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SIPs -vs- Timber Frame

SIPs Panel

Firstly some basics - SIPs, is the acronym for Structural Insulated Panels.

SIP's have some form of expanded polystyrene/foam insulated core within two sheets of structural Oriented Strand Board (OSB) board either side that forms the internal and external skin. The internal cores are chemically bonded to the sheets. When formed, the components create structural panels. The exact components differ throughout the various manufacturers. The panels can be used for both the walls and roof. SIP's have been used in Eco Homes, Green Homes, Passive Homes, what ever you want to call them for years, but will they become mainstream to become a standard building feature?

SIP Panel Description

So, why would you use a SIP system over a traditional timber frame system?

What are the Pros and Cons?

We will look at the following factors:

  • Thermal Efficiency
  • Speed of Construction
  • Cost
  • Structure
  • Durability
  • Future Maintenance
  • Fire
  • Other

Thermal Efficiency

Panels range in thickness, with the increase in thickness resulting in an increase of the thermal efficiency of the panels and system as a whole.
SIP panels range in thickness from circa 110mm to 310mm which result in some impressive 'R' ratings from R2.8 to R7 and upwards being claimed for walls and over R9 for roofs.
When compared to standard timber framed wall insulation batts which range for a standard 90mm wall thicknesses up to circa R2.8 and for a 140mm framed wall up to R4 (comparable when adding standard plasterboard and Rigid Air Barriers (RAB)), or up to R7 for roofs, are all less than than the equivalent sized SIP panel. These R values are firmly in the 'Eco Home' and 'Passive Home' arena. 


Note: 'R' rating is a measure of how well a component (floor, walls, ceilings, glazing etc) resists the conductive flow of heat. The bigger the 'R' rating the better the insulation.
When comparing the two systems, the stats provided by the various manufacturers provide assessments of best case installations. Due to the design and installation methods of a SIP system, with sealed panel junctions together with a reduced number of junctions and penetrations, in real life, there is many more opportunities to leave un-insulated penetrations in a traditional timber framed system - think behind and around electric sockets (you can often feel the drafts from plugs sockets), insulation batts cut short and not snugly fitted, omitted insulation in hard to acces or small cavities, including the eaves of roofs, which are notoriously hard to get into, and sometimes don't permit the full thickness of insulation at the eaves, in order to prevent breaching the roof itself.


Air tightness capabilities of the SIPs systems are therefore a great attribute, although the offset is that mechanical ventilation is likely to be required to ensure adequate ventilation. This however adds the potential for control and heat recovery.


Estimates suggest that SIP systems are up to 15 times more airtight than their timber framed comparables with traditional fiberglass batt insulation and estimates suggest potential savings in energy costs of 50%-70%. This saving is year after year and with energy costs only ever realisitically heading in the upward direction, there is a huge benefit to the bill payers.


When traditional framing is installed, without specific attention or request, the level of sealing to ensure airtightness will be low.


We do have to acknowledge that all systems can be installed at increased thicknesses to facilitate more insulation and every joint and function sealed, but we have to be realistic to ensure that any potential energy cost savings are economical against build cost. So, we put SIPS systems considerably ahead against traditional timber framed construction on thermal efficiency terms.

Speed of Construction

SIP panel systems arrive on site generally in circa 1200mm wide panels. Once the floor is prepared the first step is to install the bottom plates, before the panels are stood, starting from a corner, foam sealed and fixed to the bottom plate and each other. Walls are installed quickly, with little additional effort on site around window sills and lintels as all the structure is integral to the system. First floor walls are stood quickly with first floor joists installed shortly after, followed by the first floor, then the process repeated before the roof installation to provide a weathertight envelope.

Traditional timber framing also has a good efficiency of progress at this early stage with the standing of pre-nailed framing at a similar speed (if not quicker than the SIP systems) with the structure of the ground, upper floors and then truss roof (if used) quickly and efficiently installed.

In my opinion, this is where the comparisons end. The structure of both types are up, but the strapping of lintels, installation of external RABs, internal bracing panels, drilling or routes and first fix for the services is required, prior to the insulation installation, which are all hugely time consuming, with the separate trades each having an affect on the critical path of the construction programme. With SIPs, first fix services can genuinely begin to be installed the day after the walls and roof structure go on - no waiting for the roof cladding required.

The SIP system, with either factory pre-routed service paths or internal strap and plasterboard lining which provides the void for the service runs, and negates the need for plasterboard being screwed and glued in the onerous manner required by the various manufacturers to provide the required bracing requirements.


