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GENERAL
F1 A ground floor may have a U-value of 0.45 W/m2K or 0.35 W/m2K without additional insulation if the floor is sufficiently large. Diagram F1 shows the range of floor dimensions for which insulation is required.
Diagram F1 Floor dimensions for which insulation is required
F2 Floor dimensions must be measured in accordance with J1.3 h.i.
The floor area for semi-detached, terraced or flatted dwellings may be
for the building as a whole. When considering extensions to
existing buildings the floor dimensions can be taken as those of the
complete building including the extension.
F3 Care should be taken to control the risk of condensation caused by thermal bridging at the floor edge. See BRE Report BR 262: 1994.
DETERMINING THE U-VALUE OF FLOORS WITH NO INSULATION
F4 The U-value of an uninsulated floor may be determined from the ratio of its exposed perimeter to its area, using the equation:
where:
Uo = U-value of uninsulated floor (W/m2K)
P = exposed perimeter of floor (m)
A = area of floor (m2)
F5 The equation in paragraph F4 applies to all types of uninsulated floors constructed next to the ground including slab-on-ground, concrete raft, suspended timber and beam-and-block.
F6 Unheated spaces outside the insulated fabric, such as attached garages or porches should be excluded when determining P and A but the length of the wall between the heated building and the unheated space should be included when determining the perimeter.
F7 The data in Table F1 has been derived from the equation in paragraph F4. For the purposes of regulations for the conservation of fuel and power it will be sufficient to use the table using linear interpolation where necessary.
Table F1 to Part J Appendix F: U-values of uninsulated floors
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Ratio P/A |
Uo |
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0.1 |
0.21 |
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0.2 |
0.36 |
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0.3 |
0.49 |
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0.4 |
0.61 |
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0.5 |
0.73 |
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0.6 |
0.82 |
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0.7 |
0.91 |
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0.8 |
0.99 |
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0.9 |
1.05 |
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1.0 |
1.10 |
U-VALUE OF INSULATED FLOOR
F8 The U-value of an insulated floor is obtained from:
where:
Rins is the thermal resistance of the insulation, and Uo is obtained from Table F1 or the equation in paragraph F4.
F9 In the case of suspended floors Uins includes the thermal resistance of the structural deck and Rins should only include:
a. the resistance of added insulation layers; and/or
b. any extra resistance of the structural deck over and above 0.2 m2K/W.
F10 For further information on floor U-values see BRE Information Paper IP 3/90. BRE IP 7/93 shows how the U-value of a floor is modified by edge insulation (including low-density foundations), and BRE IP 14/94 gives procedures for basement storeys.
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