Insulation Above And Between The Rafters – Design Guidance

Where condensation risk analysis indicates a risk of condensation a vapour control layer should be installed behind the finish.

The underlay should have a vapour resistance of less than 0.25MNs/g and may be laid directly over the insulation or draped over the counter battens.

A rigid eaves carrier may be required to prevent ponding and avoid UV degradation. Counter battens should be a minimum of 38 x 50mm and should be fixed through the Ballytherm boards to the rafters with fixings such as Helfix Inscrew or Proctor PR nails: consult the fixing manufacturer for information on loadings and spacings. There should be a stop batten fixed across the rafters at eaves to prevent the insulation slipping down the roof (figure 28).

There is no need to ventilate beneath the underlay, but there must be sufficient air movement between the batten space and outside air to allow moisture to disperse. Roof coverings such as tiles and slates will allow enough airflow through the laps, however, air spaces beneath tight coverings should be vented.

To avoid thermal bridging the roof insulation should meet that in the walls: if a cavity wall is finished with a closer, additional insulation should be fitted from the top of the closer to the upper face of the rafters, where it will butt the over-rafter insulation. At verges the wall insulation should run to the top of the wall and butt the underside of the over- rafter insulation, which is extended across the wall head.

At ridges, hips and valleys where roof planes intersect, the over-rafter insulation should be cut to form a continuous layer of insulation. Junctions may be sealed with expanding foam insulation.

Further Advice & Information

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Floor Insulation