Retrofit and ASHPs: Fabric First or Just Enough?

Air Source Heat Pumps (ASHPs) have become a key technology in the journey to low-carbon homes. But when retrofitting a property, an important question often arises: how much fabric improvement is really needed to make a heat pump work efficiently? Should you go “Fabric First” and insulate to the max, or do “Just Enough” to get a heat pump running viably?

At Luths Services, we work across the UK to deliver sustainable building services that are both practical and energy-efficient — and this balance between carbon impact, cost, and performance is one we consider every day.

What is “Fabric First”?

The Fabric First approach prioritises improving insulation, airtightness, and glazing before installing low-carbon tech like ASHPs. The logic is simple: the less energy a building needs, the smaller and more efficient the heating system can be.

LETI’s guidance supports this – especially in deep retrofit situations or when targeting net zero carbon. They note that heat pumps work best when paired with a well-insulated, airtight building envelope, keeping heat inside where it belongs.

So Why Consider “Just Enough”?

The “Just Enough” approach, discussed in LETI’s Retrofit and Heat Pump guides, looks at the practicality of staged upgrades — especially in older homes where full retrofit might be unaffordable or disruptive.

In many cases, it’s not about perfection – it’s about progress.

LETI recognises that partial fabric upgrades, like loft insulation, basic draught-proofing, or improving heating controls, can be sufficient to make a heat pump viable today — especially when sized and commissioned correctly.

This approach allows households to start decarbonising now, rather than waiting for a whole-house retrofit budget or perfect insulation levels.

What We Recommend

There’s no one-size-fits-all answer — and that’s where Luths Services can help. Every building is different, and our role is to assess what’s needed to:

• Deliver comfortable indoor temperatures

• Keep running costs down

• Ensure a heat pump system works efficiently

• Reduce operational carbon

We use tools like IES-VE and dynamic simulations to give data-led advice — whether it’s helping you assess how much insulation is enough, or what emitter upgrades are required for lower flow temperatures.

We also look at the embodied carbon of deep retrofit materials and suggest reloved options where possible, in line with our adaptive sustainability ethos.

The Takeaway

• Fabric First is ideal when aiming for deep retrofit or net zero.

• Just Enough is viable for making progress sooner, especially when cost or disruption is a barrier.

• Either way, the system needs to be designed properly, sized carefully, and matched to the building’s heat loss.

At Luths Services, we support clients at every stage — from feasibility to installation — to ensure heat pumps are not only installed, but perform efficiently in real homes, year after year.