Insulation for Converting a Loft into a Room in the Roof

Undertaking the conversion of a loft is a distinctly different proposition from simply providing additional loft insulation along the flat ceiling, the primary aim of which is to reduce heat loss through the roof.

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Turning the loft space beneath a pitched roof into a habitable room can be an attractive proposition, especially when the temporary disruption is contrasted with the cost and hassle of moving house to gain the extra space. A loft conversion is about creating an insulated ‘room in a roof’, utilising what might otherwise only be a storage space with the aid of PIR insulation.

Is the pitched roof suitable for conversion?

If the ceiling joists that will become the new floor are able to take the weight of people and furniture and if there’s enough space to stand up in, then a loft conversion is a distinct possibility.

There are other things to consider - such as providing suitable access into the space, fire safety and means of escape, and ventilation - which are easier to plan and make allowances for in a new-build project. In existing buildings a lack of space might prevent the construction of a new staircase into the loft, for example.

Getting professional design advice at an early stage is important, whatever the type of project. Checking that existing ceiling joists can bear increased loads might mean involving a structural engineer as well, especially as many modern houses are constructed with slender roof joists that are not intended to be walked on.

What insulation should I use for a loft conversion?

When choosing insulation for a loft conversion, be sure to consider:

  • Thermal efficiency.
  • Acoustic performance.
  • Ease of handling, cutting and installing.
  • The weight of the insulation.
  • The height of the insulation and the amount of vertical space available.
  • Fire performance.

What insulation should I use for a loft conversion?

Improving the thermal performance of the existing roof space without encroaching too much on floor space or head height is a delicate balancing act. Rigid polyisocyanurate (PIR) insulation is one of the most thermally efficient, widely available materials on the market, so it offers some of the thinnest possible solutions.

Considerations when converting a loft space

Choosing where and how to position any insulation product in relation to the pitched roof timbers is largely a matter of practicality and what makes most sense to your individual project.

Installing insulation over the top of pitched roof rafters (known as sarking insulation) helps keep the structure warm and can prolong its life while minimising condensation risk. It only makes sense, however, if the roof is new, or if an existing roof is having its covering replaced.

Even then, the thickness of insulation might make the overall height of the roof too great, so an alternative option is to reduce the insulation layer above the rafters and put some insulation between the rafters as well.

Assuming the roof covering is staying in place and work can only be done from inside, the next most efficient option to achieve thermal targets is installing insulation between the rafters with another layer of insulation fixed to the underside. If the available headroom is so limited that fixing insulation below is impossible then installing it between the rafters only might be a possibility.

It’s common for a roof space to feature low-height timber stud walls - and these need to be insulated too.

The building detail is for illustrative purposes only. It does not constitute advice and should not be relied upon.

Insulation products
Product Name Thickness Range Sizes Lambda
Celotex XR4000 110-200mm Width 1200mm, Length 2400mm 0.022
Celotex GA4000 50-100mm Width 1200mm, Length 2400mm 0.022
Celotex PL4000 25-65mm (+12.5mm Plasterboard) Width 1200mm, Length 2400mm 0.022
Celotex TB4000 20-40mm Width 1200mm, Length 2400mm 0.022