lay mineral wool insulation between the joists as per manufacturer’s installation guidelines.

2. Lay Mannok Therm Roof / MR PIR insulation boards on top of the joists with joints staggered between rows. Ensure all board ends are supported on joists.

3. Where the loft is going to be boarded out it is good practice to mark the line of joists on the surface of the boards.

4. Where the loft is boarded out, the Mannok insulation boards will be restrained by the screws used to fix the chipboard, which should have a minimum 30mm penetration into the joists. If the insulation is not being covered then the boards should be fixed by nailing to the joists, with two nails for each board at each joist.

5. When upgrading the insulation in an existing pitched roof, ensure all the pipes and the cold water storage tank are properly insulated to avoid the risks of freezing.

6. Installing insulation over electrical cables will reduce the rate of heat dissipation, which may result in overheating. This is a particular risk with cables to showers and immersion heaters which usually run at a greater proportion of their capacity. Cables may need to be de-rated to prevent overheating. Alternatively, a clear air space may be left between the top of the fibrous insulation and the Mannok Therm Roof / MR insulation.

Thickness of insulation and U-Values

The below U-Value table shows the required thickness of Mannok Therm Roof / MR insulation boards required to achieve the desired U-Value in a typical pitched roof construction. Our free, online U-Value calculator can be used to calculate the required thickness of insulation for other pitched roof constructions.

Required insulation thickness of Mannok Therm Roof / MR:

Mannok Therm Roof / MR above joists U-value (W / m2K)
60 0.16
70 0.15
80 0.14
90 0.13
100 0.12
120 0.11
140 0.10

U-value results above based upon pitched roof construction shown:

12.5mm plasterboard; 150mm mineral wool insulation between joists; 150mm deep joists @400mm centres; Mannok Therm Roof / MR; timber flooring. Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

Need technical support?

For help and advice on any of our products and how they might fit your needs please contact our Technical by Mannok team.

  • Floor Insulation
  • Pitched Roof Insulation
  • Flat Roof Insulation
  • Wall Insulation