Understanding DFEE: Why Fabric Matters More Than Renewables

If you’ve looked at a SAP compliance report you’ll have seen the compliance metric ‘DFEE’ and probably wondered what it means. You’re not alone. It’s one of the new metrics in Part L compliance, and getting it wrong can leave you with a building that’s expensive to fix.

What is DFEE?

DFEE stands for Dwelling Fabric Energy Efficiency. It measures the energy efficiency of the building envelope itself, the walls, floors, roof, windows, and doors. Unlike other SAP metrics, DFEE is purely about how well the physical structure of the building retains heat.

Your SAP calculation produces a DFEE value (measured in kWh/m²/year), which must be lower than the Target Fabric Energy Efficiency (TFEE) for the building to pass.

Why DFEE is Different From DER

Most people understand DER (Dwelling Emission Rate), the CO2 emissions metric. If your DER is too high, you can often solve it by adding solar PV, upgrading to a heat pump, or specifying a more efficient boiler. Renewables and efficient heating systems can offset a multitude of fabric sins.

DFEE doesn’t work like that.

DFEE is calculated before any heating system or renewable technology is considered. It’s a measure of the raw thermal performance of the building fabric. No amount of solar panels will improve your DFEE score. No heat pump will save you.

This catches people out regularly. They design a building with average fabric performance, assuming they’ll throw some PV on the roof to pass Part L. The DER passes. The DFEE fails. And now they have a problem.

What Affects DFEE?

DFEE is influenced by:

Walls – U-values, thermal bridging at junctions, and the continuity of insulation. Cavity walls, solid walls, and timber frame all perform differently.

Floor – Ground floor U-value and edge insulation details. Often overlooked until it’s already poured.

Roof – Insulation thickness, continuity at eaves and verges, and how well it’s actually installed (not just specified).

Windows and doors – U-values, solar gain (g-value), and the ratio of glazing to floor area. Large expanses of glass hurt DFEE.

Thermal bridging – The psi values at junctions. Poor detailing around windows, at corners, and where floors meet walls can significantly drag down performance.

Air tightness – While this sits slightly separate in the calculation, a leaky building loses more heat and performs worse overall.

The Backstop Problem

Even within DFEE, you can trade off elements against each other. A poorly performing wall might be offset by excellent windows and a well-insulated roof.

But there are limits.

Each element still has minimum U-value standards it must meet—the backstop values. You can’t have a catastrophically bad wall and expect to compensate with triple glazing. The wall still has to meet minimum standards before any trade-off is allowed.

Why This Matters for Builders

The practical implication is simple: you need to get the fabric right from the start.

If a building is already constructed and the DFEE fails, your options are limited:

• Windows and doors – Expensive, but these can usually still be upgraded if needed
• Roof insulation – Often accessible and improvable, especially in loft spaces
• Walls – If insulation is already sealed in a cavity, you’re stuck with what you’ve got
• Floor – If the slab is poured, there’s no going back

We’ve seen cases where builders have assumed they’ll pass, only to find out at as-built stage that the DFEE is failing. At that point, you’re looking at replacing windows, adding roof insulation, and hoping it’s enough to offset the unchangeable elements.

If it’s not enough? You may need to have a difficult conversation with Building Control or face the reality that the building cannot be signed off in its current form.

Getting It Right at Design Stage

This is exactly why you should get a design SAP early. Running the calculation before you build tells you whether the fabric specification will achieve compliance.

At design stage, changes are cheap. Upgrade the insulation spec, revise the window U-values, improve the thermal bridging details—all of this is just numbers on paper.

At as-built stage, changes are expensive, disruptive, or sometimes impossible.

Key things to consider at design:

• Specify better than minimum – Meeting backstop U-values exactly leaves no margin for error. Real-world installation is rarely perfect.
• Watch the glazing ratio – Large glazed areas look great but hurt DFEE. If you want floor-to-ceiling windows, you need to compensate elsewhere.
• Get thermal bridging details right – Default psi values in SAP are punitive. Using accredited construction details or calculated values can make a significant difference.
• Don’t rely on renewables to save you – PV and heat pumps affect DER, not DFEE. They’re not a get-out-of-jail card for poor fabric.

The Bottom Line

DFEE is the metric that can’t be gamed. It forces you to build a genuinely thermally efficient envelope rather than papering over poor fabric with technology.

Get it right at design stage, and you’ll have a building that’s efficient, comfortable, and compliant. Get it wrong, and you’ll be facing expensive remediation work, or worse, a building that simply can’t pass.

If you’re unsure whether your design will achieve DFEE compliance, get a SAP calculation done early. It’s far cheaper than finding out when the insulation is already behind the plasterboard.