SIP’s the building method for our small sustainable house

Getting excited that we are finally making some progress again and we need to decide what the building method will be for our home.

It needs to be able to achieve proper insulation levels and air tightness. It needs to be available, affordable and suitable for our site. Above all the performance should last  50-100 years so no battens that sag over time. The last months I have investigated different concepts and products. Below some of the options.

Rammed earth / cob houses

Our site is very steep and it would become too expensive to excavate a flat section to be able to build a high mass heavy home so rammed earth or related methods are just not suitable for our site. Our site requires a light weight build on poles to keep it economical.

For more level sites in Northland I consider it a good building method with local availability of clay, sand, lime and cement to achieve a well functioning natural, comfortable, energy efficient home.

Plant based insulation

I would love to use materials that are plant or biobased and have a low or negative embodied energy. There are 2 or 3 houses being build in NZ using Hempcrete. The downside for us is  the walls need to be fairly thick; 300 mm at a minimum. This would reduce the size of our 71 m2 small home with about 5 m2. We need something light with a high insulation to thickness ratio. Also the price mentioned on Hemp technologies’ website of $2000-3000 /m2, which is 10-20 times more than some other alternatives, doesn’t fit the affordability aim.

Bio foams for rigid insulation do not seem to be commercially available here, I could only find some research programs so I haven’t been able to find anything in this category that would suit.

Cross laminated timber

X-lam is a cross laminated timber which are solid timber panels. They look great, have a negative carbon footprint, can be precision prefabricated and come in large sizes. They have great spans and are fairly light to build with. All good characteristics. On the downside the timber itself doesn’t have a high insulation value, less than R1 for most common thicknesses for walls. Additional insulation would be required which together with an already pretty high price for just the X –lam is too expensive compared to some other prefab panel systems.

Structural Insulated Panels

There are different types of structural insulated panels (SIPs). They consist of a core of insulation mostly EPS or PUR.  Two outer layers mostly OSB, metal or magnesium oxide sheets are pressed onto the insulation core making it a structural panel to build floors, walls and roofs. For our climate in Northland all of them have insulation values that will suffice so that is not a direct selection criteria anymore. The building code requires a minimum of R1.9 for walls and 2.9 for roofs. I would prefer a value above 3.5 for walls and 5 for roofs. The panels range between R3.8 to R5 for walls and between R5 and R8.7 for roofs.

Each of them have different pro’s and cons. Magnesium oxide panels are a high quality product but for several reasons I discarded them. Their price was higher compared to the other SIP panels. Their maximum length was below the highest wall we have so the wall would have to be spliced and would need cladding to reduce wind loads for our very high wind zone. This would mean the budget option to only batten and paint the panels would not be possible.

For our home I currently have the preference to build it with OSB and/or metal SIP panels. Cost comparisons between regular stick build and SIPs range from equal costs to 8% more including or excluding savings in build time. In our total budget this would mean a 0% to 2% increase in cost but it results in a home that has quality, is comfortable and energy efficient.

The biggest negative is the fact that the insulation material EPS is an oil derived product and you really don’t want EPS to end up in the environment or in landfills. With a minimum of 50 years but hopefully over 100 years of service and energy performance of the panels in our home these negative impacts are hopefully offset. It is one of the few options we have and  a choice that had to be made in our aim to achieve a quality, comfortable, air tight design. Hopefully advances in bio foam SIPs are finding their way to the market soon.

Roof selection

For the roof my preferred option is metal sip panels. They have long panel lengths available and can make the unsupported  5,8 m span. It’s relatively cheap and we could leave the interior ceiling exposed so that saves cost for lining or painting. Drawback is they are noisy when it’s raining and electrical wiring is more difficult to run. One thing to investigate is condensation.

Some data on other panels is listed below. They can’t be compared directly since they have all different characteristics. Some need building wrap or additional hardware or timber, some don’t need additional roofing which will lead to costs or savings that go with the different types but it still gives a good indication.

 

Roof options:

Name	Material	R value	t	Cost
Magroc	MGo EPS SIP	R8.7	335 mm	$174/m2
Formance	OSB EPS SIP	R6.9	265 mm	$160/m2
NZSIP	OSB PUR SIP	R6.9	165 mm	$150/m2
Kingspan Tek	OSB PUR SIP	R5	142 mm	$250/m2 incl installation
Metalcraft Thermospan	Steel EPS SIP	R5.3	200 mm	$90/m2
X-lam twinskin	CLT and ?	? R4- R5?	200 mm?	$178/m2

 

Walls

For walls my preferred option is OSB SIPS. At some point in coordination with our structural engineer we have to find out if it makes more sense to use one type of SIP or if we can mix and match metal and OSB SIPS.

Although metal sips are cheaper and houses have been built with them for some reason I have a feeling they won’t perform as well as OSB SIPS. Maybe just too light, too much of an unbreathable barrier and I prefer the more solid structural characteristics of the OSB sips.

Just as with the roof it is difficult to compare the different available suppliers and systems only on the data in the table and there are other parameters that will drive the decision. For OSB panels transportation costs will most likely be the dominant parameter in selecting a supplier. Shipping panels in to Auckland is more economical than trucking them in from the South Island where the only manufacturing facility is unfortunately.

Wall options:

Name	Material	R value	t	Cost
Magroc	MGo EPS SIP	R3.8	165 mm	+/-$150/m2?
Formance	OSB EPS SIP	R4.2	165 mm	$135/m2
NZSIP	OSB PUR SIP	R4.5	115 mm	$130/m2
Kingspan Tek	OSB PUR SIP	R5	142 mm	$250/m2 incl installation
Metalcraft Thermospan	Steel EPS SIP	R4	150 mm	$66/m2
Xlam	CLT	R1.2	75 mm	$107/m2

 

Floor

For the floor construction I don’t have a real preference. Insulation values don’t have to be as high as walls and floors and I’ll just see what makes sense from a structural point of view.

 

All choices still have to be discussed and evaluated with the architect, structural engineer and a team in Auckland experienced with SIP buildings but at least we have a way forward again to continue with our design.