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Celotex is an insulation material that can be installed in external walls as well as multiple other application methods such as walls, floors, flat roofs, pitched roofs and lofts. The method of application will depend on the type of insulation required and the location of the installation. Read the below guide to find out what insulation material will fit your project needs and what installation methods can be used.

Before you begin insulating your home:

  • Please note this is only a guide and a professional builder should be consulted before any insulation works are started on your home / property.
  • Make sure your home / property is in the correct condition to have insulation installed. Insulation should not be applied to damaged walls, rotted wood or walls containing damp. This could worsen the condition, so it is important any problems are treated first.
  • Insulating your home correctly can save money on your energy bills and make you home more energy efficient. By making you home more energy efficient you are helping to prevent climate change and global warming.

How to cut Celotex insulation boards:

Protection is always recommended when installing insulation such as protective clothing, safety goggles and face masks to avoid any irritation caused by insulation materials. Dust and other particles can be produced when insulation is cut so avoiding coming into contact or inhaling the particles is a must.

Celotex is generally cut with a hand saw. Make sure you have accurately measured the insulation and marked where the cut should be made. An easy way to cut insulation is to place the board onto a table so there is room underneath for the saw to go through. A ruler should also be placed over the insulation to use as a cutting guide. By taking time to cut the board accurately and neatly and cut offs can be re-used for smaller spaces and gaps.

How to fit Celotex insulation into masonry partial fill cavity walls

You will need:

  • Celotex Insulation boards
  • Wall ties and retaining clips (check they are the right type for your insulation needs).
  • Hand saw
  • Ruler
  • Mechanical fixings

Masonry partial fill cavity walls is installed externally and is a very common method. The insulation is placed in between the outer leaf and inner leaf of the wall. The board should be a minimum of 50mm.

When building and installing the insulation make sure the area is kept clean and excess mortar is wiped so it does not affect the insulation being installed.

Fitting the Celotex insulation

Step one: As the inner leaf is being built wall ties should be applied at the appropriate position. This should be at a maximum of 600mm centres horizontally. The first row of wall ties should be installed at least one course below the damp proof course.

Step two: The second set of rows and all rows to follow should be fixed at 450mm centres vertically with a maximum of 900mm horizontally. Once wall ties are positioned build up the inner leaf, so the ties are secure. Continue to build this up in sections.

Step three: When the wall ties are in place, begin to place the boards between the wall ties. Secure each board with a retaining clip on each tie. All boards full and cut should be retained with a minimum of three ties per board fitted around the board. When fitting boards horizontally and vertically together ensure they are tightly butted together to avoid thermal bridging and heat loss.

Step 4: When building the outer leaf of the wall ensure there is a 50mm gap between the outer leaf and the insulation. The outer leaf should be built up to the top of the insulation boards.

Step five: For cavities closed at damp proof course level by a methane barrier membrane. Mechanical fixings can be used to secure the board to the brickwork above the damp proof course.

If you choose to go onto to insulate the internal wall, please see the below guide on ‘timber frame wall lining’ and ‘top up cavity wall’

If you come to an angle, make sure the thickness of the boards continues around he angles using the suitable wall ties.

Other material should be used when insulation around doors and windows.

How to fit Celotex Insulation into a timber frame wall lining

What you need:

  • Celotex insulation board
  • Board cutting tool
  • Plasterboard lining
  • Plasterboard nails/screws

Fitting the Celotex insulation

Step one: Measure the spaces in between the stud rails. Measure the insulation material and cut to size if necessary. Push the insulation into the space all the way back to the plywood sheathing with no projection. Use any left-over insulation or cut off to fill gaps.

Step two: Tape can be applied where the insulation is butted to connect the edge of the insulation and the stud frame to avoid thermal bridging. A vapour control layer can be installed on top of the warm side of the stud work.

Step three: Plasterboard should then be fitted over the vapour control layer and fixed to the stud frame using the necessary fixings.


Make sure all walls are flush with no projection before installing insulation.
Place the unprinted foil face surface towards the air cavity for high quality thermal performance

How to fit Celotex insulation into timber frame wall sheathing

What you need:

  • Celotex insulation board
  • Galvanised clout nails
  • Studs
  • Sole plates
  • Head rails
  • Stainless steel wall ties

Fitting the Celotex insulation

Step one: Make sure the external sheathing has been applied. Fix Celotex sheet to external sheathing using the appropriate fixings at 400mm centres in the centre of the board. Apply the fixings around the parameter at 300mm centres. Face unprinted foil layer towards the air cavity for optimum thermal performance. Any fixing that are applied should align with the stud frame, sole plates, and head rails beneath.

Step two: Fill in any gaps around waist rails with off cuts and tightly align the boards together.

Step three: Use the necessary wall ties to tie the brickwork to the timber framing. This should pierce through the insulation boards into the studs. Inserting ties approx. 10mm to 20mm above brick course so a slight ‘crank’ can be applied, making sure the tie slopes downward from the sheathing to the outer brickwork.

How to install Celotex insulation for single timber frame wall lining and dormer cheeks

What you need:

  • Celotex insulation boards
  • Board cutting tool
  • Insulation tape
  • Jointing material
  • Sealant

Fitting the Celotex insulation

Step one: Ensure the stud rails are fixed with no projections and everything is correctly installed.

