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What input is needed?

Envelope constructions

Opaque constructions

Regardless of whether an opaque construction is a wall, a floor slab or a roof, it is treated in the same way in IDA ICE, only the inclination of the construction differs. Such a construction can optionally interact with ambient or an indoor space (zone) on both of its sides, making it either an internal (with a connection to a zone on both sides) or external (with a connection to ambient on one of the two sides) construction.

In the 3D view, external constructions can be visualized either with its thickness as a 3D object (which gives a visualisation closer to the model itself), or as a 2D surface (which gives a nicer picture of the building in most cases). However, the model of an opaque construction itself will always be 1-dimensional, describing the heat conduction between its two surfaces, along the thickness of it. The model itself therefore no longer has the exact geometric information as shown in 3D, but only the thickness of different layers with the same area.

Note: The external surfaces of external constructions usually have a larger area than their internal surfaces. The user can decide what area the constructions of the model should have. However, the area of the surfaces themselves will ever be the effective ones.

Depending on what effects need to be modeled, the layers of an opaque constructions can use different wall models from the IDA ICE library:

• Standard construction layer: Heat conduction and storage only
• PCM layers ("Phase changing material" = layers with temperature dependent heat storage capacity)
• both heat and humidity transport and storage (not yet implemented to IDA ICE)

The three standard material properties that need to be known for opaque construction layers in a dynamic building simulation are:

• The thermal conductivity l [W/mK] indicates how well the material conducts heat
• The density r [kg/m3] indicates how densely the material is packed
• The specific heat cp [J/kgK] indicates how much heat can be stored within the material

Glazed constructions

In an IDA ICE model, all glazed envelope constructions are described in the same way as "windows". Windows are inserted into opaque constructions and consist of a glazing and a frame.

Note: For e.g. glazed curtain walls, opaque walls need to be defined with a window with 0 % frame fraction that cover the complete surface of the wall. As the frame will always take its depth from the wall thickness, the completely covered wall needs to have a thickness of 0.

The glazing of a window can be modeled with two alternative model approaches:

1. The standard window model describes the glazing properties with 6 parameters which apply under standard conditions (20 °C and a radiation perpendicular to the window surface with a spectrum close to that of the sun. 3 of the 6 parameters (g, Te and Tv) describe for the whole glazing what percentage of the solar radiation hitting the window from outside will be transmitted to the inside in what form. 1 parameter quantifies the conductive heat flow caused by the air temperature difference between both sides of the glazing. And the 2 other parameters quantify the long wave heat emission on internal and external glazing surface.

2. The detailed window model describes the same glazing transmittance effects for radiation, conduction and convection in much more detail and with many more parameters. The main advantages with this are:

• Angle dependent radiation transmittance can be modeled more accurately, which is useful mainly for non standard angle dependencies such as venation blinds, prismatic glazings, lamellas etc.
• Wave length dependent radiation transmittance can be modeled more accurately, which is useful for electrochromic and other selective glazing.
• Higher accuracy can be achieved, in particular for non standard cavity situations such as ventilated glazings or coated glass panes.
• Surface temperatures are calculated more accurately on both sides, which is useful for internal glazings e.g. within double glaze facades or conservatories.

The parameters for the detailed window model need to be given for each glass pane, shading layer and cavity between these layers. They are so numerous that they can no longer be entered manually. Instead, they are stored in product data files, where they can be retrieved and used via the IDA ICE glass database.

Window integrated shading

A "window integrated" shading changes the optical properties of the window model so that a smaller proportion of the solar radiation reaching the outside of the window enters the zone on the inside of the window. In contrast, an external shading object (or "external window shading") reduces the solar radiation even before it reaches the outer surface of the window. This means that an external awning is regarded as an "external window shading", while an external shading screen is regarded as a "window integrated shading", even it is "external" (outside the window).

Building integrated PV

The question of when a photovoltaic system (PV) can be considered building-integrated is the subject of controversial debate. In addition to objective aesthetic aspects, there is also the purely technical criterion of whether the PV modules perform at least one other function of the building envelope (e.g. weatherproof layer or sun protection) in addition to electricity production.

If an opaque envelope construction includes integrated PV, a part of the heat from solar shortwave radiation that is absorbed on the external surface is converted into electrical energy that can be used or stored in the simulated system.

Thermal bridges

In a typical heating situation, a building looses heat through its envelope due to the temperature difference between the warmer indoor space and the colder outdoor environment. In a typical cooling situation, a building is burdened with heat flowing in the other direction from the warmer outdoor environment to the cooler indoor space. In a dynamic situation with time varying temperatures both inside and outside the building envelope, the heat flow can be different towards both construction surfaces, when heat is either stored to or extracted from the construction.

Beside this general heat loss or burden through the building envelope, a building can exchange heat with the ambient space due to construction details near envelope corners, or where internal walls and slabs abut against external walls or roofs. Such details affecting the heat exchange are called "thermal bridges". They describe how much heat is passing the envelope at a given temperature difference through the envelope, and therefore are given in W/K. IDA ICE offers features to calculate thermal bridges for each zone, using centrally given thermal bridge coefficients (like "W/K per meter of thermal bridge length"), that can be related either to internal or external dimensions.

Infiltration and natural ventilation

Airflows

This section covers the models and input which determine the airflows through the building. IDA ICE enables the user to take account of natural ventilation, i.e. flows driven by wind pressure and the stack (chimney) effect.

In the simplest case, each zone has three paths for airflows: through the supply and exhaust terminals and via leakage through the envelope. When two zones are placed adjacent to each other and there is an opening in the common wall between them or when windows are open or when additional leaks have been added, additional flow paths are created. However, let us start with the simplest case.

The mechanical ventilation terminals are always of VAV (Variable Air Volume) type. This means that as long as there is sufficient pressure head from the fans, a given flow is maintained as requested by the zone itself. In CAV (Constant Air Volume), the control signal is kept constant and a constant flow will be maintained, irrespective of pressure. In a VAV system, a controller regulates the flow with respect to temperature, carbon dioxide, humidity or pressure levels in the zone.

Since, in the simplest case, flows through two of the three paths are given, the size of the third flow, through the leak, is important only for the pressure in the zone, and not for net flows. Note that if the size of the leak is much too small, an unrealistic pressure may build up in the zone. Such a pressure may become so large as to affect the air psychometric calculation routines and may then be reported as a condensation problem, when, in fact, it is a pressure problem.

It is also possible to define additional given in/exfiltration flows, i.e. balanced flows into and out of a zone. Since these incoming and outgoing (mass) flows are always equal, they will not affect the pressure of a zone. The given in/exfiltration will only act to exchange heat, moisture and CO₂ with the ambient.

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