Installation of Acoustic/Thermal Insulation

In an ordinary partition between rooms, the insulation is mainly there for sound. The wool in the cavity dampens voices, the television or music from the next room. It will not work miracles, though. If sound travels through the ceiling, the floor or leaky joints, the wool alone will not stop it. A combination of wool, a double layer of boards and a flexible connection to the surrounding structure makes a much bigger difference.

In a loft or on a lining in front of an external wall, the main concern is heat. Here it is not enough to put the wool in. You also need a vapour control layer on the room side. Without it, moisture from inside the home gets into the insulation, where it can condense and gradually lead to mould.

Why does plasterboard construction almost always use mineral wool and not polystyrene? Wool is soft, fills the cavity without gaps, does not burn and absorbs sound far better. Polystyrene is rigid, tends to pass sound on and is not normally suitable for drywall systems.

The process is similar for a partition and a loft, and differs mostly in the details:

1. The metal frame is built and one side is boarded with plasterboard.
2. The electrician runs cables through the cavity and fits the back boxes. All services must be in place before the wool goes in.
3. The mineral wool is cut slightly wider than the clear gap between the studs, usually by roughly a centimetre. This way it wedges between the studs and does not slip down.
4. The wool is fitted from the bottom up, with slabs or batts butted tightly together, without gaps and without squashing them.
5. Around back boxes, pipes and cables the insulation is cut to fit, not left out.
6. In a loft or on an external wall, a vapour control layer is fixed on the room side, the joints are taped and the membrane is sealed tightly to the surrounding walls and any penetrations.
7. Finally the second side is boarded.

The thickness of the insulation usually follows the width of the studs. A partition built with 50 mm studs takes wool of roughly the same thickness, wider studs take thicker wool. The cavity should be completely filled, because a half-filled cavity gives a noticeably weaker result.

• Glass mineral wool is light, easy to cut and perfectly adequate for ordinary partitions and lofts
• Stone mineral wool is heavier and denser, and is chosen where higher fire resistance or better sound insulation is needed
• Acoustic slabs or batts are made specifically for partitions and wedge easily between the studs
• Rolls suit lofts and ceilings, where they are laid between and below the rafters or joists

With fire-rated partitions it is not only the wool that matters but the whole system. The type of insulation, its density and the board build-up are set by the system manufacturer or the design. If the wall has to meet a fire resistance requirement, the specified build-up must be followed exactly.

A straight partition with regular stud spacing is the simplest case, as the wool just goes into the ready-made bays. More time is needed for:

• Lots of cables, back boxes and pipes that the wool has to be cut around
• Irregular stud spacing, where every piece is cut to size
• Sloping ceilings, corners and the space between rafters in a loft
• Overhead work in ceilings, where the wool has to be held in place so it does not fall out
• Sealing the vapour control layer around roof windows, chimneys and beams
• Several layers of insulation or a greater required thickness

In practice it often happens that the insulation is in the wall but does not work as it should. The usual reasons are:

• Gaps between pieces of wool or empty spots next to the studs, which let both sound and heat through
• Wool that is squashed too much and loses its insulating properties
• Wool cut too narrow, which slips down over time and leaves a void at the top of the wall
• A missing or damaged vapour control layer in the loft, or joints that were never taped
• Sockets placed directly back to back on both sides of a partition, creating an acoustic hole
• No sealing tape under the perimeter tracks, so the partition is rigidly connected to the ceiling and floor and sound travels through the structure

Fitting wool into a partition or wall lining inside a room is something a handy homeowner can manage. Leave any electrical work to a qualified electrician, and it is wiser to leave a loft with a vapour control layer to a professional, because the details decide the result. When handling mineral wool, wear gloves, safety glasses, a dust mask and long sleeves.

1. Check that all services in the wall are finished and signed off by the electrician.
2. Measure the clear gap between the studs at several heights.
3. Cut the wool on a board with a long, sharp knife, slightly wider than measured.
4. Fit the wool into the bays from the bottom up and gently wedge it between the studs.
5. Butt the pieces tightly together, leaving no gaps at the floor or ceiling.
6. Cut the wool around back boxes and pipes, and fill small gaps with offcuts.
7. Run your hand over the wall before boarding and check that no wool is missing or sticking out past the studs.
8. Clear away offcuts and dust and air the room.

Check the insulation before the second side is boarded, because after that you cannot see into the wall:

• The cavity is filled over its full area and height, with no gaps next to the studs
• The wool holds itself in place and does not sag or slip
• The insulation thickness matches the stud width or the agreed build-up
• There are no empty spots around back boxes and services
• Sealing tape is fitted under the perimeter tracks
• In the loft, the vapour control layer is continuous, joints and connections are taped and the membrane is not torn
• In fire-rated partitions, exactly the material specified by the system has been used