Homestar Design Guide

We are proud to launch 'A practical design guide to lower carbon, healthier homes'

This new document provides an overview of the concepts of building performance introduced in Homestar version 5, and is intended as a ‘how to’ guide for designers and professionals navigating the Homestar certification process. As well as detailed case studies to show how to achieve various Homestar ratings across the motu, the guide provides detail on the options and routes available for projects to achieve Homestar certifications of varying ambition, and outlines best practice methodology for higher performance, and healthier homes.

For easier navigation, you're welcome to review the individual chapters below, or download the full guide here.

Chapter 1: Case studies

The guide includes four case studies of typical building types in New Zealand. Each has been thermally modelled in ECCHO (Energy Carbon Calculator for Homes) in three different climate zones to assess what specification will be required to meet both 6 Homestar and 8 Homestar levels of performance in each climate.

The specifications produced show one way of achieving the required performance levels. Every project will need to be individually modelled to achieve the best outcomes for the available budget.

Further build-ups and assemblies are detailed in the Assemblies chapter.

Homester V5 Design Guide_ 20240408 FINAL Case Studies-1

Chapter 2: Moisture control

BRANZ (Building Research Association of New Zealand) studies have shown around 40% of existing New Zealand homes are mouldy, causing ongoing heath and durability issues.

In our buildings, mould tends to grow where surfaces get cold enough for high moisture levels to form, both on the surface and within the construction. To reduce the risk of this, we must prevent moisture getting into the building fabric while also allowing any that is there to escape.

Homestar credit HC4: Moisture Control addresses the need to account for this in our building.

The thermal envelope is made up of all elements that sit between the interior environment and the exterior – walls, floors, roofs, windows, doors, ducts, and pipes to the exterior. For the insulation to be effective it needs to be continuous.

Chapter 3: Assemblies

The performance of different construction assemblies (or build-ups) varies based on the location, temperature, humidity, etc. These variables also need to be balanced against costs and client requirements.

In this chapter we introduce a range of build-ups for walls, floors, roofs, and joinery installations, and set out some of the properties of each. This list is not exhaustive but includes a range of standard and best practice build-ups.

Depending on the climate, site conditions and building design it may be possible to use a range of build-ups to achieve the required Homestar and performance level. However, thermally modelling the project will be the primary way to assess which is most suitable.

Homester V5 Design Guide_ 20240408 FINAL Assemblies

Chapter 4: Embodied carbon

Under Homestar v5, all projects must carry out a full life cycle assessment. There is no mandatory target for the assessment result at the time of publishing. Instead, the aim is to provide the starting point to reduce greenhouse gas emissions associated with products and materials used to construct a home.

Although Homestar EN2 only covers embodied carbon A1-A5, a significant reduction in operational carbon will also be achieved thanks to the mandatory minimum requirements from EF4: Energy Use and EF3: Water Use. A reduction in embodied carbon is also recognised in EN3: Sustainable Materials and EN4: Construction Waste. Therefore, we have included the full life cycle assessment example in this overview before we dive deeper into embodied carbon.

Chapter 5: Overheating

The indoor temperature starts rising when the net heat gain exceeds the net heat loss. When the temperature exceeds 25⁰C, it is defined as overheating in ECCHO and the Homestar Summer Comfort credit. The maximum allowable frequency of overheating is represented as a percentage of the year for each star band.

Sources of heat gain include solar gain through windows, walls, and roof on sunny days, and internal heat from occupants and appliances.

In this design guide, we will focus on some effective ways of reducing overheating.

1. Shading design
2. Glazing selection
3. Summer window ventilation

Chapter 6: Controlling the indoor environment

New Zealanders spend around 70% of their time indoors, during which time they cook, shower, sing, breathe, and generally live! In doing so we are producing excess moisture and CO_², and if the levels of either get too high this can cause health issues for people, and issues for the building fabric itself.

At the same time, many elements within our buildings are ‘off gassing’ as they age, leading to volatile organic compounds (VOCs) being released into the air. Many of these can become harmful to people if they reach certain levels.

Studies have shown that relying solely on opening windows isn't enough. To this end openable windows should be used alongside continuous mechanical ventilation, where one or more fans create airflow to remove the moisture, CO_², VOCs and other pollutants in the air. For this reason, constant mechanical extract ventilation is the minimum requirement for all Homestar levels.