The shop manufactured nature of the SIP systems also result in straighter and more consistent substrates, that reduce the amount of time straightening and packing warped timber framing, although if laminated traditional timber framing is used, this should also be straight and plumb direct from the saw mill.

Again, we give SIP's the advantage on speed of build.

Cost

Cost difference for the physical materials between SIP systems and traditional timber frame construction seems to be pretty consistently estimated at SIP's being circa 7-15% more than the same design of timber framed building.
Although difficult to quantify, the general consensus in the SIP world is that this increased upfront cost is offset by the reduced labour cost and time.
The real cost benefits are realised once the building is used and the thermal efficiencies are realised.

Arguably traditional timber frame is cheaper, but the speed of build and end user running costs cheaper.


Structure

Traditional Timber Framed House

Traditional timber frame - structures supports their loads using regularly spaced studs (usually 400-600mm), whereas a SIPs panel supports loads via the facing laminate and jointing studs which are at each panel joint. The bond between the insulating core and the external laminate creates a composite system, prevents the panels from buckling and provides the structure. Additional structural timber or steel can be added as required to support significant point loads on purlins, bearers, lintels etc, but for simple structures it isn't required.

There is no question that traditional timber framed buildings have proved themselves over and over and have also stood the test of time.

New Zealand Standard NZS 3604:2011 - Timber Framed Buildings provides all of the structural information required for standard low rise timber buildings, which is again well proven and negates the need for Structural Engineer input on standard builds.

SIP's House

SIPs structures - are less proven, although have been used for over 40 years across the world and in different climates. New Zealand's most popular SIPs systems have Codemarks and BRANZ (Building Research Association New Zealand) Appraisals, which provide consumers and Building Consent Authorities (Councils) reasonable grounds to conclude that the requirements of the New Zealand Building Code (NZBC) have been met often following on from robust testing and Structural Engineer assessments. 

These apply to all relevant Building Code Clauses, including (but not limited to) B1 Structure, B2 Durability, E2 External Moisture, F2 Hazardous Building Materials and H1 Energy Efficiency.

SIPs houses, although much lower in numbers to compare, performed well in the Christchurch Earthquakes, obviously as did many of the timber framed ones.

For structure, tradiitonal timber framed buildings have proven themselves time and time again, all around the world. For certainty, traditional timber framed houses have the edge. For the time being SIPs will have to continue to prove themselves in larger numbers as they undoubtably will become more popular - 

Durability

In exactly the same vein as the Structure comments, in order for any of the SIPs systems to receive Building Consent approval, they need to show compliance with NZBC. Again B2 Durability is demonstrated by the Codemark and BRANZ Appraisals.


The NZBC requirement for the primary B1 Structure is 50 years and although more is expected, there is less evidence than traditional timber structures on how durable SIP structures are. Stories are probably true that if OSB gets wet, it will decay, grow moulds and fungi and turn to Weetbix, but is this not exactly the same as timber framing?

Standard framing timber is treated to H1.2, which meets the requirements of New Zealand Standard NZS3602:2003 which provides the recommended treatment levels for timber being used in buildings. The OSB boards in SIP's framing usually contain timber with similar treatment levels. Both considered to be a minimum, it is easy to increase the treatment level of the traditional timber framing, but some of the leading SIP's manufacturers do not offer any treatment upgrade. If this is actually available, or becomes available, it may assist in alleviating any concerns users may have over the durability of the SIP systems.

This may seem a bit obvious, but the key is not to get either wet. This is firstly down to simple, robust designs. I.e minimum (ideally none) internal gutters, flat roofs, parapets, complicated roof lights etc, but then down to good detailing and cavity construction.

Windows, door and service penetrations - It is widely accepted that penetrations will inevitably leak at some point in time, so in addition to the cavity cladding system, flashing all penetrations in a robust manner, like fully flashing tape around penetrations, or using products like 'DynaFlash' https://dynex.co.nz/dynexbuild/dynaflash-2/ which is a plastic back flashing that can be installed at junctions and openings, extending to the base of the cavity, which will drain any leaks that may occur.

Future Maintenance

Normal maintenance is required to both SIP's and timber framed houses. Blocked spoutings, drains and damaged claddings will always cause firstly superficial damage and then damage to the underlying structure. It never fails to amaze me how often I see simple maintenance issues that are easy to fix, left to morph into big issues.
If timber decay occurs to SIP panels, all of the affected panels need to be replaced. This will result in very significant consequential works (Supporting floor structures, external claddings, internal linings, electrics, plumbing etc etc, that will require a Building Consent to be uplifted (presuming a breach of NZBC).
So will the timber framing... In order to comply with the Building Regulations and to follow best practice, the requirements of the DBH (now MBIE) Weathertightness - Guide to Remediation Design need to be followed. In addition to replacing full height studs where sections are decayed, timber is recommended to be removed at least 1000mm from decayed timber, resulting in largely the same amount of remediation and consequential works.

