Introduction
Thermal mass is the capacity of a material to absorb, retain and release thermal energy or heat. It is an important property to consider when choosing building materials for our projects, especially as we work hard to reduce our CO₂ emissions, reduce winter heating and summer cooling demands, and battle with the effects of climate change.
A growing concern in UK homes is overheating. In 2022, an Arup report commissioned by the Climate Change Committee projected that around 90% of the existing housing stock could be at risk if average temperatures rise by 2°C. Overheating poses serious risks to health and productivity. According to the Office for National Statistics, 1,311 heat-related deaths occurred in England during the summer of 2024, with older populations being particularly affected. Additionally, research by the Global Payroll Alliance (GPA) indicates that about 61% of UK office workers feel less productive during hotter summers.
Overheating can also contribute to higher energy demand, as homes increasingly rely on cooling systems to maintain comfort. Although currently less than 5% of UK homes have active cooling systems, this number is expected to rise as temperatures increase, highlighting the urgent need for passive design strategies to reduce energy consumption.
Passive design strategies, including the use of thermal mass, can help mitigate these risks. Thermal mass can help maintain a steady temperature in a building, allowing buildings to stay cool during the summer months and keep a good balance on space heating requirements in winter. Integrating passive solar design also enables the building to harness the heat from the sun during the winter months in order to store that heat in the thermal mass to be released back into the building slowly.
When combined with other passive strategies like ventilation, and insulation, this approach reduces dependence on active heating and cooling systems. Passivhaus standard buildings can achieve up to 90 % savings in heating energy compared with conventional buildings.
In this post we will be giving you an introduction to thermal mass and how it can be effectively integrated into building design. Continue reading!
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What is Thermal Mass?
Thermal mass describes a material’s ability to store heat from surrounding air or surfaces. Generally speaking, the denser the material, the better the thermal capacity.
Floors, walls and ceilings made of dense construction materials like concrete, brick, stone and rammed earth have the ideal properties for thermal mass.
Thermal mass design is best suited for buildings that are in constant or regular use like homes or offices. In contrast, a holiday cottage, or building that is occupied on a sporadic basis would be better suited to a lightweight construction that can heat up quickly as required.
How does Thermal Mass work?
Daytime absorption
On a warm summer day, the thermal mass will absorb the heat at their surface, storing it until it is exposed to cooler air later on as night approaches. The thermal mass responds naturally to the changing temperatures, allowing the building to maintain a more stable indoor environment.
Nighttime flushing
Pairing this with nighttime natural ventilation, the thermal mass is able to cool down with the help of cooler outdoor air which flushes the stored heat during the night, after which the cycle starts again. This same effect can also be utilised during the cold winter months, where the thermal mass will capture and release heat from solar gains and appliances.
Benefits of Thermal Mass
If a thermal mass design strategy is implemented early on in the process in order to make an effective system, some of the benefits will include:
- Improved energy efficiency
- Improved daylighting
- Better ventilation and air quality
- Reduction of summer heat gains
- Reduced risk of overheating and improved occupant thermal comfort
- Supports passive design strategies
- Durable and low maintenance materials
The following images demonstrate thermal mass at work, in winter to reduce space heating demand.
And these images demonstrate thermal mass at work, in summer to prevent cooling demand.
Thermal Mass in Summer
Thermal mass can significantly reduce cooling loads on a building, in both dwellings and larger commercial buildings.
During summer months, any heat gain during the day is absorbed by the thermal mass in the walls, floors and ceilings, preventing an excessive temperature rise. At night, cool air ventilates the building which allows heat that has built up during the day to be removed.
It is important that the temperature variation between night and day is to be at least 5°C, in order for thermal mass to work efficiently. In the UK temperatures between night and peak day are typically around 10°C, making this a suitable strategy for cooling.
Thermal Mass in Winter
In winter, the design must be maximised to benefit from the natural heating capacity of the sun, using a passive solar design. It is key that orientation, glazing and thermal mass work together to provide a comfortable thermal environment and reduce energy demands.
South facing windows will allow low level winter sun to penetrate into the building and absorb into the thermal mass (walls, floor ceiling). The heat is slowly released as the overnight temperature drops, but in this case, windows are not opened for ventilation, as we want the heat to remain in the building and minimise heat loss.
Supplementary heating will most likely be required until later on in the day when the sun is at the correct angle and the building begins to heat up from the day’s activities.
Designing for optimising thermal mass
Materials
Using thermal mass as a design strategy is a whole building approach. Considerations must be taken from the outset on materials, location, orientation, ventilation and many other factors in order to achieve an effective design.
The following table shows the thermal properties of construction materials:
The build-up of construction is vital in achieving good thermal mass performance. Thermal mass cannot work alone and still needs insulation. The position of the insulation in relation to the thermal mass is particularly important. The general rule of thumb is that the thermal mass is located on the inside of the insulation. An example of this would be a simple solid wall, with external insulation and render.
A cavity wall satisfies this basic rule, by having the cavity filled with insulation, and the inner leaf blockwork acting as the thermal mass. Floors follow the same rule, with best practice being to position the insulation under the mass of the floor slab.
A large thermal mass surface area is considered to be more important than the thickness of the thermal mass. It is suggested that anything with a thickness greater than 150mm is not offering any additional benefit to the thermal mass properties. As a practical guideline, designers often aim for the combined surface area of floors and walls providing thermal mass to be roughly six times that of the glazing in the room.
