WALLS AND PANELS
This section looks at the types of wall systems that are used to construct Healthy Homes.
Introduction
Recent research by BRANZ and Beacon Pathway show that the amount of timber framing in walls is severely affecting insulation values. BRANZ study report https://www.branz.co.nz/pubs/research-reports/er53/
MEASURING THE EXTENT OF THERMAL BRIDGING IN EXTERNAL TIMBER – FRAMED WALLS IN NEW ZEALAND (AUGUST 2020)
The findings of this research are based on a survey of 1103 wall panels (across 71 levels and 47 dwellings) and indicate the following key aspects:
● The average percentage of framing in walls was 34% (over the net wall area) varying from 24% to 57% (by level).
● Some individual wall panels have very high percentages of framing (50-100%); for example, smaller wall panels that are part of a larger overall wall can have higher percentages of framing - up as high as 70-100% per panel.
● There is little additional framing added on site. Around a quarter of all panels (291/1103) have added full depth framing, with the average for panels with added framing being just under 2% of net wall area, varying between 0.04% and 8% across the case study sample.
● The average additional site-added, full-depth framing timber by level is just 0.7% (range 0.1 – 4.0%).
● The average percentage by level of un-insulated areas (gaps or spaces in the wall cavity and including additional blocking installed typically for fixing linings, cladding, fixtures and services) was 3% with the lowest being 0.5% and the highest 10% across the sample.
This is particularly concerning for the owners of these homes. What they are getting is a lot less insulation than they realise, which will lead to requiring more energy and higher power bills to heat their houses than they should expect from a new home.
The solution is to either improve construction standards with the materials we are currently using or change the design to improve the current outcomes.
For this reason, the Healthy Home Design Guide recommends 140mm framing systems as the preferred solution for the design of Healthy Homes.
However, we recognise that 90mm framing is the most commonly used framing in New Zealand and we need to start our improvements there. This requires at a minimum following the full Base (Healthy Home) system recommended in this guide.
To be a resilient wall system it needs to be well connected to the foundation system.
Foundation to Frame connections
An important element to consider in any resilient structure 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. Only MAXRaft has a proprietary tested solution for this particular situation as detailed below:
To achieve this connection 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 satisfactory detail, it is not the best practice detail. Further, to achieve this connection, construction must be accurate as there is minimal tolerance for error.
An alternative detail for a 90mm frame with light weight cladding when considering maximum insulation, connectivity space and reduced thermal bridging is shown below. Note that there is still a thermal bridge where the edge insulation chamfers to take the flashing.
The recommended detail is to use a 140mm bottom plate, preferably in a 140mm framing system 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.
Cast insitu anchors is an option, but they are time consuming to install and have therefore fallen out of favour for post fixed mechanical or chemical anchor systems. Note that expansion type anchors have been shown to pull out during the Canterbury Earthquake Sequence, so these are not suitable for resilient healthy home construction.
Framing Solutions
Traditional (NZS3064 light timber framing)
High performance housing framing is all about getting sufficient insulation into the walls to meet thermal performance requirements, while creating a frame that is strong enough to support the roof. There is an immediate limit with traditional 90mm framing. As stated above, there is much more timber used in a traditional timber frame than many designers realise. This significantly reduces the actual insulation values the walls are providing.
The use of 140mm framing gives 1.5 times the space for insulation and is a more rigid, therefore resilient frame. The extra depth of insulation can compensate for some of the thermal bridging that occurs.
Enhanced Timber Framing
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. One solution is to apply a 45mm batten at right angles to the inside of the timber frame and insulate the cavity created. Cold bridging is then limited to only where the stud and the batten cross. This is also the place to run services.
The Misuse of Dwangs or Nogs
Super design practice is to minimise the use of dwangs / nogs. See https://www.buildmagazine.org.nz/index.php/articles/show/dwangs-moving-with-the-times
NZS3604:2011 states
8.5.4 Lateral support of studs
All studs shall be laterally supported by EITHER:
(a) Exterior wall claddings complying with E2/AS1 or interior linings complying with section 12, Such material shall be fixed to the studs by direct nailing of cladding or lining material, provided that building paper or similar not exceeding 3mm thick may separate the lining or cladding from the stud; OR
(b) Dwangs, walings or metal angle walings in accordance with 8.8
The EITHER/OR is very important, its not an AND!
Traditional framing and the majority of the industry still use the standard triple stud and block corners which often doesn’t get insulation installed prior to wrap and is badly installed after. We recommend using the Stud Saver System. https://www.gib.co.nz/assets/Uploads/LiteratureFile/System-Brochures/GIBFix/GIBFix-Framing-Systems-Literature.pdf
The use of more dimensionally stable LVL framing can improve quality of construction and helps with the reduction of dwangs/ nogs.
Structural cavity battens will also reduce the amount of framing required.
Alternatively, detailing the insulation on the outside of the frame will lead to the largest reduction in thermal bridging and will allow the structural frame to act as the insulated services cavity.
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.
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. Every piece of framing while adding to the structural resilience, reduces the insulation potential of the wall system.
Panel Systems
Panelised 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 in a factory environment.
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, where there are point loads 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 timber framing and insulation.
They are often used for ceilings too, creating an airtight thermally efficient building envelope.
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
● Works most efficiently when house is designed to match panel sizes.
