MBIE Research Regarding Interstitial Relative Humidty Issues in Residential Construction
This request has an unknown status. We're waiting for Ed Grove to read a recent response and update the status.
From: Ed Grove
Dear Ministry of Business, Innovation & Employment,
In MBIE's Research Project Brief dated 19 March 2018 and headed "Research to support Code clauses H1, E3 and G4" (Project sponser Dave Robson, Project Proposer Bruce Klien" concern is expressed that "the way New Zealand's new housing stock is current specified and constructed means.a "tipping point" may have been reached......if a house is highly airtight, this may benefit energy efficiency to a degree. but could have detrimental consequences for both internal moisture and indoor air quality. This could [have] potentially severe unintended consequences - both for the health of the indoor environment and the structure of some houses".
The internal Branz/MBIE documentation associated with that Research Project Brief also records that:
(a) MBIE, ESR and Branz will (it appears from 2018 onwards) be researching "a more accurate method of determining whether mould growth, decay or corrosion is going to occur under some given conditions", with one of the benefits of the research being to attempt to "Demonstrate compliance with some of the more demanding requirements in the building code";
(b) While hopes had been that the WUFI derived models that had been used to design current cavity systems could be utilised to model, with tweaking and sufficient supercomputer time, refinements, that project had hit a problem in that the science involved was not just cutting edge, but bleeding edge, with a lack of any international consensus as to the accuracy of the science behind the models, and that as an interim measure resort might have to be had to installing moisture monitors within the cavities of BRANZ staff members homes to obtain experimental data;
(c) One of the current research objectives was to determine whether homes as currently being constructed would in fact meet (or at least have any reasonble prospect of meeting) the building code.
My concern is that there appears to be a risk we're currently constructing houses in a manner which might only not comply with the Building Code, but which are likely to give rise to "severe consequences", including as to "mould growth, decay or corrosion".
From the recorded inter-wall and inter-cavity humidity levels found by Branz in its experimental field tests of cavity construction methods, all of the cavity and intra-wall spaces were found, in certain micro-climates such as Auckland, to regularly, seasonally, experience elevated (in all but four of the five cavity configurations tested, close to 100%) relative humidity levels for 2-3 months per year.
Some obvious ramifications of concern include:
(a) Corrosion of fasteners. On a straight line analysis, from the observed periods of super-elevated relative humidity, a 25% loss in mass for fasteners such as nails could be expected over a period of approximately 25 - 30 years post construction (which, assuming that degree of mass loss is a reasonably conservative point of failure) would place the first wave of widespread structural failures attributable to this issue alone due to arise around approximately 2030;
(b) relative humidity levels above 80% will result in abnormal secondary binding of water vapor across binding sites in the cell walls, resulting in bound moisture concentrations up to 10% greater than historically assumed maximum bound water concentrations. That, coupled with the international experimental data that pinus sapwoods in particular can suffer fungal decay at below even conventional fibre saturation levels, and and the fact that juvenile sapwood also has a reduced (conventional) fibre saturation point compared to mature wood (sap or heart) raises the possibly of slow, but progressive, destructive decay occurring as a result of the elevated relative humidity levels, even without any liquid moisture (condensation or leaks) occurring. Couple that with boron loss through diffusion in elevated humidity conditions, and boron's reduced efficiency in the absence of liquid water, and there would appear to be urgent need for further research as to the durability in practice of current structural framing treatments, at least until/unless the issue of elevated inter-wall and inter-cavity relative humidity levels can be solved;
(c) Heartwood formation in juvenile wood has long (since the 1930s) been known to be abnormal, at least in Pinus species (but also recently observed in some hardwood species) in that the extractives conferring biological resistance to decay fungi (as opposed to the extractives conferring impregnability and/or refractive properties) are often present in much reduced quantities or near entirely absent. That is not a concern where timbers are not exposed to prolonged exposure to moisture, as the heartwood will still tend to have sufficient durability in practice as a consequence of the water-resistant qualities conferred by the heartwood extractives. There is a risk that it will now become an issue for so long as construction timbers, containing any material degree of unrepeatable heartwood, are being exposed to relative humidity levels sufficient to overwhelm the water-resistant qualities conferred by the heartwood extractives;
(d) the secondary binding of water vapour across binding sites within cell walls also, experimentally at least, greatly increases the rate and incidence of creep, and reduces timber strength;
(e) moisture cycling cumulatively results in increased distortion, at least in distortion prone woods, - i.e. juvenile woods, woods containing spiral grain, and compression wood, all of which are abnormally prevalent in our current wood supply. Such distortion by itself tends to then place greater strain upon fasteners and joints, and has the potential to compromise the building envelope leading to leaks and widespread failures;
(e) elevated relative humidity levels greater than 70% raises a significant risk (if not certainty) of mould growth.
As a result of my concerns, I am seeking copies of:
(a) any preliminary research and/or field observations of wall and cavity relative humidity levels, mould growth, decay, corrosion levels/rates, creep and distortion, within inter wall and/or inter-cavity area;
(b) any records of the investigations/progress to date with attempting to refine the current modelling to the point that it can explain and/or predict the relative humidity levels that have been observed in Branz's real-world testing;
(c) any risk analysis or briefing documents or other communications prepared and/or sent to the Minister of Housing (or any Ministers) or industry participants, in relation to the potential for the aforementioned issues to result in building failures including a failure to meet the current building code.
From: Ministerial Services
Ministry of Business, Innovation & Employment
On behalf of the Ministry of Business, Innovation and Employment I
acknowledge your email of the 15^th of September 2021 requesting under the
Official Information Act 1982 (the Act), the following:
“As a result of my concerns, I am seeking copies of:
(a) any preliminary research and/or field observations of wall and cavity
relative humidity levels, mould growth, decay, corrosion levels/rates,
creep and distortion, within inter wall and/or inter-cavity area;
(b) any records of the investigations/progress to date with attempting to
refine the current modelling to the point that it can explain and/or
predict the relative humidity levels that have been observed in Branz's
(c) any risk analysis or briefing documents or other communications
prepared and/or sent to the Minister of Housing (or any Ministers) or
industry participants, in relation to the potential for the aforementioned
issues to result in building failures including a failure to meet the
current building code.”
Your request is being processed in accordance with the Act and a response
will be sent to you in due course.
Due to the recent changes in Covid-19 alert levels, the Ministry is
currently focused on assisting with the pandemic response and your
response may be delayed. The Ministry will contact you to provide further
updates specific to your request once it has been assessed.
If you have any enquiries regarding your request, or its urgency, feel
free to contact us via email at [MBIE request email].
Nâku noa, nâ
Ngâ Pou o te Taumaru
Ministry of Business, Innovation and Employment
Level 4, 15 Stout Street, PO Box 1473, Wellington 6140
www.govt.nz - your guide to finding and using New Zealand government
1. mailto:[MBIE request email]