This tool performs a room-by-room heat loss calculation — sizing an emitter for every room and a heat generator for the whole property. This guide explains how to use it, how to read the numbers, what it assumes, and where it stops.
⚠ Read this first
The tool fills in construction from the age band you pick. Those are assumptions, not measurements. If the property has been upgraded — new windows, loft topped up, cavity filled — change those inputs or the heat loss will read too high. Everything you enter is stored on this device only; there is no server copy and no automatic backup.
What this tool does. It performs a room-by-room heat loss calculation, sizing an emitter for every room and a heat generator for the whole property. It is an independent tool, not affiliated with or endorsed by the bodies below; they are cited as data sources only. Every figure traces to a named source: design temperatures from CIBSE 2025 Weather Data; U-values from RdSAP 10; room design temperatures from MCS MIS 3005-D (with CIBSE DHDG Table 2-2 for room types MCS does not list); method basis BS EN 12831-1, via the CIBSE Domestic Heating Design Guide.
Your first calculation
Seven steps from a cold start to a finished report.
1 · Create a project
Tap Projects and give it the property name. Work is saved as you go. You can calculate without a project — it goes to a scratch pad called “Unsaved calculation” — but name it properly if the report is going to anyone.
2 · Set the age band
This pre-fills walls, roof, floor and glazing with the construction typical for that era. Every pre-filled value is an assumption to check on site.
3 · Check each fabric element
Walls, roof, windows, doors, floor. Override anything you know is different. A green tick means you have confirmed it; amber means it is still the age-band assumption.
4 · Set the whole-building figures
Outdoor design temperature (from the postcode), air permeability, and thermal bridging. These apply to every room.
5 · Add each room
Room type sets the design temperature. Then dimensions, how many external walls, what is above and below, then windows, doors, internal and party walls.
6 · Check the breakdown
Open Notes & photos and Heat loss breakdown at the foot of each room to see where its heat is going. If a figure looks wrong, this is where you find out why.
7 · Generate the report
PDF report… at the bottom of the main screen, or from any project in the Projects list.
Projects & saving
Each property is a project. Everything saves automatically as you type — there is no save button for the calculation itself, and the room editor’s Save & close simply marks the room done and returns.
From the Projects list you can open, rename, duplicate, export or delete a project, and generate its report without opening it.
Export is your only backup.Export project… writes an .ehe.json file containing the whole survey, including photos and your saved U-value constructions. That file is the only way to move a survey to another device, and the only copy that survives clearing browser data or losing the phone. Export anything you cannot afford to lose.
Rooms & design temperatures
Picking a room type sets its design temperature from MCS MIS 3005-D Table 1 — 21 °C living rooms, 18 °C bedrooms and halls, 22 °C bathrooms, and so on. Where MCS does not list a room type, CIBSE DHDG Table 2-2 is used. You can override the temperature for any room; the tool shows whether the value is the default or your override.
The design temperature matters more than it looks. It sets the ΔT for every external element in that room, the ground-floor ΔT, and the ΔT across every internal wall to a neighbouring room. Getting it wrong moves everything.
Exposure & dimensions
For each room: length, width and height (or the floor area directly for an irregular room), how many walls face outside, and what is above and below.
External walls
The count estimates the gross external wall area from the room size. If you have measured the actual wall run, type it in — it is more accurate, and it also improves the ground-floor U-value, which uses the exposed perimeter.
Above
Roof/loft, a heated room in this property, another dwelling, or an unheated space. Each routes a different ΔT.
Below
Ground, a heated room, another dwelling, an unheated space, or outside air. Ground floors use the reference (annual-mean) temperature, not the outdoor design temperature — the earth under a floor is far milder than the air outside.
Windows and doors are deducted from the gross wall to give the net wall area, so enter the gross wall and let the tool subtract the openings.
Fabric & U-values
Set each element once for the whole building; any room can override a single element later. An extension of a different age gets its own set of values — add it with + Extension.
Three states are shown for each element: empty, assumed from the age band (amber), or verified on site (green). The report records which is which, so a reader can see what was measured and what was assumed.
Where you know the make-up of a construction but not its U-value, the U-value builder works it out layer by layer — see Using the U-value builder. Thermal bridging is added to every U-value as a flat addition from DHDG Table 2-9.
Internal & party walls
A wall between two rooms at different temperatures carries real heat, and the direction follows the temperatures. An 18 °C bedroom flanked by a 21 °C lounge and a 22 °C bathroom gains from both — they are warmer than it is. A 21 °C lounge between that bedroom and the bathroom loses to the bedroom and gains from the bathroom. The tool works out each wall from the two design temperatures either side.
