Envelope and protection · Insulation
Wood Fibre Insulation Board and Batt
Explains the wet and dry production routes behind wood fibre insulation, why it is the natural fibre most often available as a rigid board, and what to settle before a particular grade is chosen.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What wood fibre insulation is
Wood fibre insulation is made from wood residues, usually softwood, which are chipped and defibred and then formed into a mat. In the wet route the fibres are largely bonded by the wood's own resins as the mat is pressed and dried; in the dry route a binder is added and the mat is heat pressed. The route affects the character of the finished board more than the shared name suggests.
It is the natural fibre most often available as a genuinely rigid board, and the grades are quite distinct from one another: boards intended to receive a render system, boards intended to sit behind a ventilated cavity, boards profiled with tongued and grooved edges for sarking and sheathing positions, and separate flexible batts for filling between studs, joists and rafters. They are not interchangeable.
It is also markedly heavier than a fibrous quilt or a foam board, which changes handling on site, the fixings used and what the supporting structure is asked to carry. Like the other plant fibres it takes up and releases moisture rather than shedding it, and how that behaves within a specific build-up is for a qualified professional to assess rather than something to assume.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Wood fiber insulation (US spelling)
- Woodfibre board
- Wood fibre board
- Softwood fibre insulation
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Rigid boards fixed externally over a timber frame or a solid wall as the layer a render system is applied to.
- Profiled boards laid over rafters in a sarking position, within a roof build-up designed by a qualified professional.
- Sheathing grade boards used over studwork where the board serves as a face as well as an insulation layer.
- Flexible batts friction fitted between studs, rafters and floor joists in timber construction.
- Internal lining boards applied to the inner face of solid walls, where the whole wall has been assessed first.
- Boards used in retrofit work on older buildings where a vapour open build-up has been designed by a professional.
Consideration
Where it is commonly considered
- Where a rigid natural fibre board is wanted and the grade has been matched to the position by the designer.
- Where a construction is intended to remain open to vapour movement, subject to assessment of the entire build-up.
- In work on older or traditionally built walls where a professional has designed the build-up around this material.
- Where the board is doing more than one job, such as insulating and carrying a render or acting as a sheathing face.
- Where the additional weight compared with quilts and foams has been allowed for in the fixings and the structure.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where a render carrier grade and a general insulation grade are being treated as interchangeable products.
- In positions exposed to weather for longer than the manufacturer allows before the covering system goes on.
- In loaded positions beneath screeds or slabs, unless a specific grade has been selected for that duty by an engineer.
- On walls with unresolved damp, salt staining or rain penetration whose cause has not yet been diagnosed.
- Where the structure and fixings have not been checked against the weight of the board being proposed.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- As a plant fibre it takes up and gives off moisture rather than shedding it, which is central to why it is chosen for some build-ups and why the build-up has to be designed as a whole. That assessment is work for a qualified professional on the actual construction.
- Thermal behaviour
- Thermal behaviour varies between grades, since the same material is produced at very different densities for different duties. Any figure belongs to the manufacturer's documentation for the specific board and to a calculation prepared for the real build-up.
- Substrate dependency
- Where a board carries a render or acts as a sheathing face, the fixings, the spacing of the members behind it and the flatness of what it is fixed to all become part of the specification rather than site detail.
- Fire-related questions
- No fire characteristic is stated or compared here. Fire behaviour differs between insulation families and between products, and depends on the coverings and the assembly, so it belongs to the manufacturer's documentation, a qualified professional and the relevant authority.
- Weathering and exposure
- Boards are not intended to stay open to the weather. Manufacturers set out how long a board may be exposed before the render or covering system is applied, and that window is a sequencing question worth agreeing before the work starts.
- Installation dependency
- The boards are heavy and the edge profiles only help if boards are laid tight and joints fall where the system expects. Cutting is dusty, off-cuts are bulky, and a board fixed to a member that is not straight will not sit flat.
- Documentation
- Ask for the data sheet for the exact grade, not the range, together with the system documentation where the board carries a render, and the manufacturer's stated exposure period before covering.
Exposure
Exposure and environmental conditions
- How long boards may remain uncovered once fixed, and whether the programme genuinely allows for that window.
- Whether the wall or roof is exposed to driving rain, and what the outer layers are intended to do about it.
