This section details default wall U-values where there is insufficient information to enter non-default U-values. Lookup of the defaults in Tables 2.21 and 2.22 are automatically referenced by DEAP software. Values from Table 2.23 and 2.24 must be entered into DEAP by the user. Table 2.7 sets out the age-band ranges
| Wall type | Age band | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A | B | C | D | E | F* | G* | H | I | J | K | L | |
| Stone | 2.1 | 2.1 | 2.1 | 2.1 | 2.1 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| 225mm solid brick | 2.1 | 2.1 | 2.1 | 2.1 | 2.1 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| 325mm solid brick | 1.64 | 1.64 | 1.64 | 1.64 | 1.64 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| 300mm cavity | 2.1 | 1.78 | 1.78 | 1.78 | 1.78 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| 300mm filled cavity | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| solid mass concrete | 2.2 | 2.2 | 2.2 | 2.2 | 2.2 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| concrete hollow block | 2.4 | 2.4 | 2.4 | 2.4 | 2.4 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| timber frame | 2.5 | 1.9 | 1.9 | 1.1 | 1.1 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| Unknown | 2.1 | 2.1 | 2.1 | 2.1 | 2.1 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| 425 mm Cavity Wall | 1.73 | 1.51 | 1.51 | 1.51 | 1.51 | 1.1 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
| 425 mm filled cavity | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.55 | 0.55 | 0.37 | 0.27 | 0.21 |
*If the dwelling is of age band F or G but the roof of the dwelling is shown to have no insulation, then the wall must be assumed to be age band E (no insulation).
The U-values in Table 2.21 may be used as a starting point for calculating U-values of similar walls with additional retrofitted insulation, provided evidence is available to substantiate the thickness and material used.
50mm (dins) of insulation with thermal conductivity 0.035 W/mK (λins) may be retrofitted to a stone wall from age band B.
The resistance of the original stone wall is the inverse of the U-value of 2.1 W/m2K.
Rstonewall = 0.4762 m2K/W
The resistance of the extra insulation is Rins = dins/ λins = 0.05/0.035 = 1.429 m2K/W. (this simplified example does not account for any extra cladding or plaster)
Total wall resistance, Rwall = Rins + R stonewall = 0.4762 + 1.429 = 1.9052 m2K/W
Insulated wall U-value, Uwall = 1/Rwall = 0.525 W/m2K
In cases where an existing cavity wall has been filled with insulation, this method may be applied and requires the following information:
Rwall = Rins + R cavitywall - R cavityAlternatively, the U-value may be calculated in full using the thermal conductivities and thicknesses of the different layers in the wall. If the information above is unavailable, then the wall U-value should be taken from Table S3. For semi-exposed walls, the U-value for the applicable wall area is taken as that of the external walls of the dwelling adjusted as described in Section 3 using the following default values for the effective thermal resistance, Ru:
When using default U-values from Table S3 for semi-exposed walls between an existing dwelling and an unheated space, the option ‘Wall is semi-exposed’ can be selected in DEAP. DEAP then automatically applies the above Ru values to the default U-value. Alternatively, assessors may apply the relevant default Ru value from TGD L 2021 Appendix A. Table S3 should only be used for walls between an apartment and unheated space when there is documentary evidence to support the presence of insulation.
Often, the wall between an apartment and unheated spaces (corridor or stairwell for example) will be uninsulated. Where this is the case, the U-values in Table S3a are used if non-defaults are unavailable. In these cases, the ’Wall is semi-exposed’” option is not available in DEAP as the Ru value is already built into the defaults for these uninsulated walls.
| Semi exposed wall description | U-value |
|---|---|
| Semi exposed 100mm block wall adjacent unheated circulation space | 1.46 |
| Semi exposed 215mm block wall adjacent unheated circulation space | 1.13 |
The U-value of a wall adjoining a partially heated premises is taken as half the U value of the wall. This applies typically for a flat adjoining a non-domestic premises that are not heated to the same extent or duration as the flat. The U value of the wall should be based on an uninsulated exposed element (Age band A in Table 2.21). However, if there is enough information available (and proof thereof) to calculate the actual wall U-value, then this should be done, and the result halved to attain the wall U-value.
