The house model: floors, zones, envelope and windows
EnergyFlow does not draw your house room by room. You describe it with a short list of inputs, and the wizard turns them into a few thermal zones (each a part of the house treated as one body of air with a single temperature) that EnergyPlus, the building simulation program we use, calculates hour by hour. This page explains what each input becomes and where the model simplifies.
What you describe
In the house step you choose the house type (detached, semi-detached or terraced), the number of above-ground storeys (1 to 5), the roof (flat, or pitched with an unheated or heated attic), a below-ground level (none, unheated cellar or heated basement), a garage option, the construction year, and the country and postal code.
In the envelope step you enter the outer length, width and storey height, the window area on each of the four faces, and the rotation of your plan relative to true north. The envelope is the shell of the house: walls, roof, floor and windows. You can replace three defaults: how well a surface keeps heat in (its U-value, the heat lost per square metre per degree of indoor-outdoor difference, so lower is better; it is on your energy certificate, or we infer it from the construction year), the share of the sun’s energy the glass lets through (the g-value, 0 to 1), and the airtightness.
One footprint, one zone per level
The model stacks the same footprint on every storey, using the ground-floor length, width and storey height. Only the footprint area matters: the thermal model builds a square block of that area, so a long narrow plan behaves like a square one.
Each level is one thermal zone with one air temperature. Rooms on a floor are not separated, so results are floor-level results. The one exception is a garage inside the house, which makes the ground level two zones.
Attic, cellar, basement and garage
Extra spaces are real zones, not adjustments to a U-value. Each is either heated or an unheated buffer, a space between the heated house and the outdoors that slows heat loss. Buffers stay in the heat balance but are left out of the floor area, so they cannot flatter a kWh-per-square-metre figure.
| Option | Counted as floor area | What the model does |
|---|---|---|
| Heated attic | Yes, at its usable share of the footprint (0.65 by default) | An extra heated storey on top, with its roof windows if you have them. Roof insulation sits at the slope. |
| Unheated attic | No | A buffer above the top ceiling. The ceiling carries the roof insulation, the roof is uninsulated, and the attic leaks more than the house. |
| Heated basement | Yes | A heated level below the ground floor, with buried walls and slab. |
| Unheated cellar | No | A buffer below the ground floor. Heat flows through the slab. It leaks more than the house. |
| Garage, in the house | Only if heated | Carved out of the ground floor (30 m² by default). It shares walls with the house and its ceiling with the floor above. |
| Garage, attached | Only if heated | A separate volume beside the house. It shares one wall and keeps its own roof. A garage standing apart shares no surface, so choose “none”. |
Roof windows, the kind set into the slope of a roof, belong to a heated attic. The model places them on the roof surface, tilted and facing the way the slope faces. Tilted toward the sky, they generally let in more summer sun than a wall window of the same size.
Staircase coupling between floors
Heated storeys that people move between are joined by a stairwell opening through the slab, 3.5 m² by default, and the model lets air mix between the two zones through it. We call this staircase coupling. An unheated cellar, an unheated attic and a garage are reached through a door or hatch, so they get a solid slab and exchange heat by conduction only: through the material, with no air moving between the spaces.
From construction year to U-values and airtightness
The construction year selects a row from a table of defaults. At the moment they are based on Luxembourg, whichever country you choose. Replace them with your own figures where you have them.
Airtightness is how much air leaks through the shell when windows and doors are shut. A blower-door test measures it: a fan in a doorway holds the house at 50 Pa, far more than a normal breeze, and the result, n50, is how many times an hour the leaks would replace the whole volume of air in the house (air changes per hour). A lower n50 means a tighter house.
| Construction year | Wall | Roof | Floor | Window | n50 (1/h) |
|---|---|---|---|---|---|
| 2017 onward | 0.18 | 0.15 | 0.25 | 0.95 | 0.8 |
| 2015 to 2016 | 0.24 | 0.20 | 0.28 | 1.30 | 1.2 |
| 2008 to 2014 | 0.32 | 0.25 | 0.30 | 1.50 | 2.5 |
| 2000 to 2007 | 0.45 | 0.35 | 0.45 | 1.90 | 3.5 |
| 1980 to 1999 | 0.60 | 0.40 | 0.50 | 2.00 | 5.0 |
| 1960 to 1979 | 0.80 | 0.50 | 0.60 | 2.50 | 6.0 |
| Before 1960 | 1.50 | 1.20 | 0.90 | 3.50 | 10.0 |
U-values are in W/m²K for a detached house. Semi-detached and terraced rows differ slightly in n50 only. The g-value defaults to 0.5 for every year: half the sun’s energy reaching the glass gets through.
Windows and orientation
Windows are described per face: north, east, south and west. The model turns each face’s window area into a share of the wall area and applies that share to the walls on that face. It does not place individual windows, and each window is one element with your U-value and g-value, not pane by pane.
The compass in the envelope step asks where true north points on your plan, and the model rotates the whole house by that angle relative to the sun. Rotating the house changes both the winter sun that comes through the windows and the risk of summer overheating.
Limits of the model
- The plan is a square block, and every storey has the footprint of the ground floor. What counts is the internal volume of air on each floor, so an irregular plan can be summarised as a simple box whose length, width and height approximate your house. Choose them so the floor area and the storey height match, and the heated air volume matches too. The outer wall area of the box will differ somewhat from your real walls. As long as each window is entered on the face it really looks at, the simulation works.
- Roof shape is simplified. An attic and a below-ground level are volumes on the house footprint, not drawn in detail.
- Every outer wall faces the outdoors. A wall shared with a neighbour (a party wall) in a semi-detached or terraced house is not modelled as one.
- Windows are totals per face. Overhangs, balconies, trees and neighbouring buildings do not shade them.
- One U-value applies to each element type across the house. Thermal bridges, spots where the structure lets heat bypass the insulation, are not modelled as separate elements.
- The model cannot tell you whether a wall build-up is sound or free of condensation risk.