Stay Cool in Summer – Understanding Decrement Delay

What Is Decrement Delay? Our Guide to Insulating for a Cooler Home in Summer

When planning insulation for your home, you’re very likely to consider things like U-values and airtightness. But there’s another key concept that directly impacts your comfort – especially in summer, it’s called decrement delay.

This lesser-known principle can make the difference between a stiflingly hot attic bedroom and a naturally comfortable space. In this guide, we’ll explore what decrement delay is, why it matters, and why natural insulation materials are often the best performers. 

 

What Is Decrement Delay in Insulation?

Decrement delay describes the amount of time it takes for external heat to travel through the building envelope and affect internal temperatures. It measures how long insulation and building materials can delay heat transfer, keeping your home cooler for longer and the overall temperature more consistent across a 24 hour period, as opposed to huge peaks and troughs. 

Rather than simply measuring how much heat passes through a material (U-values), decrement delay focuses on how quickly that heat gets through – making it particularly important in warm weather and summer conditions.

 

Why Decrement Delay Matters for Summer Comfort

In hot weather, the heat from the sun transferring through your roof and walls can turn upper-storey rooms into ovens. Insulation with a short decrement delay allows heat to transfer inside quickly, leading to discomfort. 

In contrast, insulation with a long decrement delay slows down the heat transfer. This creates a buffering effect, keeping your interior cooler during peak outside temperatures. This is especially beneficial in:

• Loft bedrooms and conversions

• South-facing rooms

• Homes with limited mechanical cooling options 

Even if two materials have the same U-value, the one with a longer decrement delay will feel cooler and maintain a more stable indoor temperature.

 

How Decrement Delay Affects Homes

Let’s say it’s a hot July afternoon in the UK. Outdoor temperatures peak at around 3pm.

• In a home insulated with a synthetic insulation (for example)  which has a low thermal mass (ability to store and release heat slowly) the internal temperature will likely follow the same trend and peak around 3 – 4pm, making the space uncomfortable and because the maximum amount of heat is already in the home, it’s going to be very difficult or even impossible to lower the temperature significantly without air conditioning. 

• In a home insulated with natural insulation offering high thermal mass and long decrement delay, internal temperatures may peak hours later – and likely not at the same level – this will also give time to ventilate and cool the space naturally once external temperatures are lower in the evening/at night. The temperature will be much more consistent over the whole day and night and feel significantly cooler. 

 

What Affects Decrement Delay?

Several key factors influence how long heat takes to travel through a building element:

1. Thermal Mass

Heavier materials (e.g., dense wood fibre, hempcrete) can absorb and store more heat, significantly slowing its movement through the structure.

2. Material Type

Natural insulation materials like wood fibre, sheep’s wool and hempcrete generally outperform synthetic options like PIR or EPS in terms of heat transfer. 

3. Construction Layers

The combination and order of layers (insulation, plasterboard, cladding, etc.) affect the total delay. Using a properly designed insulation system will improve the internal temperatures inside the home. 

4. Insulation Thickness

Up to a point, thicker insulation can increase the delay – but this depends on the material’s density and specific heat capacity, not just bulk. 

 

Sustainable Insulation Performs Better in Decrement Delay Tests

We’ve conducted decrement delay calculations across a range of common insulation materials, both at:

• 100mm thickness

• The thickness required to achieve a U-value of 0.015 W/m²K

As shown in the tables below – in both calculations, natural, sustainable insulation materials – like wood fibre, sheep’s wool, and Hemp – outperformed their synthetic counterparts. These natural materials provided significantly longer delay times, meaning less heat enters the home during the hottest part of the day.

This reinforces the case for sustainable materials – not only for their low environmental impact, but for their superior performance in hot weather. 

Is Decrement Delay the Missing Piece in Your Insulation Strategy?

If you’re planning:

• A home retrofit

• A loft or roof insulation upgrade

• A new build project

…then it’s worth looking beyond U-values alone. 

Ask your architect or builder about materials with more thermal mass and therefore improved decrement delay. You’ll not only reduce energy use but also create a more comfortable living environment year-round. 

Our technical department is headed up by a chartered architectural technologist who has specified our materials on many build projects over the years, we are always on hand to assist with planning your insulation build-up. 

 

Conclusion: Choose Insulation That Works With the Seasons

The majority of insulation advice focuses on keeping heat in during winter. But with UK summers getting hotter, we also need to consider how to keep heat out.

Decrement delay is a key indicator of whether an insulation system will help or hinder you in hot weather. Natural insulation materials don’t just support sustainability, they actively improve summer comfort, reduce overheating, and support better indoor air quality.

 

Practical Tips to Keep Your Home Cool in Hot Weather

Even with well-insulated walls and a good decrement delay, your day-to-day habits can make a big difference in how cool your home stays during summer. Here are some simple but effective strategies:

• Keep windows and blinds closed during the hottest part of the day – If the outside temperature is higher than inside, opening windows will let in more heat. Instead, keep them closed and covered with blinds or curtains to block the sun out! 

• Ventilate in the evening and early morning – Once outdoor temperatures drop below indoor levels (usually in the evening), open windows on opposite sides of the house to create cross-ventilation and let cooler air flow through.

• Use natural shading – Although not always practical for everyone – well positioned trees, climbing plants, or even external blinds can reduce the amount of direct sunlight hitting your windows and walls. 

• Minimise internal heat sources – Avoid using ovens, tumble dryers, or other high-heat appliances during the day. Switch to energy-efficient LED lighting to reduce unnecessary heat output. 

• Use fans strategically – Fans don’t cool the air, but they help move it around. Position them near open windows at night to help pull in cooler air or or vice versa to push warm air out.

• Close unused rooms – Shutting doors to rooms not in use can help contain heat and prevent it from spreading throughout the house.

Combining these techniques with good insulation and smart material choices (like those with high decrement delay) is the best way to create a naturally cooler, more comfortable home, even during the UK’s increasingly hot summers.