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10382 posts

Uber Geek

  #1481260 29-Jan-2016 15:00
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Can the vapour / moisture barrier be an aluminium foil?  Reflective so should be great for thermal - just querying it around electric cables?


I'm sure we have some surplus aluminium/kraft/aluminium at work that could be a deal if suitable, just need to check it's the non-perforated.  Let me know if you're interested and I'll take a look tomorrow.



The issue with aluminium foil is real, all you need is for one staple to go though to phase wire of the wiring or an accident with some powered device, and the whole sub-floor area will be live.  If you're down there when it happens, then you're also standing on bare and damp ground, so worse than the "zap" you may have typically experienced from 230v when standing on a dry insulated floor etc.  Now maybe with a new build, then all circuits except water heater and oven may be RCD protected, but OTOH most new builds are slab on grade, so there are few cases where sub-floor insulation could be added to a new house anyway. 


Another issue with aluminium is that thermal reflectivity reduces as it tarnishes or gets covered in dust.  The sub-floor insulation they were selling was perforated, dust does eventually cover the top side.  The perforations are presumably there so that if for example there's a leak or spillage onto the floor above, then there's still some air circulation so it can dry out, otherwise you'll have water trapped between the floor and insulation layer - the floor structure will rot. For that reason, you should never put an impervious film (ie plastic) attached to the floor joists etc.   IMO absolutely avoid unperforated alumium foil, but also avoid the "proper" sub-floor foil for electrical safety reasons and also for the reasons that it doesn't work well long-term, and if you're going to the hassle of installing sub-floor insulation then do it properly. Plastic film over the ground might be of some benefit if the soil is damp, but also make sure that sub-floor ventilation is adequate (someone might have blocked off many of the sub-floor vents by landscaping etc).


I have installed sub-floor insulation in our 1962 house.  But also increased ceiling insulation to R3.6 (+ existing old batts), plus wall insulation in all external walls, and as part of that work (full reclad) installed flashing systems to meet current building code around all window/door joinery (external).  The house was previously brick veneer, houses of this era the cavity is open to the sub-floor and may also be open to the roof space.  In this case, the only seal around joinery (stopping airflow from the open cavity) is from the architraves, in 50+ years even if those architraves had been flush, they've probably worked loose and/or warped.  On a windy day, then hold a candle near architraves to check for airflow.  Also check sealing of doors and windows, fit rubber seals as required.  Issues like this should be addressed before considering sub-floor insulation - IMO that (sub-floor) is a minor cause of heat loss in most older houses.

5271 posts

Uber Geek


  #1481278 29-Jan-2016 15:35
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After you have purchased the house, try this: place a piece of plastic sheet on the dirt and anchor it down. Wait 24 hours and lift it. If it's damp, that gives you your answer about needing a vapour barrier.


This experiment might be more effective in winter.


146 posts

Master Geek


  #1481380 29-Jan-2016 17:39
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Legend! Great advice.




I felt the ground and it was dry as, plus no mold. Good idea to have a look in winter. Reckon I should go ahead and install underfloor now or wait and see if i need it?

2216 posts

Uber Geek

  #1481428 29-Jan-2016 19:01
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Installing it will make a difference.

If you wait you'll know exactly how much difference, if you do it now, you won't have to suffer not having it.

Location: Dunedin


1116 posts

Uber Geek

  #1482274 31-Jan-2016 15:58
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Aluminium foil on the ground is going to rip and hole too easily.


With an enclosed under floor cavity the relative humidity is going to reach 100% unless it's dry or there is enough ventilation. If it is highly ventilated or not enclosed it isn't going to benefit as much from a vapour barrier. The dirt is going to become wet nearly anywhere in New Zealand during winter.


Wool insulation works on dampness by absorbing water vapour during times of high humidity and deabsorbing it when humidity drops. An enclosed humid cavity with no vapour barrier is going to be 100% humidity for months at a time so the type of insulation won't help much unless that problem is solved first.


Fletchers was unhappy about Knauf's using of the "earthwool" name. It's a fibreglass product like Pink Batts. I doubt Pink Batts and Earthwool are unsafe to for a DIY installer.

146 posts

Master Geek


  #1482296 31-Jan-2016 16:18
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Really interesting comment BF -


I'm surprised by the claims on their website then; nothing about fireglass, and claims easy to DIY install.


Manufactured using recycled glass bottles, naturally occurring raw materials and bonded using a bio-based technology with no added formaldehyde, phenols, acrylics, artificial colours, bleaches or dyes
 'Super-soft' fibre for easier installation - virtually itch free
In this video we show you how quick, clean and easy it is to install Earthwool® glasswool insulation in the ceiling of your home.

After reading your comments I managed to find this burried in the Safety instructions: 


  • Avoid eye contact with dust or fibres to minimise eye or skin contact and inhalation during handling.

One thing I'm unsure of with a vapour barrier, are you supposed to close off vents to the below floor cavity or check that they are clear? It sounds like you recommend a wool insulation combined with a vapour barrier to reduce moisture but also soak up anything that does happen? Or would it be better to aim for zero moisture and then use polyester?



72 posts

Master Geek

  #1482323 31-Jan-2016 17:18
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We put black polythene with good quality duct tape on the ground under our house and it made a huge difference. We also got Pink Batts put in and had the cost added to our rates.


2216 posts

Uber Geek

  #1482331 31-Jan-2016 17:21
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Willuknight: Manufactured using recycled glass bottles

Location: Dunedin


10382 posts

Uber Geek

  #1482493 31-Jan-2016 22:44
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One thing I'm unsure of with a vapour barrier, are you supposed to close off vents to the below floor cavity or check that they are clear? It sounds like you recommend a wool insulation combined with a vapour barrier to reduce moisture but also soak up anything that does happen? Or would it be better to aim for zero moisture and then use polyester?







No - you must not close off sub-floor vents.  Check that they're clear from time to time.


BTW, I call BS on claims/counterclaims about health risk of the various current insulation materials, biosoluble (recycled) glass fibre, wool, polyester, expol.  You don't want to breathe dust from anything.  Knauf "Earthwool" is not like the old style pink batts - it's not irritating to skin at all for most people.  If you want to get pedantic and extreme, then expol will contain free styrene monomer, polyester from recycled PET bottles will contain antimony and release traces of acetaldehyde as it breaks down, wool will contain traces of pesticides used in drench, and in any case will be treated with a synthetic pyrethroid like permethrin so that moth larvae don't eat it - just like wool carpet is treated.  Don't worry about those things in general, how easy it is to install (look at the manufacturer's install videos, then understand that in reality it's probably going to be much worse) and take into account that cutting polystyrene is a very nasty job plus the hassle of separating all wiring from it with tape, some of the polyester materials are rigid enough to be simply friction fitted when cut to size, so easy fitting but expensive, glass fibre probably needs support to hold it in place, but it might (or might not) be easy to do this with tape and a staple-gun with SS staples.


I also think that in an average NZ house with suspended floor, typically pre-1980s, sub-floor insulation is the job to make only the last bit of a difference, ceiling, walls, and windows as well as general draught-stopping should be addressed first.

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