Loft and roof insulation with mineral wool

In the NUMIQ price list, this item covers the labour of laying mineral wool insulation in rolls or slabs, priced per m² of insulated area. It applies to two common situations:

• Ceiling below a cold, unheated loft. The insulation is laid from above, from inside the loft, between the ceiling joists and then across them, or directly on a concrete ceiling in one or two layers.
• Room in the roof. The insulation is fitted between the rafters, with a further layer below the rafters, then closed from the inside with a vapour control layer and a lining, usually plasterboard.

The work includes cutting and fitting the insulation, laying it in layers with staggered joints, filling in around walls, chimneys, flues and service penetrations, and basic fixing so the insulation stays in place. Small consumables such as knives, tape and staples are included in the labour price.

What the work does not normally include: the mineral wool itself, the vapour control layer and its installation, timber battens and loft legs, the plasterboard lining, crawl boards or a boarded storage deck in the loft, roof timber repairs, replacing the roofing membrane, and removing and disposing of old insulation. These are priced as separate items.

The first decision is which part of the house will be warm and which will stay cold. This determines where the insulation goes.

The loft is not used as living space

If the loft is used only for light storage, the ceiling below it is insulated. The loft stays cold and unheated, and only the rooms below the ceiling are heated. This is known as a cold roof and is the cheaper option, because the ceiling area is smaller than the roof slopes and the insulation is easier to lay, from above and without working overhead. With timber joists, the first layer goes between the joists and the second layer across them. On a concrete ceiling, the insulation is laid directly on the slab.

The roof space is used, or will be used, as living space

If people live in the roof space or rooms are planned there, the roof slopes are insulated. Insulation goes between the rafters, below the rafters and into any flat ceiling above the room. This is a more demanding build-up that must be closed airtight from the inside with a vapour control layer and protected on the outside by a ventilated gap or a breathable roofing membrane.

Time of year

The work can be done all year round, because it is a dry process inside the roof. Most homeowners deal with it at the start of winter, when the first heating bills arrive. In summer, an unventilated loft gets very hot and the work is physically harder, so spring and autumn are usually the most comfortable seasons for the installers.

Check the roof before insulating. If the roof leaks, if the timbers show signs of woodworm or rot, or if there is no roofing membrane or felt, these problems should be dealt with first. Wet insulation loses its insulating properties and the timber underneath decays.

Ceiling below a cold loft

1. Prepare the loft. Clear it, vacuum it, remove old insulation that is damp or damaged, and check the electrical cables on the ceiling. Fit protective covers over recessed downlights.
2. Seal the ceiling. Seal gaps around pipes, cables, light fittings and the tops of partition walls so that warm, moist air from the rooms does not flow into the insulation. With a lath and plaster or boarded ceiling with cracks, an airtight layer or vapour control layer is considered.
3. Lay the first layer. Between joists, cut the insulation 1 to 2 cm wider than the gap so it fits snugly. On a concrete ceiling, lay it over the whole area with the rolls or slabs butted tightly together.
4. Lay the second layer across. Lay the second layer at right angles to the first so it covers the joints and the joists, which act as thermal bridges.
5. Finish the edges. Take the insulation out to the external walls over the wall plate, without blocking the eaves ventilation. Keep the required clearances around chimneys and flues, and lift cables so they rest on top of the insulation, not under it.
6. Insulate and draught-proof the loft hatch. Fix insulation to the top of the hatch and fit draught strip around the frame. An unsealed hatch or loft ladder is often the biggest single heat leak in the whole ceiling.
7. Protect the water tank and pipes. Lag pipes and fit a jacket on a cold water tank, but leave the area directly below the tank uninsulated so that warmth from below helps prevent freezing.
8. Create walkways. If the loft is accessed, fix boards on loft legs or raised timbers above the full depth of the insulation so that it is not compressed.

Room in the roof, between and under the rafters

1. Check the roof from outside and the roofing membrane or felt. The membrane must be intact. With traditional bitumen felt or a non-breathable membrane, a ventilated gap of at least 50 mm is kept above the insulation. With a breathable membrane, follow the requirements of the roof build-up and the membrane manufacturer.
2. Measure the gaps between the rafters. Cut the insulation 1 to 2 cm wider than the clear space between rafters and press it in gently with no gaps. Where a ventilated gap is needed, fix battens or stops to the sides of the rafters to hold the insulation back.
3. Fix battens below the rafters. Fix the battens across the rafters. Their depth matches the thickness of the second layer.
4. Fit the second layer below the rafters. This layer covers the rafters and reduces thermal bridging, because timber insulates worse than wool. Insulated plasterboard is a common alternative for this layer.
5. Fit the vapour control layer. Fix the membrane on the room side, seal all joints, overlaps and penetrations with the system tape and seal the edges to the walls with sealant. Repair any holes with repair tape.
6. Fit the lining. A thinner third layer of insulation can be placed within a metal or timber framework, then the plasterboard is screwed on.

Insulation manufacturers describe this sequence in their installation guides for pitched roofs. As a general principle, most of the insulation should sit on the cold side of the vapour control layer so that moisture does not condense on the membrane. For less common build-ups, a condensation risk assessment by the designer or manufacturer is advisable.

Mineral wool

Glass wool or stone wool is used for lofts and roofs, from manufacturers such as Knauf Insulation, Rockwool or Isover. It is non-combustible, vapour open and also reduces noise. Rolls suit the open floor of a loft, while slabs are stiffer and stay in place between rafters without support. For fitting between rafters, manufacturers offer flexible slabs and rafter rolls that spring into place between the timbers and do not fall out.