To another room in this project
Pick the room. The ΔT follows the two design temperatures and updates if you change either. The matching wall is created in the other room automatically, so a shared wall is entered once and stays consistent.
Unheated space (garage, store)
Taken at 5 °C. Because an unheated space is outside the heated envelope, this wall counts toward the room’s air-leakage envelope.
Adjacent space — set temperature
For a space that is not a room in this project: a conservatory, a stairwell, next door’s shop, or a room you have not surveyed. You give the temperature the other side. This does not count toward the air-leakage envelope — you are describing an enclosed space, not outside air. If the space is genuinely unheated, use the option above instead.
Setting the construction
Each internal and party wall takes its U-value from a CIBSE DHDG Table 2-36 construction, from a construction you have built and saved in the U-value builder, or from Enter a known U-value… if you already have the figure. Thermal bridging is added to whichever you choose.
Party walls
To a neighbouring property. Taken at 15 °C where the neighbour is heated, 5 °C where not — DHDG assumes the adjoining space is unheated at the time of calculation.
Walls between rooms at the same temperature carry no heat and show as nil — that is correct, not a missing value.
Single-room assessments
You can assess one room without modelling the whole building. Create a project with a single room and describe its neighbours using Adjacent space — set temperature for internal walls, and Space not in this project for what is above or below.
The report states that the scope is a single room, and the whole-building charts are suppressed.
Three things decide whether a single-room result matches the full survey. The room’s own design temperature; the temperatures you assume for the adjoining spaces; and the exposed envelope — particularly the external wall area, which drives the ventilation loss. Get those right and a single-room assessment reproduces the whole-building figure exactly. Get the temperature wrong by three degrees and the emitter can move by nearly a third.
Ventilation & air permeability
Ventilation loss is driven by air leaking through the parts of the room that face outside. Air permeability is set once for the whole property, from the structure type, floor type, how much of the property is draught-proofed, and the number of storeys — or entered directly if you have a measured test result.
Each room’s leakage is then its exposed envelope × the permeability, plus any background ventilation — trickle vents, airbricks, extract fans and open flues, which you count per room.
Why the emitter figure is bigger. Emitter sizing includes an orientation factor of ×2 on the ventilation heat loss. Wind pressure, stairwell stack effects and extract ventilation cause air to move unevenly between rooms within the building — rooms on the windward side, or near extract points, see greater air movement than those on the sheltered side. That internal air movement is not heat lost from the building; it is heat redistributed within it. Each emitter must be sized to handle the air movement for its own room, so the ×2 applies to the emitter figure. The generator sizes on the building as a whole, where that redistribution nets out, so its ventilation is counted once.
Understanding U-values
The 1-1-1 rule
A U-value of 1 W/m²K means that 1 Watt of heat is lost through each square metre
of building fabric for every 1 °C temperature difference between the warmer and cooler sides.
Three quantities do the work in a heat loss calculation. They are linked, but they measure different things.
k-factorW/m·K
Thermal conductivity — how easily heat passes through a material. It is a property of the material itself, whatever thickness it is. Also written as the lambda (λ) value.
Lower k-factor = better insulator
R — thermal resistancem²K/W
How much a particular layer resists the flow of heat. It depends on both the material and how thick the layer is, so a thicker layer of the same material has a higher resistance.
Higher R value = better thermal resistance
U-valueW/m²K
The rate at which the finished construction conducts heat, once every layer and both surfaces are taken into account. This is the number that goes into the heat loss calculation.
Lower U-value = slower transfer of heat
How the U-value is worked out
First, find the resistance of each layer — its thickness divided by its k-factor:
R = L ÷ k
layer resistance = thickness (in metres) ÷ k-factor
Thicknesses are usually measured in millimetres, so divide by 1000 to convert to metres — 100 mm becomes 0.1 m.
Then add up every resistance in the construction, including the two surfaces and any air gap:
R total = Rsi + R1 + R2 … + Ra + Rsototal resistance = inside surface + each layer + air gap + outside surface
Finally, the U-value is 1 divided by that total:
U = 1 ÷ R total
What the symbols mean
kk-factor — thermal conductivity of a material (W/m·K). Also written λ (lambda).
Lthe thickness of a layer, in metres.
Rthermal resistance of a layer (m²K/W).
Rsiinside surface resistance — the thin film of still air against the inside face of the wall.
