Envelope and protection · Insulation
Cellulose Insulation (Recycled Paper Fibre)
Explains what cellulose fibre is made from, why it is placed by machine rather than cut and fitted, how it differs from the batts and boards beside it, and what to record before the hose arrives.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What cellulose insulation is
Cellulose insulation is made by shredding and fiberising recycled paper, mostly newsprint, and blending mineral additives through the fibre. Those additives are part of the product, and what they are and what the manufacturer says about them belong in the declaration for the exact material rather than in a general description.
What most separates it from batts and boards is that it is placed rather than fitted. A machine blows the fibre down a hose, so it flows around pipes, cables, uneven joists and awkward corners without anything being cut to shape, and the same material can be blown loose into an open loft, packed at higher density into a closed cavity behind netting or a membrane, or sprayed damp into open studwork.
That puts the installer, the machine settings and the density actually achieved at the centre of what ends up in the building, in a way that does not apply to a board you can see and count. It is a plant fibre, so it takes up and gives off moisture rather than shedding it as a foam does, and what that means for a particular construction is a matter for a qualified professional.
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.
- Blown cellulose
- Recycled paper insulation
- Cellulose fibre insulation
- Cellulose fiber insulation (US spelling)
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Loose fibre blown over the ceiling joists of an open loft, following the shape of the void rather than being cut to it.
- Dense packed fill blown into closed timber frame walls, floors and roof voids behind netting or a fixed membrane.
- Damp spray application into open studwork before the internal lining is fixed, trimmed flush once it has set.
- Filling irregular or inaccessible voids in older buildings where board and batt products cannot be cut to follow the space.
- Topping up an existing loft layer, where the condition of what is already there has been assessed first.
- Filling around service runs and awkward junctions that would otherwise be left as gaps in a cut and fitted layer.
Consideration
Where it is commonly considered
- Where the void is irregular, congested with services, or simply not accessible enough for anyone to fit boards by hand.
- Where a competent installer with the right equipment is available, since the method is not a general building trade.
- Where the construction is intended to remain open to vapour movement, subject to assessment of the whole build-up.
- Where the fill can be fully contained by netting, membrane or lining so it stays where it was placed.
- Where a plant fibre is preferred over foams and mineral fibres for reasons the client and designer have set out.
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.
- In positions that may become wet or that already show damp, until the source has been investigated by a qualified professional.
- In loaded positions such as under a screed or slab, where compressive behaviour is an engineering matter entirely.
- In voids that are not properly closed, since loose fibre migrates into any opening left in the containment.
- Near flues, chimneys, recessed fittings and heat producing equipment, where clearances are for the installer and the authority.
- Where nobody can confirm the density actually achieved, because that is the part of the specification that is easiest to lose.
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 releases moisture rather than shedding it, which is a different behaviour from the foam boards and is one reason the whole build-up has to be considered together. Assessing what that means here is work for a qualified professional, not a rule of thumb.
- Thermal behaviour
- How the layer behaves depends on the material, the density achieved and whether the void is genuinely full, none of which can be judged from a delivery note. Figures belong to the manufacturer's documentation and to a calculation done for the actual construction.
- Fire-related questions
- No fire characteristic is described or compared here. Fire behaviour varies between insulation families and between products, and depends on the assembly around them, so it is a matter for the manufacturer's documentation, a qualified professional and the relevant authority.
- Installation dependency
- This is an installed system rather than a delivered product. Hose reach, the number and position of access holes, machine settings and the operator's judgement all shape the result, and none of it can be checked afterwards without opening the construction.
- Maintenance
- Loose fibre in a loft is easily disturbed by anyone working in the void, and it does not go back the way it came. Later cabling, aerial and plumbing work is the usual reason a blown layer stops being continuous.
- Documentation
- Because the layer is made on site, the installation record matters as much as the data sheet: which material, which areas, what target density, how many bags went in, and who carried out the work and to what instruction.
Exposure
Exposure and environmental conditions
- Whether water can reach the void from the roof covering, a gutter, a tank, a plumbing run or an appliance above it.
- How the loft or cavity is ventilated at present, and whether the proposed fill changes that arrangement.
- Whether the containment around the fill is complete, or whether fibre could escape into rooms, eaves or service voids.