- Whether the existing construction shows damp or rain penetration that needs resolving before anything is fixed.
- How the space around and behind the boards is ventilated, and whether the work changes that arrangement.
- Whether boards can be stored dry on site, since heavy natural fibre boards take up water if left uncovered.
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The grade selected for each position, since render carrier, sarking and general insulation boards are different products.
- The fixings specified for the weight of the board and for the actual member or wall behind it.
- The straightness and spacing of the studs, rafters or battens the boards are fixed to, which decides how flat they sit.
- Where the control layers sit within the build-up, which is assembly design for a qualified professional rather than a board choice.
- How the layer is closed and detailed at eaves, verges, openings and the base of a wall.
Appearance
Appearance and finish considerations
- Where a board carries a render, the flatness of the boards is what the finished elevation will show, so the fixing matters visibly.
- Board joints can telegraph through a thin render or plaster if the boards move or joints are not treated as the system requires.
- Adding boards externally changes the depth of reveals and the projection of sills, eaves and verges on that elevation.
- In internal lining work the room loses depth, and skirtings, architraves and reveals all have to be reworked.
- Boards left exposed during construction discolour, which is a sequencing signal rather than a decorative problem.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How the board behaves in the moisture conditions of this exact position, assessed rather than assumed from the family.
- Whether the fixings will hold the weight of the boards and the covering over them in this particular structure.
- What would happen if the covering system failed and water reached the boards over a long period.
- Whether insects or rodents can reach the layer through an open eaves, a service opening or an unclosed base detail.
- How movement in the structure behind has been allowed for so that joints do not open in the finish.
Upkeep
Maintenance and repair considerations
- What would have to be removed to reach a board if a problem developed behind a render or a covering.
- Whether the same grade and profile would still be obtainable if part of the layer needed replacing.
- How damage to a render carrier board is repaired within the system, and with which components.
- What the system supplier requires so that repairs leave the documentation still describing what is there.
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Dry storage and dry site conditions, since heavy natural fibre boards take up water readily when left out.
- Fixings and fixing patterns taken from the system documentation rather than decided on site.
- Boards laid tight with profiled edges engaged as the manufacturer sets out, and joints staggered where required.
- A programme that gets the render or covering on within the exposure period the manufacturer allows.
- Dust control when cutting, and enough handling capacity on site for a board that is heavier than most alternatives.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The technical data sheet for the exact grade and density supplied, rather than the manufacturer's range brochure.
- System documentation where the board carries a render or forms part of a wider wall or roof arrangement.
- The manufacturer's stated exposure period between fixing the boards and applying the covering.
- Fixing schedules and guidance for the actual substrate and for the weight of board being used.
- A record of the grades installed and where, kept with the property documents.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which grade of wood fibre board is specified in each position here, and what duty is each one doing?
- Is this board carrying a render, acting as a sheathing face, or purely insulating in this particular build-up?
- How long can the boards stay exposed before the covering goes on, and does the programme actually allow for that?
- Have the fixings and the structure been checked against the weight of the boards proposed?
- Where do the control layers sit in this construction, and who has designed and signed off that arrangement?
- How is the existing wall or roof being assessed for damp before anything is fixed over it?
- What are the details at eaves, verges, openings and the base of the wall where this layer stops or turns?
- Which parts of this work need confirming with the relevant authority before anything is ordered?
Blind spots
Commonly overlooked points
- Wood fibre is one name covering several genuinely different products, and the grade does most of the work.
- The weight surprises people, both in handling on site and in what the fixings and the structure are asked to carry.
- The exposure window before rendering is a real constraint and is regularly lost to weather and programme slippage.
- A board fixed to a member that is not straight will not sit flat, and a render will show that for the life of the wall.
- Adding boards to an elevation changes the reveal depth and the eaves projection, which is design work rather than detail.
What this page does not do
- This entry makes no fire statement or comparison of any kind; fire behaviour is a matter for the manufacturer's documentation, the assembly, a qualified professional and the relevant authority.
- No thermal value, energy outcome or moisture performance is claimed anywhere in this entry.
- Whether a vapour open build-up is appropriate for a particular wall or roof requires professional assessment of the actual construction.
- Build Design Hub does not test, approve, certify or recommend any board, grade, render system or supplier.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
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