Annex 6.C U-values of ground floors and basements
6.C.0 Introduction
6.C.1 Example of how to use tables
6.C.2 Solid ground floors
6.C.3 Suspended ground floors
6.C.4 Basement floors
6.C.5 Basement walls

annex 6.C

U-values of ground floors and basements
[Appendix C]

Insulation not Necessary

6.C.0 Introduction

A ground floor of a non-domestic building should not have a U-value exceeding 0.25 W/m²K, if the Elemental Method is to be used (subject to the carbon intensity of the heating). This can normally be achieved without the need for insulation if the perimeter to area ratio is less than 0.12 m/m² for solid ground floors or less than 0.09 m/m² for suspended floors. However, some ground floor insulation will be needed for the majority of buildings. For basement floors the recommended Elemental U-value is also 0.25 W/m²K but for basement walls it is 0.30 W/m²K (again both of these figures are subject to the carbon intensity of the heating system). For exposed floors and for floors over unheated spaces the reader is referred to Annex B.

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Full details about how to calculate the U-value of a ground floor, a basement floor or a basement wall are given in BS EN ISO 13370 and in CIBSE Guide Section A3 (1999 edition). This Annex gives a summary of how to determine the U-value which will suffice for most common constructions.

Soil type

For ground floors and basements the U-value depends upon the type of soil beneath the building. Where the soil type is unknown, clay soil should be assumed as this is the most typical soil type in the UK. The tables which follow refer to this soil type. Where the soil is not clay or silt, the U-value should be calculated using the procedure in BS EN ISO 13370.

Calculation of areas

Extensions

Floor dimensions should be measured in accordance with 6.0.10. In the case of semi-detached or terraced premises and similar, for example a row of industrial units, the floor dimensions can either be taken as those of the premises themselves, or of the whole building. When considering extensions to existing buildings the floor dimensions may be taken as those of the complete building including the extension.

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Care should be taken to avoid thermal bridging at the floor edge. See BRE Report BR262 "Thermal insulation: avoiding risks".

Areas outwith the insulation envelope

Unheated spaces outside the insulated fabric, such as attached garages or porches, should be excluded when determining the perimeter and area but the length of the wall between the heated building and the unheated space should be included when determining the perimeter.

Interpolation

The following tables have been derived from BS EN ISO 13370. For the purposes of Section 6 it will be sufficient to derive the U-values from the tables using linear interpolation where appropriate.

6.C.1 Example of how to obtain U-values from the tables

The following example shows how to use Table 1 for a solid ground floor and serves as an illustration of how to use the tables supplied in this Annex, interpolating between appropriate rows or columns.

A proposed building has a perimeter of 38.4 m and a ground floor area of 74.25 m². The floor construction consists of a 150 mm concrete slab, 75 mm of rigid insulation (thermal conductivity 0.04 W/mK) and a 65 mm screed. Only the insulation layer is included in the calculation of the thermal resistance.

The perimeter to area ratio is equal to 38.4 ¸ 74.25 = 0.517 m/m². Table C.1 gives values for perimeter/area ratios of 0.50 and 0.55 but not for any values between 0.50 and 0.55. In this case, the U-value corresponding to a perimeter to area ratio of 0.50 should be used since 0.517 is closer to 0.50 than to 0.55.

The thermal resistance of the insulation is obtained by dividing the thickness (in metres) by the conductivity. The resistance is then 0.075 ¸ 0.04 = 1.875 m²K/W.

The relevant part of Table C1 is shown below:

The U-value corresponding to a thermal resistance of 1.875 m²K/W is obtained by linear interpolation as below:

The U-value of this ground floor is therefore 0.29 W/m²K.

Note: In the example for Table C.1 the appropriate row was chosen and interpolation was carried out between the appropriate columns. For all of the other tables, however, the appropriate column in the table should be selected and interpolation should be carried out between the appropriate rows.

Ground floors with all-over insulation or no insulation

6.C.2 Solid ground floors

Solid ground floors are taken to mean ground floors in which there is no significant air layer separating the building from the ground. Listed in the table below are U-values for solid ground floors. U-values are given in the following table for various perimeter-to-area ratios for a range of insulation levels. Where the floor is uninsulated the column corresponding to a thermal resistance of 0 should be used.

Table 1: U-values for solid ground floors (W/m²K)

Note:

As an alternative to the above table, the methods described in BS EN ISO 13370 may be used.

Ground floors with edge insulation

Where horizontal or vertical edge insulation is used instead of all-over floor insulation, the U-value of the uninsulated floor (obtained from the column corresponding to thermal resistance of 0 in Table 1) is adjusted by adding Y ´ P/A to account for the effects of edge insulation, where P/A is the perimeter (m) to area (m²) ratio and Y is the edge insulation factor obtained from either Table 2 or 3 below. As Y ´ P/A is negative, the effect of this addition will be a reduction in the U-value.

Table 2: Edge insulation factor (Y ) for horizontal edge insulation

Table 3: Edge insulation factor (Y ) for vertical edge insulation

Note:

When floors incorporate both all-over and edge insulation, the procedure in BS EN ISO 13370 may be used.

Ground floors with no insulation

6.C.3 Suspended ground floors

The following table gives U-values of uninsulated suspended floors for various perimeter to area ratios and for two levels of ventilation (expressed in m²/m) below the floor deck. The data applies to a floor deck at a height of not more than 0.5 m above the external ground level where the wall surrounding the underfloor space is uninsulated.

Table 4: U-values of uninsulated suspended floors

Note: As an alternative to the above table, the methods described in BS EN ISO 13370 may be used.

The U-value of an insulated suspended floor should be calculated using:

U = 1 / [(1/U₀ ) — 0.2 + R_f]

where U₀ is the U-value of an uninsulated suspended floor obtained from Table 4, above or another approved method. R_f, the thermal resistance of the floor deck, is determined from U_f, the U-value of the floor deck, where:

and where U_f is calculated using the Combined Method, as described in BS EN ISO 6946, assuming thermal resistances of 0.17 m²K/W for both the upper and lower surfaces of the floor deck.

6.C.4 Basement floors

The U-value of an uninsulated basement floor should be calculated by using Table 5 below, or the methods described in BS EN ISO 13370.

Table 5: U-values of uninsulated basement floors

Determine the U-value of an insulated basement floor from:

U = 1 / [(1/U₀) + R_ins]

where U₀ is the U-value determined from Table 5 (or other approved method) for uninsulated basements and R_ins is the thermal resistance of the insulation in m²K/W. The value of R_ins may be calculated from the thickness of the insulation divided by its conductivity.

6.C.5 Basement walls

Table 6 below gives the U-value of a basement wall for a given basement depth and basement wall resistance.

Table 6: U-values of basement walls