The stamford brooke study
Stamford Brook is a 650 dwelling development on the edge of the National Trust's Dunham Massey estate in Altrincham, Cheshire. The estate was left to The National Trust in 1976 by Lord Stamford and is now run by the National Trust as a 'Special Trust in Credit'. This means that the estate has to generate all the income required for its upkeep from its own resources. This includes financial income from visitors and rents from its farm tenants. It receives no external funding. This was the wish of Lord Stamford, he was very passionate about his 'traditional country estate' and wanted it to remain that way. With some foresight, Lord Stamford identified certain areas of land on the estate as investment land, which if necessary, could be sold to raise funds for the future upkeep of the Dunham Massey Estate. One of these was the 25 hectare parcel of land at Brookside Farm which now forms the Stamford Brook development.
Stamford Brook is a development that has been set up and constructed by Redrow Homes (North West Ltd.), Bryant Homes and the National Trust. The buildings within the development have been designed to exceed the required levels of environmental performance in the Building Regulations, and dispute the common view that it is not possible to merge reasonable building costs with sustainability goals.
The project represents one of the largest field trials of energy efficient and sustainable housing undertaken in the UK for many years. The design, construction and occupation of the dwellings are being studied by a research team from Leeds Metropolitan University, funded by the DTI and ODPM. A set of Robust Standard Construction Details designed to minimise thermal bridging have been developed. The dwellings are also designed to meet an air-tightness target exceeding proposed Building Regulations. The air-tightness of each of the dwellings will be tested by air pressurisation.
The overall project objective was to support future reviews of Part L of the Building Regulations. This was to be done by evaluating how a range of improvement measures that could be used to meet the requirements of an advanced energy performance standard would affect a large scale masonry housing development. The impacts and issues that the project was designed to assess included the following:
- Technical Impact
- Economic Impact
- Regulatory Issues
- Design Process Issues
- Site Project Management and Construction Process Issues
- In-use Performance
The results of the study found a significant discrepancy between the designed performance and the real performance of dwellings. The magnitude of the discrepancy is such that the BRE is in the process of rewriting the software used for SAP calculations to take account of factors that had previously been overlooked.
The following report will be looking at the Stamford brook study with regard to four main issues:
- The design stage - design improvements and recommendations prior to construction
- The differences between the design intentions and the as built performance
- Buildability and build quality
- How these findings impact on the construction industry
The Design Stage - Design Improvements and Recommendations Prior To Construction
The National Trust has always embraced the idea of sustainably designed and built homes. In 1998 the necessity to sell surplus land in order to obtain funds for the continuous upkeep of the Dunham Massey estate presented an opportunity for such a development The plan for this development was a significant step forward in terms of sustainable energy use, reduced CO2 emissions and minimisation of overall environmental impact. Furthermore, these objectives were to be achieved in such a way that the scheme was capable of being replicated in mainstream housing development.
In order to fulfil the objectives set out, the National Trust required the assistance of a professional research team. The Leeds metropolitan University were working on a similar project involving timber framed construction on a site in York named Nichol Court. It was decided that a similar type research project involving the proposed load bearing masonry construction at Stamford Brook would be an excellent comparison project. Discussions between the two bodies resulted in Stamford Brook adopting the dwelling energy standard used in Nichol Court.
The Trust's Vision and the research team's dwelling energy standard were written into an Environmental Performance Standard (EPS) in June 2001. This document was then used as the energy and environmental standard for Stamford Brook. The EPS had four main areas:
- Minimisation of Energy Use and Greenhouse Gas Emissions - This would be one of the key aspects of the report.
- Minimisation of Water Use -Houses included low flush toilets, aerated low pressure taps, A-Rated appliances, water metering and rainwater butts in gardens.
- Minimisation of Waste - Provisions were made for compost bins, recycling boxes and a recycling plan for all household waste.
- General Environmental Standards -This section specified the use of materials with a high recycled content, materials from local sources, materials with low embodied energy, low VOCs and zero ozone depletion potential. Waste on site is to be minimised during construction. House design is to optimise living space, an example being room-in-the roof houses. Durable materials and materials which can be reclaimed at the end of the building's life are to be used wherever possible.
