Iso-Mount Type2 installation guide – option 4

Fixing directly on to concrete using the Concrete Hanger Extension fixed to the concrete.
Creates a new isolated ceiling with a larger variable void for additional insulation and services.

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SonaSpray K-13 grey in London workspace for Global NGO client. BW Workplace Experts, TP Bennett & Tom Green Photography

T2 - Large void concrete

Calculate the weight of the ceiling

Ceiling boards, downlights etc, insulation, plaster and add 5% for fasteners etc.
NOTE: there will be a movement of several millimetres as the weight of the ceiling sits on the Iso-mount rubber blocks: if ceiling height is critical allow for up to 5mm.
Do not fit heavy items to the ceiling boards without the approval of the board manufacturer. Increase the amount of Iso-Mounts appropriately to support the additional weight. The maximum loading of each Type2 Iso-Mount is 18kg therefore 3 mounts per evenly spaced to spread the load, supports up to 54kg/

A typical load calculation for 200mm void with insulation – kg/

2 x layers of 12.5mm (0.5 inch) @ 10kg each – 20
200mm of 50kg/ mineral wool insulation – 12.0
2 x mtrs GL 1 ‘C’ Channel – 0.5
3 x Iso-Saddles – 0.1
Fixings, tape and plaster (nominal) – 0.9

Total – 33.5

Calculating the positions and quantities of Iso-Mounts

This is easier if done to scale on paper first but for a quick calculation take the square meterage of the room and multiply by 3 + 5% for the quantity of Iso-Mounts.
E.g. a 10m2 room x 3 = 30 + 5% =  32 x Iso-Mounts

1. Decide which way you want the ‘C’ Channel to run.
2. The Iso-Mounts nearest the walls should be fixed at a nominal 100mm from the walls and no further than 150mm from the walls.

T2 Fixing Centres Concrete

3. Divide the space equally between the mounts positioned at 100mm in from the walls.  The above example has 5 equal spaces of 902mm.  Reducing to 4 equal spaces would result in less than 54Kg/ maximum loading capability.
When the iso-Mount positions are marked drill pilot holes for accuracy. Then drill a 6mm diameter hole 60mm deep and tightly fix an 8mm Concrete Socket Bolt, which forms its own thread in the concrete.
The length of M8 studding will be approximately 70mm less than the depth of the void.
E.g. 250mm void = 180mm long studding

Insert the studding into the M8 Concrete Socket Bolt with a locking (half) nut underneath.
Rotate a half locking nut onto the lower end of the studding followed by the M8x24 connector.
The ‘C’ bracket of the Type2 Iso-Mount is then screwed to the M8x24 connector using an M8x12mm domed head socket screw.
The distance from the underside of the existing ceiling to the top of the ‘C’ bracket should be 50mm less than the required void depth, when the required positioning is achieved tighten the locknuts.

4. Ensure the ‘C’ Brackets are firmly fixed and levels are satisfactory.

5. When all of the Iso-mount ‘C’ brackets are in position you are ready to assembly the other 2 components of the Iso-Mount: the rubber block and the saddle. Entry of these into the ‘C’ bracket is restricted so assemble the rubber block into position on the saddle first and then slide the assembly into position on the ‘C’ bracket.
THERE IS A TECHNIQUE to doing this which is quickly mastered, it involves compressing the rubber against the pronged upstands to achieve good location.

6. Once assembled and seating correctly slide the Iso-Wedge into position, this will prevent the Iso-Mount coming apart when the ‘C’ channel is installed.  Note: the Iso-Wedges are removed after the ‘C’ Channel is installed.
7. Before Clipping the ‘C’ Channel Into Position.
The ‘C’ channel clips into place in seconds but is difficult to remove if you find that you need to reposition some Iso-Mounts, you may have to cut a section of ‘C’ Channel away to allow the rest to slide off the hangers – if ‘C’ Channel becomes distorted don’t re-use it.
8. Check again that you are happy with the fixed positions of all the Iso-Mounts.


Fixing the ‘C’ Channels

(Proprietary 45mm drylining channel)
These are the channels that the plasterboard will fix to with proprietary screws.  Our preferred channel is British Gypsum GL1 and GL3 Channel connectors.
NOTE: Do not snap-fix the channel into position until you are satisfied it will not have to be removed again.

How to soundproof a ceiling

NOTE: The ends of the ‘C’ Channel must be no closer than 20mm (0.75inch) to the walls: the ceiling does not touch the walls, this could transmit sound.

Soundproof ceiling

Place Iso-Pegs between the top of the saddles and the ‘C’ brackets to hold the saddles in position whilst the ‘C’ channel is clipped on.

Hook a length of C Channel onto 1 side of the saddle of 2 Iso-Mounts that are at least 1 metre apart, then squeeze the C Channel and saddle between your thumb and fingers and the channel will snap-fix into position.

Repeat this along the rows of fixed Iso-Mounts.

Finally check

  • The fixed C Channel is level and clear of the walls at both ends
  • The Iso-Mounts are still correctly assembled
  • The Iso-Pegs have all been removed

Fixing Oscar Foam

This fixes around the walls and the plaster board abuts to it, isolating the soundproof ceiling.

