Resilient Superstructures for High Performance Homes

A natural progression from my article on resilient foundation types is to talk about resilient superstructures. I specifically wish to focus on walls and panel construction types.

As an engineer designing high performance homes, the priority is designing for resilience.

1. Background

In my article, https://www.dhirubhai.net/pulse/designing-resilience-damien-mcgill/, I outlined the functional and performance requirements of the Building Code that result in AS/NZS1170.0 stating that;

SLS1-the structure and non-structural components do not require repair after the SLS1 earthquake, snow, or wind event:

Rigid adherence to this directive is the basis for designing all resilient superstructure and the cornerstone of all resilient design for high performance homes.

It all starts with the ground, then the foundations. 

My article https://www.dhirubhai.net/pulse/assessing-ground-conditions-design-resilient-damien-mcgill/, outlines the assessment required for geotechnical and structural engineers to collaborate in designing the appropriate foundation for the ground conditions on which it is expected to bear.

My article https://www.dhirubhai.net/pulse/resilient-foundations-types-high-performance-homes-damien-mcgill/ discusses insulated foundation options to cover a range of soil types.

2. Foundation to Frame connections

The next most important element is the connection between the foundation and the superstructure.

The use of the traditional 90mm frame is problematic if the requirement is to have insulation in line with the edge of the frame as this leaves insufficient room to install the hold down bolt. This connection cannot be proved by calculation as far as I’m aware. Only MAXRaft has a proprietary tested solution for this as detailed below;

To achieve this connection of course the insulation is required to be thinner at the top than at the bottom. However, it is the top that will be most exposed to the elements and therefore temperature fluctuations, so while this is a good detail, this is not the best practice detail. Also to achieve this connection construction must be accurate as there is minimal tolerance for error.

Below are four foundation options from Kainga Ora’s Homestar 6 MultiProof set. Clearly, Option 2 is the preferred high performance option for a 90mm frame with light weight cladding when considering maximum insulation, connectivity space and reduced thermal bridging.

My preference would always be to use a 140mm bottom plate, preferably in a 140mm frame to provide security of connection and a stiffer frame, the least chance for thermal bridging and the benefit of additional insulation to the wall above.

The option of cast insitu anchors is still available, but they are time consuming to install and have therefore fallen out of favour for post fixed mechanical or chemical anchor systems. Note that I have seen expansion type anchors pull out as a result of the Canterbury Earthquake Sequence, so no longer consider these suitable for resilient high performance house construction.

3. Framing types

3.1 Traditional (NZS3064 light timber framing)

High performance housing framing is all about getting sufficient insulation into the walls to meet thermal performance requirements. There is an immediate limit with traditional 90mm framing.

More often 140mm framing is used. This gives 1.5 times the space for insulation and is a more rigid, therefore resilient frame as well.

3.1.1 Improved Traditional

A solid section of timber right through the wall cross-section still provides some cold bridging as timber is not as thermally efficient as insulation. The solution is to apply a 45mm batten at right angles to the timber frame and insulate the cavity created. Cold bridging is limited to only where the stud and the batten cross. This is also a good place to run services.

3.1.2 Advantages

The entire residential construction sector is familiar with light timber frame construction so with minor tweaks to site process all tradesmen can work with this system.

3.1.3 Disadvantages

There are limitations in terms of speed of erection and quality control. Nearly all efficiencies should have been achieved in a system that has been used in New Zealand since day dot.

3.2 Panel Systems

3.2.1 Panellised light timber framing

There are companies taking the pre-nail frame to the next level by adding a structural lining or rigid air barrier, weathertight membranes, insulation and internal airtight membranes. One company that I’m familiar with is www.Ecopanel.co.nz

Ecopanel prefabricate entire wall sections in their factory in Amberley, using dimensionally stable LVL framing. They have the ability to produce a range of framing options, which can come installed with exterior cavity battens, insulated service cavities and pre-clad or pre-glazed. Ecopanel’s point of difference is that they do not contain petrochemical based insulation.

We are currently working with Ecopanel on a number of projects.

3.2.2 Structurally Insulated Panels (SIPs)

SIPs panels are a composite panel comprising two rigid skins bonded to an insulating core. They are thermally efficient with timber studwork only required at openings, corners and for specific bracing elements.

For residential use the rigid skins are usually orientated strand board (OSB) or ply. The insulation core is either polystyrene or polyurethane foam, either PIR or PUR. This generally gives better insulation values than traditional insulation.

They are often used for ceilings too, creating an airtight thermally efficient building envelope.

3.2.2.1 SIPs manufacturers

There are three main manufacturers of SIPs panels in New Zealand that I am aware of; Formance, NZSIP and Kingspan. Other suppliers come and go and product is occasionally imported from offshore.