It is important that the thermal mass materials are internally exposed. Carpet or plasterboard and dry lining will have a negative effect on performance and must be avoided. Instead, floor finishes of tiles would be most preferable, and walls are best finished in wet plaster which conducts the heat freely but also helps to minimise air leakage. Underfloor heating works well with the thermal mass. Ceilings can be wet plaster finishes or just painted concrete.
Passive solar design
A passive solar design takes advantage of the low winter sun and allows the sun’s rays to penetrate the building and warm the thermal mass. During the warm summer months however, the building needs to be protected from overheating.
The orientation of the building is the most important factor in ensuring a successful passive solar design strategy. A south orientation is key to maximising the required gains from autumn through to spring. In the middle of winter, the sun reaches a maximum altitude above the horizon of about 17° at noon in the south of England. In the middle of summer at noon, in the same location, the sun reaches a maximum angle of around 64°. Usually, a simple overhang can prevent the sun during the hottest part of a summer’s day but still enable the sun to penetrate the building during the cooler months. The south elevation should have a relatively clear view of the sky in order to allow the solar radiation to pass into the building during the winter months.
Rooms should be positioned according to the orientation also. Most used rooms like living areas, kitchen, dining etc should be on the south side of the building or within 30° of south, and secondary areas such as bathrooms, utility, circulation should be on the north side of the building. This arrangement allows the more frequently used rooms to benefit from solar gain and good daylighting. The bedrooms should be positioned to maximise cross ventilation which is particularly good for reducing the internal temperatures.
South facing windows will maximise solar gains, which as a general rule should be no more than 40-50% of the facade to avoid overheating. There should be minimal north facing windows as these will aid heat loss. 15% window of a room’s floor area on the north elevation is sufficient to provide daylighting while minimising potential heat loss.
Ventilation
Ventilation is an important part of the design to avoid overheating. Nighttime ventilation removes the heat that has been absorbed during the day, replacing it with cool fresh air.
The best strategy for smaller buildings such as dwellings, is to ensure well-designed user-friendly windows which enable natural ventilation maximising the advantage of prevailing wind conditions. Cross ventilation is preferable where possible.
In larger commercial buildings, ventilations strategies are often more complex.
Shading
It is important that the sun is controlled to allow winter heating, but to avoid summer overheating in a passive solar design. This is where shading comes in. There are many options and systems available, depending on requirements. Sometimes, a simple overhang will suffice, but in the following table we take a look at some of the alternatives:
Thermal Mass Checklist
Here is a quick thermal mass checklist to help you with your projects:
- Thermal mass provided by floors, ceilings and walls that have a good density with suitable finishes
- Keep thermal mass materials as exposed as possible
- Maintain a large surface area of thermal mass
- Avoid plasterboard where possible
- High level of insulation on the external side of the thermal mass
- Southern orientation of the building to allow passive solar gains in winter
- Place frequently used rooms (living areas, kitchen, dining) on the south side and secondary spaces (bathrooms, utility, circulation) on the north side.
- Suitable shading strategy to avoid overheating in summer
- Clear view of the sky from the south
- Well insulated glazing, with a good amount of glazing on the south elevation to maximise solar gains in winter
- Suitable ventilation from windows, while considering any security and noise issues
Helpful Links
Addressing overheating risk in existing UK homes (Arup + Climate Change Committee)
Climate change insights, families and households, UK (Office for National Statistics)
61% of UK Office Workers See Productivity Slump Due to Hotter Summers (FM Business Daily News)
Why Is Air-Conditioning Rare in U.K. Amid Heat Waves? (TIME)
Heating load in Passive Houses (Passipedia)
Cooling in the UK (GOV.UK)
Avoiding Summer Overheating (Passivhaus Trust)
Risks to health, wellbeing and productivity from overheating in buildings July 2022 (Climate Change Committee)
Public health impacts of heat 2024 (UK Parliament)
Embodied and operational carbon dioxide emissions from housing (ScienceDirect)
Thermal Mass for Housing (The Concrete Centre)
Books
Here are some recommendations for further reading:
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Conclusion
Thermal mass plays a key role in regulating indoor temperatures. When thoughtfully integrated into a wider passive design approach, it can improve comfort levels while lowering energy demand. As climate change continues to drive higher temperatures and increase pressure on energy resources, considering thermal mass in building design is becoming more important than ever.
We hope you found this post useful.
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Your Comments
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Author
Written by Valanne Fernandes, a Part 2 Architecture graduate. Valanne is a content creator with First In Architecture, spending her time researching, writing and designing inspiring new content for the website.
Wow, your website is soooooo helpful! Thank you so much. This is the first time that I understand what I’m reading about. Everything is easy and logical explained and I can understand it even as a no English native speaker. Once again, thank you!
Thank you Joanna. :)
Thank you for the article but it is not correct. You state avoid plasterboard. If you were to build a house with a good thermal time constant, the longer this is the better the building performs, then the list of building materials that perform the best are:
1. Wood
2. Plasterboard
And the list goes on. Water does out perform the above but you cannot build from it. You can of course include large volumes though in you heating system.
It seems that the term thermal mass which really does not exist, because it has no unit of measure, is the feature of many an articles but rarely is the information correct.
Look at the buildhub website. One member J S Harris writes about the information above in more technical detail under a thread called the myths of thermal mass. One other member reports that he lives in a wooden framed house with no traditional thermal mass as you describe and yet it has an excellent thermal time constant.
Interestingly adding two layers of plasterboard to your walls will significantly improve your thermal time constant…..
Useful publication
Thank you Jonathan :)