● Structural elements need careful detailing and must be inspected before the panel is fully enclosed.
● Decisions need to be made as to whether there is a service cavity or services are installed inside the panels. If the decision is the latter then it needs to be right first time as alterations are difficult.
● The Contractor needs to plan ahead as many of the connections are only possible in one sequence of operation.
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.
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. Hopefully this might change soon.
● 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.
● A large volume of timber used per square metre of wall / floor / roof.
● They still require strapping and lining with insulation to give them a suitable insulation value. This strapping and lining can of course be on the outside of the panel, allowing the timber grain to be exposed internally.
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.
Structural Implications
For thermal efficiency timber structural members should be used where possible. In all cases when designing a Healthy Home, structural steel members should be avoided or kept either inside or outside the thermal envelope but not within it when designing a healthy home to prevent cold bridging and condensation build up.
Suppliers
Panelisation
● 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 and insulated service cavities. Ecopanel’s point of difference is that they do not contain petrochemical based insulation and no materials used are on the DECLARE Red list of toxic building materials. www.Ecopanel.co.nz
● Metra Panel construction panels are an engineered construction-grade particleboard. They are CNC cut and rebated in their factory, flat packed and assembled on-site. A Metra Panel building is very airtight due to the rebated connections and ceiling system. Thermal insulation is added to the outside of the external walls. There are no frames or additional linings in a Metra Panel building and the footprint of a Metra wall is only 36mm. Metra also has a range of intertenancy walls and fire-rated ceilings. https://www.metrapanel.co.nz/
● Potius™ Panels offer an engineered timber panel system that is a lightweight and fast alternative to conventional flooring, wall and roof structures for residential homes. Panels are constructed from a variety of locally produced engineered wood products, made from trees sustainably forested in New Zealand. Potius™ Panels are insulated with polyurethane foam (Similar to SIPs panels).
SIPS
There are three main manufacturers of SIPs panels in New Zealand; Formance, NZSIP and Kingspan. Other suppliers come and go and product is occasionally imported from offshore.
● NZSIP manufactures their panels in Cromwell from OSB and PUR polyurethane foam. The PUR provides a higher insulation level and more rigid inner core compared with polystyrene. A visit to their factory gives a insight into the manufacturing process and what is possible. They have a unique cam-lock system for joining panels to reduce thermal bridging. https://www.nzsip.co.nz/
● 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/. They are looking to start manufacture in NZ soon.
● The Kingspan Knight Built Tek Building system is constructed of OSB with a urethane insulation core. https://www.knightbuilt.co.nz/
● There are companies also combining SIPs panels in their prefabrication factory into full wall and roof elements. CBG Construction is one such company who has started doing that recently. For more information. https://www.buildersinwanaka.co.nz/
Recommended Healthy Home guidelines for Wall and Panel design are set out in the table below:
** Standard 90mm framing is not the preferred option due to the reduction in insulation values shown in recent research. However it may be possible with care and attention to detail. This requires at a minimum following the full Base (Healthy Home) system recommended in this guide.
Bracing Design
If possible reduce the number of nogs or dwangs within the bracing system with Specific Engineering Design (SED) that considers the unrestrained capacity of the studs to resist compression loads. A nog or dwang is a horizontal timber element that spans between the studs. Alternatively, a thicker lining or rigid air barrier can be designed to reduce the need for nogs.
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. For more information https://www.eqc.govt.nz/sites/public_files/390-Design-guidance-bracing-systems-in-light-timeber-framed-res-bldgs.pdf
To ensure that the design is resilient the assumed ductility for the system should be reduced from µ =3.5 to around µ = 2. This requires that the bracing for Healthy Homes is designed by a Chartered Professional Engineer (CPEng).
Consider the effect on your airtightness barrier if significant movement was to take place during a large wind or earthquake event.
With traditional NZS3604 design the bracing element, usually plasterboard is fixed direct to the timber frame. Where external rigid air barriers are used there is a move in the industry to use this as the bracing element.
The rationale for using plasterboard as a bracing element instead of ply or RAB board with traditional framing is that following a SLS earthquake event, the plasterboard can be easily checked for damage. However, the rigid air barrier is hidden behind building wrap and cladding, so can’t be checked so easily.
The rationale for not using plasterboard is that it gets damaged in earthquakes. However, a more resilient bracing design would use the plasterboard, as it can be checked for damage and repaired/replaced in events larger than SLS for example.
If internal battens are used to specifically limit thermal bridging, then the internal wall lining can’t be used as a bracing element, so the bracing element becomes the exterior sheathing layers. (RAB)
The use of 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, for additional resilience.
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 internal wall linings, damage, if any, is visible.
Consider that it is statistically possible to have at least two Serviceability Limit State wind, snow and earthquake events over the design life of the home.
The use of internal service cavities makes this difficult as the plasterboard is fixed to the battens, which are not designed to be a structural member.
Note that it is not the presence of the sheet bracing material that makes the walls stiffer, it is the number and type of fixings used. This is why it is important to following prescribed nailing pattens and hold down locations.
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.
Always make sure wherever possible that the structure is braced as evenly as possible across and along the dwelling and that material compatibilities (i.e. Steel v timber v concrete block) are accounted for.
Recommended Healthy Home guidelines for Bracing design are set out in the table below: