Components of the Energy Model
A designPH model is an energy model; not all of the details that may be modelled for a full architectural design model are required unless they have thermal significance. This means that some elements such as non-thermal internal structures, internal fittings and any purely cosmetic external details can be excluded. This will usually speed up the analysis process and also reduce the likelihood of user mistakes.
Thermal Envelope (heat transfer elements)
The thermal envelope is the conceptual boundary that thermally separates the conditioned (heated or cooled) internal environment where a stable comfortable temperature is required, from the external environment which is subject to climatic variations. In terms of the building construction, the thermal envelope is usually formed by the insulated walls, roofs, floors, windows and doors. For an effective Passive House building, this layer should form a continuous enclosure around the conditioned internal spaces of the building. Note that there can be enclosed spaces such as basements, garages, loft spaces, conservatories, atria, plant rooms and so on that are unheated / uncooled, so are considered to be outside of the thermal envelope. Spaces outside of the thermal envelope may not need always to be modelled (at least not for the purposes of the designPH energy model).
The heat transfer surface areas that are required for the energy model in designPH are the external surface areas of the thermal envelope; this is the same convention as In the PHPP. This means that an energy model can be created just by drawing a single-skin model comprised of SketchUp faces. SketchUp is therefore very well suited in this respect for creating an energy model; walls and other building elements need not be drawn in SketchUp as 3D volumes. This does not mean that a building cannot be analysed using SketchUp+designPH with the walls drawn as 'solid' elements, but there is no inherent need for that additional complexity.
Opaque Surfaces
The SketchUp faces are treated as opaque heat transfer surfaces for the energy model and transmission heat losses are calculated according the surface area, the assigned U-value and the thermal boundary conditions. Note that the visual appearance of the faces from applied SketchUp materials has no effect on the energy model.
Transparent Surfaces (Windows)
Unlike some other energy analysis tools which simply tag certain surfaces as transparent, windows in designPH are modelled using a realistic 3D component comprising a frame area, glazing area and reveal depth. Windows are initially assigned a default frame and glazing type which allows the installed U-value to be calculated for each separate window, and this will be responsive to changes in the frame to glazing area ratio, perimeter length and coupling to other windows. See the Windows section below for further details on inserting windows.
Thermal Bridges
Edges in the SketchUp model can optionally be marked as linear Thermal Bridges, this serves as an adjustment factor for junctions or discontinuities in the thermal envelope where the heat flow cannot be adequately modelled using the U-values of surfaces. Thermal bridges for window installations and for the glazing spacer are automatically incorporated into the calculation of the installed window U-value, so these do not need to be identified separately.
Area Groups
PHPP uses numbered Area Groups to categorise and identify the elements above and assign the appropriate boundary conditions (temperature reduction factor). The same system of numbered area groups is used in designPH, although the assignment is usually done automatically.
| Area Group number | Area Group name |
|---|---|
| 1 | Treated Floor Area |
| 2 | North windows |
| 3 | East windows |
| 4 | South windows |
| 5 | West windows |
| 6 | Horizontal windows |
| 7 | External Door |
| 8 | External Wall - Ambient |
| 9 | External Wall - Ground |
| 10 | Roof/Ceiling - Ambient |
| 11 | Floor slab / Basement ceiling |
| 12 | Not used |
| 13 | Not used |
| 14 | Temperature zone X |
| 15 | Thermal Bridges Ambient |
| 16 | Perimeter Thermal Bridges |
| 17 | Thermal Bridges Floor Slab / Basement Ceiling |
| 18 | Partition Wall to Neighbour |
Energy Balance
An annual heating energy balance is calculated within designPH, based on the balance of the heat losses (by conduction through the building fabric and through air movement) and free heat gains (solar gains and internal gains from occupants and equipment). A utilisation factor is applied to the free heat gains because they do not always occur at times when heat is required. In designPH, the losses and gains are calculated for each building element, according to their individual properties and the specified climate.
- Fabric losses are calculated according to the area, U-value and thermal boundary conditions of each individual surface in the model
- Ventilation losses are based on an assumed level of airtightness (0.6 ac/h, Passivhaus minimum), heat recovery ventilation (75% efficiency, Passivhaus minimum) and a ventilation volume extrapolated from the Treated Floor Area
- Solar gains are calculated for each window and take into account the individual shading situation of the window
- Internal heat gains (IHG) use a range of default values that are determined according to the building type, as in the PHPP. In designPH versions <= 1.5, internal heat gains were fixed at a default level of 2.1 W/m2. See Internal heat gains for further details.
Treated Floor Area
The Treated Floor Area (TFA) is a measure of the usable internal floor area, contained within the Thermal Envelope, which is calculated according to a prescribed method – see the PHPP 9 manual (section 14.9) for further explanation. The TFA is a very important metric because the annual space heat (or cooling) demand is divided by the TFA to give the Specific Annual Space Heat (or Cooling) Demand, which is the basis of the 15 kWh/m2 target metric for Passivhaus. This figure is therefore sensitive to changes in the TFA so it is important that it is calculated accurately for certification.
designPH now has three possibilities for defining the TFA:
- Estimated TFA calculator
- Directly drawn and assigned TFA surfaces
- Direct entry box for TFA (since designPH v1.6)
See Treated Floor Area later for details on how to use each method.
Projected Footprint Area
The Projected Footprint Area is the reference area that is required for the calculation of the PER (Primary Energy Renewable) factor in PHPP9. This is defined as the vertical projection of the thermal envelope onto a horizontal plane, so it may be larger than the actual footprint where the building touches the ground, for example if there are cantilevered floors, or parts of the Thermal Envelope over unheated garages. The Projected Footprint area can be generated using the tool of the same name, provided in designPH.