— reviewed against the regs as they stand.
U-Monkey — free in your browser
Build up a wall, floor or roof and read the U-value live
Assemble the element layer by layer from a library of 418 materials, with the Part L limiting and notional checks, condensation-risk screening and the thermal-mass figures applied as you build.
Completely free — the calculation, the checks and the Building Control-ready PDF. No sign-up.
The principle
The energy hierarchy ranks the ways to cut a building’s carbon footprint, in order of preference:
- Reduce energy demand — fabric first: insulation (u-values), air tightness, minimised thermal bridging, good form and orientation.
- Use energy efficiently — efficient heating systems and controls, MVHR, LED lighting, waste water heat recovery.
- Supply from low-carbon and renewable sources — heat pumps, solar PV, solar hot water.
London plans phrase it as “be lean, be clean, be green” — and many Local Planning Authorities structure their energy statement requirements around exactly this logic, expecting demand reduction to be demonstrated before renewables are counted.
Why the order matters
A kilowatt-hour you never need is cheaper than one you generate. Fabric measures are permanent, maintenance-free and invisible to occupants; renewables degrade, need replacing, and underperform on poorly designed buildings. Part L embeds the same thinking through the fabric energy efficiency target, which renewables can’t satisfy.
It’s also why we push calculated junction Ψ-values so hard: they’re a step-1 improvement that costs nothing on site, and the K1 defaults you are improving on are published so you can see the headroom before committing. The same goes for the build-ups themselves — U-Monkey will tell you what a wall, roof or floor actually achieves while the specification is still a drawing and step 1 is still cheap.