Windows – making it clear: energy, daylight and thermal comfort
(NF78)
Overview
Many considerations have to be taken into account in the selection of windows. However, achieving a good trade-off between energy performance and daylighting, while minimising the risk of overheating, is a particular challenge for today’s designers. In this guide, using the results of new modelling carried out for the NHBC Foundation, the impact of window choice on performance is shown for a range of typical home types.
It illustrates how glazing type, frame width, overall window area and orientation affect performance and includes design aids to steer towards the best options, and away from those with less satisfactory outcomes for occupants.
The guidance is developed from modelling results using the Passivhaus Planning Package (PHPP); specialist software designed to provide sophisticated analysis of energy use and comfort within homes.
Summary of content
This guide includes:
1. A review of how windows contribute to energy performance, daylighting and comfort. This section includes:
Role of glazing
Double or triple; gap width; gas-filled; low emissivity (low-e) coatings
Role of the frame
Briefly introduces different frame materials. Explains ‘frame factor’
Heat loss and how it is measured/minimised
Calculation method, including SAP
Solar gains and overheating
Explains the mechanism of solar gain and reference to g-values
Daylighting
Introduces average daylight factor and how it is calculated
Ventilation, condensation and health
Introduces these issues within the context of window selection
Window energy ratings
Introduces the BFRC (British Fenestration Rating Council) rating process and labelling scheme
2. Optimising the window design
Develops a base case scenario (Part L compliant) and shows the effect of making changes to glazing, frame, area of window and orientation of the home. Provides design aids to optimise choice for energy efficiency, maximising daylighting or minimising overheating risk. Also shows the best and worst options in terms of outcomes for occupants. These are all provided in tabular form for four typical home types, a mid-floor apartment, a mid-terrace house, a semi-detached house and a detached house.
The guide sets out some conclusions related to the modelled homes and in terms of specific features of windows. It also includes notes and references to further information.