Blown insulation for lofts and cavity walls

The item covers blowing the insulation by machine onto a prepared surface or into a closed cavity. This includes setting the machine to the specified density, laying out the hose, marking the layer height on depth gauges, spreading the material evenly and, for cavities, drilling and sealing the filling holes. At the end the installer should hand over a record of depth, area and the number of bags used, so that the actual density can be checked.

The item usually does not include ceiling repairs, fitting a vapour control layer, walkways, removing old fill or scaffolding. These are priced separately, and it is worth agreeing at the survey who will do them.

Ceiling below an unheated loft

This is the most rewarding use. The loft is not lived in, heat escapes through the ceiling and the flakes are simply blown onto the open surface to the required depth. Under a low pitched roof where there is not even room to crawl, it is often the only sensible way to get insulation in at all. On a timber joist ceiling with old fill between the joists, the fill does not always have to be removed. If it is dry and firm, it is enough to level it and blow the insulation on top.

Pitched roofs and converted lofts

In a loft that is already lined with plasterboard or boards, the insulation is blown into the closed bays between the rafters. The space must be clearly defined: a vapour control layer and lining below, a breathable roofing membrane above and a ventilated gap under the roof covering. Without this build-up, blowing is not recommended.

Cavity walls and timber frame

Many houses built across Europe in the twentieth century have external walls made of two leaves with an air gap between them. Blown wool can be injected through holes drilled in the mortar joints without any demolition. Walls and floors of timber-frame buildings are filled the same way: a mesh or board is fixed before closing and each bay is blown full. With cavity walls the condition of the gap is decisive. It must be continuous, dry and largely free of mortar droppings and rubble that would stop the flakes.

Where blown insulation is not suitable

• in damp structures and under a leaking roof, until the cause has been fixed,
• in a pitched roof without a ventilated gap under the covering, for example under bitumen shingles on solid boarding without ventilation,
• in cavity walls on exposed elevations where driving rain penetrates the outer leaf, without an assessment by a specialist,
• on surfaces people walk on without walkways or a load-bearing frame, because the flakes are compressed underfoot.

There are three main types of blown insulation on the market. Glass wool, for example Supafil from Knauf Insulation, Isover InsulSafe or Ursa PureFloc, is light, non-combustible with reaction to fire class A1 and, according to the manufacturers, hardly settles. Isover's technical data sheet gives a density of about 15 kg/m³ for open blowing, a thermal conductivity of 0.041 W/(m·K) and settlement class S1, meaning less than 1 %. Another advantage is that rodents and insects avoid it.

Stone wool is heavier and somewhat more resistant to high temperatures, so it is sometimes chosen around chimneys or where fire safety requirements are higher. Cellulose made from recycled paper stores heat well, but it contains additives against fire and mould and, according to mineral wool manufacturers, settles more noticeably. It is therefore installed at a higher density or with a larger depth allowance. Which material to choose depends on the structure, the fire requirements and the recommendation of the designer or an experienced installer.

The depth should not be guessed. It should come from a calculation or at least from the manufacturer's tables. On a loft floor, 30 to 40 cm of glass wool is common today. As a guide, the Isover InsulSafe data sheet gives a depth of about 31 cm for a thermal resistance of 7.5 m²K/W and about 41 cm for 10 m²K/W at a density of 15 kg/m³. At that depth, roughly 29 to 38 bags are used per 100 m².

Closed cavities are filled at a higher density than an open loft floor, so that the flakes fill the whole space and do not compact over time. The exact value is in the data sheet of the specific product. A good installer can work back from the number of bags and the cavity volume to the density actually achieved.

In practice it happens that a crew blows by eye and a year later the layer looks thinner than agreed. It is therefore worth insisting on depth gauges spread over the whole area and on a record of the number of bags.

• clear the loft of stored items so that the crew can reach the eaves,
• check the roof for leaks and have damaged tiles or flashings repaired,
• make sure the roof ventilation at the eaves and ridge will stay open,
• find out where electrical cables, junction boxes and recessed ceiling lights are,
• decide on walkways to services, the chimney and the aerial before blowing,
• provide access for the machine, which usually stands outside the house or in a van, and a power socket if the installer needs one.

Ceiling below an unheated loft

1. Inspect the ceiling and roof, measure the area and choose the depth from a calculation or the manufacturer's table.
2. Check the airtightness of the ceiling. On timber ceilings without a continuous layer underneath, add a vapour control layer or at least seal the penetrations.
3. Box off the area around the chimney, the loft hatch and any lights that can get warm, following the manufacturer's instructions and fire regulations.
4. Fit eaves stops that keep the flakes out of the ventilated gap under the roof covering.
5. Place depth gauges with the layer height marked, roughly every few square metres.
6. Set the machine to the density from the data sheet and blow from the furthest point back towards the hatch.
7. Spread the layer evenly up to the marks on the gauges, without walking on the finished surface.
8. Insulate and seal the loft hatch so that heat does not escape through it.
9. Record the area, depth and number of bags and hand this information to the owner.

