Last edited 13 May 2021

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Insulation for ground floors

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[edit] Introduction

Insulation is an important component of a floor design and there are a number of factors which need to be taken into account:

[edit] Position of the insulation within the floor structure

Ground bearing floors can include insulation either below or above the concrete slab, the choice the designer makes will have an impact on the temperatures inside the building, as follows:

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.

[edit] Thermal performance

The thermal performance achieved by the floor is critical for the overall energy efficiency of the building. Approximately 15% of heat is lost through the floor and insulation can reduce this.

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.

TER – The Target CO2 Emission Rate is calculated and expressed as the mass of CO2 emitted in kilograms per square metre of floor area per year

TFEE – Target Fabric Energy Efficiency Rate is calculated and expressed as the amount of energy demand in kilowatt-hours per square metre of floor area per year.

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.

The table below shows the target U values as given in the relevant Tables of the Building Regulation Approved Documents.

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.

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

[edit] Applied floor loading

Materials will compress when a load is placed on them. is one of the important factors to consider when designing a ground floor and specifying the insulation for it.

The insulation used must be capable of accommodating the applied loads with the minimum of compression.

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 applied by the screed and the floor slab should also be allowed for when calculating the total load applied to the insulation. This is a useful point to explain the difference between:

[edit] Active and dead loads

The actual applied floor load acting on the insulation material has two components:

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.

[edit] 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.

[edit] Thermal bridging

The overall heat loss from dwellings is measured using SAP 2009 or SBEM. Both require the heat loss from the total amount of linear thermal bridging to be taken into account.

Heat is lost through the edges of the floor where it meets the wall. This can lead to cold spots and potential condensation problems if the floor insulation does not overlap the wall insulation.

[edit] Air leakage

Air leakage is an important factor in the overall thermal performance of the building envelope. As much as 10% of the overall heat loss of the building can be caused by air leakage.

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.

[edit] Condensation

[edit] Surface condensation

In order to prevent localised surface condensation the temperature factor (known as the f-factor) should be established.

BRE Information Paper IP 1/06 provides guidance and limitations on the types of buildings and the f-factor required in order to prevent surface condensation and mould growth from occurring.

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 risk of surface condensation can be reduced or eliminated by following the good practices for continuous insulation shown in the Accredited Construction Details.

Surface condensation is expected to occur at points where the surface is less than 75% of internal air temperature (see BRE Information Paper IP17 for further information and method of calculating).

With increasing insulation performance of the building fabric this is more likely to occur where gaps in insulation are evident

Condensation within the fabric of the building is not a problem except when it occurs within or adjacent to a moisture sensitive material such as timber or mineral wool insulation.

Building fabric condensation occurs when moisture from inside the building escapes through the fabric and is trapped by a moisture resistant barrier.

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:

[edit] Applications

[edit] Suspended floors

Suspended floors are specified when there is a soil contamination issue or a stability issue with the soil or land. The void under the floor can be ventilated to avoid build up of radon or methane.

[edit] 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.

For information about the characteristics of the site you can contact the Local Authority. In any case, it is likely you will be required to undertake a site survey to find out:

  1. Contamination of the soil
  2. Stability
  3. Soil type

If there are any doubts about the soil or its California Bearing Ratio (CBR), the best option is a suspended floor.

[edit] Above slab below screed

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.

[edit] 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.

[edit] Cold store floors

The main function of the insulation in a cold store floor is to prevent the ground and main structure from freezing due to temperatures inside the building being set as low as -32ºC.


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