What U-Value Should I Target For My Retrofit Project? – Can I Just Ask? | Ep.14

When upgrading the thermal performance of an existing building, U-Values are often the first metric people reach for—but what target should you be aiming for in a retrofit? The short answer is: it depends. Here’s how to navigate the regulations, understand the risks, and choose a realistic, effective U-Value for your project.

If you’d prefer to watch the video, just click play below to watch Chris and Andy from our technical team talk through this question in Can I Just Ask? – Ep.14 👇

What U-Value Should You Target in a Retrofit?

There’s a lot of confusion around U-Values in retrofit, and with good reason. While new builds have clear targets, retrofitting introduces a complex set of variables: age and construction of the building, space constraints, budget, and – perhaps most importantly – moisture risk. So what’s a good U-Value to aim for when upgrading walls, floors, or roofs in an existing home?

Let’s start with the basics.

Regulatory Guidance: The Numbers

Building Regulations (Part L in England and Wales) set a notional target U-Value of 0.3 W/m²K for thermal elements in retrofit. That’s the number most commonly quoted by building control officers and energy modellers alike.

However, the actual threshold U-Value required to meet compliance is 0.7 W/m²K. This more lenient baseline comes into play if certain criteria are met – namely, if achieving 0.3 W/m²K would reduce the internal floor area by more than 5% or have a payback period longer than 15 years. In these cases, a higher U-Value (i.e. lower performance) is permitted.

Starting Point: Existing Wall Performance

An uninsulated masonry wall can have a U-Value anywhere from 1.5 to 3.0 W/m²K, depending on its construction. Even a modest amount of insulation – say 20mm of natural fibre board – can halve the conductive heat loss, bringing it down to around 0.9 to 1.5 W/m²K. That’s a meaningful improvement, but still well above the regulatory 0.3 target.

This is where things get nuanced.

Internal vs External Insulation

The way you insulate, (internally or externally) makes a big difference, not just to the U-Value you can achieve, but also to the risks involved.

External insulation is relatively straightforward. With enough space and budget, you can easily hit the 0.3 W/m²K target using around 120mm of wood fibre board, and the moisture risks are relatively low – especially when detailed correctly.

Internal insulation, on the other hand, brings more complexity. You’re now altering the building’s thermal gradient, which raises the risk of interstitial condensation within the wall build-up. This can lead to moisture damage, mould, or long-term degradation of the structure – especially in solid wall buildings without cavities.

Because of this, more conservative U-Values are often advisable for internal upgrades. Thinner layers of insulation, while technically less effective, pose fewer risks and still deliver substantial comfort gains. Even a small amount of internal insulation can decouple the interior surface from the cold masonry, improving surface temperatures and thermal responsiveness.

So What Should You Aim For?

There’s no one-size-fits-all number. But as a rule of thumb:

• Externally insulated elements: Aim for 0.3 W/m²K or better if feasible.
• Internally insulated elements: A U-Value of 0.5–0.7 W/m²K may be more appropriate, depending on the wall construction, vapour control strategy, and overall risk profile.

In all cases, context matters. Take into account the building’s existing performance, fabric condition, and the interplay of moisture, airtightness, and ventilation. In heritage buildings or moisture-sensitive constructions, less may be more.

U-Values are a useful guide, but they’re just one part of the retrofit puzzle. Chasing a number without considering the building’s unique dynamics can do more harm than good. A carefully considered, risk-aware strategy is almost always better than a blindly ambitious one.