How to use U-Monkey — the complete guide

Everything in the free U-value calculator, explained: from your first layer to a Building-Control-ready PDF with a condensation check. Five minutes to read, less to do.

Open U-Monkey — free

No sign-up. Everything below happens in your browser.

Quick start — a U-value in under a minute

  1. Open the calculator and pick your element — wall, roof or floor.
  2. Tap a starter template (or “See a worked example”) to load a typical build-up.
  3. Swap layers and thicknesses to match your construction. The U-value and the Part L traffic lights update live.

That's genuinely it for a simple element. The rest of this guide covers each feature properly.

1. Choose your element and building-work type

U-Monkey handles external walls, pitched roofs (insulated at ceiling or rafter level), flat roofs, exposed floors and ground floors. Ground floors follow BS EN ISO 13370, so you'll also enter the exposed perimeter, floor area and wall thickness — a ground-floor U-value depends on plot geometry, not just the layers.

Then set the building-work type, because it decides the pass mark:

Not sure what you need? See What U-value do I need?

2. Build up the layers, outside → in

Add layers in construction order, outermost first, with ＋ Add layer. The picker is organised in tabs — Insulation, Insulated plasterboard, Boards, Masonry, Concrete & screeds, Timber, Plasters & renders, Membranes, Coverings & cladding, Air cavities, Metals and Special — and the search box looks across everything, including brand names.

The library holds 300+ materials: BR 443 / CIBSE generic values plus real UK products from Kingspan (including the full Kooltherm K-series), Celotex, Recticel, EcoTherm, Unilin, IKO, Rockwool, Knauf, Isover, Superglass, Jablite, Thermalite, Celcon, Durox and many more — each with its declared λ and the source it came from. Entries marked ✓ verified have been human-checked against the manufacturer's datasheet. Every thickness is editable, layers drag to reorder, and each shows its own resistance (R) so you can see what is doing the work.

Can't find a product? ＋ Custom material takes any name, λ and thickness.

Insulated plasterboard

Thermal laminates (Kingspan K118, Celotex PL4000, EcoTherm Eco-Liner, Unilin XT/TL, Gyproc ThermaLine, Knauf laminates) live in their own tab, and picking one adds two layers at once — the insulation core and the plasterboard facing. That keeps the U-value and the condensation profile physically correct. Set the core thickness to the overall board minus the facing (a 52.5mm board = 40mm core + 12.5mm plasterboard), and remember the dot-and-dab air gap behind the board if that's how it's fixed.

3. Bridging, cavities and foil facings

Bridged layers

Insulation between studs, rafters or joists is bridged: the timber conducts more than the insulation. Open the layer's ⋯ menu, choose bridging, and either type the fraction or enter the stud width and centres — U-Monkey computes the percentage and combines the two paths per ISO 6946. (Steel framing needs the BRE Digest 465 method; U-Monkey warns rather than pretending.)

Air cavities

Cavities come from the Air cavities tab — masonry cavity, dot-and-dab gap, service void, the ventilated loft of a cold roof. Ventilation is set per cavity: unventilated, slightly ventilated (with the opening area in mm²/m, interpolated per ISO 6946 §6.9.3) or well-ventilated — a well-ventilated cavity and everything outside it is disregarded, exactly as the standard requires.

Low-emissivity (foil-faced) cavities — automatic

A cavity bounded by a shiny foil surface loses far less heat by radiation: its effective surface emissivity drops from about 0.9 to 0.2, and BR 443 §4.7.2 raises its resistance accordingly (0.44 instead of 0.18 for a 50mm wall cavity). U-Monkey detects this automatically — put an unventilated cavity next to a foil-faced PIR, phenolic or multifoil board and the cavity is flagged “low-e (auto — foil-faced neighbour)”, with a toast explaining what happened. Tissue-faced boards (like Kingspan TR27, K5 or K103) are correctly excluded. It stays editable: open the cavity's ventilation sheet and untick it, and your choice is remembered.

4. Read the result

The headline U-value is computed by the full combined (upper/lower limit) method of BS EN ISO 6946:2017 with BR 443 conventions — not a simple resistance sum — including ΔU air-gap corrections and the standard's own reliability check. It reproduces the official gov.scot worked example exactly. Under the number you get a plain-English verdict, traffic-light pills for each applicable target, the cross-section drawing, and any warnings (oversized layers, multifoil caveats, steel bridging and so on).

Missing the target? The target solver tells you what thickness of your chosen insulation would get you there — try it before redesigning the whole element.

5. Check condensation risk

Every build-up gets a free interstitial condensation screening to BS EN ISO 13788 (monthly Glaser method): pick the nearest climate station and the humidity class (class 3 suits most dwellings), and optionally the BS 5250 1-in-10 severe year. You get a month-by-month verdict — no condensation, forms but evaporates, or accumulates — plus the vapour-pressure chart showing exactly where the build-up runs closest to its dew point, the fRsi surface-mould check, and a robustness scenario for cold-roof conversions. Vapour data comes from BS 5250/ISO 10456; where a value is genuinely product-specific you can enter the declared μ or sd on the layer. If condensation would land on structural timber, U-Monkey also interprets what the moisture uptake means against the BS 5250 decay threshold. For the full method, see the condensation risk calculator page.