Step two: Fit the Celotex insulation boards between studs up to plywood sheathing. Place the unprinted foil surface face against the air cavity in the studwork frame for improved performance.

Step three: Use cut off boards for to fill in any gaps where possible and make sure there are no air gaps in the wall abutment. Foil tape can be applied to butted boards to avoid thermal bridging.

Step four: Install insulated plasterboard over the studs and apply the necessary fixing into the stud frames. Make sure all boards are fitted tightly together.

Step five: Then create a vapour control layer by applying the appropriate tape and jointing materials. Vapour seal all perimeter abutments using sealant. Seal around all gaps and holes including electrical outlets, switch boxes and others.

How to upgrade cavity walls with Celotex insulation

It is important now more than ever to make your property more energy efficient and meet the target U-value set for properties.

What you need:

  • Celotex insulation board
  • Adhesive
  • Fixings
  • Timber battens
  • Insulation tape

Fitting Celotex insulation

Step one: Clear all existing walls that require insulation and clear away any wallpaper, fixtures, and fitting.

Step two: Apply adhesive all the way round the edges to ensure a constant seal is at skirting, ceiling level and openings. Additional adhesive can be added where necessary. Timber battens that have a thin width can be applied to the walls instead of adhesive.

Step three: Fix the insulation onto the adhesive to secure it into the correct position. Make sure the boards align with any edges.

Step four: Once the adhesive has set and the insulation has been applied, secondary fixing can be fitted to make the boards secure. Make sure the fixing is the right kind for the boards. Apply tape and jointed materials to the joints between The boards to create a vapour control layer.

How to fit Celotex insulation into solid walls

Solid wall insulation is applied by layering the insulation onto existing walls however this is not always a possibility.

There is a method to apply insulation where some solid masonry walls can be found narrower than 200mm. In this case, the timber studwork might need to be fixed away from the wall by 50mm instead of applying directly to the wall. This is done by applying timber batten before insulation to create the space.

What you need

  • Celotex insulation boards
  • Timber Battens
  • Insulation tape
  • Sealant

Fitting the insulation – GA4000

Step one: Ensure all walls are in the correct condition to have insulation installed and remove any fixture and fittings such as wallpaper, shelves, and skirting boards. Make sure all the walls are wiped free of any chemicals and dry.

Step two: If needed cut the assigned 1200mm x 2400mm insulation board to confirm they fit floor-to-ceiling room height.

Step three: Place unprinted foil surface facing batten cavity for the best quality thermal performance.

Step four: At the top and bottom of the wall, keep the insulation board in place by applying timber battens horizontally using the correct fixtures. Seal up each joint between the boards.

Step five: A vapour control layer should then be created using the necessary tape.

Step six: Place vertical battens over the insulation at the suitable spacing to provide suitable fixings for the required lining method. Make sure a batten meets with every lining board joint.

Step seven: Use an angle fillet at the frame and an angle bead or scrim tape at external corners to finish.

Step eight: Apply sealant around the edges of the insulation this will provide a vapour seal.

Step nine: Use more battens with a minimum 50mm x 25mm treated softwood around all openings to provide fixings for linings, grounds for skirting etc. If cables are located within the cavity produced by the batten, increase the batten depth to ensure tubing is considered.

For heavy wall mounted parts independent horizontal battens should be used. You may also need longer fixings to fit heavier fixtures to the masonry, separate of the battens.

When fitting windows and door reveals the Celotex boards should be used to avoid thermal bridging. A batten should be installed the edge of the opening and scribe the board to fit the reveal. Cut the dry lining to suit and fit into the masonry reveal – use proprietary fixings.

How to upgrade solid walls with the dot and dab method

What you need:

  • Celotex insulated plasterboard
  • Adhesive
  • Timber battens
  • Insulation tape

Fitting the insulation

Step one: Adhesive should be applied in a continuous band at skirting, ceiling level and all other openings. Thin timber battens can replace adhesive at perimeter edges.

Step two: Add additional adhesive where necessary and then line the insulation against adhesive to secure it into the right position.

Step three: Once adhesive has set, secondary fixings can be applied to the plasterboard. Make sure you use the correct fixing details for the board you are using.

Step four: Then create the vapour control layer by ensuring all the joints between the boards are tightly butted together, taped, and jointed with correct tape and joint material.

Note: Make sure existing walls are in suitable condition and able to take internal linings. Any vinyl wallpaper and gloss paint should be removed as well as fixtures and fitting and the surface should be clean and dry.

How to upgrade internal solid wall systems with mechanical fixings

What you need

  • Celotex insulation boards
  • Insulation tape
  • Angle fillet
  • Angle bead
  • Scrim tape

Fitting the insulation

Step one: Fit the Celotex insulation using the correct fixing for that board.

Step two: Create the vapour control layer by tightly aligning, taping and jointing the joints between the boards with suitable tape.

Step three: For window and door reveals use plasterboards to reduce the risk of thermal bridging. Install the batten around the edge of the opening and scribe the board to fit the reveal. Cut the dry lining to suit and fit into the masonry reveal using the correct fittings.

Step four: Use an angle fillet at the frame, as well as an angle bead or scrim tape at external corners to finish.

Note: Use the correct fittings for the board applied.

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