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 + 353 (0)49 95 27000, or email us at

In warm roof constructions the insulation is fitted in the plane of the rafters rather than at ceiling line.
Warm roofs enable all the space within the building envelope to be utilised, whether that means maximising the usable floor area, creating dramatic cathedral roofs or simply providing a controlled environment for running complex services. Warm roof construction has benefits for the roof structure, protecting it from the effects of wide variations in temperature and humidity and also increases the racking strength.

Ballytherm Insulation board is ideal for forming warm pitched roofs: in new build the roof insulation can be fitted over and between the rafters, or between and beneath the rafters, while in refurbishment projects the insulation can be installed from inside the loft to upgrade the thermal performance of the roof without disturbing the existing roof covering.


The insulation in warm roof applications must be self-supporting. Where it is laid across the top of the rafters it must be able to withstand the imposed loads from the roof covering transferred through the counterbattens. Ballytherm Insulation is strong enough for that application.

Thermal performance

Ballytherm Insulation installed above or below the rafters is not affected by thermal bridging by the roof structure, whereas the thermal performance of an insulation layer set between the rafters is reduced by as much as a third.

Although using more insulation above the rafters will improve the thermal performance it can create problems: the forces upon the fixings increase, so more fixings are required; detailing at eaves becomes more complex, and it also becomes more difficult to drive the fixings accurately into the rafters. Consequently, it is important to avoid an excessive amount of insulation above the rafters by installing some insulation between the rafters.


Where Ballytherm Insulation takes up only part of the rafter depth, the resulting cavities will each have a low emissivity surface formed by the reflective foil face of the board.

Those surfaces will then reduce the rate of radiation heat transfer across the cavity and improve the thermal performance of the roof. However, in refurbishment projects the air movement through the cavities will eliminate much of the benefit from the low emissivity surface.


Roofs must be designed to avoid the formation of condensation which can threaten the structural stability of the roof and promote the growth of mould and rot. Designers should carry out condensation risk analysis to assess the likelihood of problems and modify the roof design if condensation is predicted.

To reduce condensation risk, designers should:

  • minimise the amount of moisture entering the roof: moisture generated by activities such as washing and cooking should be extracted at source and a vapour control layer – such as 1000 gauge polyethylene – should be specified on the warm side of the insulation.
  • ensure any moisture which does enter can escape to atmosphere: specify a vapour open underlay which will allow water vapour to disperse into the batten space and ensure there is sufficient air movement between the batten space and atmosphere (it may be necessary to vent the batten space beneath tight coverings such as sheet metal). In refurbishment projects where the underlay has a high vapour resistance there must be a 50mm vented cavity beneath the underlay.

Further guidance on controlling condensation may be found in BS 5250:2002.

Roof Insulation Products

Insulating above and between the rafters Insulating between and below the rafters
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 + 353 (0)49 95 27000,, email us at or use our contact form.