Construction Based Performance Gain

Published: 04 April 2011 / Amendment 4 / BCA 2011

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CONSTRUCTION: BUILDING FABRIC PERFORMANCE GAIN

In simple terms, “Modelling software” calculates the heating and cooling loads required to condition space in the house, “design” determines the capacity of the house to “harvest” energy and “construction” determines the capacity of the house to “control” the inward and/or outward flow of energy. The house star rating is a measure the gap between deemed loads and house self-sufficiency (0-Star rating = nil, 10-Star rating = self sufficient).

In the context of this project, “construction ” decisions range from the choice of type (cavity masonry, reverse brick veneer, rammed earth etc) through to decisions about insulation, glazing performance (U and SHGC) and air infiltration (sealing), to matching the extent and placement of performance improving materials to maximise design type performance.

These are typically unseen building fabric changes that drive performance gain (or loss).

MODELLING PROCESS AND OUTCOMES

For each of the 25 houses used in the project, Baseline House (see Sidebar Note) data was generated for eight orientations in each of the four nominated Climate Zones

The worst-case orientation for each house was used to generate construction based performance improvement data. 

• If ceiling insulation was not improved, then it was likely that optimum performance would not be achieved from other materials being tested.
• If the thermal performance of a material is continually improved, then there will be a “law of diminishing returns” affect whereby building thermal performance gain reduces.
• If glazing is not improved, then there is likely to be a limit to the benefit gained from adding cavity wall insulation (a “holes in the bucket” affect).
• If glazing is improved, then the building performance gain will be affected by both the type of glazing used (U and/or SHGC management) and where the glazing is used (Living and/or Bedroom areas).

• Cavity masonry.
• Reverse brick veneer.
• Rammed earth.

• Ceiling insulation increased from R3 (DTS 5-Star compliance) to R4.
• R4 ceiling insulation + R1.35 added insulation to perimeter cavity walls.
• R5 ceiling insulation + R1.35 added insulation to perimeter cavity walls.
• R4 ceiling insulation + R1.35 added insulation to perimeter cavity walls + reflective foil (0.9 outer/0.05 inner) to the underside of the roof cover.
• R4 ceiling insulation + improved glazing (U=4.79, SHGC=0.60) to living area external glazing.
• R4 ceiling insulation + improved glazing (U=4.79, SHGC=0.60) to all external glazing.
• R4 ceiling insulation + 4 x 1200mm ceiling sweep fans (2 fans to the Kitchen, Family, Meals area and 1 fan to each of the two Bedrooms with the highest cooling loads).

• Climate Zone – weather data varies between Climate Zones and this affects the benefit gained from a specific material. Project outcomes show per Climate Zone differences.
• Orientation – insulation seems to be orientation neutral (consistent outcomes across orientations), whereas glazing is highly orientation sensitive (outcomes vary with house orientation). Project outcomes are base on worst-case orientation performance gain. For orientation sensitive materials, outcomes could vary from those shown.
• Material combination – there seems to be a “law of diminishing returns” affect where higher levels of material performance can result in diminishing building thermal performance improvement and/or a “holes in the bucket” effect whereby improving the performance of the one element (the wall) achieves little or lower gain compared to that achieved when used in combination with material improvement (improved glazing, roof insulation etc).
• House statistics – while no consistent trend correlation was found between house statistics and thermal performance (see Design Based Performance Improvement), it is likely that material performance outcomes, for the same house, will vary as the “percentage of glazing to floor area” increases.

Project data is reported as follows:

• Figure 1 – material performance summary (average performance and weighted cost factor outcomes that can be compared across Climate Zones).
• Figure 2(a-d) – Climate Zone specific material performance data. 

Three material performance measures are used:

• $/MJ – measures the cost effectiveness (cost per MJ of performance gain) for a material in the worst-case orientation in a specific Climate Zone. This data can only be used to compare materials in the same Climate Zone (see Sidebar Note).
• Weighted Cost Factor – corrected for Climate Zone MJ variance, this provides a measure of cost efficiency for a material in the worst-case orientation across Climate Zones. This data compares the relative cost efficiency (low = better) of materials.
• %MJ Gain over Baseline – measures the MJ gained as a percentage of the Baseline House worst-case orientation Total MJ outcome. This data can be compared across Climate Zones.

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