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.
Quick answer: every above-ground build-up in U-Monkey now shows three extra numbers under its U-value — decrement factor, time lag and areal heat capacity (κ) — calculated to BS EN ISO 13786 from the density and specific heat of each layer. κ is the per-element thermal-mass figure a SAP 10 assessment asks for; the other two are what decide whether a room peaks with the afternoon or after midnight. They print on the free PDF. Nothing has been made paid.
Why we bothered
A U-value is a winter number. It says how fast heat leaks out when it is cold outside and warm inside, and Building Control asks for it, so every calculator gives it. It says nothing about summer, and nothing about how the room feels: whether the walls soak up the afternoon and hand it back at night, or pass it straight through.
Take the pair of walls that turns up on every solid-wall retrofit. Insulate the brick on the outside or insulate it on the inside and you land on the same U-value, around 0.25 W/m²K. On paper they are interchangeable. In the building they are not: the externally insulated one has ten times the usable thermal mass, because the brick is on the warm side of the insulation where the room can use it. U-Monkey used to show them as the same wall. Now it shows the difference.
What the three numbers mean
Decrement factor — the share of the outdoor daily temperature swing that reaches the inside face. A 12-degree day outside becomes a 3.6-degree swing inside through a wall with a decrement factor of 0.3, and nearly 10 degrees through one at 0.8. Below about 0.3 is heavyweight behaviour; above about 0.6 is lightweight.
Time lag — how many hours the outdoor peak takes to work through. Nine or ten hours puts the 3 pm peak after midnight, when the house is cooler and windows are open. One or two hours puts it at teatime, on top of the afternoon.
Areal heat capacity, κ — how much heat one square metre of the inside face stores over a day, in kJ/m²K. This is the number SAP 10 wants per element to build the dwelling’s thermal mass parameter; without it, assessors pick 100, 250 or 450 from Table 1e. U-Monkey now calculates it from the layers you actually specified.
Seven build-ups, side by side
Computed by U-Monkey on the build-ups behind our worked-example pages. Each one opens pre-filled in the calculator from the thermal mass calculator page.
| Build-up | U-value | Decrement factor | Time lag | κ |
|---|---|---|---|---|
| Solid brick wall, external insulation | 0.26 | 0.17 | 9.0 h | 120 kJ/m²K |
| Solid brick wall, internal insulation | 0.25 | 0.29 | 8.1 h | 12 kJ/m²K |
| Full-fill cavity wall, 150 mm mineral wool | 0.20 | 0.27 | 10.7 h | 33 kJ/m²K |
| Timber-frame wall, mineral wool | 0.30 | 0.54 | 5.5 h | 14 kJ/m²K |
| Warm flat roof, PIR over timber deck | 0.14 | 0.82 | 3.4 h | 25 kJ/m²K |
| Pitched roof, PIR at rafter level | 0.20 | 0.71 | 1.7 h | 12 kJ/m²K |
| 270 mm loft quilt at ceiling | 0.16 | 0.86 | 1.9 h | 12 kJ/m²K |
The roof rows are why loft conversions overheat: a PIR roof at a perfectly good U-value passes seven-tenths of the day’s swing through in under two hours, because nothing in it stores heat. Adding more of the same insulation barely moves that; a heavier internal lining or a ventilated void does. That is a design decision worth making before the plasterboard goes up, and it is not one a U-value can inform.
How it is done, and what it is not
U-Monkey builds the ISO 13786 heat-transfer matrix for your layers at a 24-hour period and evaluates it at the inside face. The hard part was never the maths — it was the data. The method needs the density and specific heat of every layer, and U-value libraries do not usually carry them. Ours now does, for all 300+ materials and products, each with a named source and an honest label: the manufacturer’s own figure where one is published, a representative value for the material class where not, and — flagged as the weakest — a transfer from an equivalent material where nobody publishes one. The working in the app shows which applies to each layer. A custom material you type in gets no thermal-mass result until you add the two properties; U-Monkey does not guess them, because an invented density gives a confident-looking time lag that is simply wrong.
It is not a Part O overheating assessment. That needs a dynamic model of the whole room — glazing, orientation, ventilation, occupancy — and we do those as a service. What the calculator gives you is the fabric’s share of the answer, for free, at the moment you are choosing between two build-ups: which one damps and delays the heat, and by how much.
Where to find it
Under the target pills on every result there is now a Thermal mass line; explain opens the working with the full Summer performance block and the data sources. The three figures also print under Result on the free U-value PDF. The thermal mass calculator page has these and three more, each with a link into the pre-filled calculator, and the how-to guide covers adding density and specific heat to a custom material.