What U-value do I actually need?
Building Regulations Part L requires 1.6 W/m2K for a replacement window and 1.4 for a new one in England. Every Sharp window beats both. The number worth arguing about is not the frame, it is how much glass you are adding: a 1.2 W/m2K window over 12 m2 loses more heat than a 1.6 window over 4 m2.
Which applies to you
- Replacing windows one for one
Anything in the range complies. Uw runs from 1.2 on a standard casement. Spend the difference on acoustic glass instead if you are on a main road.
- Building an extension
1.4 W/m2K is the limit, but the whole-element calculation matters more. A large rooflight at Uw 1.0 offsets a big glazed wall; your building control officer will want the SAP or SBEM output, not the window figure.
- You want the lowest number available
Triple glazing, Ug 0.6, Uw from 1.0 on a frameless rooflight and 1.2 on a casement. It is heavier, so check the sash weight limits: 130 kg in aluminium, 80 kg in uPVC.
- A conservation area with slim units
14 mm slim double glazing gives Ug 1.4, which is worse than standard. It is normally accepted because the alternative is single glazing at Ug 5.7. Say this to the conservation officer in those terms.
The figures behind it
- Part L replacement window
- 1.6 W/m2K maximum
- Part L new window
- 1.4 W/m2K maximum
- Sharp casement
- Uw from 1.2 W/m2K
- Sharp frameless rooflight
- Uw from 1.0 W/m2K
- Standard double / triple
- Ug 1.1 / Ug 0.6
- Slim 14 mm double
- Ug 1.4, fits a 24 mm rebate
Products this points at
Projects that used these products
Rear of a Victorian terrace at dusk, the three-pane slim-sliding door open to the garden, warm interior light, camera low on the paving so the 20 mm interlock reads in silhouette.
4:3
Wandsworth side return
The finished block from across the street in flat overcast light, all four floors of matched aluminium windows and balcony doors square in the frame, the steel lobby screen at ground level.
4:3