As we head towards sending in the planning permission (we’ve had 2 positive pre-planning meetings), the design has been sent for a preliminary SAP analysis.
A fundamental objective is to create a thermally efficient building, so that over it’s lifetime, the amount of energy to keep the interior at a comfortable temperature and humidity more than offsets the cost (money and environmental cost) to achieve this efficiency.
In crude financial terms, the cost of heating an uninsulated house is nearly three times that of heating a modern well insulated property of the same living area.
Heated buildings loose energy in 3 ways:
- Air leakage through holes (hence an airtest and an “air-tight” building).
- Through the fabric of the building. The u-values of the materials measure how much heat is lost through them. This is primarily the walls, floors, windows, doors and roof of the building.
The lower the U-value, the better that section of the structure. For example, a wall with a U-value of 1.0 will lose heat twice as fast as a wall with a U-value of 0.5. - Through the cold bridges between the different elements. These are the Ψ (psi) values.
– “Thermal bridging occurs where the insulation layer is penetrated by a material with a relatively high thermal conductivity.”
The SAP assessor will look at all of these. They will multiply the Ψ (psi) values by the total length of their construction in the building to get a y-value. The y-value is analogous to an aggregated u-value for all the junctions in the building.
What y-values are used in the SAP calculations can have a big impact on the end figure.
Either:
- SAP assessor can use default value of 0.15, or
- Calculated value using the Ψ (psi) values listed by the Building Regulations for Accredited Details (normally 0.8 or higher, or
- A calculated value using thermally modelled junction Ψ (psi) values, which can come out as low as 0.04 depending upon construction details used.