Heat loss calculator for a house: room-by-room heat loss in seven steps

The heat loss module is a heat loss calculator for a house that works room by room. It calculates the building's design heat loss to EN 12831-1:2017, a European standard, with the thermal resistance of building elements to EN ISO 6946 and heat exchange with the ground to EN ISO 13370. It calculates heating only - there is no cooling. The wizard has seven steps. You can fill them in with Algo or by yourself, and the result updates live as soon as you enter the area and the rooms. This article explains each step, the compliance status and what a report unlocks. What a heat loss calculation is and what depends on it is in a separate guide: what a room-by-room heat loss calculation is.

Units: what you see in feet and °F

An account created from the US starts in imperial units. You see and type lengths in feet, areas in ft², temperatures in °F, heat loss in Btu/h and the heat loss per square foot in Btu/h·ft². Layer thicknesses show in inches. You can switch between metric and imperial at any time: in Settings, on the "Profile" tab ("Units"), or in the module's settings ("Units across the app").

Only the display changes. The calculation always runs in SI units to EN 12831-1, so a value you type in feet is stored in meters. A few things stay metric by design: kWh, %, the air change rate (1/h) and n50. The PDF report prints metric units for now. The screenshots in our help articles show the metric screens.

Before you start: data from the client form and from Algo

If your client filled in the client form, the step 1 start screen shows the line "Ready to bring in - Client form" with the figures from the form and the "Bring it into this module" button. The building data and the room list then fill in by themselves. You can also work with the Algo assistant, starting from step 1 below. Both sources are explained in data flows between modules.

Step 1: Project

The first step is the start screen: a picture of the calculation and three buttons, "Start with Algo", "I'll fill it in myself" and "Guide". There are no fields here and no bottom bar - at the top there is only the arrow back to the project.

• "Start with Algo" opens the assistant beside the wizard (on a narrow screen, on its own page, with this project) and asks what it needs. Algo answers with a list: the building's location, the heated area, the floors and the rooms with their areas and ceiling heights (from the floor plans), the wall, roof and floor constructions as layers or U-values (from the sections and the specifications), the windows and doors with sizes and U-values (from the window and door schedule), what lies on the other side of each element, the ventilation type and, with heat recovery, its efficiency (from the mechanical notes), and the n50 result of a blower door test, if there was one. You type the answers in the conversation or attach documents, and Algo enters the values into the steps. Check each value on its step before you read the result. How Algo reads a whole set of plans is explained in the section below.
• "I'll fill it in myself" goes on to step 2.
• "Guide" opens this article in a new tab.

You can ask Algo the same question on any step - the answer is the same. From step 2, the bottom bar holds the step list, the module settings and the warnings.

You see step 1 while the project is still empty. A project that already holds data - typed in, entered by Algo or taken from the client form - opens on step 2, and you go back to step 1 from the step list. After "Clear data", the start screen comes back.

Algo AI reads the house plans

Instead of copying the plans by hand, you can give them to Algo to read:

1. On the step 1 start screen, press "Start with Algo". Algo asks for the documents.
2. Attach them in the conversation: the floor plans, the sections, the specifications and the window and door schedule. Algo reads PDF files and photos (JPG, PNG, WEBP) - up to 5 files and 20 MB at once.
3. Algo starts reading as soon as you attach the files. While it reads, the wizard shows the overlay "I'm reading the building design" with "It usually takes 1-3 minutes." and a "Stop" button.

Algo enters the rooms (names and areas), the building elements (layers or the U-value) and the windows and doors, placed in the walls from the marks on the plan and the schedule. In "Propose" mode, the values wait in Algo's changes panel and you save them yourself. In "Automatic" mode, they are saved when the reading ends. "Undo" reverses the write.

Algo never overwrites your value with one the documents do not state. When two documents disagree, Algo asks which value to use. Values that break hard rules (for example a U-value, λ or layer thickness out of range) are not written into the form.

After the reading, Algo lists what is left to complete, in groups: windows, room floor areas, the U-values of building elements, room temperatures and other data. Each group has "show", which takes you to the right step and field. The values Algo assumed without a document are in one line: "I assumed: ... You can change this."

What it costs: a reading uses the same monthly Algo limit as the chat and does not use a report. The limit grows once you buy a report and is largest with a subscription. When it is used up, Algo tells you before it starts. One reading also fills heat pump sizing and heat recovery ventilation at no extra cost. One account can run up to 10 readings per 24 hours. Administrative pages (decisions, applications, pages about a person) are deleted as soon as they are recognized and are never read. We delete the files 24 hours after the reading starts, and its result after 30 days. More about the assistant in Algo, the AI assistant.