- How cold and humid the void becomes in winter, which is a characteristic of the building rather than the material.
- What clearance the relevant installer and authority require between the fill and any flue, chimney or recessed fitting.
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.
- Whether the cavity or void is genuinely closed, since dense packing depends on containment that will hold the fill in place.
- The netting, membrane or lining used to retain the fibre, and who has specified it for this particular construction.
- Where the control layers sit within the build-up, which is assembly design for a qualified professional rather than a fill decision.
- Existing wiring, downlighters, pipework and flues within the void, all of which have to be located before filling starts.
- Access for the hose and for the operator, which can determine whether an area gets filled properly or only partly.
Appearance
Appearance and finish considerations
- In a loft the fill stays on view and is easily disturbed, which matters where the space is also used for storage.
- Access holes drilled for the hose in walls and ceilings have to be made good, and that making good is visible work.
- Damp sprayed applications are trimmed flush before lining, so the flatness of the trimmed face affects the lining over it.
- Loose fibre finds its way into rooms and roof spaces during installation and needs containing and cleaning up afterwards.
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.
- Whether the fill can settle in vertical or sloped positions, and what density and containment are intended to prevent that.
- How the layer would be affected by a slow leak that nobody notices for a long period.
- Whether pests or birds can reach the void through eaves, vents or service openings in this particular building.
- Whether anyone walking or storing items in the loft will compress the layer permanently.
- How the containment behaves over time, since the fill only stays where it was put if what holds it stays intact.
Upkeep
Maintenance and repair considerations
- Who is responsible for restoring the fill after later cabling, plumbing or aerial work in the void.
- How wet or contaminated fibre would be identified, removed and replaced, which is a decision for a qualified professional.
- Whether the same material and density could be reinstated in a small area, and by whom.
- What record exists of where the fill was placed, so that a future owner or trade knows what is behind the surface.
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.
- A competent installer with the correct machine, since the method depends entirely on equipment rather than hand tools.
- A survey of the void beforehand to find services, flues, existing damp and anything the fill must be kept clear of.
- Complete containment in place before filling, because loose fibre will follow every opening that has been left.
- Dust control and protection of the occupied parts of the property while the work is under way.
- A written installation record made at the time, since nothing about the layer can be verified afterwards by eye.
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 and declaration for the exact material being installed, including its additives.
- The manufacturer's installation instructions covering the application method actually being used here.
- The installer's own record of areas filled, target density and quantity used, made at the time of the work.
- Manufacturer guidance on clearances, containment and any positions the material is not intended for.
- Confirmation of who carried out the work and under what instruction, 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 application method is proposed here, and why that one rather than blown, packed or damp sprayed alternatives?
- What density is being targeted, how will it be achieved, and how will anyone know afterwards that it was?
- Is the void properly contained, and what is holding the fill in place in vertical and sloped positions?
- How is the ventilation of this roof, wall or floor being handled once the void has been filled?
- How are the control layers arranged in this build-up, and who has designed that arrangement?
- What survey of the void has been done to locate services, flues, existing damp and recessed fittings?
- What happens to this construction if water reaches the fill, and how would that come to light?
- Which parts of this work need confirming with the relevant authority before it starts?
Blind spots
Commonly overlooked points
- This is a layer made on site by an installer, so the quality of the work matters more than the choice of brand or bag.
- Once the fill is in, nothing about it can be checked by looking, which is why the record made on the day matters.
- Access holes in an external wall have to be made good properly, and that is often treated as an afterthought.
- A dense packed cavity depends on the containment holding, and containment is the part most often improvised.
- Anyone working in the loft afterwards will disturb the layer, and it rarely gets put back the way it was found.
What this page does not do
- No fire statement of any kind is made here, including about any additive; fire behaviour is a matter for the manufacturer's documentation, the assembly, a qualified professional and the relevant authority.
- No thermal figure, saving or performance outcome is stated or implied anywhere in this entry.
- Condensation and interstitial moisture risk require professional assessment of the actual construction and cannot be inferred from the material.
- Build Design Hub does not test, approve, certify or recommend any insulation material, installer or installation method.
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.
Insulation Materials
Reference entries for thermal insulation materials — mineral and glass wool, rigid foam boards, natural-fibre insulations, blown and sprayed products and reflective systems.
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