nakedgreenbuilders.com - Design critique

Design critique

Buildings are responsible for one third of the UK's carbon dioxide emissions, through their construction and energy in use. Lack of thought with inappropriate materials and construction processes can make them hazardous to the occupant as well as the broader environment. Buildings should be efficient to build and operate, enjoyable places to live and work. We can advise your project from the start with a design critique.

Tuesday, 5^th July, 2005

Re; Self build, central terrace

This development has the potential to be a showcase for environmental building: to show how it should really be done. Unfortunately, we feel that some of the environmental potential will be sacrificed through poor design and the involvement of commercial interests. The design falls short of making full use of building orientation, day lighting, thermal mass and internal space. Would these houses really be affordable in the long term, or are they just cheap, at the expense of the environment?

Building orientation

Scant attention has been paid to solar orientation. The decision to homogenise the external envelope of each building, presumably for ease of timber frame construction, is inappropriate to the solar aspect of each individual dwelling and has resulted in poor design details such as inappropriate aperture location and size. Positioned correctly, windows, doors and conservatories can function to environmental advantage: capturing solar gains, reducing heating load, creating air locks to reduce heat loss, providing cross-ventilation for cooling, and maximising light penetration and uniformity, thus reducing bulb-dependency. However, wrongly positioned they are environmentally negative, and will result in additional heating load.

To make the most of a conservatory or porch at the rear of the houses, there would need to be rear access to the gardens. This would have other benefits, for example when entering with a bicycle and in terms of meeting the neighbours.

Internal day lighting

The terraced houses are all deep plan. Any building deeper than 6m needs to be artificially lit, so these houses need to have some way of allowing daylight to penetrate into the middle of them to reduce the energy in use. The simplest way to do this would be to put in cloister windows. Cloister windows needn’t be parallel to the front of the house. For this terrace they could be positioned to capture solar gain and increase available sky component: perpendicular, over the party walls.

Thermal mass

Lack of inclusion of thermal mass in the walls would be a major oversight, since lightweight buildings have no way of capturing and taking advantage of diurnal temperature fluctuations. The best place for the building to absorb heat is from the top down as the heat stratifies. Thermal mass on the ground is ineffective as it will never be in contact with the warmer air. Thermal mass on the floor only has value when used in conjunction with an underfloor heating system. Underfloor heating reduces the amount of energy required for heating because the thermal comfort provided by having warm feet means that ambient air temperatures can be lower.

Masonry, rammed earth, limecrete or concrete could all be used as thermal mass, but as the embodied energy of both limecrete and concrete is high, rammed earth or at least reclaimed masonry would be better. A masonry chimney or pillar would be an easy way of introducing a small amount of thermal mass to a timber frame construction and could provide a vital structural element. This could be used in conjunction with a Phase Change Material as a surface finish to a lightweight party wall. PCMs can increase the thermal capacity of a lightweight building. However, as this is an ‘affordable’ terrace, and acoustic insulation is important, rammed earth partition walls would be our preference.

Maximising internal space

As these buildings are small, it is vital that the best use of space is made. Although these buildings were originally intended as single-occupancy, it would be more energy efficient to increase the occupation density. Any increase in occupancy levels reduces the energy requirement of each occupant, with five people each using only 40% of the energy of a single occupant. Clearly these houses are not big enough for five people, but there might be room for two! Even if it is not possible to raise the roof line or build down to the concrete below, to accommodate a storey and a half, a loft storage or bed area could be included. Currently, the design of the terrace allows very little storage space despite the proposed flexibility of internal layouts. Various roofing options were discussed at the design meeting. A curved roof gives maximum headroom over the largest area and makes the best use of internal space whilst being the least intrusive externally.

Affordability

The original aim for the terrace was to create affordable social housing, intended to be single person units. Affordable housing is not just about the cost of construction, but the cost to the occupants throughout the building's life cycle. It is essential to get the design of the building envelope right, as building design will have an impact on the energy in use of the building. Decisions made now will affect something which should outlive any of the services or people in the buildings. Service decisions, should be left for the occupants to have a say in, for example whether to site expensive photovoltaic cells on the roof.

The design of this development should be led by a desire to do the ‘best’, rather than what seems to be the ‘easiest’. Some of the techniques discussed, such as rammed earth, or the detailing of EDPM green roofing membrane, might be off-putting because they are new, but they can be learnt easily. This development will receive a lot of publicity, and has great potential, but if the houses do not meet the environmental potential that they should achieve, then they will be expensive to live in. To do this, more attention should be paid to providing a sound footing from which new residents can build, than to specifying services that will be obsolete long before the fabric of the building.

Simon Lewis

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