FIBO 850/950

Fibo 850/950 – An ultra lightweight, loadbearing block, suitable for general purpose walling applications. Ideal for use in housing and extensions projects. Easy to handle, one hand lift.





Ultra lightweight, loadbearing units, suitable for general purpose walling applications. Ideal for use in housing and extensions.

General Properties – Table 1

Face Size 440mm x 215mm
Dimensional Tolerances Category: D1
Mean Unit Strength 3.6N/mm2
Net Dry Density 850 kg/m3
Thermal Conductivity 0.25W/mK @ 3% moisture content (internal use)
0.28W/mK @ 5% moisture content (external use)
Moisture Movement <0.5mm/m
Reaction to Fire Class A1
Configuration Solid Blocks: Group 1
Specific Heat Capacity 1000 J/kg/K
Water Vapour Diffusion Coefficient µ =  5/15  (Tabulated value from BS EN 1745)
  • High thermal insulation reducing the amount of added insulation required to comply with energy efficiency standards
  • Lightweight, making Fibo 850/950 an easy to handle – onehand lift – and quick to lay
  • Good background for direct application of plasters and renders – no bonding agents required.
  • Fixings can be easily made and held securely. Ideal back ground for direct nailing
  • High levels of fire protection – up to 2 hours for 100mm loadbearing walls

Fibo 850/950 is an ultra lightweight concrete block manufactured from expanded clay aggregates and a mixture of other naturally occurring raw materials and cement. The clay aggregate is produced from carefully selected clays which through heat expansion are bloated to create a low density porous aggregate with numerous cavities. This is what makes Fibo 850 so incredibly light and thermally efficient.

Fire Resistance

The fire resistance periods of Fibo 850/950 loadbearing and non-loadbearing walls are shown in Table 5. This data is only valid for walls complying with BS EN 1996 Part 1-1, Part 2 and Part 3. For walls designed in accordance with BS 5628, fire resistance values can be confirmed with our Technical Department.

The thicknesses given in Table 5 are for masonry alone, excluding finishes. For the fire resistance of walls with finishes, refer to the Lignacite Design Guide – Fire Resistance.

Fire resistance of Fibo 850/950 blocks – Table 5

Solid blocks (Group 1 units) –

no finish

Non-loadbearing wall

(criteria E1)

Loadbearing wall

(criteria RE1)

a 1.0 a 0.6
100mm 3 hour 2 hours 3 hours
140mm 4 hours 3 hours 4 hours


  1. These Tables are only valid for walls complying with BS EN 1996 Part 1-1, Part 2 and Part 3. For walls designed in accordance with BS 5628, fire

resistance values from that Standard are available on request.

  1. Criteria E1 refers to walls with a separating function. Criteria RE1 refers to walls with a separating and loadbearing function.
  1. This Table is derived on data from the National Annex to BS EN 1996-1-2. References to a ≤ 1.0 and a ≤ 0.6 refer to the proportion of load on a wall. If unknown, we suggest the values for a ≤ 1.0 are used as these are ‘worst case’ values.

Thermal Bridging

A significant factor in thermal assessments is the heat loss through thermal bridges (known as non–repeating or linear thermal bridges).

These occur at junctions between elements or where the continuity of the external fabric insulation is interrupted (e.g. at junctions with external walls, floors and roof). Assessors will need to apply a PSI (y) value to the particular junction being measured.

The Concrete Block Association (CBA) have developed a comprehensive set of junctions that have been independently assessed. The results clearly demonstrate that constructions using Fibo850 aggregate blocks can be assigned improved performance when compared to the Government’s Accredited Construction Details and Default values shown in Appendix K of SAP 2012.

As a member of the CBA, Lignacite Ltd is able to advocate the use of these enhanced bridging details. This information will be of interest to designers and SAP assessors as well as builders who will have the responsibility for correctly constructing the various junctions.

Junction details and PSI (y) values can be accessed at


The mortar type for work above ground level should be designation (iii) / Compressive Class M4. Stronger mixes may be used only with the permission of the designer. Stronger mixes may also be required for work below ground in accordance with PD 6697.


Fibo 850/950 has an open textured surface which is ideal for applying plaster and render. It has a face size of 440mm x 215mm in 100mm and 140mm widths and is produced in solid form only.


Fibo 850/950 is suitable for use in housing and extension projects. It can also be used to construct walls in other buildings where there is a requirement to specifiy blockwork with a low selfweight, eg., partition walls on floor slabs.

Fibo 850/950 can be considered for use in the following locations:

  • Inner and outer leaves of external cavity walls
  • Internal walls, including fire break walls
  • Internal walls below ground, such as the inner leaf of external cavity walls and interior walls

For use in separating walls meeting the requirements of Part E of the Building Regulations, we recommend the use of products from the Ashlite, Lignacite or Lignacrete ranges.

Unit and laid weight

Typical units and laid weight for Fibo 850/950 are shown in Table 2.

Block Weights – Table 2



Form Unit Weight (kg) Laid Weight


100 Solid 8.7 98
140 Solid 12.2 143
Note: Weights are based on 3% moisture content by weight.

Sound Insulation

Sound insulation values for Fibo 850/950 blockwork are shown in Table 4. For use in party walls satisfying Part E of the Building Regulation, other products from Lignacite Ltd are recommended.

