Loose-fill (blown-in) insulation
Loose fibres blown by machine into cavities, conforming without seams around obstacles. Ideal for retrofit where batt installation is not feasible.
What is loose-fill insulation and how is it installed?
Loose-fill insulation consists of small, discrete fibres or granules conveyed by a blowing machine into cavities where they settle to form an insulating layer. The machine draws material from a hopper, controls the flow rate, and propels it through a hose into the target space: typically attic floors, wall cavities, or closed roof panels. Installation is rapid and requires only small access holes, making loose-fill particularly suited to retrofit work where opening walls or roofs to install batts or boards would be impractical or expensive.
Two installation methods exist. Open blowing fills attic floors to a predetermined depth, allowing material to settle naturally over time; this method is simpler and leaves ample space for maintenance access. Dense-pack compresses material into closed cavities at higher density, minimizing settling and air movement but requiring skilled technique and careful coordination to avoid over-pressurizing the cavity or shredding the insulation.
What materials are used in loose-fill insulation?
Several materials can be blown as loose fill, each with distinct properties suited to different applications and climates.
| Material | Source | Characteristics | Typical use |
|---|---|---|---|
| Cellulose | Recycled newsprint and cardboard, treated with fire retardant and borate | Good acoustic damping; higher thermal mass; settles 10–20 percent; hygroscopic; benefits from vapour control | Attic floors; walls in dry climates; retrofit where existing framing is irregular |
| Mineral wool (stone and glass) | Blast furnace slag, trap rock, or silica sand melted and spun into fibres | Non-combustible; resists settling (2–4 percent); will not support mold; vapour permeable; slightly higher density | Attic floors; walls; dense-pack in cavities; high fire-rating requirements |
| Wood fibre | Softwood particles (spruce, pine) with minimal binders | Bio-based; good acoustic properties; hygroscopic; lower density; suitable for interior applications; less common in UK and Central Europe | Attic floors in well-ventilated conditions; walls with adequate vapour management |
| Perlite or expanded glass beads | Volcanic glass (perlite) or recycled glass expanded by heat | Lightweight; will not rot or settle; incombustible; lower thermal mass; less common; can migrate in windy cavities | Specialized applications; retrofit in situations requiring inorganic material and minimal settling |
| Expanded polystyrene (EPS) beads | Expanded polystyrene produced as loose granules | Lightweight; will not absorb moisture; combustible (requires careful handling); settles minimally; good performance in cold climates | Retrofit cavities; attic applications where settling must be minimal |
What are the advantages of loose-fill insulation in retrofit projects?
Loose-fill insulation excels in retrofit applications because it conforms to existing geometry without requiring structural modification. Unlike rigid boards or batt products, it fills irregular spaces created by non-standard joist spacing, crossbracing, plumbing runs, and electrical conduit. It leaves no compression gaps and performs well around obstacles, a critical advantage where existing framing is congested.
Retrofit installation is non-invasive; in attics, material is blown from an access hole and settles in place, with no need to open ceilings or remove finishes. In walls, one or two holes allow cavity filling, and the holes can be patched neatly afterward. The speed of installation also reduces labour costs and occupant disruption, making loose-fill economically attractive for upgrading existing buildings to higher insulation standards.
How does settlement affect performance and density control?
Settlement is the gradual compression of loose material under its own weight and vibration. Cellulose settles 10–20 percent, while mineral wool and glass fibre settle roughly 2–4 percent. Higher settling rates require installers to overfill attic spaces to account for future compression, or to place material at a higher initial density so that after settlement, it reaches the target thermal resistance.
In walls and enclosed cavities, dense-pack placement is preferred because it achieves final density immediately and virtually eliminates settling. The installer blows to the density specified in the product's technical approval, which varies depending on cavity geometry and structural design. The installer must control blowing pressure and rate carefully; excessive force can crush the material, reduce its thermal performance, and potentially distort or damage the cavity structure.
Overfilling in attics, initially placing more material than the target final depth, is standard practice to account for settling over years. The amount depends on the material type, initial packing density, and acceptable final thickness.
How should vapour control be managed with loose-fill insulation?
