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19 April 2017

Thermal Performance Aspects of Low-Rise Apartment Design

Ensuring that medium-density low-rise apartment complexes comply with NZBC-H1 (Energy Efficiency) is more involved than for stand-alone dwellings.

Low-rise apartment buildings are becoming popular as society and urban planners accept that the residential zones of our inner cities need to move towards having housing at higher densities. This involves new forms of residential construction, both processes and materials, but also brings opportunities for architects and designers to be innovative with the inter-relationships of each dwelling to those around it. Because of this connectiveness and closeness, the thermal performance of each residence is quite different and more varied than for the standalone house in a traditional suburb.

Complex Thermal Performance

Instead of having elevations exposed to all points of the compass, for medium-density developments there may only be one orientation for the exterior wall of an apartment, and this could be to the south. Also, not all units will have a roof — there may be a neighbour above the ceiling, and also below the floor, and some of the boundary walls will be party walls with a neighbour or a passageway on the other side instead of outdoor air. This leads to an objective assessment of the thermal performance being much more difficult to calculate than for a single building, and more complex than the rule-of-thumb methods often used to give a rough guide for individual homes.

For low-rise medium-density developments there is usually a common structural system and material for the primary frames and floors, with maybe timber framings for intermediate floors within an apartment. The party walls may be part of the structure, but if not, they are usually of a standard construction throughout the project. When it comes to the exterior facade, it is usual to have a small range of cladding materials and a standard form of window unit. These are then arranged in accordance with the designer's aesthetic requirements, but it is this portion of the building fabric which has the greatest effect on the overall thermal performance.

For most aspects of the design and construction of the whole apartment complex, the standardising of components is not a problem except when thermal performance needs to be considered. While there could be a single floor plan with one standard arrangement for the services, (be it single or multi-storied), throughout a project the passive thermal performance for each apartment will not be the same. The end units will have a greater exterior wall surface than the inner ones, as well as differing orientations to the sun. The inner units will have neighbours on each side and therefore fewer heat losses and gains. If the standard dwelling modules are stacked vertically, then there will be some with a roof while the others will have a neighbour above; and equally with the floors where the lowest will be on the ground, or often above a car-parking area usually open to the outside air. Each of these typologies will have different passive thermal performances despite the common plan.

Recently I undertook a New Zealand Building Code - Clause H1 (Energy Efficiency) analysis of a four-storey plus car-park apartment complex. There were twenty-three individual dwellings within a common concrete structure, but only two plan types — each mirrored. One plan was two-storied, and the other single. While there were only two floor plans, there were fourteen typologies when thermal performance was considered. This was due to different orientations; shading from lift/stair-towers, return walls, and differing balconies depths; some had a roof and the others only ceilings; and, floors were either over neighbours or the open carpark.

Another project of several multi-storey blocks, had a central corridor so that the apartments generally faced either east or west. This meant that the east units had good solar gains during the morning, with less heat loss in the warmer afternoons when the sun had moved away. The west units had no solar gain in the morning when the outside air was cool — or cold in winter — and on a summer's afternoon the solar gain added to the already warm air so that the units overheated. The blocks, and surrounding buildings, shaded each other further complicating the assessments.

Key Influence on Thermal Performance

Generally, within any one apartment the interior wall, floor and ceiling components can be manipulated without too much difficulty so as to adjust the thermal performance, but this has only minimal effect. It is the exterior wall, floor, and roof elements which have the most influence on thermal performance. Adjustments cannot be determined by simple rules or assumptions. If a conservative non-calculated approach is used, e.g. double-glazing or tinted glass, then the results often will not be as expected and unnecessary costs are incurred. Double glazing often causes over-heating in apartments unless the area of opening ventilation is balanced. Tinted glass will decrease solar gain, but also provide no additional insulation on cold days/nights to balance the lowered passive heat gains. A small change in window area can have a disproportionate effect on thermal performance.

While not part of the thermal envelope, fixed or movable shading to portions of the facade can be of greater importance for thermal smoothing of apartments than for standalone houses, and also can generate elevational design variations.

Paying specific attention to maximising passive thermal design of the units within a residential complex can give substantial finance and comfort gains. My other blogs in this series give information on the various aspects of specific thermal performances which may inform design and construction decisions, especially where the project has repeating modules.

Keith Huntington, (of Ecorate Ltd – Architect), has been using approved thermal simulation software to provide NZBC-H1 compliance assessments for Building Consents since 2010. His Clients, and their architects, are increasingly using the objective analysis to better inform their designs so as to maximise the passive thermal performance before decisions are made as to which heating/cooling units may be suitable, or even necessary.

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