How to insulate your loft? Things to consider.20 years ago you could walk into your local newsagent and spend your hard earned pocket money on a chocolate bar. For your shiny 20 pence piece you could buy a bar that would be more than big enough to spoil your dinner. However, when you walk into your newsagents today not only is the cost of the chocolate bar much higher but the bar itself is a lot smaller. Despite these obvious changes we still expect the same tasty experience.

This trend, to achieve the same with less space, is seen in other areas of modern life with a good example seen when we compare new housing developments with developments of 25 years ago. The need for plot optimisation sees today’s developments with 3 storey ‘room in roof’ houses, whereas developments of 25 years ago tended to have only 2 liveable storeys.

Similarly this general trend has transferred itself to the refurbishment and retrofit markets with homeowners choosing to extend their houses not only outwards but also upwards.

Celotex Insulation offers a range of solutions designed to achieve compliant U-values in the thinnest possible thickness.

How to insulate your loft. Things to consider.

When insulating a loft, a number of possible options exist with the following proving popular in both the new build and refurb arena:

Insulating a pitched roof (Unventilated – Breather Membrane over Rafters)

The target U-values in a roof for a new build domestic property ultimately stems from the SAP assessment, however in the absence of a SAP we would generally work to a U-value of 0.13 W/m2K. A typical example build-up to meet this target is to position 120mm of Celotex FR5000 between a 150mm rafter at 400 centres with a 72.5mm Celotex PL4000 insulated plasterboard fixed to the underside of the rafters.

By taking the breather membrane approach the clear cavity between the rafters is reduced to a 25-30mm drape space for the breather membrane rather than a minimum 50mm well ventilated cavity required in ventilated pitched roofs. This in turn enables the thickness of insulation below the rafters to be reduced also.

Insulating a pitched roof (Ventilated – Bituminous Sarking Felt)

Most common in the refurb market is to add insulation at the pitched level in order to create a heated room in the roof of a previously cold loft space. When a bituminous sarking felt is already in position a minimum 50mm well ventilated cavity is required between the felt itself and the insulation.

Under Approved Document Part L1B a U value target of 0.18 W/m2K should be achieved for loft conversions.

To meet this target with the same 150mm rafters at 400 centres 100mm of Celotex FR5000 should be positioned between the rafters, leaving the 50mm well ventilated cavity above the insulation, a further 62.5mm of Celotex PL4000 insulated plasterboard should be fixed to the underside of the rafters.

In both the unventilated and ventilated pitched roof examples the vapour control layer is created by the continuous layer of Celotex PL4000 insulated plasterboard negating the need for an additional membrane.

Insulating an attic wall

A popular approach to completing the insulation envelope for room in roof constructions is to run the line of insulation vertically down the attic or ashlar walls and then across horizontally to create a junction with the wall insulation leaving an unheated storage area at the eaves.How to insulate your attic wall? Things to consider.

Again, SAP assessments drive the U-value required for the walls in new build domestic properties but a sensible U-value to help pass a SAP is 0.18 W/m2K. To achieve this 80mm of Celotex FR5000 would be positioned between a 100mm stud with a further 62.5mm Celotex PL4000 insulated plasterboard fixed over the face of the stud.

To achieve the usual target of 0.28 W/m2K for a loft conversion, 70mm of Celotex FR5000 would be positioned between the 100mm stud with a 27.5mm Celotex PL4000 insulated plasterboard fixed over the face of the stud.

Again, the inbuilt vapour control layer within the Celotex PL4000 negates the need for an additional membrane.

The small area of horizontal ceiling at the eaves would usually require a U-value of 0.16 W/m2K with the thinnest solution achieved by positioning 90mm of Celotex FR5000 between the joists and 50mm of Celotex FR5000 over the joists.

These solutions are a small snapshot of viable options to create an additional room within the roof of a domestic property whether new build or refurbishment. All of the products mentioned are readily available from our merchant and distribution partners across the UK. The Celotex Technical Centre advisors are also on hand to guide you through the process of making the most of the available space.

Find out more about how you could use Celotex FR5000 or Celotex PL4000 to improve loft insulation.