How should I insulate my loft conversion?

Many families decide to extend their properties rather than move house and a loft conversion is one way to get more living space and increase the value of your home. But not all lofts are suitable or economically viable for conversion.

A viable loft conversion

You’ll need the following to be sure that a loft conversion is viable in your home:

  • at least 2 metres clear head height, once the insulation and new finishes are in place, is required in the room and at the head of the staircase from the floor below
  • the net floor area with enough head height needs to make a useable room
  • tanks, pipes, and chimneys may need to be moved.

Warm loft insulation

Most lofts will only have ‘cold loft’ insulation immediately above the ceiling of the top storey. This is aimed at stopping the 25% of heat from your home that can escape through the roof and should be sufficient if you just want to use your loft for storage.

Converting a loft into living space requires ‘warm loft’ insulation, where you insulate the walls (internally or externally) and immediately under or over the rafters, so that the loft space retains heat and draughts are minimised.

How much insulation do I need for my loft conversion?

The amount you’ll need depends on the type of insulation you use, the breathability of the structure, the installation and the finish.

In this video Superhomer Maria Hawton-Mead talks about the eco-conversion of her loft which exceeds current regulations to achieve a high level of insulation.

The Building Regulations set out general standards which your insulation must achieve, measured in U-values. U-values determine how effective a material is as a heat insulator: the lower the U-value, the better the insulation.

Building Regulations in England require that the U-value of insulation on pitched or flat roofs, for example, be 0.18W/m2 or lower. So, to insulate a pitched roof, this can be achieved with any of the following materials installed between the rafters:

  • 250mm wood wool board
  • 240mm layer of fibre or wool insulation
  • 165mm of rigid foam (known as PIR) board insulation
  • 75mm rigid foam board with one layer of reflective foil insulation under the rafters
  • 65mm foil face rigid insulation with one multi-layered quilted product under the rafters.

You might want to think about environmentally friendly insulation materials.

Good quality installation

Getting the insulation installed correctly is also important to save energy and prevent condensation:

  • gaps between the wall insulation and wooden framework you insert it within, and between the insulation and floor, should be filled for airtightness, while maintaining ventilation to the roof structure
  • joints between boards should be tightly butted and finished by taping over the jointing materials, while some installations require air gaps between the layered elements
  • insulation should be installed under a vapour control layer to reduce the risk of condensation and mould
  • make sure you include battens in internal wall insulation for fitting curtain poles or pictures.      
Insulation batts ready to go in loft conversion

Insulation batts ready to go in loft conversion











If you do not have the height in the room to insulate under the rafters, then insulating above the rafters, by taking off the roof tiles and insulating beneath, is an option. It is also an expensive option, unless you are already having the roof covering replaced. External wall insulation is also an option to reduce loss of floor area.

What is the vapour control layer (VCL)?

While the loft insulation will stop heat escaping from the loft, warm moist air from human activity, such as cooking and showers, can condense on cold surfaces and cause unhealthy mould. If the loft is not airtight moisture can also get into the roof structure and cause damage to the structural components.

You can prevent this by

  • installing a continuous vapour control layer on the room side of the insulation
  • making the loft as airtight as possible, and
  • ventilating the roof structure above the insulation.

The vapour control layer may be integral to the insulation material you use, for example many foam insulation boards will have a foil VCL. If not, as in the case of mineral wool, a separate polythene membrane should be fixed under the plasterboard. You can get insulated plasterboard but it may not have the VCL.

In older properties which are designed for the moisture to evaporate through the structure, for example those built before 1919, then the breathing elements should be reinstated using breathable materials. For example, a Victorian house should have lime render externally and lime plaster internally. Wood fibre insulation used internally works well with breathing walls.

Completed loft conversion with roof windows

Completed loft conversion with roof windows











Can I retrofit insulation to an existing loft conversion?

If you have an existing loft conversion or attic room there may not be much insulation in place, but increasing the amount of insulation can be expensive and disruptive, so you will need to balance the costs and benefits.

You could add an extra layer of insulated board to the existing walls and ceiling, but this will result in a loss of floor space and head room.

If you are putting a new roof on the house then this is a good time to add insulation between the ceiling of the loft and the new roof. It is also a Building Regulations requirement in some cases.

Can I have solar panels with a loft conversion?

Maria Hawton-Mead also wanted to install solar panels and she recommends you:

  • make sure your roof is structurally sound to take the weight of solar panels
  • record the exact location of rafters before they are covered up with insulation as the support brackets for the panels need to be fixed to them
  • make sure your panels don’t overshoot the ridge (peak) of the ridge

What else do I need to think about before I convert my loft?

Converting your loft to living space is a major building job that can affect the appearance, structural stability and fire safety of your property. Here are four things you will need to check before you finalise your plans:

  • whether you need planning permission, especially if you live in a Conservation Area
  • that the work complies with Building Regulations
  • for semi-detached or terraced properties, that you have Party wall agreement from your neighbours
  • if you have bats breeding or resting in your loft. Bats are a Protected Species and you’ll need a licence for the work.

If your loft shares a wall or floor with another property then you will probably need to install sound insulation.

If you live in Wales, Scotland or Northern Ireland, the Building Regulations may have different U-value requirements.

Check out Maria’s SuperHomes page


  1. Forest of Dean Council Building Control Loft Conversion Guidance [1]
  2. Forest of Dean Council Building Control Guidance for Updating Traditional Buildings [2]
  3. Brighton and Hove Council’s Householder guidance on external wall insulation [3]
  4. Brighton and Hove Council’s Householder guidanc eon energy efficiency for historic houses in Conservation areas [4]
  5. Vapour Control Layers , SuperHomes

More information on:

  1. Planning permission
  2. Building Regulations
  3. Party Wall Agreements
  4. Bats in the attic
  5. Cold loft insulation and Boarding out your loft for storage

Written by Trish O’Flynn with thanks to Tony Gwynne of Forest of Dean Council for his input and supporting guidance.