Thermal Mass for Part L1A 2013

In this post we look at the role thermal mass plays in how buildings pass Part L1A of the Building Regulations.

Timber frame constructions are typically associated with lower levels of thermal Mass (a TMP of 100 is commonly used where a dwelling is predominantly low mass construction, but dwellings using aircrete blockwork and minimial dense constructions can also fall under this category). Building materials with higher thermal mass (more dense and higher heat capacity), such as concrete, brick and stone, have a greater ability to absorb daytime heat gains (reducing the cooling load and helping to avoid summer overheating) and in turn, they release that heat during the night (reducing the heating demand), which can make the internal temperature of a building more stable.

Temperature fluctuation

Image source unknown (borrowed from

The downside to higher thermal mass, is that it can make a dwelling less responsive to deliberate temperature changes.  Dwellings with high thermal mass in the UK climate take longer to reheat once cooled down.

A low thermal mass construction allows a dwelling to heat up more quickly, with less thermal lag – although it does then also release that heat more quickly.  Good insulation levels should help to ensure that any released heat is largely retained within the dwelling’s envelope.

The Government’s dwelling energy model, SAP2012 and earlier versions, assumes a dwelling is heated to set periods and internal temperatures throughout the year, with the heating demand based on these assumptions, together with the entered specification.

Under the new SAP2012 model for new build dwellings, slower thermal lag from a higher thermal mass construction is of minimal benefit when compared to better responsiveness from low thermal mass constructions; after initially heating up in the morning, by the time the next heating period is reached, the background temperature for both will typically have dropped to similar levels.

Thermal Mass comparison

The above represents two identical dwellings  – the only difference between these two dwellings, entered into the new energy model, is the Thermal Mass Parameter, one medium dense, one lightweight construction. In this example, the medium dense construction house fails to pass TER/DER, while the lightweight construction one passes.

The ‘Notional Building’ uses a Thermal Mass Parameter (TMP) of 250 kJ/m²K for setting the targets under SAP 2012 for Target Emission Rate and Target Fabric Energy Efficiency for compliance via Approved Document L1A2013 (England).  This is considered a ‘Medium’ (250) TMP and is associated with typical modern, masonry house constructions.

Higher thermal mass is known to help with minimising summer overheating, with good design (good night ventilation, correct materials in the correct places), it can be very beneficial – which is why in hotter climates it’s use is so much more prevalent.   It is however, also possible for lightweight housing to provide similar thermal comfort levels in the summer, using appropriate ventilation and solar shading.

A high thermal mass may be more beneficial in circumstances with a more constant, lower level heat demand, but as far as the  SAP2012 energy model is concerned, for a modern dwelling in the UK climate, a quick heating response (from lower thermal mass) is considered more beneficial within the calculations – and that makes it easier to pass.

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Jon Ducker is a qualified energy assessor working for Kingspan Insulation Ltd. He has an extensive knowledge of energy efficiency, renewable energy systems and sustainability in buildings with an expert knowledge of the relevant sections of buildings regulations and standards and their interactions with SAP. He provides authoritative advice regarding energy assessments for a wide range of public and private sector clients.

0 comments on “Thermal Mass for Part L1A 2013
  1. A great article highlighting yet another shortfall in the use of SAP. The higher the envelope efficiency the more favourable a more constant, low-level heat source becomes. Surely SAP should at least have provision for an alternative heating schedule!

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