Rsooutside surface resistance — the same on the outside face. Sometimes written Rse.
Rathe resistance of an air gap or cavity within the construction.
Uthe U-value of the finished construction (W/m²K).
The surface resistances
Every construction has a thin layer of still air clinging to each face, and that still air resists heat flow too. These are fixed values, added automatically by the tool — you never enter them.
They differ because heat moves differently depending on direction — sideways through a wall, upwards through a roof, downwards through a floor.
A note on rounding
The tool keeps full precision through every layer and rounds only the final U-value. A hand calculation that rounds each layer as it goes may differ by about 0.01 — both are correct, it is just where the rounding happens.
Using the U-value builder
The U-value builder — linked at the bottom of the tool — calculates a U-value from the actual make-up of a construction, to BS EN ISO 6946. Use it when you know what a wall, roof or floor is built from but not its U-value, or when the age-band default does not describe what you have found on site.
1 · Pick the element
Wall, roof, floor and so on. This sets the internal and external surface resistances (Rsi and Rse), which differ by element and by the direction of heat flow.
2 · Add the layers
Work through the construction from one face to the other — render, block, cavity, insulation, plasterboard. For each layer give its thickness and material; the builder holds conductivities for common materials, or you can enter one you know.
3 · Add any airspace
An unventilated cavity is a resistance in its own right, not a material. The builder carries the standard airspace resistances so a cavity is entered as a cavity rather than guessed at.
4 · Read the result
The total resistance and the resulting U-value update as you build. The working is shown, so you can check it or copy it into a report.
5 · Save it to your library
Give it a name and save. It is then offered in the tool wherever that element is chosen — including internal and party walls, grouped as My U-values (built up).
Getting the value back into a calculation. Open the builder from the element you are setting and the result is offered back to that element when you return. Or save it to the library and pick it from the construction list later — the library is shared across every project on the device.
The builder gives the U-value of the construction itself. The tool then adds the thermal bridging allowance (DHDG Table 2-9) on top, which is why a wall entered at 0.28 shows as 0.38 in a room's breakdown when bridging is set to +0.10. Do not add bridging yourself in the builder or it will be counted twice.
⚠ A built-up value is only as good as the build-up
A U-value calculated from assumed layers is still an assumption. If you have not opened up the construction or seen a reliable drawing, say so in your survey notes — a calculated figure can look more authoritative than the evidence behind it.
Minimum ventilation (0.5 ACH)
Every room needs a minimum amount of fresh air. The method works from a room's air leakage through its exposed envelope — but a well-sealed building, or a room with few external walls, can come out below that minimum. A fully internal room can come out with almost no ventilation loss at all, which would size an unrealistically small emitter.
The tool handles this in two separate ways, so the calculation stays faithful and you still get a usable answer.
1. The flag — always shown
Any room whose estimated air leakage falls below 0.5 air changes per hour is flagged, on the room card, in its heat loss breakdown and in the report. The flag changes no figure. Assessing whether ventilation is actually adequate is outside the scope of this tool — advise the client to seek expert advice and stress the importance of maintaining sufficient ventilation in the room.
2. The switch — your choice
Apply 0.5 ACH minimum on the building tab is off by default. Off, every room is sized on its own estimated air leakage rate. Switch it on and any room whose estimated leakage falls below 0.5 ACH is sized to the 0.5 ACH minimum instead, which gives an internal room a realistic emitter. The report states which basis was used.
The 0.5 ACH minimum air-change rate is from CIBSE DHDG Table 2-18 (derived from BS EN 12831-1:2017), where it is a ventilation-sufficiency check rather than a sizing input. Switching it on is a deliberate, documented departure so that internal rooms return a usable emitter size. Comparing the two settings on the same building is a useful way to show what purpose-provided ventilation costs in heating terms.
The three figures
Every calculation ends in three numbers. They answer different questions and are not interchangeable.
Emitter totalkW
Sizes the radiators. Fabric loss plus ventilation with the air change counted twice. Use the per-room emitter figure to size that room’s radiator.
Generator (design)kW
Sizes the heat generator. The coldest-day load at the outdoor design temperature: fabric plus ventilation counted once. This is the design heat loss.
Typical loadkW
The milder average-day figure, at the reference (annual-mean) temperature. The generator must be able to modulate down to this, or it will short-cycle for most of the season.
A room surrounded by warmer rooms can gain more than it loses, giving a negative design loss. That is arithmetically correct but no emitter is required on those figures — the report flags it, and it is worth checking the assumed temperatures of the adjoining rooms and whether the room will hold its design temperature when they are not calling for heat.
