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What Information Is Needed for a Heat Loss Calculation?
A practical checklist of what to gather before your heat-loss calculation.
- Last updated: September 2026
- Heat Loss Hub · Advice Centre
A room-by-room heat-loss calculation is only as reliable as the information behind it. Whether that information is collected during a site visit or provided by you remotely, the calculation ultimately needs to describe your property accurately, room by room.
You do not need to be a heating engineer to provide this information. You mostly need to be observant, measure carefully, and gather whatever documents you already have.
This page sets out what is normally needed, why it matters, and what to do if some of it is missing.
KEY TAKEAWAY
The calculation is only as good as the information that goes into it.
GET STARTED
Get an independent room-by-room heat-loss calculation for your property.
Why Does the Information Matter So Much?
A heat-loss calculation is not a single number pulled from a table. It is built up room by room, surface by surface, from the actual dimensions and construction of your property.
Get the information wrong, and the calculation will faithfully calculate the wrong house. This is why gathering good information is at least as important as the calculation method itself.
KEY TAKEAWAY
A calculation can be mathematically correct and still describe the wrong house if the information behind it is wrong.
Basic Property Information
Before looking at individual rooms, a designer needs some general information about the property. Useful basics include property address and type, approximate age or construction period, number of floors, total floor area where known, orientation (which way the property faces), and whether it is detached, semi-detached, terraced or a flat.
Some of this can be found on an EPC or estate agent listing if you do not know it offhand.
KEY TAKEAWAY
These basics set the context for everything else. A Victorian terrace and a 2020s new build will usually have very different construction, even if their floor areas look similar.
Room-by-Room Information
This is the core of a room-by-room calculation. For every heated room, useful information includes room name or reference, length, width and ceiling height, which walls are external and which are internal, what is on the other side of each wall (another room, outside, a garage, a neighbouring property), what is above the ceiling (another room, a loft, the sky), and what is below the floor (another room, a void, the ground).
The aim is to describe not just the room itself, but what surrounds it in every direction.
KEY TAKEAWAY
A room does not lose heat in isolation. It loses heat according to what is on the other side of each of its surfaces.
Windows and External Doors
Windows and doors typically lose heat faster than the surrounding wall, so their size and type matter. Useful information includes approximate width and height of each window and external door, glazing type where known (single, double or triple glazing), approximate age, and frame material (timber, uPVC, aluminium).
If you are not sure of the glazing type, a photograph showing the window from inside is often enough for a professional to identify it.
KEY TAKEAWAY
Window and door performance can vary considerably. Two similarly sized windows of different ages and specifications can lose very different amounts of heat.
Wall Construction
Wall construction has a major effect on heat loss. Useful information includes solid wall or cavity wall construction where known, wall thickness if measurable, whether any wall insulation has been added (internal, external or cavity fill), and approximate age of construction, since this often indicates typical wall type for the period.
If you are unsure, a photograph of an exposed edge (such as a window reveal) can sometimes help a professional identify the construction.
KEY TAKEAWAY
Wall construction is one of the most influential pieces of information in the whole calculation.
Floor Construction
What is beneath each room matters. Useful information includes whether the floor is solid (concrete) or suspended (timber over a void), what is directly below (ground, an unheated cellar, another heated room, an integral garage), and any known floor insulation.
Ground-floor rooms typically need more careful floor assessment than upper-floor rooms sitting above another heated room.
KEY TAKEAWAY
A floor above an unheated garage behaves very differently from a floor above a heated room below it.
Roof and Ceiling Construction
What is above each room also matters. Useful information includes whether the ceiling is below a loft, below another heated room, or below a flat or pitched roof with no loft space, approximate loft insulation depth if known, and whether any room is within a loft conversion, since sloped ceilings need particular attention.
KEY TAKEAWAY
A bedroom below an insulated loft loses heat very differently from a loft-conversion bedroom with sloped ceilings directly below the roof.
Insulation Information
Where you know about insulation work, it is worth mentioning specifically, since it may not be visible or obvious otherwise. Useful information includes cavity-wall insulation (installed or not, and roughly when), external or internal wall insulation, loft insulation depth and coverage, floor insulation, and any certificates or installer paperwork from previous insulation work.
KEY TAKEAWAY
Insulation you cannot see is easy to forget about, but it can make a significant difference to the calculated result.
Extensions and Alterations
If your property has been extended or significantly altered, this needs to be clearly identified. Useful information includes when the extension was built, whether its construction matches the original building or is different, any drawings or planning documents from the extension, and any rooms that have been knocked through, subdivided or repurposed since the property was built.
An extension that is not identified may accidentally be calculated using the same construction as the original building, which can be inaccurate.
KEY TAKEAWAY
An unidentified extension is one of the most common sources of error in a heat-loss calculation.
What Can an EPC Provide?
Your Energy Performance Certificate can be a useful supporting document. It can often provide approximate floor area, property age band, general construction type, existing heating system, and some insulation information as recorded at the time of assessment.
An EPC is a helpful starting point, but it is not usually detailed enough to replace room-by-room measurements and information.
KEY TAKEAWAY
An EPC is a useful clue, not a substitute for a proper room-by-room calculation.
