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]

Heat loss. Building fabric performance.

As I already mention on my previous post, UK Government set the climate change targets to reduce the CO2 emission by at least 80% by 2050 for the UK’s housing stock. The ways to do this, build completely new zero carbon houses or refurbish already built dwellings and improve its energy efficiency. Both, refurbished and new built houses, has to face the same challenges: heat loss reduction, prevention of overheating and integration of low carbon technologies.

This time, I will cover only heat loss problems.  

Heat loss from a dwelling could be separated into two main categories:

Fabric heat loss – heat loss which is transmitted through the materials of the building.

Ventilation heat loss – heat loss through the ventilation and openings.

Building fabric performance

Traditional buildings are considered to be built in 20^th century by using load-bearing masonry wall, with pitched, slate (or another natural roofing material) covered roof. Windows are generally single glazed with timber frame. Houses have timber and lime plaster finishes. However, these houses do not produce a good thermal insulation as the materials been used has a high U-value (the measure of heat lost. The lower the number of U-value, the better thermal insulation). To reduce heat loss in these houses we need to improve fabric performance which requires a good knowledge about materials.

 

Stone. It is strong, durable and low carbon building material. Stone was one of the first building materials. It is strong in compression and weak in tension. Mostly it is used for load-bearing walls and columns. Dimension stone – crushed stone can be used as flooring, exterior cladding, solid surfaces and walls. However, stone as material does not have high thermal performance, which means that the better insulation would be required.

Timber. It is easy material to work with, strong in both tension and compression, sustainable and has high thermal resistance. Thus, timber frame is widely used in new zero carbon house construction. However it has some disadvantages, for example, it is moisture sensible material which, according to the weather conditions, change its shape and size. It is not suitable for high structures, because of its limited strength. Also, it has low fire resistance and an extra protection should be taken to avoid rotting.

Steel. It is very strong material which can be used in both residential and commercial/industrial construction. It has high fire resistance and is 100% recyclable material. Nevertheless, steel is good heat/cold conductor. It can absorb heat and release to cooler space, if not well insulated. This, also, means that the U-value of the steel is really high. It is not easy to manipulate steel, all the parts for the structure should be made accurately off site. Also, steel can be damaged by weather conditions, and it starts corrode. To prevent this, steel needs to be painted, plated or galvanized.

Bricks. Strong in compression, easy to care, affordable material. Nowadays, there is already sustainable type bricks such as wool bricks. But, to build a brick work requires more time than timber or steel construction and high workmanship.

Concrete. It is, as well, strong and cheap material and can be made with different characteristics for different proposes. For construction mostly used reinforced concrete (made by pouring concrete over steel mesh/bars). Nevertheless, but concrete is not very sustainable material. It is energy intensive to make and transport, and produces a significant amount of greenhouse gas emission. Also, it is poor insulator.

Glass. It is stable material, long-lasting, efficient and recyclablematerial. Nowadays, seeking to reach the high U-values is used double or triple glazing. Glass in an expensive material and energy-intensive in production.

Plastic. It is light and durable material resistant to damp and pests. Plastic does not decompose which might be both advantage and disadvantage. However, plastic can be recycled, but by doing this more harmful gasses can be released.

Wool. It used for insulation in new and retrofit homes. It is stable, durable material. It is renewable and potentially recyclable depending on blended content.

Straw. Can be used as wall insulation in new houses. It is renewable material with quite low U-value, however, not very stable of the low resistance to moisture. Thus, the protection from moisture is a critical requirement.

Cellulose is ‘green’ material made of 80% post-consumer recycled newspaper and is treated with non-toxic borate compounds which makes material fire, rot and pests resistant. It has low U-value and is mostly used for ceiling and wall insulation for retrofit houses. Cellulose insulation is non-recyclable material and not very long-lasting as the thickness may decline over time.

I have mentioned only few main materials that are used in construction. However, to conclude I can say, there is no perfect material to prevent the heat loss totally. The best way is to combine few materials and make a new strong construction. For retrofit homes, the only solution is to install better insulation.

Heat loss though the ventilation

The highest rate of heat loss are through the natural ventilation such as openings, cracks in structure, droughts and thermal bridges. All this appears because of poor insulation and workmanship. To reduce this, draught proofing is required around the windows, doors, water pipes and tanks. Also, all the cracks and thermal bridges in the structure needs to be fixed. Moreover, instead of using simple mechanic extract ventilation, install mechanic ventilation with heat recovery which prevents heat lost in air change between inside and outside. 

Case study

Greenwatt Way development of zero carbon homes – a total of 10 new dwellings has been constructed since autumn of 2010 in Chalvey, Slough, United Kingdom.



Construction consist of 2 different wall types: masonry and timber frame.

Masonry construction:

20mm of rendering;

102.5 brickwork;

150mm fully filled cavity;

100mm blockwork; 

8mm render coat; 

65mm ThermalLine board;

Total wall thickness 445,5mm

Timber frame construction:

102mm brickwork;

50mm vented cavity;

10mm OSB panel;

2x100mm fibre insulation;

10mm OSB panel;

25mm vertical battens;

15mm plasterboard;

Total wall thickness 412mm

Both reached identical U values (0.12 W/m2K). The roofs(covered with solar tiles) and floors achieve a U-value of 0.1 W/m2K. The ground floor has a floating screed on concrete slab and insulation. Windows are triple glazed (U= 0.8 W/m2K). Door's U-value is 0.88W/m2K.

Building Element	Greenwatt way	Code for sustainable homes level 6/zero carbon
Floor U-value	0.1	0.15
Roof U-value	0.1	0.13
Wall U-value	0.12	0.15
Window U-value	0.8	0.7
Door U-value	0.8	0.8

References:

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

'Greenwatt way' [Online] Availabe at: http://www.thisisconcrete.co.uk/home_page/case_studies/greenwatt_way.aspx [Accessed at 14th of November, 2013]

Fisett, P., 2005. ‘Cellulose installation a smart choice' [Online] Availabe at: http://bct.eco.umass.edu/publications/by-title/cellulose-insulation-a-smart-choice/ [Accessed at 14th of November, 2013]

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

‘Material characteristics’ [Online] Available at: http://www.consumer.org.nz/reports/insulation/material-characteristics [Accessed at 14th of November, 2013]

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 14th of November, 2013]

‘Greenwatt way’ [Online] Available at: http://www.house-builder.co.uk/documents/WILFORD-Chris.pdf [Accessed at 14th of November, 2013]