Enabling Retrofit

A case study by Ben Walsham, ARKATA, in collaboration with Tom Petty, builder at Petty Projects, and Co-Founder of CarbonTrace.

The Project

I met Tom Petty through an Australian Institute of Architects working group called “AU 2050 Carbon Cooperative”.

Tom engaged ARKATA to become the NatHERS partner for a Victorian alteration and addition project. The northern aspect of the home was proposed to be rebuilt, while the southern part was proposed to be retrofitted.

The home itself had several impressive features at the outset. It was proposed to salvage and re-use existing materials from the northern demolition, where bricks would be re-used for the proposed reverse brick veneer walls. Lightweight foundations were proposed with minimal reliance on new concrete pours, where the existing piles would be re-used for the new suspended timber floor.

This was a project where carbon consciousness and thermal performance were goals from the start, not bolted on at the end.

ARKATA’s Role

Our role was to develop the energy efficiency strategy, where optimisation of thermal performance happened alongside understanding cost and carbon implications. We were to provide the data that enables Tom to make informed decisions about where performance and carbon intersect.

The Baseline

The project is a typically old Victorian home that is draughty, cold, and vulnerable to consistently high energy usage through all periods of the year.

The home started at an existing, pre-renovated Star Rating of 2.5 Stars, with heating loads at 207 MJ per m2 per year.

This house was leaky, sitting at a considerable NatHERS-calculated air leakage rate of 80+ air changes per hour at 50 Pascals.

The proposed scope of renovation works moved this initial Star Rating from 2.5 Stars to 6.9 Stars.

    
      
          
          Star Rating
          Heating Load (MJ/m².yr)
          Cooling Load (MJ/m².yr)
          Total Load (MJ/m².yr)
          Reduction
        

      
          Existing
>80 ACH@50pa
          2.5
          207.6
          29.7
          237.3
          —
        

          Proposed
>10 ACH@50pa
          6.9
          49.8
          13.8
          63.6
          -73.2%
        

    
  

The Optimisation Map

    
      
          Potential Upgrades
          Star Rating
          Total Load (MJ/m².yr)
          Load Reduction
          Carbon Implications
        

      
          R2.0 insulation to existing suspended timber floors
          6.9 → 7.4
          63.6 → 52.6
          -17.3%
          Minimal — into existing structure
        

          Replace existing windows with high-performance uPVC (Avg. U-Value 1.9)
          6.9 → 7.4
          63.6 → 54.5
          -14.3%
          High — new manufacturing, disposal
        

          Increase ceiling insulation to R6.0
          6.9 → 6.9
          63.6 → 62.6
          -1.6%
          Low — additive to existing
        

          Reduce existing window to 1800mm wide
          6.9 → 7.1
          63.6 → 60.8
          -4.4%
          High — new manufacturing + infill
        

          Target airtightness of 5.0 ACH@50pa
          6.9 → 7.3
          63.6 → 55.6
          -12.6%
          Low to Medium — some new infill to air seal
        

    
  

The Carbon Lens

Tom didn’t implement everything that was possible, because when we begin to think about carbon consciously, the upgrades we take on become obvious.

Underfloor insulation alone moves the home from 6.9 Stars to 7.4 Stars with minimal impact to the home’s overall carbon footprint.

Further to this simple intervention, Tom was keen to air seal and minimise air leakage, which as shown in the table above, provides a considerable thermal improvement with low carbon impact.

The Outcome

This process gave Tom a clear map: where the home could go thermally, and what each move costs in carbon terms. When those two dimensions are overlaid, a sweet spot emerges: high-performance, low carbon impact, informed by data.

The end results:

      Heating loads
      Cooling loads
      Star rating
    
MJ/m² per annum · Retrofit progression from pre-renovation to optimised with airtightness

Tom’s Words

Words to be inputted.

The Site’s Evolution

Image Source: “Why don’t you just push it over?” — Tom Petty (LinkedIn)