For more on why the two are different:
What Can Architectural Drawings Provide?
If you have scaled floor plans, elevations or building specifications — from a new build, an extension, or a renovation — these can be extremely valuable. They may already show room layouts and dimensions, wall positions, window and door sizes and positions, and in some cases construction build-ups and U-values.
Good drawings can remove much of the need for separate measurement, though the property should still be checked against them where possible in case anything has changed since they were drawn.
KEY TAKEAWAY
If you have drawings, always share them. They are usually the single most valuable piece of information you can provide.
What Can Photographs Provide?
Clear photographs of each room can help confirm window type, general room proportions, visible wall or floor finishes, and radiator positions and rough sizes. Photographs taken from a corner of the room, showing as much of the space as possible, are usually most useful.
Photographs work best as supporting evidence alongside measurements, rather than as a replacement for them.
KEY TAKEAWAY
A photograph can confirm what a designer suspects from the measurements. It rarely replaces the measurements themselves.
Existing Heating System Information
Although not strictly part of the building heat-loss calculation, it is often useful to note your existing heating type (gas boiler, oil boiler, electric heating, existing heat pump), the type and approximate size of existing radiators in each room, and any known issues with particular rooms being difficult to heat.
This context can help a designer sense-check the results and can be useful later if the heat-loss figures are used for further design work.
Design Conditions — What You Don’t Need to Provide
Some parts of the calculation are the designer’s responsibility, not yours. You do not need to provide the outdoor design temperature for your area, the appropriate indoor design temperature for each room type, U-values for different construction types, or ventilation rates and assumptions.
These come from established reference data and professional judgement, applied to the property information you provide.
KEY TAKEAWAY
Your job is to describe the building. The designer’s job is to apply the engineering standards to it.
A Simple Checklist You Can Use
| Category | What to Gather |
|---|---|
| Basics | Address, property type, approximate age |
| Rooms | Dimensions, ceiling height, what is on every side |
| Windows/doors | Sizes, glazing type, approximate age |
| Walls | Construction type, known insulation |
| Floors | Construction type, what is beneath each room |
| Roof/ceilings | What is above each room, loft insulation |
| Extensions | Age, construction, drawings if available |
| Documents | EPC, architectural drawings, photographs |
You do not need every single item to get started. The more complete the list, the more confidently the calculation can proceed.
What if I Don’t Have All This Information?
Do not worry if you cannot answer every question. Many homeowners do not know their exact wall construction or loft insulation depth, and that is normal for an existing property.
Where information is genuinely unknown, a professional designer can use a reasonable, clearly documented assumption based on the property’s age and typical construction of that period, and flag it as an assumption rather than presenting it as a confirmed fact.
KEY TAKEAWAY
Missing information does not usually stop a calculation. It just needs to be handled honestly rather than guessed silently.
Providing Information for a Remote Calculation
If your calculation is being completed remotely rather than through a site visit, the information above becomes even more important, since there is no separate opportunity for somebody to check the property in person.
Frequently Asked Questions
You typically need room-by-room dimensions, window and door sizes, construction details for walls, floors and roofs, known insulation, and information about any extensions or alterations. Documents such as an EPC, architectural drawings or photographs can support this.
No. You need to describe the construction (solid wall, cavity wall, insulated or not) where you can. The designer applies the appropriate U-values based on that description and relevant reference data.
That is common for existing properties. A professional designer can make a reasonable, documented assumption based on the property’s age and typical construction of that period.
An EPC can provide useful background information, but it is not usually detailed enough to replace room-by-room measurements for a proper heat-loss calculation.
Not necessarily, but they are very helpful if you have them, particularly for new builds, extensions or renovations. Without them, careful measurements and photographs can often provide enough information instead.
As accurate as reasonably possible. Small errors in individual measurements are unlikely to change the overall picture significantly, but consistently rough estimates across many rooms can affect the reliability of the result.
Ideally every heated room, since a room-by-room calculation is designed to show the heating requirement for each individual space, not just the property as a whole.
Mention it separately and provide whatever information you can about when it was built and how it was constructed. An unidentified extension is one of the most common sources of error.
Yes, always. Insulation work is not always visible, so it is easy for a designer to miss unless you mention it specifically.
Photographs are useful supporting evidence, but they generally cannot replace measurements. A photograph can show what a wall looks like; it usually cannot reliably show how long it is.
You do not need to provide design temperatures, U-values, or ventilation rates. These come from established reference data and professional judgement, applied to the property information you supply.
The underlying information required is the same. The difference is simply how it is collected: measured and observed on site, or supplied by you through drawings, photographs and a guided self-survey.
Good Information Makes a Good Calculation
A room-by-room heat-loss calculation is built from the property information behind it. You do not need to be a heating engineer to provide that information — you mostly need to measure carefully, gather what documents you already have, and flag anything unusual such as extensions or known insulation work.
The calculation is only as good as the information that goes into it.
A calculation can be mathematically correct and still describe the wrong house if the information behind it is wrong.
An unidentified extension is one of the most common sources of error in a heat-loss calculation.
Missing information does not usually stop a calculation. It just needs to be handled honestly rather than guessed silently.
GET STARTED
Get an independent room-by-room heat-loss calculation for your property.