In order to supersede the minimum requirements, other design features were adopted throughout the site. These included:
- Parging of the Walls - This was in an attempt to fill air holes and complete the surface of the wall. A successful parge coat is effective when referring to the air tightness of a building.
- 142mm Cavity - The wider the cavity, the greater the isolation between the two leafs. This has a positive impact on both the thermal and acoustic insulation performance of the wall. (This width was only used in terraces where staggering occurred. This was to ensure that the U-Value of the exposed wall where the two dwellings met remained constant. The standard width of 100mm was used where terraced houses were not staggered in order to keep the footprint of the building minimal.)
- Plastic Wall Ties - Stainless steel is a conductor and as such, can have a negative effect on the U-Value of a wall. As with heat, vibrating sound waves can be transmitted along the wall ties. In an attempt to combat these issues, plastic wall ties were used in the construction.
- Separate Lintels - One for the inner leaf and one for the outer leaf, this removes thermal bridging. The use of a sufficient cavity tray is still required.
The Difference between the Design Intention and the as Built Performance
In keeping with research protocol, a series of "co-heating" tests were conducted on two separate dwellings (One mid-terrace and one semi-detached) at Stamford Brook between December 2005 and February 2006.
The main finding of these experiments was that the measured whole house heat loss coefficients in both cases severely exceeded the predicted values. Results showed a 75% increase between predicted heat loss values and measured heat loss (adjusted for solar gain) values in the tested semi-detached dwelling house and a 103% heat loss increase using the same calculations in the tested mid-terrace dwelling.
Further analysis of the data, along with thermal imaging in the loft space and off the cuff measurement of cavity temperatures in the party wall indicated that a large section of the excess heat loss could have arisen due to a thermal bypass associated with bulk air movement up the party wall cavity.
A theoretical analysis confirmed that a thermal stack driven bypass in the party wall cavity could potentially give rise to significant heat loss with a magnitude equivalent to an effective single sided party wall U-value in the order of 0.6 W/m2K.
Further testing was carried out on the dwellings by removing the horizontal cavity sock and repeating the co-heating test. This was done by removing a block in the loft space and extracting the mineral wool cavity sock. When the results were compared it was found that a mineral wool-filled cavity sock positioned horizontally in the party wall cavity at the level of the ceiling insulation was partially successful in mitigating the effect of the thermal bypass and reduced the size of the effective U-value to between 0.1 to 0.2 W/m2K.
As a result of these tests it was it was discovered that there is potential for considerable carbon savings for both newly constructed and existing dwellings built with unfilled cavity masonry party walls if measures were implemented to reduce or eliminate the party wall thermal bypass.
Buildability and Build Quality
The investigation and research of this project indicates that the construction process should have been as simple as going through the motions. The performance requirements of the development as set out by the National Trust and the two developers meant that a substantial amount of time was spent by the design team when detailing materials and construction techniques in order to achieve these goals.
Cooperation from all members of the development meant that buildings of the highest standard could be achieved and any problems that arose could be ironed out and an amicable solution could be quickly found all the while keeping the performance goals at the forefront of everyone's minds.
Robust Standard Details and Specifications were provided to Subcontractors with the tender documents. The main items of documentation were the construction specification, materials schedule, trade specifications and Stamford Brook Working Details. These required the sub-contractors and suppliers to examine and cross reference the information to fully understand what was required to meet the standard.
Documents such as these that were meticulously designed and researched made the building of each housing unit easier as they eliminated the uncertainty. In autumn 2004 a project issue checklist was devised. This checklist was an opportunity for everyone involved on site to add any issues that may have arisen. This was then circulated by the project managers each week for remarks. The checklist was a very successful tool in the early months of construction and helped to communicate buildability issues and solutions to all staff involved. Later in the construction the checklist was then replaced by fortnightly site meetings attended by a core of site management members and subcontractors where relevent.
As with any building it is virtually impossible to complete everything inch perfect. Unfortunately some lulls in the buildings performance were as a result of bad workmanship and sloppy building quality control.
Most of the defects were highlighted during tests the selected buildings. During the co-heating tests it was observed that a substantial amount of mortar had collected on top of the cavity socks during construction. This has a dramatic negative value on the acoustic performance of the wall as the mortar acts as a bridge between the two leafs.
