Kelvin House is a heritage building in the heart of Wellington. It was originally constructed in 1928, and later converted into an apartment building in 1998, with the top two storeys added in 2001. Recently, Inside Design undertook a project to reclad those two upper storeys to address weathertightness issues and introduce exterior insulation. Sto’s new StoVentec R Ventilated Rainscreen Façade System was specified to meet the NZBC requirements while maintaining the classic plaster finish. 

“Initially we were called in to look at the condition of the upper two floors to get a feeling for what the existing direct fixed cladding was like, and whether there were any issues,” explains Seb Bernhardt, Director, Inside Design. “Our building surveyor found that there were a number of weathertightness issues during their survey of the existing cladding and let the body corp know that they needed to look at recladding those two levels.”  

Dealing with a heritage building meant that the new cladding would need to replicate the original plaster finish while meeting the performance requirements of a modern commercial build. When researching potential systems, Seb came across the StoVentec R ventilated rainscreen façade system and got in touch with Sto to learn more. 

“When we approached Sto, we had been on a search for a cladding that could handle the specific engineering design wind pressures at the top of the building,” explains Seb. “It needed to be suitable to be installed in a commercial tower build, but also needed to be a monolithic plaster cladding that would look similar to what was there beforehand. Sto Ventec R fit the bill.”

A smart option for refurbishment projects with uneven substrates and mixed materials

StoVentec R is a ventilated rainscreen system featuring an adjustable aluminium bracket and rail sub-construction, and lightweight StoVentec Carrier Boards — which at 6kg m² are more than 50% lighter than other carrier boards. The system is topped with the StoArmat Render System to provide a seamless plaster finish.

The StoVentec R system is especially beneficial for refurbishments where the existing walls and building elements may be irregular or unsuitable for the direct application of render or standard cavity-based cladding, as the rails can be aligned independent of the underlying wall’s condition, eliminating the need for costly and time-consuming levelling work.

The subframe design of the system means the outer cladding is not rigidly fixed to the structural wall and can move independently from the main structure, allowing the building to deflect under seismic activity, vibration and general building movement without cracking or stressing the outer surface. This is an advantage in refurbishment projects and those with mixed materials as the system can adapt to different expansion behaviours in the wall structure. 

Incorporating exterior insulation with a ventilated cavity

While working on initial details for the job, the design team ran into an issue with the internal insulation of the building. The façade engineer from Oculus suggested adding external insulation. As StoVentec R is designed with external insulation as an option, this was a straightforward solution — insulation boards can be fixed over the rigid underlay and moisture barrier, providing the benefit of continuous exterior insulation and reducing thermal bridges while maintaining a ventilated cavity behind the StoVentec Carrier Board.

“We were working with a large 120mm - 150mm cavity. With the help of Oculus we revised details to put thermal insulation on the outside of the building,” says Seb. “So we then had an externally insulated cavity system with adjustable rails to handle the cladding alignment, a heritage friendly monolithic façade on the outside, and we were able to vastly improve the thermal performance of the building.”  

Design and installation backed by expert technical support

As the Inside Design team hadn’t used the StoVentec R system before, and it was relatively new to New Zealand, the support offered by Sto became key to ensuring a smooth project. This included an initial review of concept details, supplying compliance statements and technical information, and assisting with a peer review of the final site-specific building consent construction details, alongside the façade engineer. 

The Sto team also created a 1:1 scale model of the façade in their warehouse, so they could go over any complex details with installer Dion from STS Contracting, before installation on site. “That 1:1 scale model based on site details was really invaluable in drilling down on those finer points,” says Seb. 

“While the system had been used in Europe for a number of years, it hadn’t been fire and BRANZ tested yet,” says Seb. “Sto followed up with all technical and practical assistance and really supported Dion too. I think that’s what we look for with suppliers, we really want suppliers to back their product from spec to project.” 

The Sto team have completed a number of BRANZ tests on the system. It meets B1 structure requirements, is tested for wind loads of +/- 4.5 kPa ULS and seismic deflection of +/- 56 mm. It has also been tested to C3 spread of fire (BS 8414-2) and E2 external moisture (E2/VM2) which will make future specification for large multi-storey projects such as Kelvin House even more straightforward. 

A seamless render finish for a heritage building

Preserving the aesthetic of the original façade was crucial for this project — something that was achieved thanks to the seamless StoArmat Render. 

“The Sto Ventec R system was great to work with in terms of maintaining and enhancing some of the original architectural features like recessed windows reveals,” says Seb. 

“They have some standardised solutions using extruded profiles that work well for that, so we were able to get those deep architectural reveals to windows and enhance the building,” he explains. “The extra depth of the system, rather than a drawback, became an asset as we were able to level out the façade and resolve structural issues within the existing framing.” 

The end result is a successful reclad of Kelvin House that has resolved all weathertightness issues, improved thermal performance and rejuvenated the façade — all while preserving the original aesthetic. 

“The final finish of the plaster is lovely, all the corners and junctions are really well done and it stands up to inspection at close quarters,” says Seb. “The building now has a 21st century cladding system — with a cavity system, rigid air barrier, peel and stick membrane and external insulation, but it looks like the original extension.” 

Kelvin House, Wellington
Architects: Inside Design
Facade Engineers: Oculus
Main Contractor: Cemac
Sto Contractor: STS Contracting

Sto System: 

StoVentec R Facade System