Recommended depth

For a loft insulated at ceiling level, the widely recommended minimum depth of mineral wool is 270 mm, typically a 100 mm layer between the joists and a 170 mm layer across them. This gives a U-value of roughly 0.16 W/m²K, which is the improved value usually expected when an existing roof is upgraded. Many installers now lay 300 mm. At rafter level, the rafters are usually too shallow to reach a similar value with mineral wool alone once the ventilated gap is allowed for, so it is combined with a layer below the rafters, often PIR board or insulated plasterboard. The exact thickness depends on the product and the build-up, and a calculated U-value from the supplier is more reliable than a rule of thumb.

Vapour control layer and roofing membrane

The vapour control layer is a membrane with high vapour resistance on the warm, room side of the insulation. The roofing membrane is on the cold side and, where it is in contact with insulation, it must be breathable so that moisture can escape from the structure. Confusing these two membranes is a serious mistake. For more detail, see the article on vapour barrier installation.

Other options

Besides rolls and slabs, blown insulation is used in lofts and fills uneven and awkward spaces better. For rooms in the roof, sprayed foam applied directly to the underside of the roof is sometimes chosen, but it needs careful assessment, because it can trap moisture in the timbers and some lenders and surveyors are cautious about it. Expanded polystyrene is rarely laid on a loft floor, because it is not vapour open and it is combustible.

Rolls and slabs, or blown insulation

Laying rolls and slabs suits lofts that are high enough to move around in comfortably and have a flat ceiling. The advantage is that the owner can see exactly what depth has been installed, and the insulation can easily be lifted later if something needs repairing in the ceiling or the wiring. Between the rafters of a room in the roof, there is practically no alternative to rolls and slabs, because the insulation must stay in place on a slope.

Blown insulation makes sense in a low loft where standing is impossible, where there are many joists, struts and awkward spaces, and where carrying material up a ladder is difficult. The machine stays outside the house and the insulation is delivered through a hose. On a clear, open loft floor, both methods are comparable, and the decision comes down to price and installer availability.

• Gaps between rolls or slabs. Insulation is sometimes cut exactly to the gap or laid with spaces between pieces. Heat escapes through the gaps and the insulation loses part of its effect.
• A single layer only between the joists. The joists remain exposed and act as thermal bridges. A second layer laid across covers them.
• Holes and untaped joints in the vapour control layer. Moist air flows into the insulation, condenses and the timber gets damp. Typical weak points are electrical boxes, penetrations and junctions with walls.
• Vapour control layer on the wrong side. A membrane with high vapour resistance under the roof covering traps moisture in the structure.
• Compressed insulation. If people walk on the insulation or store things on it, it loses depth and insulating performance.
• Forgotten loft hatch. An uninsulated, unsealed hatch loses a lot of heat and condensation forms around it.
• Blocked roof ventilation. Insulation pushed tight against the roof membrane at the eaves blocks the airflow, and the roof becomes damp from the inside.
• Cables buried under insulation. Cables, especially those feeding electric showers or immersion heaters, can overheat. They should rest on top of the insulation.
• Insulation tight against a chimney or flue. The clearances to combustible materials and the requirements of the chimney or flue manufacturer and the insulation manufacturer must be respected.

Check with your local authority building control whether your project needs approval, particularly if the roof space is being converted into a habitable room or the roof covering is being replaced. Laying loft insulation does not require a specific licence, but for work funded by government energy schemes the installer must be TrustMark registered. When accepting the work, it is worth having documentation of the product used and its depth. Protective clothing, gloves, goggles and a dust mask are recommended when working with mineral wool.

Inspect the insulation before it is covered by plasterboard or boarding. Afterwards, defects can no longer be seen. Go through these points:

• measure the insulation depth in several places and compare it with what was agreed,
• check that there are no gaps between rolls or slabs or along the joists, and that the joints in the layers are staggered,
• look at the edges along the external walls to see whether the insulation reaches the wall plate,
• confirm that penetrations, chimneys, flues and the loft hatch have been dealt with and that cables rest on top of the insulation,
• check the vapour control layer for taped joints and sealed edges to the walls, with no holes or tears,
• make sure a ventilation gap has been left at the eaves and the insulation does not block the airflow,
• ask for details of the product used and its depth.

During the first winter, it is worth checking the ceilings from below for damp patches or mould in the corners. If they appear, there is a thermal bridge somewhere or a leak in the vapour control layer. A thermal imaging survey in cold weather can also help by showing where heat is escaping.

The labour price in the NUMIQ price list is around €10 per m² of loft or roof insulation. The price covers labour and small consumables, without the insulation itself, the vapour control layer or the lining. Prices vary by region and city, roughly from €9 to €11.50 per m².

The final cost is mainly affected by:

• whether the work is on a flat loft floor or on roof slopes, where the work is overhead and slower,
• the number of layers and the total insulation depth,
• access to the loft, its height and carrying material up,
• the complexity of the roof, the number of rafters, dormers and penetrations,
• removal and disposal of old insulation,
• additional work such as the vapour control layer, battens, lining and walkways.

Some households can get loft insulation funded or part-funded through government energy schemes, which change frequently. Check current conditions on the official portal.

When comparing quotes, compare the same depth and the same type of insulation. If you are also planning a lining for the room in the roof, see the article on installation of plasterboard sloped surfaces and ceilings in attics. For a room in the roof, an alternative is sprayed closed-cell foam insulation. Other insulation items can be found in the section on building insulation.