Overheating

Nowadays planners, designers and property owners has to face new difficulties. Appears to be growing evidence of overheating in homes, as the new houses satisfy more demanding standards of energy efficiency. Additionally, climate change leads to global temperature rise which affects home temperature as well. Overheating can cause a discomfort or sometimes over longer periods can be harmful to occupants, in extreme cases even put there life at risk. People who are more likely to be occupying their homes during the hottest periods at the day (daytime), are most vulnerable to overheating, such as the elderly or sick.

Overheating – it is conditions in building, when the warmth cause discomfort and heat stress to occupants. After some tests was found out that most people begin to feel ‘warm’ at 25C and ‘hot’ at 28C, above internal temperature of 35C there is a significant danger of heat stress.

External gains

Sunlight through the windows heats the interior surfaces. This heat remains trapped inside the building. It works the same as greenhouse, thus sometimes the process is called ‘greenhouse effect’. On the new built dwellings the ‘greenhouse effect’ is even bigger. Modern houses with double or triple glazed windows and high insulation will tend to retain the heat indoors and allow it to build up. On winter time it will help to keep the warmth inside the building, however, on summer period it can cause overheating.

Internal gains

Inside the well-insulated and poorly ventilated houses internal heat gains can have serious consequences. There are 4 types of internal gains:

Occupants and their activities – people inside the house emit the heat in form of ‘metabolic’ gains. More people move, more energy (in form of heat) they radiate into the environment. Also, all the activities like cooking, bathing, showering contribute to heat gains.

Appliances – fridges, washing machines, TV’s, computers, microwaves etc emits heat even in standby mode.

 

Building services – mechanical ventilation system, hot water distribution and storage systems poorly insulated can have a massive impact on heat emission inside the building.

Lightening – even low energy lights can add heat gains.

Other factors which might increase overheating

Site context – if house is surrounded by noisy objects like busy roads, industrial buildings, railways etc it may prevent occupants from opening the windows enough for ventilation. Also, the temperature is always higher in cities to compare with the rural environment.

Orientation – it makes a significant impact on solar gains. Houses with large amount of west-facing windows will gain more heat than the ones facing north.

Building designs – Modern houses are high insulated, which means that the heat gains are retained inside and needs to be removed actively seeking to avoid overheating.

Ways to reduce the overheating

There are many ways to reduce the overheating such as:

Orientation – intentionally selected orientation can help to control solar heat gains.

Shading – It is effective way to reduce the sun heat gains only by using curtains, blinds, shutters. Also fencing and planting could work as valuable shades for the house.

Purge ventilation

Cooling ventilation strategies – homes with the installed ventilation systems (including purge ventilation) helps to release heat from the inside the building. However, occupants should have great knowledge how to use it, to avoid heat loss.

Heat reflective finishes – light colour finishes, reflective or green/brown roof has a huge impact on keeping temperatures down.

Appliances – trying to use energy efficiency appliances at home, do not keep them turned on when not using etc

Lighting – use low energy light bulbs, do not keep lights on when not in use. 

Case study

Again for the case study I will use Greenwatt Way development. As I have already mentioned on my previous posts all the houses on this development are well-insulated and heat loss parameters achieved requirements of Code for sustainable homes level 6 (0.8W/m2K). However, such a low heat loss might cause overheating problems, especially on summer time. Thus, it is essential to find some ways to get rid of warmth inside. For this, the high level rooflight was installed to ensure a good purge ventilation. Moreover, high performance triple glazed windows with draught resistant seals allow larger openings fore natural ventilation. Finally, whole house ventilation with heat recovery, which allows fresh, pre-heated air get into all living areas and bedrooms.

References:

'Greenwatt way' [Online] Availabe at: http://www.thisisconcrete.co.uk/home_page/case_studies/greenwatt_way.aspx [Accessed at 15th of November, 2013]

‘Greenwatt Way. A zero carbon homes newbuild case study’, 2011. Energy saving trust.

Historic Scotland Alba Aosmhor. ‘Fabric improvements for energy efficiency in traditional buildings’ [Online] Availabe at: http://www.historic-scotland.gov.uk/fabric_improvements.pdf [Accessed at 15th of November, 2013]

‘Greenwatt way’ [Online] Available at: http://www.house-builder.co.uk/documents/WILFORD-Chris.pdf [Accessed at 15th of November, 2013]

‘Fabric first’, October edition, 2010. Energy saving trust

Richards Partington Architects, 2012. ‘Understanding overheating – where to start’ NHBC Foundation

Grater London Authority, 2008. ‘Your home is in a changing climate’ [Online] Available at: http://www.ukcip.org.uk/wordpress/wp-content/PDFs/3Regions_Retrofitting.pdf [Accessed at 16th of November, 2013]

Zero Carbon Hub, ‘Overheating in homes’ [Online] Available at: http://www.zerocarbonhub.org/resourcefiles/OverheatingInHomes8pp_2013_8March.pdf [Accessed at 16th of November, 2013]

Thermal Bridging

Thermal bridge (cold bridge) – a junction where thermal insulation is not continuously and cause heat loss (air leakages). It usually occurs when structural element passes through insulation layer or the insulation is not thick enough where two construction elements meet.