In the PDF report, the room summary table reads left to right as the method works: Fabric + Vent = Design loss, with Emitter alongside as the larger figure carrying the ×2 orientation factor.
Notes & photos
Each room has a Notes & photos panel at the foot of its editor, and the building has Survey notes & photos on the main screen. Both print in the report.
Photos are taken with the phone camera or chosen from the gallery, then reduced to about 1200 px and re-encoded, so a survey stays a sensible size and still prints cleanly on A4. Tap a photo to view it full size, add a caption, or delete it.
Where photos are stored. In this browser’s own storage on this device (IndexedDB), alongside the projects — not in your phone’s photo gallery, and not on any server. They travel with the project when you export it, and they print in the report. Anything you photograph in a client’s home is your responsibility: get their agreement, and avoid capturing people or documents you do not need.
Saving the report as a PDF
The PDF report… button — on the main screen below the totals, and on each project in the Projects list — builds a printable A4 report of the whole assessment.
What it contains:
Cover
Property, client, assessor, company and job reference, the date, and the three dwelling figures.
Basis of assessment
Climate zone and percentile, outdoor design and reference temperatures, total floor area, the fabric U-values used, thermal bridging, air permeability, and the system flow/return as set in the Radiator Sizing tool. Any room below the 0.5 ACH minimum is flagged here, naming the rooms and stating which sizing basis was used.
Room summary
Every room: temperature, area, fabric, ventilation, design loss, emitter — with columns for the required output at ΔT50 and the selected radiator size, filled by the Radiator Sizing tool. The Dwelling row totals area, fabric and ventilation, so Fabric + Vent = Design loss can be checked at a glance.
Room detail
For each room: every element with its area, U-value including bridging, ΔT and watts; the fabric subtotal; a pie chart of where that room’s heat goes, with any heat gains listed separately; the room’s notes and photographs; and any ventilation advisory.
Survey notes & photos
Building-level notes and photographs, where you have added them.
Appendix
Whole-building loss intensity in W/m², a pie of loss by element type, a pie of loss by room, and the rooms ranked by W/m² — which shows the leakiest rooms regardless of their size. For a single-room assessment the whole-building charts are suppressed and the scope is stated on the cover.
Sources
The named standards and tables behind every figure.
Tapping PDF report… hands the report to your phone's built-in print screen. From there you save it as a PDF. The exact steps depend on the phone.
Android
1. Tap PDF report… — the print preview opens.
2. At the top, tap the printer dropdown and choose Save as PDF.
3. Set paper size to A4 if it isn't already.
4. Tap the blue PDF / download button, choose where to save, and confirm.
iPhone / iPad (Safari)
1. Tap PDF report… — the print preview opens.
2. Tap the Share button (the square with an arrow).
3. Choose Save to Files, pick a location, and tap Save — it is saved as a PDF.
Some iOS versions place the PDF option slightly differently. Apple's own guide, "Annotate and save a webpage as a PDF in Safari," has the current steps for your version.
The report is set to A4. Choosing A4 as the paper size gives the cleanest page breaks — each room's breakdown stays together on one page.
Tool limitations
What this tool does not do, and where it can bite. Read this before relying on it for anything that matters.
Your work is stored on this device only
Projects, the U-value library and photos are held by the browser on this device. There is no account, no server copy and no automatic backup. Two different storage areas are used: projects and your U-value library in local storage, photos in IndexedDB. Both belong to the browser.
Clearing your history is safe
Browsing history is just the list of pages you have visited. Clearing it does not touch your projects.
Clearing “cookies and site data” deletes everything
This is the item that wipes projects, the U-value library and photos. It sits next to “history” in the same dialogue and is often ticked with it. So does “clear site data” in site settings, and uninstalling the app.
There is no “protect this site” tick box
A bulk clear does not let you exclude one site. If you want to clear up without losing your surveys, delete data site by site instead — in Chrome, Settings → Site settings → Data stored, pick the site and delete it; on iPhone, Settings → Safari → Advanced → Website Data, then swipe to delete individual sites. Leave this tool alone and clear the rest. If you do use the bulk clear, export your projects first.
Blocking site data stops it saving
The tool sets no cookies at all, but browsers group local storage under “cookies and site data”. If that is blocked — or you are in a private window — the tool cannot save. It will tell you if this happens; export immediately if it does.
Storage is tied to where you opened it from
Data saved while running the tool from one address is not visible from another. Install the app or use the same address every time — do not work from a downloaded copy of the file.