One area showed where a vertical cavity sock had been torn, perhaps by a falling tool or block during construction leaving a 200mm gap. This gap in the envelope would have allowed some air to bypass the sock in the first half of the co-heating test.
How these findings impact on the construction industry
The results from this experiment will change the face of the construction industry in the UK forever. Many would believe that the study undertaken at Stamford Brook was ahead of its time and that sustainable housing developments were a couple of years down the line. The efforts of the national trust, Redrow Homes (North West Ltd.) and Bryant Homes have achieved everything that they set out to do and in doing so they have paved the way for large scale sustainable developments and there is absolutely no reason the industry cannot follow.
The entire project from start to finish was well documented. Any construction issues that arose were dealt with in a professional manner by all key bodies involved. Solutions were analysed and put into action. These revised construction methods and materials were then implemented around the site and included in all construction managers' specification and detail handbooks.
Common faults that occur every day on building sites across the UK will no benefit for tried, tested and approved methods of construction as a result of this study. Furthermore, previous to this study the idea of sustainable and green development went hand in hand with added costs and inconvenience. The team behind this development have succeeded in dispelling this myth. Areas that seem to be costing more such as windows and doors are offset by other areas such as the loft insulation, where money is saved.
The discovery of the extreme heat loss in the partition wall cavity is a big step forward. This can now pave the way for renewable solutions to minimise the heat that can be lost through stacking air in partition cavities. The findings in the co-heating tests on the party cavity with and without the horizontal mineral wool cavity sock will also pay an important role in any future solutions.
The work of Leeds Metropolitan University was recognised in 2007 when the team were chosen as finalists in the Carbon Trust/Daily Telegraph Innovation Awards for their work on the party wall thermal bypass.
In conclusion, the development at Stamford Broke as both a large research project and a practical housing development was an inspiring idea by the national Trust. It was all made possible by the generous donation of land by Lord Stamford and the hard work of the research team at the Leeds Metropolitan University.
The findings of this report indicate how traditional house building on a large scale can convey high environmental standards, such as important reductions in water and energy consumption.
There is no renewable energy at Stamford Brook but through good quality design incorporating building orientation and making use of passive solar heating techniques this project has demonstrated that no matter how large the required volume of housing, with the proper mind set the development can still be energy and water efficient. This type of thinking and construction has a knock on affect on the occupants of each dwelling, saving homeowners money as well as reducing their carbon footprint.
If the Government's low carbon housing targets are to be achieved. It is imperative that we improve and constantly update the production process as a whole and continually ensure that what we design in theory is in fact what's being realised in practice.
Tests carried out in this study produced results that evolved some building techniques and took a positive step in the right direction. It is up to the house builder to ensure adequate testing is carried out on all future developments. Without these test results and many more like it, it will be impossible to move forward. The house builder must build to specification, carry out tests and submit any results back to the suitable building body. This is a necessity if we are to ensure that no stone is left unturned in the pursuit of a fully sustainable build.
Modern Methods of Construction and Innovation for Masonry & Concrete Construction
"The construction industry accounts for 5.2% of the GDP (gross domestic product) of the United Kingdom, 8% if related construction products and related components are included. Additionally the construction industry produces, maintains and adapts 60% of all fixed capital investment - the buildings, structures, and infrastructures upon which most economic activities depend. The UK Government therefore, has a vested interest in ensuring that the quality and efficiency of construction as it has a bearing on the long-term economic growth and industrial competitiveness in the UK and in export markets. Construction processes and the function, desirability, cost, sustainability and the utility of the finished products affect the quality of life of everyone living in the UK."
Modern Methods of construction are defined as "A range of products and techniques aimed at improving efficiency in constructions that includes off-site manufacturing of components, on-site fabrication and improved management methods, often imported from the commercial construction sector." (Sustainable Concrete - Methods of Construction -2007). The aim of Modern Methods of Constructions (MMC's) is to improve the quality of all new homes and meet consumers needs by speeding up their construction.
Most modern concrete systems fall under this definition. Ready-mixed concrete isnow batched off-site and delivered in exact quantities to fill ready-made formwork with minimal waste. Other methods include traditional building blocks, packaged and crane-delivered from modern factories direct to the floor requiredand then installed from new forms of ultra safe scaffolding.