Repeating thermal bridges

Repeating thermal bridges occurs where the structural elements with the low thermal conductivity repetitive cross the higher thermal conductivity layers. For example, timber studs bridge the layer of insulation, steel wall ties in the masonry cavity external wall, mortar joints in lightweight concrete blockwork (because the mortar has higher thermal conductivity compared with the blocks). These should be included when calculating within the main building element U-values.

Non-repeating thermal bridges

This mostly appears around loft hatches, around openings (doors, windows), where internal walls or floors penetrate the thermal envelope, etc. These bridges should be considered separately from main building element U-values.

Geometric thermal bridges

Geometric thermal bridges are result of complex building shape. They can be 2-dimentional or 3-dimentional, depending on where they occur. Mostly geometric thermal bridges appears at the junction of wall/roof, at the corner of external walls, at the wall/floor junction.

Building Regulations

At the part L1A of Approved Documents it is said that ‘The building fabric should be constructed to a reasonable standard so that:
a. the insulation is reasonably continuous over the whole building envelope; and
b. the air permeability is within reasonable limits.’

Ways to avoid thermal bridging

• Keep the house design as simple as possible. The smaller external area and amount of junctions the less air leakage routes in the house.

• Where possible, do not interrupt the thermal envelope (Do pen on section test to check if there is no passes through the insulation).

• If the thermal envelope is interrupted by water pipes, vents, windows, doors, etc, thermal resistance in the insulation should be as high as possible.
• At the junctions of building elements there should be no gaps
• For regular thermal bridges such as wall ties or mortar joints, the fabric been used should be high thermal resistance.

To conclude thermal bridging has a significant impact on the thermal and energy performance. New build houses (sustainable/zero carbon) are highly insulated, so, any heat loss is essential when trying to achieve the highest U-values.

Case study


For example I will use the same Greenwatt way development as for my previous posts.





The houses were design with very carefully detailing to avoid air leaking. For this they used special tapes and seals, done airtightness testing through the construction process, etc. Nevertheless, they had to deal with few challenges, such as north facing roof light/natural ventilation, which penetrates the ceiling cassette and cause heat loss. It needed to be very well insulated. The most difficult to insulate was 1 bedroom flat above the bin and bicycle store as the dwelling has 4 external walls, an exposed floor and an exposed roof. Also, in order to achieve high air tightness levels, post boxes had to be taken out from the design, external mailboxes are used instead.

Insulated roof light/natural ventilation

The balconies are supported on an independent structure to avoid thermal bridging

As a result, all these houses has a very limited heating demand 80% less than homes built to 2006 Building Regulation standards and 90% less than a typical existing home.

References:

'Greenwatt way' [Online] Availabe at: http://www.thisisconcrete.co.uk/home_page/case_studies/greenwatt_way.aspx [Accessed at 15th of November, 2013]

‘Greenwatt Way. A zero carbon homes newbuild case study’, 2011. Energy saving trust.

Historic Scotland Alba Aosmhor. ‘Fabric improvements for energy efficiency in traditional buildings’ [Online] Availabe at: http://www.historic-scotland.gov.uk/fabric_improvements.pdf [Accessed at 15th of November, 2013]

‘Greenwatt way’ [Online] Available at: http://www.house-builder.co.uk/documents/WILFORD-Chris.pdf [Accessed at 15th of November, 2013]

‘Fabric first’, October edition, 2010. Energy saving trust

Richards Partington Architects, 2012. ‘Understanding overheating – where to start’ NHBC Foundation

‘Thermal bridging’ [Online] Available at: http://www.leedsmet.ac.uk/teaching/vsite/low_carbon_housing/thermal_bridging/introduction/index.htm [Accessed at 15th of November, 2013]

Row, M., 2012, ‘Thermal bridge- what is it and how to avoid? [Online] Available at: http://www.insulationshop.co/Thermal_bridge_-_What_is_it_and_how_to_avoid%20_it [Accessed at 15th of November, 2013]