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Slab insulation for new build in Auckland

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Slab insulation for new build in Auckland

Posted 12 Sep '19 06:47 PM

Hi everyone

First post, but this is a fantastic forum so I hope this is received well.

I'm building a new single story house (215m2) in west auckland (north facing living) and looking to build a quality home and build in efficiency (within cost/benefit).

According to UpSpec from Branz for Auckland ( it recommends some basic improvements which are quite different to what I would have considered as sensible and I'm keen to hear people's thoughts.

1) Ceiling / wall insulation to be the same as code; I would have thought upgrading was an absolute quick win. Keen to hear people's perspective if there is a noticeable difference between R2.2 > R2.8 walls (90mm) & R2.9 > something higher in ceiling.

2) 'Living areas have exposed concrete slab (with finish), other areas tiles = very good.' This is a non insulated concrete slab (under / perimeter) and I'm keen if anyone has a house in auckand with this setup and how comfortable it is being uninsulated vs insulated.

Be really keen to hear people's comments so many thanks in advance :)

RE: Slab insulation for new build in Auckland

Posted 12 Sep '19 10:51 PM


I agree that some of these recommendations seem odd, but they do state that their recommendation are considered to be the most cost effective, so would give 'optimum' returns in terms of thermal efficiency V cost i.e diminishing returns... They do caveat this recommendation whilst electric is cheap. That's only going to go one way.

1. Walls/Ceilings - They are probably right theoretically that the increased cost of upgrading, but I fully agree with you that upgrading wall and ceiling insulation is an easy upgrade - Especially the walls, when you consider once its built, there's no possibility to upgrade once built. Depending on whether there is access to ceiling voids, these can obviously be upgraded later.

2. Not so sure about the slab, but I would again look to put the maximum in at the time of construction due to it being the only chance. A large exposed slab with no or little insulation, in my opinion would not make for a comfortable living space. You may find yourself heating the air more to compensate for a cold floor?

RE: Slab insulation for new build in Auckland

Posted 14 Sep '19 02:08 PM

The cost of going max R value in conventional 90mm timber frame stud walls is minimal. Likewise with the ceiling but that can be done later with better insulating products.

Expect slab insulation to be a lot more costly as it involves considerably a lot more work. Depending on the perimeter insulation method, installing EPS on the outside perimeter (EPS exposed) is a lot more costly than on the inside of the concrete footing/foundation. It all depends on what you're trying to achieve - ie if you looked at underfloor in-slab heating systems? By far the biggest complaint with exposed concrete and tiled floors is the cold comfort and the hard standing of the feet / joints (by seniors) vs carpet. Summer months won't pose a problem but it's in the winter months where it's more of a challenge.

Having 'some' EPS in the concrete is better than none and to go with extra EPS (ie 50mm vs 100mm) sheets offers little payback or benefit, especially in Auckland climate (but definitely beneficial in Canadian climate of sub -30C temperatures). In NZ, if you had a choice to have no perimeter insulation but with insulated slab insulations in (sheet form), I would prefer this than having no EPS insulated sheets but only perimeter insulation. Adding to more complexities is the choice of foundation you're going with? Here in Christchurch the most common is rib / raft type foundations to suit earthquake risk. However, these rafts foundation do not offer much of "insulation" in the slabs as the EPS pods are non-continuous. That is the concrete is continuous around each pod offering no thermal break between the concrete surface and to the bottom of the DPM. That's the crux of the issue, when you pay 3 times the price for TC3 land rib raft foundation, you get a very strong slab but offers relatively little to no insulation.

RE: Slab insulation for new build in Auckland

Posted 15 Sep '19 07:24 PM

Regarding wall insulation, R2.8 vs R2.2 will have some benefit, but cost is over double (e.g. earthwool is $6 for R2.2, $17 for R2.8). First step though should be dealing with windows as they're the lowest R value in the house. North or East facing are fine, but check eaves. You want minimum 600mm eaves to north and east (depending on height from eaves to window head). South facing should be minimal, as should West, as you barely get any sun in South at all (so net loss) and West you get lots of hot afternoon sun in summer (requiring cooling) and no decent sun in winter, so net loss. PVC if it works for the style/look you're going for. Double glazed AL with low-E otherwise if you can. I wouldn't bother thermally separated as the way they're installed defeats it (they sit on an alu WANZ bar which bridges the thermal break).

