Soffit INSULATION
Many clients perceive that rooms above exposed upper floors will be cold. However, it is possible to achieve high levels of thermal performance in exposed upper floors. The architect has a choice of where to position the insulation: for concrete floors, either above or below the floor, or for timber joist floors, between the joists.
If insulating below the structural floor, the insulation can be laminated to a variety of materials so the floor can be insulated and finished in one process.
Insulating above the structural floor reduces the thermal mass of the floor allowing for a fast warm up. However, if only part of the floor on that storey is exposed it may cause problems with steps between finished floor levels.
When insulating between timber joists, the whole depth of the joist can be utilised as an insulation zone. If Mineral Wool insulation is installed between the joists, a very high insulation value can be achieved without increasing the depth of the floor construction or adding significant weight to the structure.
Shelter Factors
Where an exposed floor is over an enclosed but unheated space, such as an integral garage in a dwelling, the shelter factor of the garage improves the U-value of the exposed floor.
BRE publication BR 443 (Conventions for U-value calculations), sets out the shelter factors to be applied to separating floors in these locations.
There are specific additional requirements that need to be taken into consideration when calculating the U-values for elements that are adjacent to unheated spaces. The U-value of an exposed floor (soffit) which separates a heated area from an unheated area, can be calculated by including an additional amount of thermal resistance (Ru) due to the sheltering effect of the unheated area.
Standard default values of Ru can be taken from BR 443 (Conventions for U-value calculations) or SAP 2009. For situations not covered by default values, or where additional guidance is required contact our Technical Support Team on 01744 766 666.
Concrete Upper Floors
Insulation can be positioned above or below the concrete floor. When above the floor, the insulation is usually located below a floating screed or chipboard deck. If only part of a concrete slab is insulated, careful detailing may be needed to avoid steps in the floor level.
Insulation located below the floor should be covered, usually with plasterboard or other fire resistant boards.
Suspended Timber Upper Floors
In timber joist floors, the insulation is usually located between the joists. A vapour control layer is not usually included because of the risk of trapping water spillages within the floor construction.
Care is needed to minimise air leakage. The floor perimeter should be sealed by applying expanded foam tape under the skirting and a continuous bead of sealant to the back of the skirting prior to fixing.
The perimeter of the ceiling should also be sealed with a flexible sealant. Gluing the joints between the floor deck boards can also reduce air leakage through the floor.
Heated floors
When the floor is to be heated by hot water pipes or electrical cables, then the position of the insulation will be determined by whether the floor slab or the screed is to be heated.
If the screed only is to be heated then the insulation is laid between the floor slab and the screed. The insulation should be overlaid with a vapour control layer/building paper which acts as a slip layer and prevents wet concrete screed penetrating between the boards. Alternatively, pre-channelled insulation boards can be used.
British Standards
Designers should consult BS EN 312 : 2010 and BS EN 300 : 2006 when selecting floor decking boards.
Compressive Strength
All materials are compressed under load. Insulation materials used below screeds and floating chipboard decks should be capable of accommodating the applied loads with minimum compression. The applied load has two components:
- The dead load, which is due to the weight of the materials laid on the insulant, and
- The design load
The guidance in BS EN 1990 : 2002+A1: 2005 and BS EN 1991 -1 - 1 : 2002 should be followed when determining the relevant loading factors for floors. However, in a sector where future use is potentially unknown and point loads can be large, the structural engineer is often required to assess the service needs of the floor on a project by project basis.
Weather resistance
The soffits of exposed floors may need to be finished with a weather resistant finish. Although not exposed to direct driving rain, wind turbulence can result in wind-driven rain being blown up onto soffits.
Another consideration is that soffits, being sheltered from the washing effect of wind driven rain, tend to collect dirt and grime over time.
Fire resistance
The fire resistance of the exposed floor should comply with Building Regulation requirements. This will be between 30 and 120 minutes, depending on the purpose group of the building and its height above ground.
Solution Optimiser and Pathfinder
| Knauf Insulation Solution | Product | Construction Code | U-value Range (High) | U-value Range (Low) |
| Below concrete soffit | Rocksilk® Soffit Linerboard Standard | Es01 | 0.26 | 0.19 |
| Between timber joists |
OmniFit® Slab 35 or Rocksilk® Flexible Slab |
Es03 |
0.29 | 0.13 |
Exposed Soffit Below Concrete Soffit
Product:
Rocksilk® Soffit Linerboard Standard is a non-combustible Rock Mineral Wool slab with a black tissue facing. Rocksilk® Soffit Linerboard Extra is a Rock Mineral Wool Slab with a fibre cement flat sheet facing.
Overview:
- Installed without the need to access areas above the floor.
- Provides a solution to upgrade thermal performance of existing floors without reducing floor height.