There are cases where an assessor may need to infer whether defaults from table 2.23 or table 2.24 apply. Table S2.23 indicates default U-values where internal drylined insulation is retrofitted to one of the wall types in Table 2.21 This table assumes that the insulation added has thermal conductivity 0.038 W/mK and the assessor has insufficient detail on the actual thermal conductivity of the installed insulation. Table 2.23 applies where the insulation is an addition to that of the original wall and none of the original insulation has been removed. If the original insulation was removed and the starting point before drylining is an uninsulated wall, then Table 2.23 can be used by assuming the ’original wall U-value‘ of an uninsulated wall - select the appropriate wall type from Table 2.21 and use age band E or earlier. Where the actual installed insulation thickness for the new insulated drylining does not match one of the columns in the table below, the next lowest insulation thickness should be assumed from this table. For example, if the installed drylining insulation is 63mm thick, the column ’Plasterboard + 60mm insulation‘ should be referenced if using Table 2.23.
Table 2.24 applies where the BER assessor comes across information1 indicating that walls or roofs have been upgraded but appropriate supporting documentation is not available. The BER assessor should confirm these upgrades through a mixture of onsite tell-tale signs and record-type indicators. For example, for external wall insulation, this could be confirmed with a photo of a larger exterior reveal depth at the window and a contractor confirmation. Acceptable verification indicators are listed here.
If there are sufficient verification indicators of upgrades, the age of the dwelling or the extension is used to select the appropriate “onsite-observable” defaults from table 2.24. Depending on the dwelling’s age, there are two sets of defaults, pre-1978 and 1978 onwards. These age-bands refer to the age of construction of the dwelling or extension, and not to the age-band of the upgrade. This aligns with the introduction of regulations in 1976 and related transition period. 1978 also aligns with age band F in the DEAP manual.
| Original wall U-value (Table S3) | Plasterboard + insulation thickness | ||||||
|---|---|---|---|---|---|---|---|
| + 0mm insulation | + 20mm insulation | + 30mm insulation | + 40mm insulation | + 50mm insulation | + 60mm insulation | + 70mm insulation | |
| 2.4 | 1.53 | 1.12 | 0.91 | 0.77 | 0.67 | 0.59 | 0.53 |
| 2.2 | 1.45 | 1.08 | 0.88 | 0.75 | 0.65 | 0.58 | 0.52 |
| 2.1 | 1.41 | 1.05 | 0.87 | 0.74 | 0.64 | 0.57 | 0.51 |
| 1.9 | 1.31 | 1.00 | 0.83 | 0.71 | 0.62 | 0.55 | 0.50 |
| 1.78 | 1.25 | 0.96 | 0.81 | 0.69 | 0.61 | 0.54 | 0.49 |
| 1.64 | 1.18 | 0.92 | 0.78 | 0.67 | 0.59 | 0.53 | 0.48 |
| 1.1 | 0.87 | 0.72 | 0.63 | 0.56 | 0.5 | 0.46 | 0.42 |
| 0.6 | 0.53 | 0.47 | 0.43 | 0.39 | 0.36 | 0.34 | 0.32 |
| 0.55 | 0.49 | 0.44 | 0.4 | 0.37 | 0.34 | 0.32 | 0.30 |
| 0.37 | 0.34 | 0.31 | 0.3 | 0.28 | 0.26 | 0.25 | 0.24 |
| 0.27 | 0.25 | 0.24 | 0.23 | 0.22 | 0.21 | 0.20 | 0.19 |
Plasterboard is not likely to have been used in buildings constructed before 1950. So, for a building originally constructed prior to 1950 (that is, where age band A, B, or C applies), if plasterboard is present, it is acceptable to assume that the plasterboard has been retrofitted.
On that basis the U-value for ’Plasterboard+ 0mm Insulation‘ from Table 2.23 can be used. Where plasterboard is present in a dwelling constructed after 1950, evidence must be provided to substantiate that it is an addition to the original wall build-up.