In Central European climates, moisture management below insulation is essential. Warm interior air carries moisture, which must not condense within the insulation layer or on cold structural surfaces. A vapour retarder (low-permeance membrane) installed below loose-fill insulation in attics provides this control, limiting vapour diffusion from interior air while allowing any moisture trapped during construction to escape gradually through the upper side.
The vapour retarder must be intact and continuous; tears or gaps allow moisture to bypass it. In wall cavities, vapour control placement follows the structural design and climate zone rules (STN standards). The retarder sits on the warm side (interior) of the insulation, creating a clear boundary between the conditioned space and the insulation itself.
In unheated or semi-heated spaces, or in cold-roof construction above non-habitable attics, vapour management shifts: the insulation may be placed above a vented layer, or ventilation is designed to remove any moisture. The specific strategy depends on whether the space is heated, ventilated, or cold, and on local climate.
What about maintenance access and protecting loose-fill from disturbance?
Once blown, loose-fill insulation in an attic is vulnerable to disturbance. Walking on it compresses and damages the material, creating gaps and voids. To preserve performance and maintain access to electrical, plumbing, or HVAC systems, permanent walk-boards must be installed above the insulation surface.
Walk-boards are typically made from plywood, spanning across the joists and sitting on top of the final insulation layer. Sides or low railings prevent material from sliding off. These boards allow service personnel to move through the space without damaging the insulation, and they define safe walking paths, reducing the risk of foot slips.
| Access consideration | Requirement |
|---|---|
| Walk-board material | Plywood with grain oriented to direct moisture outward; secured to joists or blocks to prevent shifting |
| Walk-board coverage | Full-length catwalks along principal paths; branching to mechanical equipment and electrical panels as needed |
| Side rails or trim | Lumber edge trim prevents material spillage and serves as a visual boundary |
| Equipment access | Air handlers, boilers, and electrical boxes must remain unburied and accessible for service; blown insulation must not bury or restrict airflow around active equipment |
| Ventilation preservation | Ensure eaves vents and rafter vents are not blocked by blown material; maintain airflow paths in ventilated roofs |
What are the typical installation sequences and quality checks?
Installation begins with access-point setup and air sealing before blowing. For attics, depth is marked; for cavities, the operator watches for resistance and material backing out, signalling fullness. Quality checks include visual inspection for uniform coverage, confirmation that joists and obstacles are fully bridged, and measurement of depth and density as specified. Any voids should be topped up. In dense-pack, the design specifies target density; the installer verifies the cavity reaches it without over-pressurization. Final steps include sealing temporary access holes and installing walk-boards in attics.
Frequently asked questions
- What is the difference between open blowing and dense-pack?
- Open blowing fills an attic floor with loose material to a settled density, typically used where no structural constraints exist and settling is acceptable. Dense-pack compresses the material into cavities (walls, rafter spaces, closed roof panels) at higher density, minimizing settling and requiring careful technique to avoid damaging the cavity or the insulation itself.
- How much does loose-fill insulation settle over time?
- Cellulose typically settles 10–20 percent over years due to gravity and vibration, while mineral wool and fiberglass settle much less, roughly 2–4 percent. Settlement rates determine the final R-value and influence the need for initial overfilling or dense-pack placement.
- Can I walk on loose-fill insulation in an attic?
- Walking directly on loose fill compresses and damages it, reducing performance. Permanent walk-boards or catwalks made from plywood are essential for access to electrical, plumbing, or HVAC systems; these boards sit on the joists above the insulation surface.
- Is a vapour barrier required below loose-fill insulation?
- In Central European climates, a vapour retarder (typically a low-permeance membrane) below loose-fill insulation in attics helps manage moisture from the interior, especially in winter. The exact requirement depends on the climate zone and building assembly; local building standards (STN) and the structural design should guide this decision.
- Why doesn't loose-fill leave gaps like batt insulation?
- Loose material conforms to all surfaces, bridging around joists, pipes, and electrical conduit without compression. This continuous coverage eliminates the gaps that occur when batts are poorly fitted or compressed around obstacles, a major retrofit advantage in existing structures.
- What is the ideal density for loose-fill insulation?
- Attic floors typically use lower settled densities, while walls and enclosed cavities require higher densities to minimize settling and air movement. The product's technical approval and the structural design specify the target density for each application.