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Thermal Mass - Explained

30th March 2010

Thermal Mass in Insulating Concrete Formwork Buildings

Thermal Mass in a building is probably the single most important element used in the design and construction. Unfortunately it is also likely to be the least understood. Early cave dwellers understood the benefits of thermal mass so does the wine industry. Now I am not suggesting we all rush off to develop underground cities but I am proposing the benefits of thermal mass be given very early consideration in the design process.

The energy used for space heating accounts for up to 50% of the buildings energy consumption depending on type, and around a third of the carbon emissions from all UK buildings. To lesson this impact, revisions to Part L of the Building Regulations, along with other codes and standards, have done much to reduce fabric heat loss through requirements for greater levels of insulation and reduced air leakage. These are very effective and well understood measures. Something less well known is that reducing heat loss from a building also enhances the ability of thermal mass to further lower the space heating load when used in a low energy design. This point is acknowledged in the current proposed changes to Part L1 for dwellings, which reflect the increasing significance of thermal mass as we move towards more highly insulated, air tight, low energy buildings.

With the advent of a warming climate, summertime performance is also a driver for thermal mass. When it is used in combination with good ventilation and shading, it helps buildings adapt to the effects of hotter weather by reducing both the risk of overheating and the cooling load in air conditioned houses.

Until recently, the use of thermal mass was often disregarded in favour of largely services based solutions to the heating and cooling of buildings, which is not surprising in an age when cheap energy was plentiful and the effects of climate change had yet to be felt. However, the time to re-evaluate the contribution that thermal mass can make to building performance has come. To do this, it is important to have a basic working knowledge of thermal mass and how best to use it.

Exploiting thermal mass on a year-round basis is not difficult, but it does require consideration at the outset of the design process when requirements for the building form, fabric, and orientation are being established. Providing this is done sympathetically a more passive approach to design can realise benefits which include:

• Enhanced energy efficiency and carbon savings over the life of the building.
• Improved daylighting.
• Improved ventilation and air quality.
• Optimal decrement delay (time lag) and decrement factor (heat flow) for reducing heat gains in the summer.
• Good summertime comfort and reduced risk of overheating.
• A measure of future proofing against the effects of a warming climate.
• Reduction in the need for more expensive low and zero carbon technologies to meet CO²  targets.
• Enhanced property resale value


Thermal mass is the ability of a material to store heat.

For a material to provide a useful level of thermal mass a combination of three basic properties are required:

1. High specific heat capacity – to maximise the heat that can be stored per kg of material.
2. High density – to maximise the overall weight of the material used.
3. Moderate thermal conductivity so that heat conduction is roughly in synchronisation with the heat flow in and out of the building.

Timber has a high heat capacity but a low thermal conductivity. This limits the useful heat absorption rate and so provides a low thermal mass.
Steel also has a high heat storage capacity but it also has a very high rate of thermal conductivity which means that heat is absorbed and released too quickly for any meaningful thermal mass efficiency.
Concrete with its high heat capacity and density but moderate thermal conductivity offers a good balance.
Cast in-situ concrete has a highly effective thermal mass with a typical density of 2300 kg/m³ a thermal conductivity  of 1.75 W/mK and a specific heat capacity of 1000 J/kg.K

Concrete steadily absorbs heat that comes into contact with its surface, conducting it inwardly, and storing it until the surface is exposed to cooler conditions and its temperature begins to drop. When this occurs, heat will begin to migrate back to the cooler surface and be released. In this way, heat moves in a wave-like motion alternately being absorbed and released in response to the variations in day and night-time conditions.

The ability to absorb and release heat in this way enables buildings with thermal mass to respond naturally to changing weather conditions, helping stabilise the internal temperature and provide a largely self-regulating environment. When used appropriately, this stabilising effect helps to prevent overheating problems during the summer and reduces the need for mechanical cooling. Similarly, the ability to absorb heat can reduce fuel usage during the heating season by capturing and later releasing solar gains and heat from internal appliances.

Insulating Concrete Formworks (ICFs) are large lightweight moulds for creating cast-in-situ concrete structures; they themselves do not directly contribute to the structural performance of the building but they do provide the thermal insulation of the structure.

The ICF blocks are made from flame-retardant expanded polystyrene foam (EPS), which is designed to withstand the pressure of placing and compacting up to 3 metres of concrete in one pour.

Buildings incorporating ICFs are in essence standard concrete buildings, which are structurally designed to comply with Eurocode 2 or (BS8110).

Because the ICF blocks remain in place during the concrete curing process accelerated drying of the surface is avoided, allowing the wall to cure homogeneously. This reduces internal stresses thus preventing surfacing cracking. Thermal bridging can be reduced as it is practical to build structures without the need for anti-crack steel or expansion joints within the concrete core.
Once the concrete has cured the EPS remains in place to, (a) provide insulation to the building and (b) to provide an ideal long-term protective environment for the concrete.

Walls constructed using ICFs can provide a significant contribution to preventing summer time overheating and a stable internal environment.

Thermal mass can be spilt into two parts:

1. Admittance - the ability of a wall to rapidly absorb and release energy.
2. Decrement - the ability of a wall to delay the passing of heat through the building.

Admittance - The main mass of concrete is contained within an insulated blanket giving poor admittance values, and preventing rapid heating and cooling into the core. The actual performance of an ICF wall is similar to lightweight frame buildings or modern masonry in this respect. The insulated inner surface does however give additional advantages.

• The internal wall always feels warm o the touch, reducing the tendency of the occupants to compensate for the cold surface, and turn the heating up.

• The warm internal surface prevents the formation of drafts created by the difference in temperature between the air and the wall surface.

• The risk of the condensation in eliminated.

• Improving the active thermal mass in ICF constructions Walls built using ICFs have the strength and flexibility to support a variety of concrete flooring options.  This provides thermal mass were it can be best used. e.g. in the ground floor under a window, or in the ceiling to catch heat from rising warm air.

Tying the floor construction into the walls also has the benefit of linking the concrete cores increasing the available mass.

Decrement - ICFs provide an ideal combination of decrement delay (8hrs or more) and decrement factor (0.1 or lower). This means that if the external surface temperature of the wall varies by 20 degrees over the day, the internal surface temperature will only vary by 2 degrees, and there would be an 8 hr delay in the peak temperature occurring.  For the thicker ICF blocks the delay can be up to 11 hrs, with only   0.4 degree increase under the same conditions.

Steady State Thermal Mass - over the course of a year the core temperature of a typical ICF wall will vary only by 1 or 2 degrees C, minimising internal temperature fluctuations, and the total heat input required to maintain a comfortable and even living environment.

In an ICF structure the external concrete element is disproportionably insulated on both sides, the external insulation is normally considerably thicker than the internal to meet the design criteria and maximise the benefit of the protected thermal mass. Bear in mind that internally the building will have substantial amounts of thermal storage in the concrete floors and ceilings.

In the winter and summertime this balance of protected thermal mass provides a unique level of energy efficiency and thermal comfort. ICF buildings are the way forward to meet the current and future demands for fast and economic construction combined with a long lasting and sustainable  building method.

 

 

 




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