The Stretch and Grow House

Phil McLean is a secondary school teacher who’s passionate about design. He was given the opportunity to take six months immersing himself in a new topic, in part to experience being a beginner learner again, to empathise with his students. His chosen new field was building and the result is his open source design of the Stretch and Grow House.

I was intrigued by Phil’s journey when he got in touch with me, explaining he was researching innovation in the New Zealand building industry. Then we caught up for breakfast in Christchurch and Phil showed some of what he’d achieved. I was impressed by his early design concept, the number of people he’d met in such a short space of time, and most of all, his enthusiasm.

Map of Innovative Housing in New Zealand
Phil McLean has produced a shared Google Map of Innovative Housing in New Zealand

Small, efficient and affordable are the main parameters that Phil’s tried to design for. His fresh perspective on these desired outputs of the building industry have led to some real creativity and innovation. One of the best features is that Phil’s made his thinking and his ultime Stretch and Grow concept, freely available. It’s currently in the form of an open source SktechUp model. You can also view a presentation of Phil’s design process.

Housing Innovation in New Zealand

When we met for breakfast, Phil was just at the beginning of his six month journey. But he was already reeling off names such as Lawrence McIntire from the The Little Greenie, Tim Bishop of SHAC, Philip Ivanier of Australasia’s first certified Passive House, Earth Song, and more. By reaching out with passion and genuine interest, Phil McLean had managed to connect with some of New Zealand’s most innovative building thinking.

The output of Phil’s background research is also freely available and I encourage people to get in touch and add their own innovation to the map. Quite literally. Phil’s created a Google Map of Innovative Housing Projects in New Zealand.

The Stretch and Grow House

The Stretch and Grow House
Phil McLeans SketchUp model of his Stretch and Grow House concept

Phil’s Stretch and Grow House is a design for a small house with an adaptable loft space. The principle is that the usable floor area within the house can be increased and then adapted to meet the changing needs of a family.

For cost, quality and efficiency, Phil’s building material of choice is a structural insulated panel (SIPs) system from KnightBuilt. A great prefab solution.

Growing the floor area inside the envelope without increasing the foot print of the house is very intentional. Phil’s idea is that this would keep future costs to a minimum as a building consent wouldn’t be required in many cases.

Next Steps

It would be great to see a full working example of Phil’s concept. Right now he’s working on a small-scale physical model to complete his project. I hope though that this is just the beginning for both the map of innovation and the Stretch and Grow concept.

Phil’s learnt, connected, been inspired and created. He’s now also shared and put his ideas out there, free to all. So the question of ‘what next’? is really up to all of us.

The Stretch and Grow House


  • Another very interesting podcast thanks Matthew. I love Phil’s project and the google Map of projects across NZ is a fantastic idea.

    We tend to avoid using SIPs for 2 reasons –
    1. The nature of the insulation and/or adhesives used.
    The insulation is either EPS or Urethane, while both of these are relatively benign in use, have low or no off-gassing issues and GHGs are no longer used in their production, they are both petrochemical products and they are both closed cell or non-breathable materials. This means that moisture control in the building fabric has to be very carefully designed (no bad thing in itself, but if it goes wrong it could be!) and it means that ventilation inside the building really must be designed to be adequate for the occupants. Adding to this SIPs are usually very airtight (which is a good thing!) meaning ventilation must be through design and not through infiltration to ensure adequate fresh clean air for the occupants.
    2. Questions over structural durability of heavier cladding fixed to the outside surface eg brick. They are incredibly strong as a panel form but not necessarily so strong for pulling away forces. We found this to be an issue on a multi-storey building though and may not be an issue on a single story height at all.

    In terms of the Stretch and Grow House and the use of SIPs, I would be very concerned about the ventilation. It is quite possible that natural ventilation could be seriously inadequate at night when people are sleeping and tend to close windows, especially in the heating season when people open windows less generally. (Although saying that, in the summer it could easily be inadequate also as internal heat gains and the stack effect could make the loft a very warm and stuffy place!) Once people start sleeping in the loft space this could exacerbate the issue evening further. While natural ventilation may seem like a cheaper upfront option, the running costs could be costlier if the building is ventilated adequately (heating all the cold fresh air being brought into the house) and/or there could be a heavy comfort & health penalty.

    It will be interesting to see where Phil’s work goes at the end of his sabbatical.

  • phil McLean

    Thanks for the feedback Elrond. I am working on the ventilation aspects. Building codes stipulate that rooms are required to have windows that open to the outside to provide that.
    Mechanical systems are recommended by SIP manufacturers: “SIP buildings are extremely airtight and require mechanical ventilation. Ventilation systems bring fresh air into the building in controlled amounts and exhaust moisture laden and stale air to the outside. By limiting air exchange to controlled ventilation systems, SIP homes allow for all incoming air to be filtered for allergens and dehumidified, creating better indoor air quality. Proper ventilation is important in all homes to preserve indoor air quality.”

    Yes, as you say, creating moisture barriers is critical, as in other building materials. I hope to have a good response from SIP panel manufacturers and designers in response to your comments.

    Thanks again

  • Dear Phil,

    Great, definitely good advice from the SIP website there. Although it doesn’t say, heat recovery is implied also which is worthwhile.

    You probably know already, but I would warn that Building Code ventilation requirements are generally woefully inadequate! Not just because they might not require a suitable amount but also because they take no account of user operation / behaviour.

    Best wishes, Elrond

  • Matthew Cutler-Welsh

    Hey Elrond and Phil, great comments (as always).

