Keeping cool, while staying warm

Careful attention to building design is essential to meet the challenges of warmer summers, while also reducing the amount of energy that we use during the colder times of the year…

Homes are constructed to higher standards, with improved levels of energy efficiency, reducing heating demand, fuel costs and carbon emissions, which means that houses are now designed and built to be better insulated and more airtight to limit heat losses in the winter.  However improved insulation levels and better air tightness could lead to overheating in homes during the summer months unless appropriate measures are taken.

Good design is essential. Designers should consider building orientation, glazing areas and types of construction to avoid overheating as well as ventilation, shading and thermal mass.

Ventilation 

A good ventilation strategy is essential (with summer bypass for heat recovery where necessary), particularly with the much better levels of air tightness that are now being achieved, which helps to reduce heat losses in the winter months, but which requires adequate input and extract ventilation. Designed-in features such as purge ventilation (where the hot air inside the building is rapidly removed), must also be explained to occupants to ensure it is effective.

Shading

Features such as recesses, overhangs and balconies can be integrated into the building elevation as a means of providing solar shading. These features can be seen on the Victorian colonial style bungalows which incorporated deep verandas to help shade the internal space, but do tend to make those internal spaces darker. The glazing can also be optimised to reduce solar gain when and where necessary.

The original town hall and the first administrative building in Blantyre, Malawi showing shading verandah and use of thermal mass.

Thermal mass

In hot temperatures, higher thermal mass constructions can help to reduce daytime temperatures inside a building by absorbing some daytime heat gains and releasing that heat during the night. This has the effect of making the internal temperature range potentially more stable (but potentially needing more overnight purge ventilation if well insulated). The downside is that high “thermal mass” homes are typically considered to be less responsive to heating input during the heating season and require more heat energy to achieve thermal comfort (combined air and surface temperatures) and take longer to reheat once cooled down.

Higher levels of thermal mass buffer the temperature gain to the evening, but in a long heat wave it is possible for this thermal mass to stay warmer for longer.

In a highly insulated and airtight home where there is insufficient secure night-time ventilation, that built-up heat can increase the risk of overheating. If a higher thermal mass were required this can be achieved via various means such as the use of stone, concrete, brick, but a dwelling’s Thermal Mass Parameter is also affected by the floor finish, internal walls and internal floors, as well as the internal wall finish. Higher levels of thermal mass can be useful in climates that have large diurnal temperature swings (eg in the Middle East), where sufficient night time ventilation is incorporated into the building design, but is usually of less benefit in the UK. Some materials can act as both low and high thermal mass, via a phase change effect, although this would add cost.

A low thermal mass dwelling tends to have better heating responsiveness, with less lag, which can mean lower and more easily purged internal temperatures in the summer months at night, but does mean that daytime temperatures can be higher, which means that heat gains may need a little more consideration. A dwelling built using the Kingspan TEK building system for example combines a good level of air tightness, thermal insulation and good junction detailing to limit losses, and is typically considered a low thermal mass construction. With consideration of the various design issues discussed above, this will typically result in a highly efficient, comfortable building (but with low air tightness, a good level of air exchange ventilation would be important eg MVHR).

Summary

• The thermal protection of the external envelope (U-value) is mainly responsible for the heating consumption of a house.
• The prevention and reduction of thermal bridges from junctions is essential for the effective functioning of the insulation.
• Solar gains and losses from openings are significant and limiting these gains in the summer months, whilst maximizing them for the winter can be a key consideration.
• Heat losses from air exchange can be significant to the energy efficiency of a dwelling. In the summer months, adequate purge and cross ventilation needs to be considered.
• The influence of the thermal storage capability of the external leaf of walls is extremely small.
• The heat capacity of the building components facing towards the interior, which includes internal walls and intermediate floors (i.e. the thermal mass) can have a perceptible influence on the temperature stability and thus on summer comfort.
• Lower heat capacity wall constructions can result in higher temperatures in the summer months during the day (unless specific measures are undertaken to limit solar gains by shading and removing heat by adequate ventilation), but are balanced by quicker cooling and lower temperatures at night.
• During a heat wave, when the heat absorbed by building materials is re-emitted into the living space, a higher thermal mass construction could contribute to uncomfortably high internal temperatures and overheating unless specific measures are undertaken to limit solar gains by shading and removing heat by adequate ventilation.
• Within energy modelling calculations such as SAP, over a year’s demand, higher thermal mass construction is of minimal benefit when compared to the better responsiveness within the model for a low thermal mass construction. 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.

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 its 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.

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