For walls starting from U values lower than 0.27, any benefit from applying drylining with insulation will need to be calculated on the basis of the actual thickness and thermal conductivity of the insulation.
| Verification Route | EWI | CWI | IWI | Roof - Ceiling level | Roof - rather / flat |
|---|---|---|---|---|---|
| Onsite verification Observable (1978 Onwards) | 0.33 | 0.37 | 0.55 | No changes | Current approach (Table S5) |
| Onsite verification Observable (pre-1978) | 0.55 | 0.6 | 0.8 | No changes | 0.49 |
Where insulation is observed in the walls or flat/rafter roof sections of a dwelling constructed before 1978, it can be assumed that these elements have been retrofitted with insulation, and the age-band default for pre-1978 in table 2.24 can be selected. Where insulation is observed in the elements of a dwelling constructed after 1978, evidence must be provided to substantiate that it is an addition to the original wall build-up.
| Wall type and available data: examples | How to select the appropriate wall U-value |
|---|---|
| Full detail on all wall layers available from documentary evidence | Enter actual wall U-value based on U-value calculation into DEAP rather than using default value. |
| No detail available on exposed wall other than wall type and age band | Select wall type and age band in DEAP. DEAP automatically selects U-value from Table S3. |
| No detail available on semi-exposed stone wall other than wall type and age band | Select wall type and age band in DEAP and tick “wall is semi- exposed" in DEAP. DEAP selects U-value from Table S3 and automatically applies an Ru value. |
| Semi-exposed uninsulated 100mm block wall between apartment and corridor | Select wall type in DEAP. DEAP automatically selects U-value from Table S3a. |
| Original dwelling stone wall, age band B, retrofitted with drylining plasterboard (with 20mm insulation) | Identify U-value from Table S3 (2.1W/m2K) and identify retrofitted U-value from Table S3b. User enters resulting U-value (1.05W/m2K). |
Curtain walling is used sometimes for apartments and requires a special procedure to get the heat loss and the solar gains correct simultaneously. For curtain walling, a site-specific thermal transmittance calculation should be undertaken by a competent person (such as a façade engineer) and calculated in accordance with IS EN 12631:2017 taking on board the specific geometry of the installed product/ system.
The U-value of curtain walling is that for the whole façade, which is an average value including mullions, transoms, glazing and spandrel panels. Calculations should be made by:
The façade U-value includes all effects of thermal bridging within the façade. It is therefore permissible to calculate the thermal bridging heat loss with the lengths of window surrounds set to zero. All other junctions are included as normal.
Guidance in relation to rainscreen cladding and their effect on U-values is given in BRE 443 Section 4.
A wall consisting of glass blocks can sometimes be used in a dwelling in place of a normal wall or window. The thermal conductivity of glass, 1.05 W/mK taken from CIBSE guide A, may be used in place of certified thermal conductivity for the glass in a glass block wall. After the glass thickness is measured, the U-value can then be determined.
The glass block wall may be entered as a window or a wall in DEAP. If there is a certified solar transmittance value and U-value available for the glass blocks, it may be entered as a window in DEAP. Where there is no certified U-value and solar transmittance data, the glass block wall should be entered as a wall - thick glass will absorb more light than thin glass, so the values in Table 6b of the DEAP manual should not be used.
In the absence of other information, the thermal bridging effect of the adhesive between the glass blocks may be ignored as the adhesive can be assumed to have a similar thermal conductivity to that of the glass blocks.
A semi-exposed wall is one that is adjacent to an unheated space. This affects the rate of heat loss through the wall – i.e. the U-value - a fact accounted for by adding an extra thermal resistance.
The way this adjustment is made depends on the source of the U-value of the wall:
In buildings incorporating flats, where corridors and stairwells are heated, walls between the flat and heated corridors/stairwells should be treated as non-heat loss walls (i.e. assuming the same temperature on either side of the walls). Otherwise these walls are treated as elements adjacent to an unheated space and their U-values should be calculated accordingly."
In existing apartments with semi-exposed walls there are a number of options to choose from:
If using a default and the wall is semi-exposed uninsulated block, choose one of the walls from Table 2.22 in DEAP from the DEAP dropdown. This automatically applies a default Ru value to an uninsulated block wall between a dwelling and a corridor.