If you’ve ever spoken to someone about new windows, you’ve probably heard the terms U-value, R-value and even G-value thrown into the conversation. They sound technical (and they are), but they’re also key to understanding how well your home holds onto heat and how comfortable it feels all year round.
At CR Smith, we like to keep things simple. So here’s a straightforward guide to what these values mean, how they differ, and how they can help you choose the best windows for your home.
R-values – resistance to heat flow
R-value measures how well something resists heat escaping. The higher the R-value, the better the insulation.
Think of it like a cosy jumper:
- A thin summer T-shirt has a low R-value.
- A thick woollen jumper has a high R-value.
The same principle applies to windows. A high R-value means the glass and frame are keeping more heat inside your home.
U-values – how quickly heat escapes
U-values are the opposite. Instead of measuring resistance, they measure how quickly heat can pass through a system. Here, the lower the number, the better.
If you’re trying to remember the difference, think of the “U” in unwanted heat loss. Lower U-value = less heat escaping from your home.
Unlike R-values, U-values take into account the whole window — the glass, the frame, and even how it’s fitted. That’s why installation matters so much. At CR Smith, we install our Lorimer windows from the inside so the frame sits just inside the exterior wall. This helps reduce gaps, which means better energy efficiency to your surrounding external wall.
G-values – solar heat from the sun
G-values measure how much of the sun’s heat passes through the glass and into the building. It’s shown as a scale between 0 and 1.
- 0 means no heat from the sun gets through.
- 1 means all the heat from the sun gets through.
Here in Scotland, a higher G-value can be handy in winter because it lets more free warmth in.
How they all work together
Here’s the quick version:
- High R-value = more resistance to heat loss.
- Low U-value = less heat escaping.
- Balanced G-value = just the right amount of solar heat gain.
R-values tend to be more consistent, as they measure the material itself. U-values of a building can fluctuate depending on the installation and insulation of surrounding elements. The better items have been installed, the higher the overall U-value will be. For windows alone, the U-Value will remain unchanged, but installation is vital to ensure an overall increased property U-Value. G-values are all about the glass and controlling the heat gain into your home.
What’s “good” for windows?
For most homes in Scotland:
- R-value: 7–10 is excellent.
- U-value: 1.4 W/m²K or lower is very good and the minimum standard required by Building (Scotland) Regulations 2004.
- G-value: depends on your property orientations to the sun and whether you want a room to gain more heat or control the amount of heat entering the space to avoid overheating.
Our Lorimer uPVC windows are designed to score well in all three. That means warmer winters, cooler summers, which will help to lower energy bills.
The takeaway
It’s not about picking just one number. The best windows balance all three values to suit your home and lifestyle.
If you’d like to see how well your home could perform with new Lorimer windows, give us a call on 0800 200 444 or book a free design appointment. We’ll help you choose the perfect combination of looks, comfort and efficiency.