The U-value library travels with an export
Exporting a project includes its photos and your saved U-value constructions, so an export backs up both. On import, constructions are added to whatever library is already on that device — nothing you already have is overwritten, and a construction whose name is already present is skipped.
⚠ Export or lose it
A cleared browser, a lost phone or a reset device takes every survey with it. Export project… after any survey you would be upset to lose, and keep the file somewhere else.
What the calculation does not cover
Ventilation adequacy
The tool flags a room whose estimated air leakage is below the 0.5 ACH minimum, but assessing whether ventilation is actually adequate is outside its scope. Advise the client to seek expert advice.
Warm-up and intermittent heating
These are steady-state design figures. No allowance is made for reheating a building from cold, which BS EN 12831-1 treats as a separate calculation.
Solar and internal gains
Not included. Design heat loss is calculated without them, which is the conventional and conservative approach.
Pipework, system volume and hot water
Not included. Distribution losses, system volume and hot-water demand are separate calculations.
Radiator selection
This tool gives the required emitter output per room. To choose actual radiators, use the Radiator Sizing tool — it reads this survey’s per-room emitter figures and corrects them for your flow temperature and pipe connection, then fills the report’s radiator columns.
Accuracy depends on what you enter
Age-band construction, default U-values and derived air permeability are assumptions. They are reasonable starting points, not measurements. A retrofitted property loses far less than its original fabric suggests, and a single wrong design temperature moves every figure in that room.
Method & sources
Method basis
BS EN 12831-1, via the CIBSE Domestic Heating Design Guide 2026 — fabric per Worksheet A2, with thermal bridging added to every U-value (Table 2-9).
Design temperatures
CIBSE 2025 Weather Data (Table 2-3), by postcode zone and percentile. Ground floors use the reference annual-mean temperature.
Room design temperatures
MCS MIS 3005-D Table 1, with CIBSE DHDG Table 2-2 for room types MCS does not list.
Fabric U-values
CIBSE DHDG 2026 tables and RdSAP 10. Internal and party walls per Table 2-36. Ground floors calculated to BS EN ISO 13370 using the exposed perimeter; constructions built layer by layer follow BS EN ISO 6946.
Ventilation
Air permeability and its conversions per CIBSE DHDG 2026 Tables 2-11 to 2-17. Minimum air change rate from Table 2-18, derived from BS EN 12831-1:2017.
This is an independent tool. It is not affiliated with, endorsed by, or approved by CIBSE, MCS, BSI or any other body; those documents are cited as data sources only.
About & Terms
Data privacy. This tool runs entirely on your device. Everything you enter — projects, calculations, notes, photographs and your U-value library — stays in this device’s browser storage and is not sent to any server or third party. There is no account, no tracking and no analytics. Nothing is shared unless you choose to share it yourself by exporting a file, producing a report, or sending an email. Photographs you take of a client’s property are personal data and your responsibility to handle appropriately; exported files and reports carry those photographs with them.
Disclaimer. This tool performs a room-by-room heat loss calculation to assist competent persons in sizing heat emitters and heat generators. It is a calculation aid only. It does not constitute engineering certification, a design, or a substitute for professional judgement, and it does not by itself satisfy any MCS, Building Regulations or other regulatory requirement.
Results depend entirely on the information entered and the assumptions made. Fabric U-values, air permeability, adjoining space temperatures and construction details taken from age bands or default tables are assumptions, not measurements, and must be verified on site. Where the tool applies a minimum ventilation rate, or estimates a value you have not supplied, this is stated on screen and in the report and remains the user’s responsibility to accept or override.
The user is responsible for checking all inputs and outputs, for working within their demonstrated scope of competence, and for compliance with current UK legislation, standards and manufacturers’ instructions. Assessing ventilation adequacy, and any matter outside heat loss calculation, is outside the scope of this tool.
To the fullest extent permitted by law, the author accepts no liability for any error, omission, or any direct or indirect loss or damage arising from use of this tool or reliance on its output.
Backups. Your work is stored only on this device. Clearing the browser’s site data, uninstalling the app, or losing or resetting the device will delete every project, the U-value library and all photographs, with no means of recovery. Exporting is the only backup — an exported project file carries the survey, its photographs and your U-value library. See Tool limitations.
If a calculation looks wrong, the tool misbehaves, or you have an idea to improve it, please get in touch. Tap the ? button on any screen and choose Email support, or use the link at the bottom of the tool. Your build version is added to the email automatically.