There’s a commonly known misconception that insulation “traps heat” and turns your home into a hotbox in summer. It sounds believable on the surface. Insulation keeps warmth in during winter; wouldn’t it also keep heat in during summer?
Fortunately, that’s not how insulation works. And thanks to a simple but clever experiment, we have clear evidence that proves this myth wrong.
In this article, we’ll break down the study, explain how insulation actually behaves in hot conditions, and explore why air leakage (not just insulation) is a real reason homes overheat. We’ll also look at how polyurethane solutions like YetiFoam deliver far better thermal performance than traditional batts or foil insulation, especially in sheds and underfloor applications.
The Bradford Hotbox Experiment: Three Boxes, One Clear Result.
To address the myth directly, CSR Bradford ran a controlled experiment in western Sydney. They built three identical mini “rooms”, each representing a different type of wall system:
- Box 1: No insulation
- Box 2: Lined with Bradford Thermoseal™ reflective wall wrap
- Box 3: Insulated with Bradford Gold R2.5 wall batts
Each box was placed in the sun on a hot summer day. Inside each one, the team placed the same amount of ice.
The goal: see how quickly each box warmed up and how fast the ice melted.
You can view the Bradford summaries here:
How Does Insulation Actually Work?
Insulation doesn’t generate heat. It doesn’t store heat. And it doesn’t “decide” whether your home is warm or cool.
Insulation drastically slows the movement of heat.
- In summer, insulation helps keep hot outdoor air from heating your indoor environment.
- In winter, insulation helps keep warm indoors
- The bulk insulation box (with glasswool batts) stayed the coolest for the longest, showing the slowest temperature increase and the slowest ice melt.
Without insulation, your air conditioner is constantly battling the rapid flow of heat through your roof, walls and subfloor. With insulation, your indoor environment becomes more stable, more efficient and far more comfortable.
Air Leakage: A Key Benefit of Insulation
One of the biggest drivers of discomfort in Australian homes isn’t actually the insulation; it’s air leakage.
Air leakage is the uncontrolled movement of outdoor air entering the home and conditioned indoor air escaping through gaps, cracks and unsealed construction joints. This can happen around floorboards, wall and floor junctions, subfloor vents, ceiling penetrations, window frames and more.
Many building-science sources note that the subfloor plays a bigger role than most people expect. In some guidance, it’s estimated that up to 42% of the air that moves into a home may originate from the subfloor or crawl space.
What does this mean in practice? It means the air beneath your house, which can be hot, humid, dusty or moisture-laden, can be pushed into your living spaces.
What Airtight Homes Teach us About Insulation (USA & Canada)
If you look at some of the most efficient homes in the world (particularly across the northern United States and Canada) airtightness is treated as non-negotiable.
Those climates experience extreme cold, so builders design homes to be:
- well insulated
- air-sealed from top to bottom
- wrapped in continuous thermal and air barriers
- ventilated with controlled, mechanical fresh-air systems
The entire structure is treated as a single, continuous envelope that must remain airtight for comfort and efficiency.
The result? Homes that maintain consistent internal temperatures with far less energy use and without the draughts, humidity swings or pressure imbalances common in older Australian houses.
Underfloor Insulation: Stopping Heat at One of Its Biggest Entry Points
Most Australian homeowners know they need ceiling and wall insulation, but few realise that the subfloor is a major source of summer heat gain and loss.
In homes with raised timber floors, you typically have:
- gaps between floorboards
- cracks at the wall and floor perimeter
- open subfloor ventilation
- exposed joists and framing
In summer, hot, humid outside air pushes through these openings and rises into the home. At the same time, your cool conditioned air sinks downward and escapes through the gaps.
This means your air conditioner is fighting a losing battle, driving your cooling bills sky high.
Traditional batts, for example, placed between joists can help with R-value, but they:
- leave air gaps around the edges
- sag over time
- still allow hot air to circulate through the subfloor cavity
- don’t provide any meaningful air sealing
A polyurethane system like YetiFoam solves the problem at its source. It adheres directly to the underside of the floorboards, creating a continuous layer that:
- blocks draughts
- stops outdoor air infiltration
- prevents conditioned air from escaping
- stabilises the internal temperature
- acts as a condensation and vapour barrier
Instead of heat flooding in from below, your home finally holds onto the cool air your AC produces.
The Verdict: Insulation Protects Homes in the Summer
In summary, insulation doesn’t create a hotbox effect. It prevents it. Bradford’s experiment showed that insulated spaces stay significantly cooler than uninsulated ones, proving insulation slows heat transfer rather than trapping heat.
The real issue behind overheated homes is uncontrolled air leakage, primarily through the subfloor, where a surprising amount of indoor air can originate. When a home isn’t sealed, hot outside air pushes in and conditioned air escapes, forcing your cooling system to work harder.
Modern, airtight approaches used in the USA and Canada show how powerful proper sealing can be for year-round comfort, and Aussie homes need to start taking example.
With underfloor systems like YetiFoam, which insulate and air-seal in one, homeowners can finally stop unwanted heat from entering from below and hold onto the cool air they pay to produce.