Insulation boards by Celotex can be installed in pitched roofs, flat roofs, solid masonry walls and more. It’s easy to install when you’ve got a bit of guidance, so we’ve broken down the steps for installation in many applications.


Table of contents:


What is Celotex?

Celotex is a form of insulation typically installed within external walls. They can be applied internally as well depending on the type of insulation. The type of walls (also known as leafs) Celotex insulation can be installed into can be tricky to comprehend. So this guide can make sure you’re all set for suitable insulation whatever the weather! Not sure about what the best Celotex insulation is for your needs? Check out the comparison guide here.


How to cut insulation board

First, make sure you have the correct protective gear on hand. Clothing and goggles or safety glasses. As a result of cutting the board, dust and particles can be produced. So it is important protective assets are worn to ensure that you do not inhale the debris.

The most efficient way to cut Celotex is with a hand saw. Not only does this support minimal dust creation, but in many cases you can use off-cuts to fill any gaps that may still be there during installation.


How to fit Celotex insulation into masonry partial fill cavity walls

Masonry partial fill cavity walls are a popular type of external wall construction. In fact, they have been used for over a century. So here is what you will need to do to when it comes to installing Celotex insulation boards in masonry partial fill cavity walls.

Tip: Make sure wall ties used are suitable for structural requirements and have a retaining clip to hold the insulation permanently in place – BBA-approved is recommended. Although manufacturers advice may vary, butterfly ties are not considered suitable for partial-fill cavity projects by Celotex.

What you need:

Note: You will need to install the PIR cavity board between two types of masonry. As a standard measurement, 50mm residual cavity is a must to maintain between the outer face of cavity insulation and the inner face of the outer leaf.

Fitting the Celotex insulation

Step one: Install the first row of board-retaining wall ties at least one course below damp proof course, positioning them at maximum 600mm centres horizontally. This will provide minimum support of two ties per 1200mm board.

Step two: Install the second and following rows of ties at 450mm centres vertically with a maximum 900mm horizontally. If structural purpose affects this, you may need to install ties at closer centres.

Step three: Place the boards between the wall ties, securing each one with a retaining clip on each tie – ensuring both full and cut boards are retained by no fewer than three ties around their perimeters. Make sure horizontal and vertical joints are tightly butted to minimise heat loss. For internal and external angles, ensure the thickness of the board continues around the angle – continuing to make sure suitable wall ties are used.

Note: For windows and doors or other openings in the property, use proprietary insulated cavity closers.

Step four: For cavities closed at or below DPC level by a methane barrier membrane, use mechanical fixings to secure the board to the brickwork above the DPC.

Note: Avoid puncturing the gas barrier membrane

If you want to upgrade the walls using PL4000, see here.

Always consider manufacturer or professional installation advice.


How to fit Celotex Insulation into a timber frame wall lining

Tip: For high thermal performance, place the unprinted foil face surface towards the air cavity.

What you need:

Fitting the Celotex insulation

Step one: Make sure all services are installed and that all studs and rails are flush with no projections.

Step two: Insert the insulation into the framing, pushing back the plywood sheathing. If necessary, fit services into the cavity and cut the boards to fit firmly between the stud rails. Ensure the wall insulation continues with floor perimeter

Step three: Cut leftover boards for infill panels and use cut-offs when possible to make sure there aren’t any gaps in the wall structure.

Step four: Install a vapour control layer over the warm side of the studwork

Step five: Fix plasterboard lining over the vapour control layer using suitable plasterboard nails or screws.

Always consider manufacturer or professional installation advice.


How to fit Celotex insulation into timber frame wall sheathing

What you need:

Fitting the Celotex insulation

Step one: Fix Celotex sheet to external sheathing using galvanised clout nails – 400mm centres in the centre of the board and 300mm centres around parameter. Face unprinted foil surface towards the air cavity for optimum thermal performance. Ensure to carefully align fixings with underlying studs, sole plates and head rails.

Step two: Tightly align boards together using off-cuts to fill in around waist rails.

Step three: Use helical stainless steel wall ties to tie back brickwork to timber framing – drive through Celotex insulation into studs. Inserting ties approx. 10mm to 20mm above brick course so a slight ‘crank’ can be applied which,  ensuring the tie slopes downward from the sheathing to the outer brickwork.

Always consider manufacturer or professional installation advice.


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

What you need

Fitting the Celotex insulation

Step one: Make sure studs and rails are flush – no projections. Services must be correctly installed.

Step two: Install Celotex insulation tightly between studs, pushing it up to plywood sheathing. For improved thermal performance, place the unprinted foil surface face against the air cavity within the studwork.

Step three: Cut leftover boards for infill panels; use cut-offs where possible and make sure there are no air gaps in the wall abutment.

Step four: Install the PL4000 board insulation over the studs, ensuring edges of the boards are packed close together and not leaving any gaps.

Step five: Fix back the boards at the stud lines as well as the top and bottom rails. Ensure joints between the boards are tightly aligned, taped and jointed with correct tape and jointing material – this will create a vapour control layer.

Step six: Use sealant to vapour seal all perimeter abutments; make sure to seal around all penetrations for electrical outlets and switch boxes.

Always consider manufacturer or professional installation advice.


How to upgrade cavity walls with Celotex insulation

A brief overview of cavity walls can be seen here.

Due to increased care being taken regarding insulation standards, today it’s important to ensure the target U-value set for properties is met with proper insulation.