Step 2: Building

At the top of the step you enter two figures for the whole project:

• the heated area, the sum of the heated rooms' areas. If the sum of the rooms from step 5 is larger, you get a warning.
• the design outdoor temperature for the building's location, from the house plans or local weather data. HeatAlgo has no US climate data. Algo can only set this temperature from the Polish national table, for the nearest of eight Polish cities (Warsaw, Kraków, Gdańsk, Wrocław, Poznań, Szczecin, Łódź, Katowice). For a house in the US, type your own design temperature.

When the project has a wall to a neighboring building (a duplex or a row house), an "Annual mean outdoor temperature" field appears beside them. The other side of such a wall is taken at this temperature, never below 41 °F (5 °C) (EN 12831-1:2017, § 6.3.2.5, Table 7). Without it, the calculation does not run.

Below them, the data for the whole building:

• the dimension basis - internal (between walls, the most common for detailed plans), external (outside dimensions) or measured. Measure all envelope elements on the same basis.
• thermal bridges - the construction class adds a blanket ΔU_TB: very good 0.004 Btu/h·ft²·°F (0.02 W/(m²·K)), standard 0.009 Btu/h·ft²·°F (0.05 W/(m²·K)), retrofitted 0.018 Btu/h·ft²·°F (0.1 W/(m²·K)), poor 0.026 Btu/h·ft²·°F (0.15 W/(m²·K)).
• airtightness - classes I-IV suggest a typical value. If you have a blower door result, enter n50. Above 3 1/h, the app suggests the detailed method with per-room airflows.

Step 3: Ventilation

Choose the type: natural, mechanical exhaust, or mechanical with heat recovery. With heat recovery, the recovery efficiency field appears (85% is typical for a good unit), and so does the "Insert default airflows" button. It fills the supply and exhaust airflows in the rooms with typical values and does not overwrite fields you already filled. The air change rate defaults to 0.5 1/h. If the project has a ventilation system designed in the heat recovery ventilation module, the heat loss calculation checks that both agree: the ventilation type, the efficiency and the room airflows.

Step 4: Library

Here you save the project's constructions, so you do not enter them in every room from scratch. The step has five groups as rows: Walls, Roofs, Floors, Ceilings, and Windows and doors. Beside each group's name are two counts - a folder icon for the constructions in this project, a book icon for the entries in "My library" - with the first names below. The plus on a row creates a new construction in that group, and the pencil (or the whole row) opens its list, with search. The first row of the list is Algo, which answers questions about the elements and layers in this project.

You make a new construction in one dialog, in three steps:

• "Kind" - for example an external wall, a wall to an unheated space or a wall to another heated room. Roofs have one kind, so they skip this step.
• "Starting point" - "Describe it to Algo" (the dialog closes and Algo asks for the element's description from the plans, a photo of the section or its layers, computes the U-value and, when you ask, saves the construction in "My library"), "Ready-made construction" (a typical European layer build-up from the catalog, for example a 10 in ceramic block with 8 in of graphite EPS: search by name, material or maker, and narrow the list by building period, construction, insulation and whether it meets the Polish WT 2021 limits; on a phone, the filters are behind the "Filters" button), "My library" (a copy of one of your entries), "Enter a U value" (from the plans or an energy audit, no layers) or "From scratch" (an empty list of layers).
• "Layers and name" - the element's section and a table of layers from the inside: material, thickness (inches in imperial) and λ, with moving and removing layers. The "Non-homogeneous" switch adds the λ of the framing for walls with studs. The U-value is computed as you type, to EN ISO 6946.

A window or a door is one step: its kind, width, height and U-value. "Choose from ready-made windows and doors" lists typical windows of each period and the WT 2021 limits. The one you pick sets the kind, the U-value and the name, and you type the size yourself. The "Use a ready-made construction" button beside the step's title opens the same dialog on the ready-made constructions. Ready-made constructions are suggestions. Check the thicknesses and λ against the building's documents, because the result matches reality only when the layers match the real cross-section.

"My library" holds the constructions, windows and doors saved on your account, available in every project. Open it from the menu on the left ("My library") - it has the same groups, lists and dialog as this step. An account without an active subscription keeps 5 entries in total. An active subscription lifts the limit, and if it lapses, the saved entries stay. Algo reads your library and, when you ask, adds, changes or deletes entries, each with an Undo.