Sound Reduction – Table 4

Sound Reduction Index Rw (dB)


Form L/tweight Plaster 12.5mm

Plasterboard on dabs

100 Solid 40 42
140 Solid 42 44
Note: 1. The Above values are based upon technical assessments and test to BS EN ISO 140-3.

Note: 2. Surface finishes are assumed to be applied to both wall surfaces.

Partial Cavity Fill and 100mm Fibo 850/950 blocks

U-values (W/m2K)

Cavity fill type 12.5mm plasterboard    on dabs 13mm lightweight plaster
Internal finish
50mm Celotex CW4000 0.26 0.27
50mm Kingspan Kooltherm K108 0.23 0.24
60m Celotex CW4000 0.24 0.24
60mm Kingspan Kooltherm K108 0.21 0.21
75mm Celotex CW4000 0.21 0.21
75mm Kingspan Kooltherm K108 0.18 0.18

Notes to tables:

 The U-values shown are based on the use of various proprietary insulation products. Alternative products can be used, provided they can achieve an equivalent thermal resistance (m2K/W).

  1. Wall ties are assumed to be stainless steel with a cross-sectional area of no more than 12.5mm2 for structural cavities up to 125mm wide.
  2. The suitability of full fill cavity insulation materials will depend on exposure conditions and should be confirmed by the designer. For partial cavity fill, a 50mm residual should be maintained. In some cases it may be possible to reduce the cavity width to a minimum of 25mm. The insulation manufacturer should be consulted for guidance

Movement Control

Movement joints should be considered in accordance with PD 6697 at approximately 6.0 metre spacings. In areas of concentrated stress, such as those above and below openings, consideration should be given to the use of bed joint masonry reinforcement.


The design of walls incorporating Fibo 850/950 should be in accordance with relevant design standards including BS 8103: Parts 2 and BS EN 1996-1-1 and the requirements of the Building Regulations.


Fibo 850/950 blocks are BSI Kitemarked approved to BS EN 771-3. They are Category 1 masonry units manufactured under a BSI certified Quality System complying with BS EN 9001.


Responsible sourcing – Lignacite Ltd operates its manufacturing plants to a BSI certified Environmental Management System (EMS) complying with ISO14001. Lignacite Ltd. complies with the requirements of BES 6001 – Framework Standard for the Responsible Sourcing of Construction Products, Certificate No: BES 580823. This independently confirmed Responsible Sourcing Certification provides re-assurance to our customers that they are procuring products responsibly and sustainably. Credits can also be gained under environment assessment schemes such as BREEAM and the Code for Sustainable Homes.

Environmental ratings – Summary green guide ratings applicable to Fibo 850 blocks can be obtained from the BRE Green Guide to Specification.

Thermal Resistance

Fire resistance of Fibo 850/950 blocks – Table 5

The thermal resistance values (m 2 K/W) for Fibo 850 are shown in Table 3. The values are derived by dividing the block thickness by its thermal conductivity (W/mK).

Thermal Resistance (m2 K/W)


Form 3%




100 Solid 0.40 0.35
140 Solid 0.56 0.50
Note: 3% moisture content (m/c) should be used for protected locations such as the inner leaf, and 5% for exposed locations such as the outer leaf when rendered.

Thermal insulation

Fibo 850/950 blocks can be used to satisfy the requirements of Part L of the Building Regulations.

U-values (W/m2K)

Presented are the U-values for a range of wall constructions based on 100mm Fibo 850 blocks in conjunction with full and partial cavity insulation. The outer leaf is facing brick, but a rendered block outer leaf will usually achieve at least the same U-value.

For constructions not shown please contact our Technical Department (tel 01842 810678) who will be pleased to provide confirmation of performance.


Cavity fill type

12.5mm plasterboard    on dabs 13mm lightweight plaster
Internal finish
100mm DriTherm Cavity Slab 32 Ultimate 0.25 0.26
100mm DriTherm Cavity Slab 34 Super 0.27 0.27
100mm Isover CWS 32 0.25 0.26
100mm Isover CWS 36 0.28 0.29
100mm Xtratherm Cavity Therm 0.18 0.19
100mm Kingspan Kooltherm K106 0.17 0.17
125mm DriTherm Cavity Slab 32 Ultimate 0.21 0.22
125mm DriTherm Cavity Slab 34 Super 0.22 0.23
125mm Isover CWS 32 0.21 0.22
125mm Isover CWS 36 0.23 0.24
125mm Xtratherm Cavity Therm 0.15 0.15

Surface Finish Recommendations

Drylining – Application to be as manufacturer’s recommendations.

Dense Plaster – Apply either 1:1:6 cement:lime:sand or 1:4 ½ Masonry cement:sand or 1;5 ½ cement;sand and plasticiser. Alternatively: Thistle Bonding or Thistle Hardwall or Knauf Ultimate backing plaster.

Finishing Coats – Thistle plaster finish or Thistle multi-finish or Knauf Multi cover.

External Rendering – Rendering to be in accordance with BS EN 13914-1. Avoid over strong mixes. Ensure the first coat of render is applied to a greater thickness than successive coats. Ensure the first coat of render is applied to a greater thickness than successive coats.

Builders considering the use of proprietary render systems must exercise caution to accurately adhere to the render manufacturers’ design and specification instructions. Detailed guidance is also published in the NHBC Standards, Chapter 6.11- Render.

Strictly adhere to the specific application instructions, paying particular attention to prevailing weather conditions and the minimum recommended thickness of single coat renders.