In this report I will be looking at three MMC's, paying close attention to how they can improve health and safety on the site and the quality / efficiency of construction. They are:
- Thin Joint Masonry using Aerated Concrete Blocks
- Insulated Concrete Formwork
- Tilt-Up Concrete Construction
Thin Joint Masonry using Aerated Concrete Blocks
Thin joint masonry is a high-speed, clean, precise system for construction combining aerated concrete blocks of precise dimensional tolerance with 2-3mm mortar joints. The mortar used is a special pre-mixed concrete based mortar that only requires water to prepare it for use. The nature of the mortar and the materials means it sets very quickly, giving early stability to the build, thus making it ideal for this type of construction. The width of the bed joint means that the aerated concrete blocks must be meticulously analysed during the manufacture process and a high degree of dimensional accuracy must be observed in order to ensure they are suitable for thin joint mortar construction.
The picture to the right shows the construction process. The mortar is placed into a serrated scoop that is dragged along the top of the previous course of aerated concrete blocks. This ensures a consistent mortar bed thickness that the next course of blocks can be laid on.
The slit in the serrated scoop is designed so that as the mortar passes through, teeth leave small even channels. These channels provide adequate grounding for the blocs to catch onto. As mentioned previously, it is important to lay the blocks as soon as possible once the mortar has been applied as it will stiffen quickly and lose strength.
Benefits of Thin Joint Construction
Thin join masonry construction boasts many advantages over traditional aggregate blocks and general use sand / cement mortar.
Independent studies carried out by a leading building surveyor have shown that a wall of aerated blocks and thinlayer mortar can be laid twice as fast as one built with aggregate blocks and general use mortar. This is as a result of the time it takes the thin joint mortar to go "off". Thin joint mortar and aerated concrete blocks will be stable after 60mins, meaning that more courses can be built in one day. Traditional aggregate blocks are limited in the number of courses as it takes longer for general use mortar joints to obtain sufficient stability and the additional weight leaves the bedding joints vulnerable to be pushed out.
Thin joint construction also possesses an impressive thermal and acoustic performance improvement over traditional methods. Heat and sound can travel through the mortar joints and as the joints are smaller by 60 - 70%, there is greater air tightness, resulting in a noticeable difference in the performance of the wall.
The aforementioned construction benefits of thin join construction also have a positive knock on affect with regard to health and safety. The majority of these "pro's" are associated with physical exertion. Aerated concrete blocks are much lighter than traditional aggregate blocks. Workmen can easily move them from the drop point on the site to their required installation location easier. For taller buildings, a load of blocks can be delivered and dropped onto the scaffolding at the require height, as they are lighter they will not have as much of an influence on the scaffolding as heavier, traditional aggregate blocks.
Insulated Concrete Formwork (ICF)
Insulating concrete forms (ICFs) are hollow 'blocks' or 'panels' manufactured from expanded polystyrene insulation (EPS) or other insulating foam materials. They are assembled or stacked like Lego bricks to form the profile of the walls of a building. Once in place, steel reinforcement is installed into the cavity to increase stability and concrete is pumped in to form the structural element of the walls. The forms stay in place as a permanent part of the wall assembly. Essentially, the structure is a sandwich consisting of a heavy, high-strength reinforced concrete between two layers of a light, highly insulated material (the EPS or foam). This combination creates a wall with numerous attractive properties such as air tightness, strength, sound attenuation, insulation and mass.
Benefits of Insulated Concrete Formwork
Speed of construction is one of the main selling points when a developer or house builder is looking at this system. Furthermore, installation is simple and there is no need for high paid skilled tradesmen such as block or brick layers. This cuts down on the costs of the project. Heavy plant equipment, normally associate with site work is cut to a minimum due to the lightweight of the forms and the speed of the modular build.
This method of construction has impressive thermal and acoustic performance. As the concrete is poured into the cavity, there will be no air gaps and the building will be completely airtight. This mass improves the U-Value and acoustic insulation value of the structure keeping it both warm in the winter and cool in the summer.
Health and safety around the site is increased with this system. This is achieved by the fact that the ICF's are lighter and easier to handle than traditional blocks, thus resulting in a reduction in injuries. Furthermore, the site is easily cleaned which minimises trips and falls and as the need for plant and other heavy equipment is minimal the chances of serious injury is greatly reduced.
