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A roof can be insulated in a number of ways dependant on whether it is a flat roof or a pitched room. This includes externally, which means the area is accessed from the outside of the property. External pitched roof insulation is usually carried out during refurbishment or during the build.
When refurbishing, all attachments will need to be removed such as tiles, slates, roof shingles etc. A solid base will then be formed from wood, it will be topped with damp-proof membrane and then the finishings reattached. During this process the insulation will be installed.
External insulation is a popular method within many historic buildings due to the risk of moisture or wood rot being lower than internal insulation methods.

Pitched roof insulation can also be installed internally, which means the area is accessed from inside of the property. This space is generally accessed through the space within the loft. Pitched roof insulation can be installed in addition to loft insulation and a width of 270mm is recommended. The 270mm thickness can be spread between materials such as insulation boards and insulated plasterboard. If your pitched roof space does not allow that depth, simply choose a thinner material.

Pitched roof insulation must have a 50mm cavity at the back to allow for airflow and must sit flush which the roof rafters.

Pitched Roof

Insulating a pitched roof from the inside

Measure all gaps

Measure the gaps between each and every rafter. Some of the spaces between rafters may differ to the others therefore not every insulation board can be cut to the same size. Use a tape measure and make markings on the board showing where to cut. You will need to choose whether to insulate between joists, over joists, or do both.

Create an Air Gap

When insulating between the joists, be sure to create an air gap of 50mm. This allows ventilation and air movement. Timber battens 50mm wide can be used to attach to the joists which will act as a block when pushing the insulation back into place.

Cut The Insulation

Cut the boards to size using a sharp knife. Mark lines for cutting with a straightedge ruler and a permanent marker. Score the marked lines with your knife and snap the board over a hard edge and score the other side. Alternatively, you can use a fine-toothed saw to cut the boards.

Install the Insulation Boards

When cut to size, push the now cut insulation boards into the space until it meets the batten or 50mm mark. If you should need to join two boards, tightly butt them together and apply foil tape to the join.

Vapour Control Layer

Some insulation boards include a built-in vapour control layer, but if not you will need to install one. Vapour barriers are sheets that can be fixed to the roof using a staple gun. You can make a fold at each joist to attach in a thicker layer. If you need to overlap sheets, staple a solid join between them.

More Boards or Plasterboard

At this point it is optional to install insulation horizontally over the joists. This can be a sheet of insulation, insulation followed by plasterboard, or insulated plasterboard. Apply the boards by attaching them to the joists meeting two boards in the centre of the joist. Start this from the top and work your way down. The addition of plasterboard is a popular option when using the loft space for a room in the roof or loft conversion as opposed to just storage space.

What are the Benefits of Insulating a Pitched Roof?

  • There is a selection of materials that can be used within a pitched roof application as opposed to just one. This includes mineral wool slabs, polyisocyanurate (PIR) insulation boards and phenolic boards.
  • Pitched roof insulation prevents heat from escaping the property by slowing the process in which heat can pass through the material. This will keep your house warmer in winter. The insulation material will also work from the outside in and prevents heat coming in. This will keep you house cooler in the summer.
  • Many insulations materials have a foil facing. The foil will work by reflecting heat back down into the building. Once the house has heated from the bottom upwards, it will then reach the insulation and heat the property from the top down. If you go into your loft space you will be sure to feel these affects, as the heat cannot escape and reflects, this space will be very warm.
  • The less heat escaping, the less your property will contribute to global warming. Preventing heat loss will create a more energy efficient and sustainable property.
  • It is optional whether to complete with plasterboard. This is a common choice when using the space for more than just storage.

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