Methods of improving the thermal performance of timber framed buildings…

The potential benefits of modern engineered timber products over traditional masonry construction are significant – improved air tightness, a reduced level of embodied energy and a greater speed of construction to name but a few. However, despite the many advantages, timber construction inherently lacks thermal mass, whereas traditional masonry construction benefits from the high thermal capacity of brick and concrete.

Commonly, in the absence of thermal mass, internal temperatures will be greater during the daytime because the building’s fabric will not absorb excess heat gain. Conversely, during the night, internal temperatures will be lower because the building fabric is unable to release ‘stored’ warmth. In order to maintain appropriate levels of thermal comfort, a suitable heating and cooling system is required. This may negatively affect cost and energy performance.

At a simplistic level, the thermal capacity of a material is determined by its density; the denser the particle arrangement, the greater the ability to absorb warmth (or ‘coolth’). In contrast, a material with a lesser density, and thus greater air content, experiences a reduced level of thermal transmission, therefore acting as an insulator. Timber construction intrinsically offers improved levels of insulation over masonry, yet it lacks the ability to absorb heat gains, as its air content is greater.

The presence of thermal mass within modern, timber buildings can significantly aid the moderation of internal temperatures and accordingly improve comfort levels. In addition, the incorporation of thermal mass offers the potential to reduce energy consumption by negating the requirement for artificial heating and cooling. Consequently, the duty of incorporating thermal mass within modern timber buildings falls mainly with designers.

Methods for Incorporating Thermal Mass within Modern Timber Frame Construction:

Exposed thermal mass floor:

Internal floors constructed from material with high thermal capacity i.e. concrete, stone, brick. Floor surface is to remain exposed, thus maximising absorption.

Ceiling blocks: Exposed terracotta ceiling blocks inserted between timber joists.

Thermal mass core:

Internal walls / floors constructed from brick and concrete. Green building methods such as rammed earth ( or clay lump ( may also be incorporated. (The building envelope remains a well-insulated, airtight timber frame construction.) Infill bricks: Compressed, unfired clay bricks used as infill within timber frame construction.

Earth shelter:

Building is constructed ‘against’ an earth bank, either existing or purpose built. The earth sheltered proportion of the building envelope thus becomes a thermal mass wall (providing that an insulation layer does not act as a thermal barrier between the earth and the buildings structure). Generally, a mass wall is ideally located on the Northern side, thus potentially maximising exposure to direct sunlight entering the building. The remaining external walls and roof can be constructed using a timber frame; a similar approach is adopted in ‘Earthship’ construction (

Thermally Massive Building Design:

In 2010, a new visitors’ centre for Gressenhall Musem was designed by NPS Group. It was conceived as a demonstration ‘eco’ building and a number of green materials and construction techniques were incorporated. The building combines an innovative timber superstructure with solid earth and straw bale wall construction. Notably, the building adopts a system for passive heating and cooling; High daytime heat gains (resulting from the glazed South West façade) are moderated internally by the thermal mass of the exposed brick and compacted chalk floor, together with the internal walls which are constructed from rammed earth and clay lump. During the night, when internal temperatures fall, stored heat is released from these thermally massive building elements. As a result, internal temperatures are moderated naturally and it was envisaged that the building would require no additional heating or cooling.

Proposed South elevation: Double glazed panels are supported by a timber frame, which is partly enveloped by rammed earth wall.

Proposed Floor Plan: Internal walls are constructed from rammed earth and clay lump, forming a thermal mass ‘core’ within the building. The external walls on the North West elevation are formed using rendered straw bale, thus providing a layer of high insulation to the ‘colder’ part of the envelope.

Detailed Section (in part): Roof structure is constructed from timber ‘Eco Joists’. Structural support is obtained via oak columns at the front and rear of the building; the solid earth, and straw bale components do not bear the weight of the roof.  The Eco Joists support a double layer of WISA plywood, thus creating a curved ‘monocoque’ roof deck.

Jonathan George Mawer RIBA

Cambridge, UK

Posted on: 2nd June 2012.