The retro-“climate”-fitting of the existing building stock is a massive challenge. The operation of our existing buildings is a major contributor to carbon emissions, and practically every building needs to be upgraded to meet climate goals. The question is: how?

Some believe the problem can be solved by simply banning gas and swapping all gas burning equipment for heat pumps. The problem here is that if we were to simply electrify our existing buildings as they operate today, replacing all gas with electricity, we would need a grid 10 times larger in terms of peak capacity! 

Even if we believed the grid will magically decarbonize by 2050 despite such huge increases in demand (and that’s before electric cars), there are no silver bullet energy sources on the horizon that could make that possible. Not to mention the huge cost, lack of materials and labor.  

Therefore, demand reduction is imperative, before everything else.

That’s where Phius REVIVE comes in: Retrofitting buildings to the right level of efficiency is crucial. The success of tackling climate change through the existing building stock begins and ends there. 

In 2012, Phius switched gears to realign passive house with the US Department of Energy (DOE) and National Lab’s Best Practices. Our goal was to design a building energy efficiency standard that met global carbon reductions to help solve our climate crisis, while also providing a cost-effective path to zero. The Holy Grail: be able to identify synergies in the building design process that would result in an economic gain overall compared to business as usual. 

For new construction, that approach is exactly right. Anything we propose on a large scale has to be economically feasible. That is even more true for retrofitting our existing buildings. The buck stops there – literally.

A little over 10 years later, the Phius new construction passive building standard is now on its way to becoming the leading solution for policymakers and municipalities to meet their climate action plans and to set themselves up for success for a 2050 zero-carbon economy. 

Massachusetts has created a Specialized Opt-in Stretch Code that has already been  adopted by 17 municipalities, including the city of Boston. Beginning in 2024, all multifamily buildings larger than 12,000 sq ft in those municipalities will need to meet passive building standards. A similar path is intended for commercial buildings as well. That development is big and will likely be replicated across the country.

That’s new construction…but what about retrofits?      

The economics for retrofits are more challenging. So is the upgrade process itself. People live in their buildings and upgrades need to be completed while minimizing disruption to the tenants. And, every existing building is different, making each project’s detailed solutions unique.

The Birth of Phius REVIVE 

To solve these not insubstantial challenges, we have created the Phius REVIVE certification program for retrofit projects. 

Our objective from the beginning has been multifaceted:

  1. Meeting climate goals
  2. Optimize for economics
  3. Create comfortable and resilient habitats that protect us not only from the elements but also from climate change events already in motion
  4. Secure grid stability 

Phius standards therefore apply to both, old and new, and the logic of the design goals is the same. This includes the same carbon emission reduction goals, the same comfort, resilience, and enhanced grid stability targets. 

The difference is that retrofits often have specific conditions that are already in place and cannot be remedied after the fact. That affects design decisions, sequencing and potentially phasing of construction and quality assurance measures.

For those conditions, there are rules in place to accommodate. Those differentiate the Phius (CORE and ZERO) REVIVE retrofit standard from the Phius CORE and ZERO standards for new construction.

Phius REVIVE is the solution for all retrofit situations, residential or commercial – from gut rehabs, retrofits with tenants in place to single-family homes.          

As I mentioned, cost-effectiveness is especially crucial for retrofitting: In 2015, Phius released the first ever climate-specific, cost-optimized passive building standard. In 2018, the standard was updated to include optimizations for typology and density. Today, we have proof of concept: the cost premium for large multifamily new construction Phius certified projects is down to between 1-3%. 

For retrofits, a slightly higher premium is to be expected to account for the existing conditions and increased effort required to calculate and mitigate existing thermal bridging. Overall, the same cost-optimization objective holds true.

Leading the Way for Retrofits

Phius has a long history of spearheading retrofit work. First, Phius retrofit projects were completed as early as 2009 and many have been completed until today. Leading practitioners presented their current Phius REVIVE projects during Day 1 of the Retrofit Summit hosted by Phius in the Spring of 2023, sharing valuable lessons learned during their projects. Projects ranged from single-family to commercial to neighborhood scale.

In 2017, Phius began working with Rocky Mountain Institute (RMI) under a DOE grant  to figure out how to apply the large-scale zero energy retrofit program Energiesprong, which had taken off in the Netherlands, to the US. The project was called REALIZE and it was the opener to a second, much larger project funded by DOE, the Advanced Building Construction Collaborative (ABCC). The ABCC is a powerful RMI-led group of leading industry experts, stakeholders, and national labs which Phius is part of.     

The ABCC was organized to solve a massive problem: retrofitting the entire building stock to meet climate goals within the next couple of decades. An unimaginably large challenge.     

The Phius REVIVE standard is one of the solutions that are being investigated as a promising optimal large-scale solution to retrofit single-family, small and medium size multifamily buildings in various climate zones and also some large multifamily projects as per a preliminary report.     

While the ABCC was evaluating Phius’ existing standards, a new concept for standard setting arose as well. We realized that if we wanted to figure out how to do 1,000’s of retrofits a day – as we must to meet our climate goals – then the added embodied carbon in the materials used would become really critical. 

There would be so much material needed to upgrade the entire building stock to highly efficient levels that the embodied carbon emitted during a very short timeframe (10-20 years) would become a critical problem. Therefore, as best as possible optimized solutions were needed, and that not only for cost-effectiveness and operational carbon but also embodied carbon. And, upon closer investigation, that optimum was different by climate zone.

Phius REVIVE 2024

From there, a new framework emerged in our minds. What if the governing design constraint could be set as a resilience target and not as an energy target per square foot? Would that allow us to optimize for less upfront investment and consequently less embodied carbon without losing the resilience and survivability benefits and low-load benefits to the grid? Could we thus optimize even more closely for the retrofit case?

Out of that thinking, Phius REVIVE 2024 was born. And as the date in the name implies, it is currently in development and testing. It is a challenging, completely new way of thinking about optimization. It means rethinking everything. 

The new pathway now optimizes based on resilience criteria – the hottest and coldest weeks – which will determine the level of envelope upgrades. In addition, a life cycle total carbon (operational + embodied) assessment balances embodied with operational carbon, which is the big challenge for massive scale retrofits.

For now, the Phius REVIVE 2024 program only applies to residential buildings up to three stories to make sure we got everything right, and is being tested on 10 volunteer projects. For a period, we will evaluate those projects that chose to go through the program and adjust as necessary. By the time the program is fully launched in 2024, hopefully the other building typologies have been researched sufficiently to release the program for them as well. 

Until then, qualifying retrofit projects will continue to certify under the existing Phius REVIVE program.

Phius REVIVE 2024, if it proves to be successful in optimizing carbon even further while putting the focus squarely onto resilience and survivability, will be a crucial path to meeting climate goals and help us transition the entire building stock to the new renewable energy economy.