Closed cavity in a wall or pitched roof

1. Check with a borescope or probe how deep, continuous and dry the cavity is.
2. Plan a grid of filling holes so that the insulation reaches the whole area.
3. Drill the holes, in masonry usually in the mortar joints.
4. Blow the insulation at a higher density, hole by hole, until the machine meets resistance.
5. Check the fill with another probe or with a thermal camera when there is a temperature difference between inside and outside.
6. Seal the holes and make good the surface.

Warm indoor air carries moisture. If it passes through the ceiling into cold insulation, it can condense there. So a simple rule applies: underneath, on the warm side, the structure should be as airtight as possible, while above, the insulation should be able to breathe freely. A plastered concrete ceiling is usually tight enough. With timber joist ceilings, board linings or plasterboard, a vapour control layer with taped joints is normally added.

A ventilated space must remain above the insulation. Vents at the eaves and ridge must not be blown shut, otherwise moisture builds up in the loft and mould appears on the rafters. Penetrations through the ceiling, such as cables, lights, ventilation ducts and chimney stacks, should be sealed before blowing. Later nobody can reach them.

• lower density than the manufacturer specifies: the layer looks thick but insulates worse and may settle over time,
• uneven layer: less insulation ends up at the edges and behind joists, creating cold strips,
• eaves ventilation blown shut: the loft can no longer breathe, water condenses and the timber gets damp,
• leaky ceiling from below: moist air passes into the insulation and reduces its effect,
• no walkways: every time someone checks the aerial or chimney the insulation gets trampled,
• blowing under a leaking roof: wet wool stops insulating and the problem stays hidden,
• cavity full of mortar and rubble: the flakes stop and voids remain in the wall,
• no record of material used: later it is impossible to tell whether the depth and density were achieved.

Owners often compare quotes only by the price per square metre. The difference then lies in depth or density, which cannot be seen at first glance. When comparing, it pays to ask for the depth, density, product and number of bags per 100 m².

Blown insulation is installed by specialist insulation contractors, often trained or approved by the manufacturer of the specific blowing system, and also by carpentry and roofing firms. It is worth asking for references, for proof of manufacturer training and whether a record of depth and material used will be handed over. In apartment buildings, work on the roof and external walls is decided by the owners' association or the building manager.

Insulating a loft floor or filling cavities normally does not change the appearance of the house. Check with your local authority whether your project needs to be notified or approved.

In many European countries loft and cavity insulation is supported by grants, energy supplier schemes or tax relief. Conditions change often and usually require a registered installer and sometimes an energy assessment. Check the current conditions on the official portal in your country before you order the work.

Blowing is a job for the machine and an experienced operator. The owner can, however, prepare the loft and save the crew time and themselves money for extra hours.

1. Clear the loft and carry old items, boards and boxes to the hatch.
2. Sweep or vacuum the ceiling and remove any building rubble.
3. During heavy rain, check the roof from inside with a torch and note where light shows through or water drips.
4. Mark junction boxes, lights and cables that pass through the ceiling with coloured tape.
5. Tape or seal the gaps around penetrations and at the hatch so that warm air cannot flow through them.
6. Build simple walkways from timber battens and boards to the chimney and aerial, higher than the future layer.
7. Photograph the prepared loft. The photos help at handover and with any grant application.

On an open loft floor the check is easy. Look at the depth gauges to see whether the layer reaches the mark everywhere, and measure it in a few places with a folding rule. The surface should be even, without hollows at joists and edges. The vents at the eaves must be clear and the hatch insulated.

Density can be checked by calculation. The bag weight multiplied by the number of bags and divided by the volume of the layer should roughly match the value in the data sheet. For cavities in walls and pitched roofs, the best check is a thermal camera during the first heating season. Cold spots show where insulation is missing, and the installer can top them up.

Handover checklist

• the layer depth matches the agreement over the whole area,
• the installer has handed over the product name, area, depth and number of bags,
• ventilation at the eaves and ridge is clear,
• the clearance around the chimney and lights follows the manufacturer's instructions,
• the hatch is insulated and closes tightly,
• walkways to the aerial and chimney sit above the insulation,
• filling holes in the walls are sealed and made good,
• there are no signs of leaks or damp patches anywhere.

In the NUMIQ price list, blown glass wool insulation is listed as labour per m² including minor consumables. The insulation itself, the bags of wool, is charged separately as material. The guide price for the labour is about €15 per m². Prices vary by country, region and city and are usually higher in large cities.

The labour price depends mainly on whether it is an open loft floor or a closed cavity. Cavities add drilling, checking and making good the holes. Access to the loft, blowing height, the distance between the machine and the work area, the need for walkways and sealing the ceiling also play a role. The material depends on depth and density: the thicker the layer, the more bags. When comparing quotes, always compare the same depth and the same product.

If the roof is being worked on at the same time, see the article Sheet Metal Works. For a habitable loft, sprayed insulation is often chosen instead of blowing, described in Sprayed Closed-Cell Foam Insulation. If you are also planning to insulate the facade, read Building insulation.