Tilt-Up Concrete Construction
This method does exactly what it says on the tin. The process is uncomplicated, quick and produces very little waste. It begins with the slab being set out and poured. Footings are installed around the slab that will support the tilted up concrete panels. The timber panel forms are then assembled on the slab. These timber forms act as a mould to pour the concrete panels. The advantage of this is that all window and door opes can be set in exact position. Next the steel reinforcement is tied into the timber form moulds along with inserts and embeds for lifting the panels and attaching them to the footing, the roof system, and to each other. It is important to ensure the slab beneath the concrete forms is clean and free of debris prior to the concrete being poured as this will affect the face of the panels.
Once the concrete panels have set and the timber forms / moulds have been removed, it is connected to a crane with cables that are attached / hooked to the inserts. This is where this method gets its name from. As the crane rises the concrete panel is tilted into a vertical position above the footings in the slab. The panel is then guided into place and the crane lowers it in place. They connect the braces from the tilt-up panel to the slab, attach the panel's embeds to the footing, and disconnect the cables from the crane. The crew then moves to the next panel and repeats this process.
When all panels are in place, the installation crew will apply finishes to the walls with sandblasting or painting. They will also caulk joints and patch any imperfections in the walls prior to moving onto the next stage of the building.
The picture to the right shows how the tilt up process is completed on site. The site operative crew are standing back a safe distance as the crane tilts the completed panel upright for fixing and installation.
Benefits of Tilt-Up
There are a number of benefits associated with this method over traditional building methods. Tilt-Up concrete construction is faster than building walls using traditional construction techniques. The trades can begin work earlier in the process on a tilt-up project, which allows greater overlapping of project phases.
This method can also have a beneficial impact on the cost of the project. As the work is done on site, raw materials can be sourced locally, cutting down on specialised material transport over large distances. This has a positive effect on CO2 emissions and can promote local business and trade. Labour costs are lower as tilt-up work crews are typically smaller than the crews associated with traditional construction methods.
The finished product ticks all the desired boxes when it comes to performance. Concrete is a very durable building material. It will also meet fire resistance standards and provide excellent acoustic and thermal resistance. Another design benefit that sets it apart from other methods such as pre-cast concrete prepared off-site is its flexibility. Each panel is custom-designed for the client's needs and preferences. A full range of building finishes, wall textures, adornments, colours, even curved walls are available with this method. Tilt-up construction provides architects and designers with practically boundless flexibility in designing a building that is functional, durable and aesthetically pleasing.
With reference to health and safety, tilt-up concrete is a proven, safe method of construction. The vast majority of the project takes place on the ground rather than on scaffolding, reducing many of these risks normally faced by site workers. All heavy lifting is done by cranes reducing the risk improper lifting injuries. Due to the weight of the various panels, it is important that suitably qualified professional install the hooking inserts as any incidents revolving around a falling panel will certainly result in death.
- Centre for the Built Environment. Leeds Metropolitan University, Buildings and Sustainability. The Stamford Broke Project. Available from: http://www.leedsmet.ac.uk/as/cebe/projects/stamford/index.htm [Accessed xx December 2009]
- Sustainable Concrete. Sustainable Design and Construction, Methods of Construction. Available from: http://www.sustainableconcrete.org.uk/main.asp?page=118 [Accessed xx December 2009]
- Sustainable Concrete. Sustainable Design and Construction, Methods of Construction, Insulated Concrete Formwork. Available from: http://www.sustainableconcrete.org.uk/main.asp?page=120 [Accessed xx December 2009]
- Sustainable Concrete. Sustainable Design and Construction, Methods of Construction, Tilt Up. Available from: http://www.sustainableconcrete.org.uk/main.asp?page=121 [Accessed xx December 2009]
- Sustainable Concrete. Sustainable Design and Construction, Methods of Construction, Thin Joint Masonry. Available from: http://www.sustainableconcrete.org.uk/main.asp?page=124 [Accessed xx December 2009]
- Tilt-Up.com. Tilt-Up Concrete Construction Articles, What is Tilt-Up Construction?, How are Tilt-Up Concrete Buildings Constructed?. Available from: http://www.tiltup.com/commercial-construction-articles/concrete-panel-building/ [Accessed xx December 2009]
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