What is thermal mass?

This post will give you an introduction to thermal mass and night cooling. 

Energy efficiency, rising temperatures and CO² emissions are very much on our minds in this current climate. As we look at the performance of our buildings, we are working hard to reduce our CO² emissions, reduce winter heating and summer cooling demands, and battle with the effects of climate change. 

Air conditioning in the UK used to be a rare thing, particularly in dwellings, but it is becoming increasingly popular at a rate of 8% annually, and in domestic properties air conditioning sales have increased by 27%. 

Overheating is clearly a growing problem. A thermal mass design strategy can help maintain a steady temperature in a building. The design will allow buildings to stay cool during the summer months and keep a good balance on space heating requirements. 

Employing thermal mass alongside passive solar design 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. It is said that a good passive solar design in a modern UK masonry or concrete building can cut heating fuel consumption by up to 11%. 

How does thermal mass work?

Thermal mass describes the materials ability to store heat from surrounding air or surfaces. Generally speaking, the denser the material, the better their thermal capacity. 

Heavy construction materials such as concrete, brick and stone have the preferable properties to store heat and allow for a good level of thermal mass. 

On a warm summer day, the thermal mass – floors, walls and ceilings made of dense materials, will absorb the heat at their surface, storing it until it is exposed to cooler air later on in the day and at night. The thermal mass responds naturally to the changing temperatures, allowing the building to maintain a more stable temperature. With natural ventilation, bringing in cool air at night time, the thermal mass then cools down, releasing the 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

The following images demonstrate thermal mass at work, in winter to reduce space heating demand and 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 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 to be no less than 5˚ degrees, in order for thermal mass to work efficiently. In the UK temperatures between night and peak day are typically around 10˚ degrees, 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 days 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. 

Some suggest a general rule to provide six times the surface area of floor and walls providing mass to 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 suns 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˚ degrees 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 64˚ degrees. Usually a simple overhang can prevent the sun during the hottest part of a summers 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 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˚ degrees 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 rooms 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. Night time 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, that 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 the 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 here we will take a look at some of the alternatives. 

Thermal mass design is best suited for buildings that are in constant or regular use. 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. 

A quick thermal mass checklist

• Thermal mass provided by floors, ceilings and walls that have a good density with suitable finishes
• Keep thermal mass materials as exposed as possible
• 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
• 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