Free on every above-ground build-up, printed on the free PDF. No sign-up.
Build the wall or roof layer by layer; the thermal-mass line appears under the U-value.
The short answer
A U-value tells you how fast heat leaks out in winter. It tells you nothing about what the wall does at 3 pm on a July afternoon, or how much of the day's heat the room can soak up and give back after dark. Those are thermal mass questions, and the standard way to answer them for a single construction is BS EN ISO 13786: treat the outdoor temperature as a 24-hour wave, push it through the layers with their density and specific heat as well as their conductivity, and read off three numbers at the inside face.
- Decrement factor — the share of the outdoor daily swing that reaches the inside. 0.1 is a castle; 0.9 is a tent.
- Time lag (hours) — how long the outdoor peak takes to arrive. A long lag pushes afternoon heat into the cooler night.
- Areal heat capacity, κ (kJ/m²K) — how much heat the inside face stores over a day. This is the thermal-mass input a SAP 10 assessment needs per element; the alternative is a guessed default of 100, 250 or 450 from Table 1e.
Same U-value, different building — ten UK build-ups compared
All computed by U-Monkey on the build-ups behind our worked-example pages. Every row opens pre-filled in the calculator, so you can change a layer and watch the numbers move.
| Build-up | U-value | Decrement factor | Time lag | κ |
|---|---|---|---|---|
| Solid brick wall, external insulation (EWI) | 0.26 | 0.17 | 9.0 h | 120 kJ/m²K |
| Solid brick wall, internal insulation (IWI) | 0.25 | 0.29 | 8.1 h | 12 kJ/m²K |
| Full-fill cavity wall, 150 mm mineral wool, aircrete inner leaf | 0.20 | 0.27 | 10.7 h | 33 kJ/m²K |
| Partial-fill cavity wall, 90 mm PIR | 0.18 | 0.28 | 10.3 h | 33 kJ/m²K |
| Timber-frame wall, mineral wool between studs | 0.30 | 0.54 | 5.5 h | 14 kJ/m²K |
| Uninsulated cavity wall, dense block inner leaf | 1.67 | 0.42 | 7.4 h | 156 kJ/m²K |
| Solid 1½-brick wall, plastered | 1.65 | 0.30 | 9.5 h | 132 kJ/m²K |
| Warm flat roof, PIR over timber deck | 0.14 | 0.82 | 3.4 h | 25 kJ/m²K |
| Pitched roof, PIR between and under rafters | 0.20 | 0.71 | 1.7 h | 12 kJ/m²K |
| Pitched roof, 270 mm loft quilt at ceiling | 0.16 | 0.86 | 1.9 h | 12 kJ/m²K |
W/m²K for U-value; κ is the areal heat capacity of the inside face over a 24 h period. Density and specific heat from U-Monkey's sourced library; the working in the calculator shows the source for each layer.
Read the first two rows twice
The solid brick wall with external insulation and the one with internal insulation have the same U-value to within a hundredth. One has ten times the thermal mass of the other. Insulate outside and the 215 mm of brick sits on the warm side, absorbing the afternoon and paying it back overnight — κ 120, decrement 0.17, the swing arrives nine hours late and cut to a sixth. Insulate inside and the same brick is stranded on the cold side behind the PIR: κ 12, and the room now behaves like a timber-frame one — quick to warm up, quick to cool down, and with nothing to blunt a heatwave. Both pass Part L. A U-value calculator that stops at the U-value cannot tell you they are different buildings.
The roof rows are why loft conversions overheat. A PIR roof at U 0.20 — a good number — passes seven-tenths of the day's swing through in under two hours; a 270 mm quilt at ceiling level passes nearly nine-tenths. Nothing in that construction stores heat, so the room under it peaks with the afternoon. Adding more of the same insulation barely moves it; a heavier board on the inside face, or a ventilated void, does.
What you do with the numbers
- SAP assessors: give your software the κ per element instead of the Table 1e default. It is the honest figure for the build-up you actually assessed, and it is on the PDF with its method.
- Architects and designers: when two build-ups both pass on U-value, the decrement factor and time lag are the tie-breaker for bedrooms, south-facing rooms and lofts. Prefer the one below 0.3 where you can; specify a heavier internal lining where you cannot.
