Achieving a 7 Star energy efficiency rating for homes is the new minimum standard in most parts of Australia, following the introduction of the National Construction Code (NCC). This step forward improves comfort for homeowners and helps to reduce greenhouse gas emissions, protecting the environment. But while insulating walls and ceilings often gets the most attention in building to achieve a 7 Star energy rating, one simple option for improvement is often overlooked: under slab insulation. In this blog, we look at why it’s time to challenge the status quo...
Why use under slab insulation?
Traditionally, under slab insulation hasn’t been the norm in Australian residential construction, but there’s a good reason for that to change. The impact of an uninsulated raft slab on a home's star rating can vary based on factors including climate zone, building design and other insulation measures. Precise data on the star ratings of homes with uninsulated raft slabs is limited, buts it's generally accepted that not using under slab insulation can negatively affect a home’s overall thermal performance.
As interest in underfloor insulation grows, some studies have highlighted the positive effect of under slab and concrete slab edge insulation on a home's energy efficiency, showing that in cooler climates like Melbourne, Canberra and Hobart, ground heat loss can account for up to 49% of a home's total heat loss.
In hotter climate zones, the opposite problem – ground heat gain – can be a challenge as heat from the ground can transfer heat into buildings, increasing indoor temperatures. Underfloor insulation can be used for passive cooling strategies, which can also include shading and natural ventilation, to minimise unwanted heat gain. These measures help maintain comfortable indoor environments without over-reliance on mechanical cooling systems.
Given the push towards higher energy efficiency standards and the need for a 7 Star energy rating, using under slab insulation is a valuable way to reduce heat loss (or gain), improve thermal comfort and help reach stricter energy efficiency targets. As a result, slab insulation is gaining traction as a practical design choice in sustainable and energy-efficient home construction.
Common misconception: building 7 Star homes is expensive and complicated
Alongside the lack of knowledge or experience of under slab and edge slab insulation, a big hurdle for many builders is the mistaken belief that reaching a 7 Star rating involves costly materials and design compromises. However, including under slab and slab edge insulation challenges this myth:
- Efficient solutions, lower costs: with the right insulation, you can avoid overinvesting in other areas, such as ceiling or wall insulation.
- Easier compliance: achieving the energy efficiency rating becomes a more predictable and manageable process.
If you’re concerned about expenses escalating out of control, you might be surprised at the outcome of a recent study – it revealed that the average cost to upgrade new homes in Australian cities to achieve a 7 Star energy efficiency rating is approximately $4,311. This finding challenges the belief that higher energy efficiency standards drive up construction costs excessively.
For builders aiming to meet the NCC's 7 Star requirements, energy efficiency improvements must be made across the entire house's envelope. Often, builders focus on features such as high-performance glazing or increased ceiling insulation, which can escalate costs and make the project more complicated to deliver.
Insulation under concrete slab, however, offers a straightforward and effective solution. Research shows that this measure can increase a home’s energy efficiency rating by 0.5 to 1 star, especially in designs where floors were previously uninsulated.
This improvement has several advantages:
- Minimising heat loss through the ground, a significant contributor to energy inefficiency in homes.
- Improving thermal performance year-round, keeping homes cooler in summer and warmer in winter.
- Reducing the need for other costly adjustments, giving more flexibility on design choices for double-glazing or lighter roof colours.
Benefits of Kingspan’s under slab and slab edge insulation
Incorporating under slab insulation alongside slab edge insulation, such as Kingspan Kooltherm K3 Floorboard and Kingspan GreenGuard GG350, not only simplifies the journey to a 7 Star energy rating but can also bring cost savings and improved energy efficiency. Here’s why you should consider this straightforward approach.
Using Kingspan Kooltherm K3 Floorboard for under slab insulation and Kingspan GreenGuard GG350 for slab edges delivers an unbeatable combination of performance and ease of use:
- Thermal efficiency: Kooltherm K3 has a low thermal conductivity, ensuring maximum insulation with minimal thickness. Kingspan GreenGuard GG350 complements this by protecting slab edges, an area often prone to thermal bridging.
- Simplifying the path to 7 Stars: adding floor insulation reduces the burden on other design elements, giving you more flexibility in areas including window orientation, glazing and ceiling insulation levels.
- Cost-effectiveness: by enhancing thermal performance at the ground level, builders can achieve the desired energy rating without resorting to expensive alternative measures.
- Compliance across climate zones: whether you’re building in warmer or cooler climates, our insulation solutions are designed to meet performance requirements in all Australian climate zones.
Under slab and slab edge insulation: impact on star rating
Having done the research ourselves, it’s clear to see the difference that insulation can make. These are the four scenarios that we looked at to measure the benefits of underfloor insulation:
- Kooltherm K12 to external walls (80mm) (in between studs)
- Kooltherm K3 to the slab (25mm)
- Kooltherm K3 to the slab (50mm) and
- Raft slab (no insulation)
What did we find? Well, the star ratings for these designs were 7.3, 7.2, 7.4 and 6.4 respectively, with all three designs using a Kooltherm product meeting the new 7 Star energy rating requirement. The raft slab (no insulation) design was unsuccessful, with the 6.4 rating not meeting the minimum requirement.
As a result of our research, we’ve found that in some cases adding under slab and slab insulation has resulted in lower requirements for ceiling insulation – another bonus for builders.
Under floor insulation simplifies 7 Star
Every project is unique, with energy efficiency calculations varying by climate zone and state. By making the most of under slab and slab edge insulation, you can simplify the 7 Star process and limit the need for more complex design choices, wherever you are. To explore how Kingspan Kooltherm K3 Floorboard and Kingspan GreenGuard GG350 can help your project achieve a 7 Star energy rating faster and more easily, get in touch with our technical service team today and let us guide you through the 7 Star era.