Formance

Formance is based in Christchurch and uses SIPs panels manufactured in China comprising of two layers of OSB board with a polystyrene core. https://www.formance.co.nz/

I have personally been involved in the design of high performance housing using Formance panels since 2015.

NZSIP

NZSIP manufactures their panels in Cromwell from OSB and PUR polyurethane foam. A visit to their factory gives a good insight into the manufacturing process and what is possible. They have a unique cam-lock system for joining panels.  https://www.nzsip.co.nz/

We have worked with NZSIP panels for a number of years and have some interesting houses on the go with them.

Kingspan

I have not worked with the Kingspan Knight Built Tek Building system but it appears to be constructed of OSB with a urethane insulation core. https://www.knightbuilt.co.nz/

Panel System Advantages

SIPs panels can be very stiff compared to light timber framing, making them more earthquake resilient when used in a properly designed wall system.

·        Improved insulation values

·        Improved airtightness

·        Quick to erect

·        Can be factory assembled as full wall elevations, with joinery added.

Panel Disadvantages

Structural elements need careful detailing and must be inspected before the panel is fully enclosed. Many times we have been to site to check a connection to find that it can no longer be seen as the panel has been fixed over or around it. This makes issuing a PS4 unnecessarily difficult.

My advice is plan your installation checking that all the connections have been allowed for rather than throwing it up and missing a bolt or a tie down.

Decisions need to be made whether there is a service cavity or services are installed inside the panels. It the decision is the latter then it needs to be right first time as alterations are difficult. Cutting of bracing panels for additional electrical boxes will be frowned upon during an engineer’s inspection!

3.2.3 Cross Laminated Timber (CLT)

CLT panels have been used for wall construction in New Zealand residential buildings for a while now. They are constructed of layers of timber glued and pressed together in a similar fashion to plywood. Being solid timber they use a lot of timber, but this makes them very stiff and good for resilient construction. They still require strapping and lining with insulation to give them an insulation value suitable for high performance homes. This strapping and lining can of course be on the outside of the panel, allowing the timber grain to be exposed internally.

Advantages

Made of renewable timber.

Solid, Stiff panels that can span large spaces.

Disadvantages

No longer made in New Zealand. Product imported from Australia or Austria.

Gaps between timber laminates can open up as the sides of the laminates are not glued together. Some products are better than others in this regard.

3.2.4 Others

There are other sandwich panels using steel or aluminium skins, commonly known as refrigeration panels, but these are generally not used in residential applications.

4.0 Structural Implications

4.1 Structural Members

For thermal efficiency timber structural members should be used where possible. In all cases structural steel members should be avoided or kept outside the thermal envelope when designing a high performance home to prevent cold bridging.

4.2 Bracing Design

The bracing design basis in NZS3604:2011 uses a ductility factor of 3.5. This means that the wall structure can be more flexible than people may realise. If care is not taken when using NZS3604 or the Gib bracing software, this will conflict with the cornerstone premise of designing a resilient superstructure; the structure and non-structural components do not require repair after the SLS1 earthquake, snow, or wind event.

Anyone who has been through the Canterbury or Kaikoura earthquake sequences will happily pay for a more rigid structure, if that means they don’t have to deal with EQC or their insurer for more than the most superficial of cosmetic repair!

More recent techniques of using a rigid air barrier or ply sheathing on the outside of the framing along with Gib internal linings will produce a more rigid building envelope than previous construction techniques, with a built in factor of safety for additional resilience. Even the old diagonal sarking is not as stiff as a correctly fixed sheet of ply.

However, if the rigid air barrier is to be used as the bracing element, consider how this will be able to be checked for damage following an earthquake. It is unlikely to be simple, by the time membranes, cavity battens and cladding are added. At least with Gib, damage, if any, is visible.

Consider that it is statistically possible to have at least two serviceability limit state wind, snow or earthquake events over the design life of the home.

The use of internal service cavities make this difficult as the Gib is fixed to the battens, which are not usually designed to be a structural member.

Sips panels are stiffer than timber frame and have proprietary bracing values provided through standard panel testing (P21 tests). The use of glues to bond them together and to the top and bottom plates make them less ductile than standard timber framing, so more easily able to withstand an SLS event assuming they are properly fixed to the foundation. This lack of ductility may cause issues in larger earthquakes as ductility can provide for some energy dissipation though nail slip etc. At the end of the day if the Alpine fault goes, there will be bigger issues to worry about!

Always make sure where ever possible that the structure is braced as evenly as possible across and along the dwelling and that material compatibilities are accounted for.

Seek professional engineering advice when designing a high performance home would be my recommendation.