Roof Insulation

Ballytherm Polyisocyanurate insulation is the ideal material to use for insulating in-between the rafters. The pages within this section provide design guidance, design details, installation instructions and advice on sitework.
Custom sizes are also available on all product ranges. For more information call our insulation experts on 0800 644 6900, or email us at

roof insulation

General considerations

Insulation installed in-between the rafters of a house provides a warmer roofspace than insulation installed at the line of the ceiling. All of the space in the upper part of a home can be used when insulation is used in-between the rafters. This “warm-roof” construction allows for cathedral ceilings used for a single story or in a two-story home. Also the warm roof approach enhances the structural soundness of the roof and its resistance to fluctuations in heat and moisture within the air.

The ideal material to use for insulating in-between the rafters is Ballytherm insulation board. The insulation installation can be done on either side (above or below) of the rafters during new construction or, during a home renovation project, from within the attic, without interfering with the external roof envelope.  This is a great way to improve roof performance for a home in a cold climate.

Structural Matters

Where insulation is installed above the rafters, it incurs an additional force from the roof covering.  Ballytherm Insulation can withstand this force and meets the requirements for self-supporting insulating materials within this type of warm roof construction.

Heat Effectiveness

Placement of roof insulation is important to the performance of the product. Ballytherm insulation may be placed on the top or the underside of the rafters. However, the most effective use of the insulation is above the rafters. While the thermal performance of the insulation is improved, this approach can be a problem as the need to use more fixings is required. One way to reduce the amount of insulation used above the rafters is to add insulation in-between the rafters.


The thickness of Ballytherm insulation may be less than depth of the rafters. It is worth bearing in mind that, as a low emissivity surface area in the cavity is created by the foil face of the Ballytherm thermal insulator, radiant heat transfer will be reduced. But heat loss remains more of an issue with renovation insulation than with new build, due to air transfer.

Moisture Considerations

One thing of concern is the development of condensation, leading to mould growth and rot. A proper risk assessment to determine potential moisture issues is recommended and measures to address this can then be incorporated into the roof design. The following measures in the overall building design should also be incorporated.

  1. Remove moisture at the source of generation
  2. Install a vapour control layer on the warm side of insulation
  3. Provide a means of escape for any moisture which does enter, by using the right underlay and batten space.

Further Information

BS 5250:2002 provides more best practice information on dealing with moisture laden air. For specific feedback on your roof project and using Ballytherm insulation  contact us at 0800 644 6900, email us at or use our contact form. One of our insulation experts is ready to advise you now!

Roof Insulation Products

roof insulation Insulating above and between the rafters roof insulation Insulating between and below the rafters
roof insulation Insulating loft conversions

Further Advice & Information

For more information about Ballytherm products, technical advice, or guidance on the suitability of our products for your project, contact us at 0800 644 6900,, email us at or use our contact form.