- Retrofit: the IWI-versus-EWI decision above, in numbers, before the scaffold goes up. Internal insulation is often the only option — but you should know what it costs in mass, and plan the ventilation and shading accordingly.
- Part O: the fabric half of the story. If the dynamic simulation later says a room fails, the decrement factors tell you which surfaces to fix first.
How it is calculated
U-Monkey builds the heat-transfer matrix of BS EN ISO 13786 for the layers of your build-up, outside to inside, at a 24-hour period, and evaluates it at the inside face — with the standard surface films for the decrement factor and time lag, and the layers alone for κ, as the standard defines it. Every layer needs three properties for this — conductivity, density and specific heat — and the last two are the hard part: most U-value libraries do not carry them. U-Monkey's does, for every one of its 300+ materials, each with a named source and a basis: the manufacturer's declared figure, a representative class value, or a transfer from an equivalent material where nobody publishes one. The working shows which applies. If a layer has no data — a custom material you typed in — the calculator says so rather than print a confident-looking time lag; add density and specific heat from the datasheet in the layer's menu and it fills in.
Two things it is not. It is not a room overheating model — that needs the glazing, orientation, ventilation and occupancy of a Part O assessment, which is a service, not a calculator. And it is not a substitute for a manufacturer's certified dynamic data where a product has it; it is the standard's method applied to sourced material properties, shown in full so you can check every line.
Frequently asked questions
What is thermal mass, in one sentence?
The ability of a construction to absorb heat, hold it and release it later — which is why a heavy stone church stays cool on a hot afternoon and a caravan does not. BS EN ISO 13786 turns that into three numbers for a wall, roof or floor: areal heat capacity, decrement factor and time lag.
What is the decrement factor?
The fraction of the outdoor daily temperature swing that gets through to the inside face. If it is 25 °C outside at 3 pm and 13 °C at 4 am — a 12-degree swing — a wall with a decrement factor of 0.3 lets a 3.6-degree swing through; one with 0.8 lets nearly 10 degrees through. Below about 0.3 is heavyweight behaviour, above about 0.6 is lightweight.
What is the time lag?
How many hours the outdoor peak takes to work through to the inside. A masonry wall with a 9–10 hour lag delivers the 3 pm peak at midnight or after, when the house is cooler and windows can be open; a PIR roof with a 1–2 hour lag delivers it at 5 pm, on top of the afternoon.
What is areal heat capacity (κ), and why does SAP want it?
κ (kappa) is how much heat one square metre of the inside face can store over a daily cycle, in kJ/m²K. SAP 10 uses it to build the dwelling’s thermal mass parameter, which feeds the gains-utilisation and summer-overheating parts of the calculation. Without a value per element, assessors pick a default of 100, 250 or 450 kJ/m²K from Table 1e. U-Monkey works it out from the actual layers instead.
Why do two walls with the same U-value have different thermal mass?
Because the U-value only counts resistance in series — it does not care which side of the insulation the heavy material sits. Put insulation on the outside of a brick wall and the brick is on the warm side, storing and releasing heat into the room (κ around 120 kJ/m²K). Put the same insulation on the inside and the brick is stranded on the cold side: same U-value, κ around 12. The room becomes fast to heat and fast to cool.
Is this a Part O overheating assessment?
No. Part O compliance for new homes needs either the simplified method or a dynamic thermal simulation of each room to CIBSE TM59 — glazing, orientation, ventilation and occupancy included. U-Monkey gives you the fabric’s contribution to that picture — which build-up damps and delays the heat better — using the ISO 13786 periodic method. It is the right tool for comparing constructions and for the SAP input; it is not a room model. If you need the assessment, we do those.
Where do the density and specific heat values come from?
Every material in U-Monkey’s library carries a density and specific heat with a named source: the manufacturer’s own datasheet where one publishes it, a representative value for the material class where not, and — flagged as the weakest kind — a transfer from an equivalent material where nobody publishes the figure at all. The working shows which applies to your build-up. U-Monkey never invents a value: a custom material with no density and specific heat simply gets no thermal-mass result until you add them.
Is it free?
Yes, all of it. The three numbers appear on every above-ground build-up in the free calculator and print on the free U-value PDF. No sign-up.
Start from the main U-value calculator, read where the numbers appear in the app, or see the sister check: condensation risk to BS EN ISO 13788.