Technical Booklet F1 Conservation of Fuel and Power in Dwellings·Page 47·3.45
Replacement Windows in Buildings with External Appearance Constraints
Where replacement windows cannot meet standard performance due to maintaining external appearance, a centre pane U-value of not more than 1.2 W/m2.K is acceptable, or single glazing combined with low-E secondary glazing may be used.
Where, because of the need to maintain the external appearance of a building, replacement windows are unable to meet the standards given in Table 3.1, the replacement windows should have a centre pane U-value of not more than 1.2 W/m2.K. Alternatively, single glazing may be used in combination with low-E secondary glazing with the weather stripping on the secondary glazing to minimise condensation risk between the primary and secondary glazing.
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