- Non-combustible with the best possible Euroclass A1 reaction to fire classification.
| Weight of Rocksilk® Soffit Linerboard Standard | ||
| Product | Thickness (mm) | Weight per slab (kg) |
| Rocksilk® Soffit Linerboard Standard | 220 | 12.7 |
| Rocksilk® Soffit Linerboard Standard | 185 | 10.7 |
| Rocksilk® Soffit Linerboard Standard | 160 | 9.2 |
| Rocksilk® Soffit Linerboard Standard | 130 | 7.5 |
| Rocksilk® Soffit Linerboard Standard Unfaced | 75 | 4.3 |
| Rocksilk® Soffit Linerboard Standard Unfaced | 50 | 2.9 |
Typical construction
A concrete upper floor slab, or beam and block floor insulated on the underside with Rocksilk® Soffit Linerboard Standard. The linerboard can either be screw fixed directly to the slab, or to timber battens either fixed directly to the soffit.
Fixing Pattern
Rocksilk® Soffit Linerboards are available in 1200x600mm slabs and it is recommended that they are fixed into position using five fixings per board. This should be done using one steel fastener in the centre of the slab and four perimeter fixings at each corner, minimum 50mm from each corner.
Typical Specification
Rocksilk® Soffit Linerboard Standard thickness.......mm to be screwed to the structural soffit using ...... fixings manufactured by ........... Fixings to be set in at least 50mm from any board edge. Boards to be butt jointed.
Installation
Mechanical fixing directly to the soffit ensure the structure is stable and suitable to support the extra load of the lining boards. If in doubt, seek specialist advice.
Due to the nature of this application, multiple fixing types are available. Any fixings approved by the fixing manufacturer may be used to secure Rocksilk® Soffit Linerboards in position. Due to the variations in the specifications of concrete and other substrates, we advise that you seek specialist advice from the fixing manufacturer prior to installation for confirmation.
Where board edges are exposed, ensure insulation is covered with either a cement board or a suitable flashing.
Use a flexible and fire resistant sealant to seal any imperfection of fit at junctions between boards and walls.
Performance
Thermal performance
Rocksilk® Soffit Linerboard Standard and Extra have a thermal conductivity of 0.034 W/mK.
Fire performance
Rocksilk® Soffit Linerboard Standard has a black tissue facing and is Class 1 surface spread of flame to BS 476: Part 7: 1987.
Rocksilk® Soffit Linerboard Standard and Extra have the best possible Euroclass A1 reaction to fire classification to BS EN 13501-1.
U-values (W/m2K) USING ROCKSILK® SOFFIT LINERBOARD STANDARD TO AN EXPOSED SOFFIT:
| Insulation Thickness (mm) | Reinforced Concrete (RC) Floor Slab 150 - 300mm 2.500 (W/mK) |
Precast plank 150mm 1.300 (W/mK) |
| 295 (220+75) | 0.12 | 0.12 |
| 270 (220+50) | 0.13 | 0.13 |
| 220 | 0.15 | 0.15 |
| 185 | 0.18 | 0.18 |
| 160 | 0.21 | 0.20 |
| 130 | 0.25 | 0.25 |
Note: Calculated with 50mm Screed finish k=1.150 and 1 steel fixing per slab. For situations not covered by default values, or where additional guidance is required contact our Technical Support Team on 01744 766666
U-values (W/m2K) USING ROCKSILK® SOFFIT LINERBOARD EXTRA TO AN EXPOSED SOFFIT:
| Insulation Thickness (mm) | Reinforced Concrete (RC) Floor Slab 150 - 300mm 2.500 (W/mK) |
Precast plank 150mm 1.300 (W/mK) |
| 295 (220/6*+**75) | 0.12 | 0.12 |
| 270 (220*+**50) | 0.13 | 0.13 |
| 220/6* | 0.15 | 0.15 |
| 185/6* | 0.18 | 0.18 |
| 160/6* | 0.21 | 0.20 |
| 130/6* | 0.25 | 0.24 |
Note: Calculated with 50mm Screed finish k=1.150 and 1 steel fixing per slab. Thermal conductivity of facing board = 0.240 W/mK. * The / 6 is for the cement fibre flat sheet **denotes standard product. For situations not covered by default values, or where additional guidance is required contact our Technical Support Team on 01744 766666
Fixing methods
There are two primary fixing methods, "permimeter fixing" and "centre fixing", that can be adopted to mechanically fix Rocksilk® Soffit Linerboard.
Five metal fixing arrangement
Using this method all perimeter, as well as the centre fixing, should be installed using metal screws and insulation washers.
Tube washer arrangement
Rocksilk® Soffit Linerboards may also be installed using one centre metal fixing and four perimeter fixings made up of tube washers.
Tube washer length
A minimum of 15mm should be left between the end of the tube washer to allow for some compression and to stop the tube washer coming into contact with the substrate.
Tube washers with Rocksilk® Soffit Linerboard Extra
When installing tube washers through Rocksilk® Soffit Linerboard Extra, a pilot hole big enough to push the tube washer through the cementitious board must be drilled. This can be done before or after the slab has been offered to the soffit.
Fixing heads
A minimum head diameter of 70mm is required to provide optimum strength of fixing between substrates.