Roof, if it's trussed you can always add more later, so that's what I'd do.

Floor, if heated, absolutely insulate. If not, then I'd just do edge insulation, as that's where the shortest path to air is. e.g. insulfound could be used if it's simple slab-on-grad - can either be dropped inside boxing, or used as boxing. EPS underneath is of benefit if the soil is wet (e.g. clays), but if dry/sandy doesn't have a huge benefit as the soil is less conductive anyway. Cost is around $30/sqm for 50mm H-grade EPS, but as @SBQ indicates there'll be labour involved. Not too much if it's a straightforward slab though.

If it's a wafflepod, then the only thing to do is go MAXraft if you want it properly insulated. i.e. don't count the EPS pods for insulation.

The BRANZ Annual loss factor site is pretty good if you have time to fill it out. Will give you an idea of total thermal losses over a year (accounting for gains etc). Takes a fair amount of time, but is quite useful for getting a baseline, and you can then change things around to see how it affects things.

RE: Slab insulation for new build in Auckland

Posted 02 Oct '19 09:08 PM

Hi all

Just thought I would drop a note to say thanks for taking the time to write and share your thoughts, it's most appreciated.

Interestingly, I've added everything into BRANZ and it's quite an eye opener as to where the maximum bang for buck could be spent. Looking at the house build, at $0.30 per kWh, the upgrade from R2.2 to R2.8 equates to a very small saving each year, i.e. $10-15 so I don't believe it's worth the upgrade at the loss rate.

Windows are the biggest loss factor - but interestingly adding a decent pair of thermal curtains does have a decent R value which would out perform thermal breaks etc & glass upgrades - fine for Auckland in the warm days.

Slab perimeter insulation works out best and the Branz tool rates pod style insulation as being slightly better than a non insulated slab.

The investigation goes on.. :)

RE: Slab insulation for new build in Auckland

Posted 02 Oct '19 09:45 PM

The thermal curtain solution is great, if you open and close them at the most optimum time of day, taking into account the season, weather cycles, sun etc, which for most people is not very realistic, in my opinion, looking at window frames that are not metal to reduce the thermal bridging is a good start.

The BRANZ tool seems pretty useful though to highlight these different factors.

RE: Slab insulation for new build in Auckland

Posted 03 Oct '19 09:45 AM

There's a lot to consider and no online tool can factor all the variables. As mentioned in another post, issues like having the window glazing set in-line with the wall insulation (which is rarely done in NZ) will provide a much better benefit than trying to go with a high performance SGU window that is sticking out. While the BRANZ tool mentions window orientation, a simple variable like the size of the window will do more (or less), than to changing the insulation specs in the walls. Draftproofing is a major concern and many can mention how leaky the flush boxes for mains and light switch outlets can be on a windy day.

The whole model stinks ; until NZ building gets to like overseas standards of whole house mechanical ventillation / HVAC, the slight improvements you make even like insulation in the slab is pointless in terms of payback. It can also be argued it's pointless installing expensive balanced pressure HRV that we see being promoted in NZ (ie Zehnder, Wolf, etc) because 1) Cost prohibitive & 2) Building costs are way too high ; timber framing in NZ is around 3 times of N. American timber at Home Depot.

I am not surprised the pod 'waffle / rib-raft' slabs are only marginally more R value than the un-insulated conventional slab however in places like Christchurch, the use of these slabs is not for R value performance but for earthquake requirements. You would think paying 1/3rd of the cost of construction of a home that goes into the foundation gets you more of a house but what i've seen over time, more regulations, etc. provides a new home that is smaller in size; yet they're no more comfortable than the houses built 20 or 30 years ago. The most compelling result... ask yourself, will the extra cost or insulation, etc, that you put into building your house be marketable when it comes to selling your home?

RE: Slab insulation for new build in Auckland

Posted 09 Oct '19 01:19 PM

We opted for a Cupolex slab without any sub/perimeter insulation (no underfloor heating, Christchurch)

Here's some technical info on R values for the 260mm domes:

It's generally assumed that a slab on ground achieves R1.3 (with a waffle slab being only marginally better)

Our floor achieves just over R2, and then a bit more if you also factor in underlay/coverings.


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