Step 5: Rooms

Most of the work is here. "Add room" creates a card with a name, a type (living room, bedroom, kitchen, bathroom at 75 °F, corridor at 64 °F and others, office and service rooms too), the area, the ceiling height, the design temperature and the story ("Basement", "Ground floor" - the first floor in US terms - or "Floor 1" and up). "Choose typical values" sets the temperature and the ventilation category in one click. You can duplicate a room, and "Add attic" runs the wizard that generates the slopes, knee walls, gables, floor and collar tie ceiling from the building width, roof pitch, knee wall and length.

In every room, the "Envelope elements" section shows its elements as rows: the kind's icon, the name, and below it a gray line with the area, the U-value or the number of layers, and the windows. The pencil opens an element for editing. The "⋯" menu beside it has "Remove" (with an Undo) and "Duplicate to…", which copies the element with its windows into this room or another one - the line below it then says "Copied to: …" with "Undo". The "Add an element" plus in the section header and the pencil on an element open the same dialog, on one screen in three parts:

• "Construction" - a list of constructions, right in the dialog: the current one first and checked, then "In this project" and "My library", each with its U-value, and a "Search constructions" field above the list when there is something to choose from. When you edit, only the chosen construction shows, with its layers, name and U-value - "Change" opens the list, and a construction made of layers has "Edit layers" beside it. The "New construction" button by the heading opens the same dialog as step 4 - from layers, from ready-made ones, from a U-value or with Algo. An element with a typed U-value has its "U value" field in the same part.
• "In this room" - the name, the gross area, what lies on the other side and the net area.
• "Windows & doors" - the rows of windows and doors, then "Add a window or door" and - when another element in the project already has windows - "Copy from another element", which adds copies of its windows and doors to this element. "Save" finishes. Picking copies the construction into the room: the library stays as it was, and you can change the copy in this room alone.

A room that borders nothing colder is marked with "This room has no elements to calculate". Only its ventilation is then calculated, and a note goes into the result and the report. You pick an element's type from ten: "External wall", "Wall to unheated space (garage, pantry)", "Wall to another heated space", "Roof / flat roof", "Floor on ground", "Floor above unheated space (cellar/garage)", "Floor over outdoor air (passage, overhang)", "Floor over a heated space", "Ceiling under unheated space (attic)" and "Ceiling under a heated space". For each, you enter the gross area (with openings - the windows and doors from the table subtract themselves, and the heat loss uses the net area) and the U-value, typed directly or from layers copied from the library. Windows and doors cannot be larger than the element they sit in - if they are, the calculation stops. Non-homogeneous walls (wood framing with studs) have their own mode with a framing fraction.

The details the standard asks about:

• floor on the ground - the perimeter P and the depth z. Without a measured perimeter, the app assumes 4·√A and lowers the compliance status. The groundwater level affects the f_GW correction.
• elements to unheated spaces - pick what is on the other side (a room with 1-3 external walls, a basement, an insulated or uninsulated attic, a circulation space, a crawl space) and the b_u factor follows. You can also give an exact temperature. A new "Floor above unheated space (cellar/garage)" starts with "Basement - with external doors/windows (b_u ≈ 0.8)", per EN 12831-1, Table B.2. Elements you already saved stay as they were.
• elements to heated spaces - on the other side, pick from the "Heated" group: "Another heated room" (a temperature; an empty field means the same as this room, so no loss, and the partition counts for this room only, not in the building's heat loss), "A neighbour's flat in this building" (temperature required) or, for a wall, "Neighbouring building (semi-detached, terraced)" (the annual mean outdoor temperature from step 2, never below 41 °F; with the first such wall, the dialog asks for it right away). Those two count in the building's heat loss too. When the other side is warmer, the partition is a gain. Elements in this group get no thermal bridge supplement.
• floor over outdoor air (a cantilever or an overhang) - calculated like an external element, at the design outdoor temperature and with no path through the ground.
• windows and doors - a table with width, height, U-value and quantity.
• the ventilation category - a habitable room or a wet room with a window (n_min 0.5) or an internal secondary room (0.0); with heat recovery, also supply and exhaust.
• high rooms (from 13' 1") - you must select a heat emission system.