Insist on robust design, simplicity, construction quality and ensure ongoing maintenance on all buildings to prevent these unwanted and unnecessary failures.

Fire

SIPs systems differ in terms of their component make up and will all therefore differ in fire capabilities. The internal cores will generally be fire retarded or made from fire retardant plastics, but the OSB and timber will behave as a combustable materials. Fire engineering.com states that SIPs are not inherently fire resistive. Tests by manufacturers have shown that under fire conditions, SIPS will burn like their component materials: wood (OSB) and plastic (foam insulation and adhesives). This in our opinion is not that much different to traditional timber framing, although as the core differs in materials between manufacturers, this is the biggest variance.
The BRANZ Appraisal and Codemark for NZSIP panels, nor the Formance Codemark cover the fire 'C' Building Code Clause, so compliance has to be demonstrated to the Building Consent Authorities as required, using their documents provided. Demonstrating compliance may also be contributed to by the plasterboard manufacturers systems and the external cladding manufacturers literature.
From our search through the various SIP manufacturer websites and literature, the information provided relating to fire related properties is limited and is something to be considered further when considering SIPs systems.

Other

  • Off-site fabrication is a significant factor to SIPs greener credentials, with the installation of structural insulated panels quicker and cleaner as using accurately pre cut panel systems reduce site waste that includes packaging, timber off cuts and bags of insulation offcuts. SIP's virtually eliminate guesswork and waste.
  • SIP designs can allow a smaller footprint, whilst providing a larger volume of space, due to the insulating envelope being contained within the roof plane, rather than the ceiling, which enables dramatic living spaces and loft areas.
  • Due to the solid sheet nature of SIPs internal fixings for anything including shower enclosures, shelfs, wall cabinets, pictures etc always have a guaranteed secure fixing, so when living in an Earthquake prone country this provides additional reassurance for those shelves above you bed!
  • SIP's homes provide the most benefits for simple designs. The more complicated the structure, the greater reliance for addition timber or steel structure, which may negate some of the potential efficiences.
  • As SIPs homes offer potentailly lower running costs there is the potentail for better Re-sale values. If you look at the Ecobob forum - Valuations for Energy Efficient/Passive Homes you will see some mixed reactions to 'alternative' construction methods certainly by valuers, so i'm not so confident that the general public would put much of a premium on a SIP house.

Handling and Storage

Stored SIPs must be protected from the rain and must not be stored in contact with the ground. The SIPs are also bulky and heavy and whilst possible to man handle small panels, large panels will often require a crane or telehandler, whilst timber framing is generally more easily managed and not as susceptible to rain damage.

HVAC Design

With a significantly more airtight building, there is a requirement to provide adequate ventilation that is properly sized to account for the inherent energy efficiency of the structure. Whilst the installation of mechanical ventilation (Example - Fantech Zehnder) perhaps is seen as a downside to SIPs, it does provide the opportunity for the installation of heat recovery systems and air filtering to increase control over the internal environment, allowing fresh air in when, where and how you want. Whilst these systems can (and are) be installed in traditional timber framed houses, greater benefits are achieved in more airtight houses due to the greater level of control.

SIPs houses are  more airtight, so they let you control the indoor environment. A modern ventilation system will let you have fresh air when, how, and where you need it, saving energy.

High humidity levels can be unhealthy and can damage your building. A ventilation system to keep indoor humidity around 40% will be ideal. Systems can automate exhaust fans in kitchens, bathrooms, and laundry rooms to expel moist air as needed, so SIP homes can often be seen as being healthier.

Summary

So, all in all I think that SIPs have the edge over traditional timber framing, but as with anything different, it will take more time for SIP's to prove their Green or Eco Home credentials, and time for the general public to become accustomed to the terminologies, all while as the years pass, more and more confidence will likely be gained over SIPS structural and durability capabilities.

There will of course always be a time and place for traditional timber framing, but with SIPs systems offering such good thermal efficiency and speed of construction efficiencies, expect to see and hear more aboute SIPs. 

Go to the Ecobob Forum - SIPs v Timber Frame and let us know your thoughts and experiences.

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