6. Save, share, resume and export

Every report can be verified — free ones included

Every report U-Monkey issues — the free U-value PDF as well as the paid condensation report — carries a unique report ID printed on the document. Anyone holding the report can enter that ID at energycount.co.uk/verify and confirm in seconds that it's genuine and matches what was originally issued.

Why that matters: a plain PDF is just a file. It can be edited after the event, a thickness can be "adjusted" somewhere along an email chain, and the person receiving it — a building control officer, a SAP assessor, a main contractor — has no way to tell. The report ID closes that gap. It gives your calculation provenance: Building Control can check it without taking anyone's word, your client can rely on it in a handover pack, and it still verifies years later — during a sale, a retrofit or a warranty query — when the original email is long gone. It's the difference between "here's a PDF someone made" and a document that stands on its own, which is exactly what you want attached to a Building Regulations submission.

Power tips

Frequently asked questions

How do I add insulation between rafters or studs (a bridged layer)?

Add the insulation layer, tap its ⋯ menu and choose the bridging option. Enter the bridge material (softwood is preselected for timber) and the fraction — either directly, or from the stud width and centres and U-Monkey works the percentage out for you. The layer then shows its combined bridged resistance.

How do I model an air cavity, and what about foil-faced boards?

Add one of the air cavity entries from the Air cavities tab (standard cavity, dot-and-dab gap, service void, ventilated loft…). Set its ventilation from the ⋯ menu: unventilated, slightly ventilated (with the opening area) or well-ventilated. If the cavity sits against a foil-faced board, U-Monkey automatically treats it as a low-emissivity cavity — the surface emissivity drops from about 0.9 to 0.2, raising the cavity resistance to the BR 443 §4.7.2 value (0.44 for walls at 50mm) — and says so on the layer. You can override this in the ventilation sheet; your choice is remembered.

How do I add insulated plasterboard like Kooltherm K118?

Open the material picker and use the Insulated plasterboard tab. Picking a board adds two layers in one go — the insulation core and the plasterboard facing — so both the U-value and the condensation profile are physically correct. Set the core thickness to the overall board size minus the facing (for example a 52.5mm K118 is a 40mm core plus 12.5mm plasterboard).

Can I use my own lambda value?

Yes — in the material picker choose “＋ Custom material” and enter the name, λ (W/m·K) and thickness. You can also edit the vapour resistance (μ or sd) of any layer from its ⋯ menu, which the condensation check then uses instead of the built-in default.

What do the traffic lights mean?

Green means the build-up beats the relevant target with margin; amber means it meets the minimum; red means it misses. The target itself depends on the building-work type you chose — new dwellings and new extension elements use the Part L 2021 limiting values (wall 0.18, roof 0.15, floor 0.18), existing elements are appraised against the Table 4.3 threshold values, and upgrades to retained elements use the Table 4.3 “improved” values.

How does the condensation check work and is it free?

Every build-up is screened for interstitial condensation to BS EN ISO 13788 (monthly Glaser method) using BS 5250 vapour resistivities and UK climate data — free, on screen, automatically. Pick the nearest climate station, the humidity class (class 3 suits most dwellings) and optionally the 1-in-10 severe year. The chart shows the vapour pressure against the dew-point limit through the build-up. A submittable Building-Control condensation report is available as a paid extra.

How do I save my work or share it with a colleague?

Use the 💾 Save button (or Ctrl+S) to download a project file, 📂 Load to reopen one, and 🕐 Recent for your latest builds — the app also autosaves and offers to restore when you come back. The Share option makes a link that reopens the exact build-up on any machine, and My build-ups keeps a named library in your browser.

Is the PDF really free, and will Building Control accept it?

The U-value PDF report is completely free — no sign-up. It shows the full layer table, the ISO 6946 upper/lower-limit working, corrections and the method statement, so a building control body can check every line. And every report — the free U-value PDF and the paid condensation report alike — carries a unique report ID that anyone can check at energycount.co.uk/verify.

What does the report ID / verify function actually do for me?

Every U-Monkey report is issued with a unique ID printed on it, and anyone — you, your client, the SAP assessor, Building Control — can enter that ID at energycount.co.uk/verify and see that the report is genuine and matches what was originally issued. A PDF on its own is just a file: it can be edited, or a number can be misread when it is retyped into an email chain. The verify check settles both in seconds, which is why a verifiable calculation is taken more seriously than an anonymous spreadsheet — and the ID keeps working years later, when the original email is long gone.

Why does my ground floor ask for perimeter and area?

Ground-floor U-values follow BS EN ISO 13370, where the result depends on the plot geometry — the exposed perimeter, the floor area and the wall thickness — not just the layers. Enter those and U-Monkey does the rest, including the soil type.

What if I see a warning or the result looks wrong?

U-Monkey guards against common slips — a layer thicker than a metre, a suspicious λ, bridged steel (which needs the BRE 465 method), multifoils quoted by certified system R — and says so rather than silently computing. Warnings appear under the result; each one explains what to check.

Ready? Open U-Monkey and build your element — or start from the U-value calculator overview.

Easiest way to learn it? Open it.

U-Monkey runs free in your browser — build a wall from a template and you'll have a U-value and a condensation verdict inside a minute.

Open the calculator or call 01202 623236