GROUND FLOORS - DESIGN CONSIDERATIONS |
Insulation Position
Suspended ground floors may be constructed from timber, concrete slabs or beam & block. In the case of suspended timber floors, insulation is installed between the joists. When suspended concrete slabs or beam & block construction is employed, the insulation can be installed between a screed or timber flooring and the concrete deck.
Ground bearing floor design can include insulation above or below the concrete slab. Insulation placed below the slab facilitates steadier temperatures in a building, whilst insulation positioned above the slab facilitates a quicker response to the buildings heating system.
Thermal Performance
The Building Regulations require that when ground floor U-values are calculated, BS EN ISO 13370: 1998 is used. This method of calculation uses the ratio of the exposed floor perimeter to the floor area as the greater part of floor heat loss occurs at the edges. Measurement of the perimeter and area of the floor should be to the finished inside surfaces of the perimeter walls that enclose the heated space. Projecting bays should be included, but unheated spaces such as porches or garages should be excluded. With the new Building Regulations emphasis on the energy efficiency of a dwelling as a whole the insulation incorporated into a ground floor has a major contribution to make towards the dwelling’s overall energy performance.
Thermal Bridging at Junctions and Accredited Details
As a result of the improving U-values for floors and the need for air tightness at junctions with external walls, the necessity to ensure that there are no cold bridges at junctions and openings has become ever more important. Now, under the name of Accredited Details, these issues are taken into account when calculating the energy performance of buildings (SAP 2005). Guidance is given in ‘Limiting Thermal Bridging and Air Leakage: Robust Construction Details for Dwellings and Similar Buildings. www.planningportal.gov.uk
Condensation
There are two sources of condensation that should be taken into account when designing a ground floor. These include–
- Surface condensation
- Interstitial condensation
Timber suspended ground floors
Surface condensation is unlikely to occur on the internal surface especially if there is adequate insulation beneath with no gaps. Interstitial condensation is unlikely to occur, however adequate sub-floor ventilation should be provided. Vapour control layers should not be used in this type of ground floor.
Pre-cast concrete (Beam & Block) suspended ground floors
Surface condensation is unlikely to occur provided that sufficient continuous insulation is included over the concrete structural floor to give a fast thermal response. Interstitial condensation is likely to occur on the upper surface of the concrete beams and filler blocks. It is unlikely to be harmful. Adequate sub-floor ventilation should be provided.
Ground bearing floors
Surface condensation is unlikely to occur on the floor surface if insulation is situated over the concrete slab. If insulation is situated below the slab, surface condensation can occur due to the slabs slow thermal response. This would normally be considered nuisance condensation and regular heating is recommended. Interstitial condensation can occur on the upper slab surface where the insulation is over the slab. If considered harmful, an adequate vapour control layer should be provided on the warm side of the insulation. Interstitial condensation is unlikely to occur if the insulation is installed beneath the concrete slab with an adequate vapour control layer which also acts as a damp-proof membrane.
For more detail refer to BS 5250:2002 ‘Code of practice for control of condensation in buildings’
Air Leakage
When dealing with suspended timber ground floors, there is a requirement to minimise air leakage from the ventilated sub floor void into the heated space. As well as gluing the joints between the floor deck panels, the floor perimeter should be thoroughly sealed. This can be achieved applying expanded foam tape under the skirting and a continuous bead of sealant to the back of the skirting prior to fixing.
Applied Floor Loading
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Different building components Dead Loads |
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Element
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Dead Load (kN/m2)
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Flooring grade chipboard
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0.1 to 0.2
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75mm flooring concrete screed
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1.75
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150mm concrete floor slab
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3.50
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Design load: general office use
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2.50
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Design load: light industrial use
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5.00
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Design load: general storage use
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4.80
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Design load: heavy storage use
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7.20
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The main function of a floor is to support the applied loads placed on it during the life time of a building. As all materials are compressed under load, insulation used under slabs, screeds and chipboard should be specified to deal with the applied loads and be subject to minimum compression.
The applied load has two components –
- The dead load, which include the weight of the structure of the floor
- The design load where BS 6399: Part 1: 1996 suggests that in offices, a design load of 2.5 KN/m2 should be allowed for in all cases.
The dead loads applied by various building components are shown in the table below. However, the designer must also take into account the level and application of the dynamic loads. BS 6399 is based on the uniformly distributed load and the floor strength must be sufficient to support any applied load over the loaded area.
For example, a heavy item of furniture resting on its feet has a significantly increased point loading compared to one resting on its whole base.
Determining the U-value
The following chart shows the thickness of Multi Roll 44 needed to achieve a u-value of 0.25 W/m2K in a suspended timber frame ground floor. The chart results are based on using default figures for several variables that are taken into account in the calculation method. These types of charts give guidance only on the expected thermal performance of a ground floor. For calculations of u-values to your own specific requirements consult the Superglass technical Hot Line.
Worked Example
The following worked example illustrates how to use the chart.
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Perimeter P = 20 + 28 + 14 + 18 + 6 + 10 = 96m |
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Area A = (20 x 28) - (18 x 9) = 560 - 162 = 398m2 |
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Ratio = P/A = 96 ÷398 = 0.24 |
Reading off from the chart shows that in a suspended timber ground floor with a P/A ratio of 0.24, a u-value of 0.25 W/m2K can be achieved using 150 mm Multi-Roll 44.