Care should be taken to ensure that mechanical fixings are not over tightened, surface compression of the product is not recommended.
This compromises the thermal performance and can impact the integrity of the facing.
Double Layering
Unfaced Rocksilk® Soffit Linerboard Standard can be used as a base layer in multi-layer systems to expand the total installed thickness and improve on potential U-values.
Approved Fittings
The table below details the combination of metal fixings and Fixfast tube washers for use with standard thicknesses of Rocksilk® Soffit Linerboards. For details on bespoke thicknesses, please contact our Technical Support Team
|
|
Perimeter fix – plastic to concrete |
Centre fix – metal to concrete |
||
|
Rocksilk® Soffit Linerboard Standard Thickness (mm) |
Tube |
Screw |
Washer |
Screw |
|
130 |
SF-T-75x100 |
SF-RS-6.1x85 |
SF-P-70-F |
SF-RS-6.1x175 |
|
160 |
SF-T-75x125 |
SF-RS-6.1x85 |
SF-P-70-F |
SF-RS-6.1x200 |
|
185 |
SF-T-75x150 |
SF-RS-6.1x85 |
SF-P-70-F |
SF-RS-6.1x225 |
|
220 |
SF-T-75x200 |
SF-RS-6.1x75 |
SF-P-70-F |
SF-RS-6.1x250 |
|
270 (220 + 50) |
SF-T-75x250 |
SF-RS-6.1x75 |
SF-P-70-F |
SF-RS-6.1x300 |
|
Rocksilk® Soffit Linerboard Extra Thickness (mm) |
|
|
|
|
|
136 |
SF-T-75x100 |
SF-RS-6.1x100 |
SF-P-70-F |
SF-RS-6.1x175 |
|
166 |
SF-T-75x125 |
SF-RS-6.1x100 |
SF-P-70-F |
SF-RS-6.1x200 |
|
191 |
SF-T-75x150 |
SF-RS-6.1x100 |
SF-P-70-F |
SF-RS-6.1x225 |
|
226 |
SF-T-75x200 |
SF-RS-6.1x85 |
SF-P-70-F |
SF-RS-6.1x275 |
|
276 (226 + 50) |
SF-T-75x250 |
SF-RS-6.1x85 |
SF-P-70-F |
SF-RS-6.1x325 |
Exposed Soffit Between Timber Joists
Product:
Overview:
- Friction fitting between timber joists closes joints, preventing air movement and infiltration
- Significantly improves acoustic performance restricting airborne noise
Benefits:
- Non-combustible with the best possible Euroclass A1 reaction to fire classification.
- A+ Generic BRE Green Guide rating.
Performance
Thermal performance
OmniFit® Roll 40 is a Glass Mineral Wool roll, designed for use in multiple applications that offers excellent thermal and acoustic performance. OmniFit® Roll 40 has a thermal conductivity of 0.040W/mK.
Rocksilk® Flexible Slab is a Rock Mineral Wool slab, designed for use in multiple thermal and acoustic applications as well as the fire protection of a loft conversion floor. Rocksilk® Flexible Slab has a thermal conductivity of 0.035 W/mK for 140mm slabs or 0.037 W/mK for 50-100mm slabs.
Fire performance
OmniFit® Roll 40 and Rocksilk® Flexible Slab have the best possible Euroclass A1 reaction to fire classification toBS EN 13501-1
Typical construction
A timber joisted floor finished with 22mm tongue and groove flooring grade chipboard or similar decking, the underside finished with either 12.5mm plasterboard, 6mm calcium silicate board or similar. OmniFit® Roll 40 or Rocksilk® Flexible Slab is laid between the joists to fully fill the void.
Typical Specification
Rocksilk® Flexible Slab* ......mm thick, supported on the ceiling to friction fit tightly between the joists and fully fill the ceiling void. OmniFit® Roll 40 */ Rocksilk® Flexible Slab* to be cut to fully fill the gap between the last joist and the perimeter wall.
(* delete as appropriate)
Flooring grade tongue and groove chipboard to be screwed to the floor joists, all as specified by the designer.
Installation
Install the floor joists so that any joists running parallel with masonry walls have a minimum gap of 35mm between themselves and the wall to allow for insulation to be installed.
Install the ceiling and seal the joint between the ceiling and the surrounding walls with a flexible sealant, to prevent air leakage.
Friction fit the OmniFit® Roll 40 to completely fill the ceiling void. Alternatively, cut slabs of Rocksilk® Flexible Slab to fit between the joists. Ensure the insulation is in contact with the underside of the floor deck and the ceiling. Between the last joist and the wall, cut insulation to size and install in the gap.
Screw the chipboard deck to the timber joists, with all joints glued using a waterproof PVA adhesive. Leave an expansion gap of at least 10mm or 2mm per metre run of floor at the room perimeter.
When fixing the skirting board, apply a self- adhesive foam strip to the underside of the skirting and two beads of sealant to the back surface. Apply pressure to ensure the foam strip is compressed immediately before fixing the skirting in place.
Contact our Technical Support Team