How to Install Celotex GA4000: A Step-by-Step Guide

What Is Celotex GA4000?

Celotex GA4000 is a double foil-faced PIR (polyisocyanurate) rigid insulation board with a thermal conductivity of 0.022 W/mK — one of the best available for rigid boards. It is suitable for pitched roofs, flat roofs, floors, and walls, making it the most versatile board in the Celotex range and the UK’s best-selling PIR insulation board.

Tools and Materials You’ll Need

  • Celotex GA4000 boards (correct thickness for your application)
  • Tape measure and pencil
  • Sharp Stanley knife or hand saw (fine-tooth)
  • Straight edge or cutting guide
  • Foil tape (Celotex or equivalent, minimum 75mm wide)
  • PPE: dust mask, safety glasses, gloves
  • Appropriate fixings (timber battens, screws, or adhesive depending on application)

Before You Start: Key Considerations

  • Confirm your U-value target — check Approved Document L for your project type before ordering.
  • Check rafter/joist depth — ensure the board thickness you’ve chosen fits within your structural depth, leaving at least 50mm ventilation gap for cold roof applications.
  • Store boards correctly — keep flat, dry, and off the ground prior to installation. Do not leave exposed to UV for extended periods.
  • Measure twice, cut once — GA4000 cuts cleanly with a sharp knife and straight edge; no specialist tools required.

Step-by-Step: Installing Celotex GA4000 Between Rafters (Warm Roof)

Step 1 — Measure and Mark

Measure the width of each rafter bay carefully. Rafter spacing can vary, even in modern builds. Mark the board face with a pencil, allowing for a snug friction fit (typically 2–5mm undersize on width to ease fitting without leaving excessive gaps).

Step 2 — Cut the Board

Using a sharp Stanley knife and a straight edge, score firmly along the marked line — one clean pass through the foil facing and into the foam. Snap the board away from the score line, then cut through the foil on the reverse. A fine-tooth handsaw also works well for multiple cuts.

Tip: Change knife blades frequently. A blunt blade drags and tears the foil facing, leaving a ragged edge that’s harder to tape.

Step 3 — Fit the Board

Push the cut board firmly into the rafter bay. The board should sit snug without bowing. If the gap between the board face and the underside of the roof deck is less than the required ventilation depth (for cold roofs), trim the board accordingly.

For warm roof installations (no ventilation gap required), boards can be fitted flush to the top of the rafters and a second layer run across the rafter faces below.

Step 4 — Tape All Joints and Perimeters

This is the most commonly skipped step and the most important. All board-to-board joints, board-to-rafter joints, and perimeter edges must be taped with foil tape to prevent warm moist air bypassing the insulation layer and causing condensation.

Use a minimum 75mm wide foil tape and press firmly to ensure full adhesion. Pay particular attention to corners and eaves where boards abut wall plates.

Step 5 — Second Layer (if required)

For warm roof specifications, a second layer of GA4000, TB4000, XR4000 or PL4000 is typically installed across the rafter faces, perpendicular to the first layer, to eliminate thermal bridging through the timber. Fix with timber counterbattens or proprietary insulated battens before plasterboarding or fitting a VCL.

Step 6 — Vapour Control Layer (VCL)

For warm roof and wall applications, fit a VCL (polythene membrane or specialist foil membrane) on the warm side of the insulation if not already incorporated into the build-up. Tape all laps and penetrations. Confirm with your architect or building control officer whether a VCL is required for your specific build-up.

Step 7 — Finish and Board Out

Install plasterboard (if PL4000 is not installed) or your chosen internal finish over the insulation layer, fixing through to the structural timber. Use appropriate screw lengths to ensure a minimum 25mm penetration into the rafter or batten.

Installing Celotex GA4000 on Floors

For ground floor applications (between joists or below screed):

  1. Ensure the subfloor is clean, dry, and level.
  2. Cut GA4000 boards to fit snugly between joists (or lay as a continuous layer below screed).
  3. Tape all joints with foil tape.
  4. For below-screed installations, lay a polythene DPM over the insulation before screeding.
  5. Do not allow screed to be poured directly onto the board without a separating layer.

Common Installation Mistakes

  • Leaving gaps at board edges: Even small gaps allow significant heat loss and can cause condensation. Fill with offcuts or expanding foam and tape over.
  • Skipping the foil tape: Untaped joints are the single biggest cause of underperforming PIR installations.
  • Using the wrong adhesive: Some solvent-based adhesives attack PIR foam. Use only adhesives specified as compatible with PIR / polyisocyanurate.
  • Not accounting for thermal bridging: Timber rafters and joists conduct heat. A single insulation layer between rafters rarely achieves the target U-value without a cross-batten layer below.

Safety Notes

Celotex GA4000 is combustible. Do not use near open flame during installation and ensure boards are covered by plasterboard or another Class 0 / A2 fire-rated lining as soon as practicable. Do not leave boards exposed in occupied spaces.

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In stock in all standard thicknesses from 25mm to 150mm, with fast UK delivery.

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