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Last edited 23 Jan 2018
Insulation for ground floors
- The position of the insulation within the floor structure.
- Thermal Performance; k and R values.
- Applied floor loading.
- Thermal bridging.
- Air leakage.
- Retrofitting floor insulation.
 Position of the insulation within the floor structure
- If the insulation is installed below the slab, the thermal capacity of the building is increased, helping to maintain steady internal temperatures.
- If insulation is installed above the slab, the building will respond much more quickly to the heating system.
Suspended floors are usually insulated in such a way that they offer reduced thermal mass and respond quickly to the heating system. In the case of suspended concrete, the insulation is installed above the deck, either under a screed or timber boarding. Suspended timber floors are normally insulated between the joists.
Choosing a suspended floor allows the designer to use the same design regardless of site ground conditions. The void below the floor can be ventilated to reduce radon or methane build up. It also allows for expansion of clay soils without affecting the structure of the floor.
 Thermal performance
The minimum standard for new dwellings is calculated as a notional building using the limiting values in Table 4 of Approved Documents L1A, L1B, L2A and L2B. Assessing the performance of the dwelling by calculation of TER and TFEE.
If the building is constructed using the notional building specification the CO2 target will be met. It is allowable for the builder/designer to vary the specification provided the same overall TER and TFEE are achieved in the calculation of the actual or as-built performance.
If the building is constructed to the notional building specification the CO2 target will be met. It is allowable for the builder/designer to vary the specification provided the same overall TER and TFEE are achieved in the calculation of the actual rates.
Whilst these give the optimum U values for target CO2 emissions, there is a statement for refurbishment and extensions that 'consequential improvements should only be carried out to the extent that they are technically, functionally and economically feasible', allowance will be made where the thickness of floor insulation in an extension or refurbishment creates issues with existing floor levels.
|BUILDING TYPE||TARGET U VALUE|
|New build domestic||0.13 W/m²K|
|New build non-domestic||0.25 W/m²K|
|Existing domestic||0.22 W/m²K|
|Existing non-domestic||0.22 W/m²K|
 Applied floor loading
If the insulation is below a slab, screed or timber boards the entire load is acting on the insulation. Point loads are spread by the layers above the insulation so that the load acting on the insulation is lower than the load applied to the floor surface.
A point load applied to a floor where the insulation is positioned below a thin screed will result in a higher applied load on the insulation than where the insulation was positioned below a thicker floor slab because the load is bearing on a smaller area of insulation under the screed.
- The dead load, which is due to the weight of the materials laid on the insulation.
- The design load associated with the use of the floor.
For specific applications, the guidance and recommendations contained in BS EN 1991-1:2002 and BS EN 1990:2002+A1:2005 should be followed, and this will help the designer ensure that the strength of the floor will be sufficient to support any applied loads over the loaded area.
 Standardised values
Standardised values are available to the designer for the dead loads applied by building components and the estimated active loads for various types of building use. These form the structural design requirements of the floor, but are of less value when considering the compression resistance requirements of the floor as the active loads are likely to be localised for point loads, not uniformly distributed loads.
Building Regulations incorporate target air tightness values to reduce the levels of heat lost through air leakage and balanced ventilation systems are recommended to provide appropriate air changes in a controlled manner.
 Surface condensation
Generally, an f-factor of no less than 0.75 is adequate for the internal environment in dwellings. Non residential values vary between 0.30 and 0.90 dependent on the activity within the building.
The best methods to eradicate or reduce this problem are to use an appropriate vapour control layer (VCL) in the correct position or to create a ‘breathable’ construction. The VCL is always on the warm side of the insulation.
Good practice for ground floors:
- Ensure all thermal and cold bridging is eliminated around the external perimeter of the floor.
- Ground bearing slabs must have a suitable damp proof membrane which can be placed above or below the insulation.
- Suspended floors should incorporate a ventilated void below the floor with a minimum height of 150mm. A vapour control layer should be positioned above the insulation layer.
 Below ground supported slab
This is a ground bearing application and the land and soil of the site must be strong and stable enough to take the weight of the building or house.
The insulation is installed on the concrete which is directly on top of the ground. Then screed is installed above the insulation. This approach avoids the ‘heat sink’ of having concrete directly under the floor, as described in ground bearing floors under slab.
This building method works well with underfloor heating. The screed over the insulation gives even temperature, with no hot spots, right across the floor span and warmth is retained after the heating is turned off. A good ambient temperature can be achieved.
 Above slab below chipboard
Without underfloor heating, this is a good option. The chipboard over the insulation prevents the heat sink, as described above and provides a warmer underfoot experience than concrete or screed and is faster to install as there is no drying time for the screed.
 Related articles on Designing Buildings Wiki
- Accredited construction details ACDs.
- Beam and Polystyrene Block Suspended Flooring - Case Study.
- Cavity wall insulation.
- Floor insulation.
- Heat transfer.
- Passivehaus Foundation Insulation - Case Study.
- Solid wall insulation.
- Thermal bridge.
- Thermal insulation for buildings.
- Types of floor.
- Types of flooring.
- Types of insulation.
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