    Phil, Solarchitect ( might be a good resource for you in Christchurch. Russell’s own house has neat passive ventilation system that utilises a stack effect. It achieves this over two storeys with low windows and trickle vents downstairs, plus a Velux roof window in the top storey.

    By all reports, it ventilated naturally pretty well during the Christchurch summer.

    Elrond. Very interesting caution about SIPS. Are there any good ones?


  • Hi Matthew,

    Two issues with this statement – “By all reports, it ventilated naturally pretty well during the Christchurch summer.” – 1) It doesn’t sound like anyone is actually checking. We, Kiwis in particular, are well adapted to putting up with things for a long time that are actually quite uncomfortable. Monitoring and reporting comfort conditions and building performance really needs to start happening in the industry across the world. 2) Winter night time ventilation is more likely to be an issue. Less chance of any stack effect, people less likely to open windows, cold draughts etc…

    What do you mean by any god SIPs? As far as I know they are all comparable. We design for off-site timber cassette construction (closed panels) with breathable materials and insulate with cellulose blown in on site. All the advantages of SIPs, none of the disadvantages. 🙂

    Cheers, Elrond

    • Matthew Cutler-Welsh

      Again, good points Elrond.

      I think you’ve answered my question about a ‘good’ SIP, suggesting timber cassettes with cellulose avoids the two issues you initially mentioned, toxicity and structural durability?


  • phil McLean

    A good read on the pros and cons of sips from the Branz website is:
    Bio SIPs are in the pipeline it seems.

    Elrond, how does your product compare (initial installed cost only thanks) to stick framed structure or SIPs. Hate to ask that ‘shallow’ question about initial cost, I know there are more important questions if you can afford to ask them.


  • Phil – Thanks for the BRANZ link, definitely very interesting.

    I have had a quick look at the cost on our projects but I don’t have any actual figures for comparison to hand. (The figures I have include various costs for external walls and I can’t see what part of the cost is specifically the cassettes) We are only ever building to standard budgets though. (Publicly funded schools and housing, privately funded houses etc)

    On some projects the contractor has suggested SIPs would be cheaper, but they have never met our performance requirements yet! Although the insulation value is good in the centre of the panel, they need perimeter framing and framing around openings which results in a thermal bridge.

    Also in some cases the timber frame subcontractor has actually chosen to stick build certain aspects on site. Usually it doesn’t go this way though as the site erection time has a big cost impact and contractors are more often looking to manufacture as much off-site as they can.

    Best wishes, Elrond

  • Elrond Burrell

    Yes it does answer those questions, better thermal performance possible with less thermal bridging also.
    Bio-SIPs that Phil has mentioned also look very interesting for addressing issues of toxicity.

  • Elrond Burrell

    Worth checking out Wikihouse also and as presented at TED:

    A lot of synergy with the Stretch and Grow house

  • Walter Savage – Knightbuilt Lt

    Hi Phil,
    Interesting commentary. I hold the NZ license for the Kingspan Tek Panel building system, which is a
    very successful SIP’s building system. Kingspan’s SIP’s panels are used in a number of countries around the world including the UK, Europe Canada, USA, Australia and New Zealand. Sip’s homes that use the panel for the walls and the roof are definitely quite close to passive, and we strongly recommend installing a ventilation system. To that end we supply Mitsubishi systems with the houses we build.

    The Kingspan Tek Panel is 142mm thick and has 16mm internal and external facing of Orientated Strand
    board with 105mm of closed cell PUR (Polyurethane) foam core. The panel is very strong and has an R rating of R5.1. The panel has over 15 years of product history and has been extensively tested in a number of countries to ensure full compliance.

    We have been building in New Zealand for 3 years and the feedback from our customers has been great, easy to heat in winter and cool in summer. People say that no one building system is perfect; each will have its pros and cons. My experience building with SIP has only been positive; building with SIP’s is quicker, the end product is a lot stronger and far more thermally efficient. SIP’s panels incorporate 3 building elements into one (Framing, Insulation & Bracing).

    The speed of the build is another advantage; we can kitset the external walls & roof of a single storey 250m2 home with a mono pitch roof in 5 days in our warehouse and erection on site takes around two days.

    Modular building is still quite new to New Zealand, but since the Christchurch earthquakes modular has become the buzz word both in Commercial and Residential building. SIP’s panels are a modular builders dream, easy to use, very strong, high thermal efficiency and quick to build with. SIP’s panels are not new they started appearing in basic forms after the Second World War, since then they have developed into a great building system.

    Phil and I have had a number of conversations regarding SIP’s and I think he has done a great job with his model and concept.

  • Elrond Burrell

    Hi Phil & Matthew – an exciting similar project here in the UK:

    • Phil McLean

      Thanks Elrond, I was talking to wikihouse people today at social housing forum. It’s a funky idea but hasn’t yet been practically tested in the world as a house – as yet there isn’t a house. They are working on cladding systems. Regarding my small design, it has been challenging to get information from SIP manufacturers in the UK or NZ for that matter. I’m trying to nut out the cost of panels and am exploring possible next steps. There are many steps ahead yet if I walk that path. I would love to ‘bat’ for those who are being shut out of housing by the current ‘racket’ which leaves people down our way severely indebted to banks. I hate to see these burdens put on young families, etc.

      • Elrond Burrell

        Yes, in terms of built projects, I’m only aware of the Wikihouse being used for an exhibition stand to date. Nice, but basic and no need top deal with the weather!!

        Totally agree with your aspirations. The trouble with a project like Dwelle is that it still looks like it’s aimed at fairly affluent people – it might be relatively small but it’s high-tech and has what look like expensive finishes.

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