What warm actually means on a window
A window is warm when it loses heat slowly. The figure that measures this is the U-value, the rate at which heat passes through a square metre of the window, and lower is better. It rolls up the frame, the glass and the sealed unit into one number, which is why it is the honest way to compare a modern sash with the draughty timber one it replaces.
Alongside it sits the Window Energy Rating, the coloured scale that runs to A++. It balances heat lost through the window against solar heat gained through the glass, so a good rating means the window is not only holding warmth in but letting useful winter sun through.
The numbers, and what they have to beat
Every ECOSlide® sash carries an A+ Window Energy Rating as standard. The U-value is 1.3 W/m²K as standard and 1.2 with enhanced glazing, with a laminated specification at 1.4.
Those figures matter because Approved Document L, the part of the Building Regulations that governs heat loss, sets a ceiling of 1.4 W/m²K for a replacement window in England. The standard sash is comfortably inside it, and the enhanced glazing further still. In other words the window is not scraping a pass, it is beating the requirement before you upgrade anything.
Where the heat was actually escaping
On an old sash the loss is rarely through the glass alone. It is the gap where the two sashes pass each other, the rattle around a shrunken frame, and the draught under the sill. A window can have decent glass and still feel cold because the air is moving through the joins.
A modern sliding sash closes those paths. The weather bar is mechanically joined into the bottom sash so rain is pushed off the sill rather than into the frame, and the whole window is made to measure and colour matched so there are no packers and no gaps to draught through. Air permeability is tested to Class 4 at 600 Pa, the tightest class in the standard, and water tightness to Class 7A, with wind resistance at Class C4 at 1600 Pa under BS EN 12211, equivalent to gusts of around 115mph.
A warm window is a well sealed window as much as a well glazed one. The class figures above are the part most quotes never mention, and they are where an old sash quietly loses its heat.
Staying warm without stopping the air
A sealed room needs a way to breathe or moisture builds up, so concealed trickle vents are fitted to meet Part F of the Building Regulations, head vents as standard and sash vents on request. They let a room change its air slowly without opening a window to the cold, which is the balance a warm and healthy room needs.
The result is a window that holds its heat, satisfies the Building Regulations on paper, and does not trade warmth for a stuffy room.
What to do next
If you are comparing windows, ask every supplier for the U-value and the energy rating in writing, then compare like with like. Ours are A+ and 1.3 as standard, 1.2 enhanced.
Across West London and the Home Counties we survey the openings ourselves before anything is ordered. Anywhere else in Great Britain the windows are made to a schedule of sizes you provide and delivered kerbside for your own fitter.
Questions we get asked
What U-value do windows need to meet in the UK?
For a replacement window in England, Approved Document L sets a ceiling of 1.4 W/m²K. Our sashes are 1.3 as standard and 1.2 with enhanced glazing, both inside that limit.
What does an A+ energy rating actually mean?
The Window Energy Rating balances the heat a window loses against the solar heat it gains. A+ is near the top of the scale, so the window keeps warmth in while still letting useful winter sun through.
Are uPVC sash windows as warm as timber?
Warmth is measured by the U-value, and a modern sealed unit in a made-to-measure frame performs to the A+ and 1.3 figures above regardless of the frame material. A tight seal and good glazing are what keep a room warm.
Will new sash windows lower my heating bills?
They reduce the heat a room loses through the window and the draughts around it, which is where an old sash leaks most. The saving depends on the whole house, but the window stops being the weak point.
Read next
Talk to us about your windows
Send a photograph of the front of the property and roughly how many windows are involved, or your schedule of sizes for a supply-only order, and we will tell you what is realistic before anyone comes out.
Call 0800 861 1450, message us on WhatsApp, or email info@sashwindowfactory.co.uk.