1. Mark a horizontal line around the wall at the calculated level of the underside of the floating ceiling, i.e. level with the underside of the plasterboard. When using 2 layers of 12.5mm plasterboard this will be 25mm, (1 inch), below the underside of the ‘C’ Channels.
2. Install 25mm wide x 10mm thick (1×3/8”) self-adhesive Oscar Foam along this line with the bottom of the foam on top of and sitting 5 mm (3/16 inch) above the line – this allows for the foam to distort when compressed by the plasterboard and space for a bead of Oscar Seal flexible acoustic sealant.
NOTE: This will prevent direct contact between the plasterboard, and the wall: a potential path for vibration and noise transfer. Oscar foam and the Oscar Iso-Mount combined create the floating soundproofing ceiling.

Soundproofing a concrete ceiling

Installing insulation

*For smaller areas*
High density (60kg/ (3.75lbs/cu.ft) or higher) mineral wool slab.


  • Can be purchased locally to the job
  • Can be cut with a wood saw
  • Does not require specialist installation machinery

1. Order material 100mm thick to part fill the depth of the void plus give an air gap between the insulation boards and the new ceiling board/s.  Mineral Wool is available in slab form in various thicknesses.
2. Cut the width to be a tight fit between the ‘C’ channels, butt joints tightly and fill all gaps.
3. Hold the boards in place using the manufacturers recommended adhesive to adhere the boards to the underside of the old ceiling.

*For large areas we recommend Celbar blown insulation – see Celbar section below*

Fixing the plasterboard

1. Fix the first layer of 12.5mm plasterboard with 30mm (1.1/4inch) drylining screws which will self-tap into the ‘C’ channel. Do not position screws under the joists or the Iso-Mounts where they may make contact and form a path for sound transmission. Keep the outer boards compressed into the Oscar Foam at the perimeter of the room. Keep gaps to the minimum and seal gaps with Oscar Seal.

2. For best results fix a second layer of plasterboard repeat the operation but stagger joints and use 38mm (1.1/2inch) drywall screws, seal gaps with Oscar seal keeping level or below the surface of the board, then tape and joint the ceiling ready for plastering.

1. Plaster to the walls, i.e. plaster over the Oscar Foam that runs around the perimeter of the ceiling separating the acoustic ceiling from the walls.
2. Before the plaster is fully dry run a flat trowel or blade flat against the wall and through the new plaster to create a gap of 3mm minimum and break all direct contact between the new ‘floating’ ceiling and the walls
3. When the plaster is dry, tidy any nibs caused by creating the gap and seal the gap with Oscar Seal. Oscar Seal is an intumescent flexible acoustic sealant that can be painted over when dry.

Celbar recycled insulation

The recycled non-hazardous cellulose insulation that is blown into the void with specialist machines – for larger areas of 100m2 plus

Celbar advantages:

  • The best performing acoustic insulation
  • Non-itchy
  • Fire retardant
  • Up to 85% recycled
  • The lowest embodied energy (the energy used to manufacture) in the insulation industry
  • Non-hazardous
  • Blown into the void completely filling all gaps
  • A very effective thermal insulator and where there are multiple habitations very little heat energy will be lost to adjoining units. Thermal transmittance through a typical ceiling will be better than U = 0.20W/m2K
  • Very economical for larger areas
  • Holiday Inn, Sheraton and Intercontinental Hotels use Celbar ceiling and wall insulation for minimum sound transmission between rooms ensuring guest comfort

Celbar is manufactured by International Cellulose Corporation, Houston, Tx
ISO 9001: 2000 certification

For non UK installations visit or email for your local installer

Component Suppliers

Oscar Acoustics

  • Oscar Iso-Mount assemblies
  • 25 x 10 x 10mtr rolls Oscar-Foam, (1 x 3/8 x 32.8ft)*
  • Oscar-Seal Acoustic Sealant
  • Celbar Acoustic Void Fill

Builders merchants:

  • Plasterboard
  • 45mm Gypframe GL1 Lining Channel and GL3 channel connectors
  • Mineral wool
  • Fasteners

Important Safety Information

  • Mineral Wool and Celbar are also thermal insulators. Bring this to the attention of your qualified electrician who may have to allow for heavier cables to prevent cables overheating and ventilation gaps around downlighters etc.
  • Fix cables securely and note the positions to avoid drilling into them later.
  • Use a suitable detector to avoid cutting into electrical cables and water pipes.
  • Consult the manufacturers of the plaster board for fixtures to be fixed directly to the plasterboard. Use fastenings as recommended by the plasterboard manufacturer.
  • Locate the joists (various tools are available from DIY stores for this purpose) and ensure that all screws that are fixing the Iso-Mounts are fixed through the plasterboard and securely into the timber joists away from the joist edges. Do not fix to plasterboard alone, this will not support the weight and is likely to lead to a failure of the ceiling, with injury or worse.
  • Only competent persons fully conversant with the use of Personal Protective Equipment should carry out the soundproofing ceiling installation.

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