Insulated plasterboard that adheres to the masonry inner leaf or can be mechanically fixed to timber battens is the way to go when it comes to upgrading the insulation of cavity walls. Celotex PIR foam boars are fantastic for this as they are easy to handle and have a minimum thickness. Not only reducing lost floor space, but also keeping high thermal efficiency.

Note: If you’re a homeowner who wants to improve performance of the property, replacing or adding more insulation in an already-treated cavity is not possible.

If you plan to upgrade the wall as a whole, you can add building fabric, air tightness and controlled ventilation for improved thermal performance. Place composite facing on the insulation board, behind the plasterboard layer, to form a type of air and vapour control layer -tape and join the tapered edges to do so.

What you need:

Fitting Celotex insulation

Step one: Make sure existing walls are permeable – strip away any existing gloss paint or vinyl for a clear space.

Step two: Apply a continuous band of adhesive to ensure a continuous seal is at skirting, ceiling level and openings. Thin timber battens can replace adhesive at perimeter edges. Add additional adhesive where necessary, according to manufacturer instructions,

Step three: Align the insulation against the adhesive to secure it into the right position.

Step four: Once the adhesive has set, apply secondary fixings to Celotex PL4000 – ensure fixing details are in line with fixing manufacturer recommendations.

Step five: Ensure the joints between the boards are tightly aligned, taped and jointed with correct tape and joint material – this will create a vapour control layer.

Note: Avoid the load being directly applied to the Celotex PL4000. Mechanical fixings suited should be used for other internal fittings. Seek advice regarding suitably fixings from the manufacturer.

Always consider manufacturer or professional installation advice.


How to fit internal lining systems using mechanical fixings

What you need

Fitting the insulation

Step one: Make sure existing walls are permeable and stripped of any gloss paint or vinyl wallpaper for a clean surface.

Step two: Cut the 1200mm x 24000 PL4000 board to fit the board floor-to-ceiling height of the room. When using softwood timber battens, make sure they are treated prior to installation.

Step three: Fix treated timber battens to the masonry, set out at a maximum of 600mm vertical centres – this will coincide with the edges of the boards. Horizontal battens, as a minimum requirement, are to be used as a support at the top and bottom of the board edges.

Step four: Secure Celotex PL4000 with appropriate mechanical fixings – details must be aligned with instructions from fixing manufacturer.

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

Always consider manufacturer or professional installation advice.


How to fit Celotex insulation into solid walls

The solid wall has a similar installation method to the upgrading cavity walls, with a few minor tweaks – such as the type of board that can be used. Once again, if you’re hoping to improve the performance of your solid wall then adding more insulation to the outer face of the wall is not always a possibility.

When it comes to the insulation, however, 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 directly to the wall.

What you need

Fitting the insulation – GA4000

Step one: Make sure walls are in a good condition and suitable for internal linings – take care to remove any vinyl wallpaper or gloss pain to leave the wall surface clean, dust-free and free from loose material(s). Ensure the wall is also dry, having any remedial work done prior to installation of the boards to remove dampness.

Step two: Where required, cut the assigned 1200mm x 2400mm board (Celotex PL4000 or Celotex GA4000) to ensure they fit floor-to-ceiling room height.

Step three: Place unprinted foil surface facing batten cavity for optimum thermal performance.

Step four: At the top and bottom of the wall, fix horizontal battens over the insulation to keep the insulation boards in place. Seal each board joint.

Step five: Create a vapour control layer using insulation tape

Step six: Place further vertical battens over the insulation at the appropriate spacing -this will provide suitable fixings for the desired lining system. Make sure a batten meets with each lining board joint.

Note: For window and door reveals, use Celotex TB4000 boards -this reduces 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 mechanically fix into the masonry reveal – use proprietary fixings.

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

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

Step nine: Use further battens -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 created by the batten, increase the batten depth to ensure tubing is taken into account.

Note: For heavy wall mounted parts, use independent horizontal battens. You may also need longer fixings to fit heavy items to the masonry, separate of the battens.

Always consider manufacturer or professional installation advice.


How to upgrade solid walls with the dot and dab method

Note: Make sure existing walls are in suitable condition and able to take internal linings. Any vinyl wallpaper and gloss paint should be removed, with debris (dust etc) to also be removed to leave a clean surface.

What you need:

Fitting the insulation – PL4000

Step one: Apply a continuous band of adhesive to ensure a continuous seal is at skirting, ceiling level and openings. Thin timber battens can replace adhesive at perimeter edges.

Step two: Add additional adhesive where necessary, according to manufacturer instructions, then align the insulation against it to secure it into the right position.

Step three: Once adhesive has set, it is advised that secondary fixings are applied to Celotex PL4000 – ensure fixing details are in line with fixing manufacturer recommendations.

Step four: Ensure the joints between the boards are tightly aligned, taped and jointed with correct tape and joint material – this will create a vapour control layer.

Always consider manufacturer or professional installation advice.


How to upgrade internal solid wall systems with mechanical fixings

Use suitable mechanical fixings to secure Celotex PL4000 insulation – fixing details should dealt with as advised by manufacturer’s fixing instructions.

What you need

Fitting the insulation

Step one: Fit the Celotex insulation in tightly using suitable mechanical fixings (in accordance with manufacturer’s instructions).

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 Celotex PL4000 boards -this reduces 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 mechanically fix into the masonry reveal – use proprietary fixings.

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

Always consider manufacturer or professional installation advice.

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