Step 6: Adjustments

For a typical single-family house you can skip this step. It holds internal heat gains (ignored by default; any value above zero is an assumption to agree with your client, with a per-room override), intermittent heating with the heating-up power φ_hu (from the auxiliary table or typed by hand, with the setback period, the air change during setback, the building's thermal mass and the heating-up time) and the heat source sizing correction. That correction is a factor that helps size the heat source and deliberately does not change the rooms' heat loss.

Step 7: Result

The result shows the building's design heat loss, the heat loss per square foot, the heated area and the split into transmission, ventilation and thermal bridges. The "Room breakdown" carousel switches between rooms, and clicking a card opens the details: the envelope elements with their layers and the U-value calculation, windows and doors, the heat loss components, ground corrections and ventilation.

When the sum of the rooms differs from the building's heat loss, one line below the carousel reads "Sum of rooms X · Building Y · difference Z", with the reason. The first reason is wind: each room is counted with the wind on its own side, the building with the wind once. The second is partitions to other heated rooms: they go into their rooms' heat loss (EN 12831-1:2017, § 6.3.1.3, Formula (3)) but not into the building's (§ 6.3.1.2, Formula (2)). You size the heat source on the building's heat loss, and radiators and radiant floor loops on the room heat loss. A room that gains more heat from warmer neighbors than it loses can come out at zero or below - its card then says "No heating needed". The "Calculation method" tab lists the assumed parameters. You download the PDF with a square button on the bottom bar (see below) - more in the article on reports.

Until there is a result, step 7 shows a sample result under frosted glass. In the middle is the step's heading and what the result still needs: rows with a "Fill in" or "Fix" button that takes you to the right step and field, and a "Complete with Algo" button. When the calculation fails on our side, you see "The result could not be calculated" - your data is saved - with "Try again" and "Ask Algo what happened". When the calculation stops at a value you entered, the list names that value, and "Fix with Algo" asks the assistant about it.

Issues, blockers and the compliance status

The icon with the "N to fix" count on the bottom bar, beside the step number, collects everything you still need to complete. With nothing to fix, it is not shown. Items labeled "Blocks the calculation" stop the calculation - for example no rooms, a room without envelope elements, the heated area exceeded, missing airflows with mechanical ventilation, the annual mean outdoor temperature missing for a wall to a neighboring building, or windows and doors larger than the element they sit in. The "Fix" button takes you to the field. The other remarks do not block the result but lower its status.

The status on the result has two values: "Result meets EN 12831-1:2017" or "Approximate result - does not meet strict EN 12831-1 compliance". The "About this result" dialog gives the reasons. Two things lower the status: an assumed perimeter of the floor on the ground, or heating-up power from the table without confirming the conditions of Annex F.2. Missing airflows, the old temperature correction factor, the missing annual mean outdoor temperature and windows larger than their element do not lower the status but block the calculation - the same dialog lists them under "To fix". A project with air transfer devices between zones (ATD) or designed airflows between rooms is outside the module's scope, and the app says so plainly.

The status describes the calculation against EN 12831-1, a European standard. It is not a statement that the result is accepted by a US building department, a utility rebate program or an inspector.

What a free account shows

The calculation runs in full. On screen and in the PDF you see the building's heat loss, the heat loss per square foot and one fully calculated room. The other room cards show "Value available once the project is unlocked", and the method details (source sizing, thermal bridges, n50, ventilation) wait for the unlock. One report unlocks the whole project - see what is free and what a report unlocks. You can also buy a report for the heat loss calculation alone, for that project.

Module settings and clearing data

From step 2, the bottom bar has three parts: on the left a square "Back" button, in the middle a glass island, on the right "Next" (on a phone, the arrow alone). The island holds, in order: the module settings gear, the step number (for example "4/7" - click it for the list of every step) and the "N to fix" count. On step 7, two square icon buttons take the place of "Next": report notes and the PDF download. Their tooltips give their names ("Report notes", "Download PDF"). Both are gray until there is a result. Algo opens from the button in the top right corner (on a phone, Algo's mark alone).

The gear opens the "Module settings" panel, with the module's name below the title. The "Data sources" section has a "Client form" row with a "Bring in" button and an "Algo AI" row with a "Fill in" button, which opens Algo with the same question as "Start with Algo" on step 1. The "Units" section has the "Units across the app" row, where you switch between metric and imperial. The "Module data" section has a "Clear data" row with a red "Clear" button, which wipes the whole heat loss form in this project after you confirm. The heat loss result feeds heat pump sizing, radiant floor heating and ventilation - after you change the heat loss calculation, those modules offer an update.

A HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.