The Roofing Industry is currently seeing a significant increase in complaints of “leaking” and “condensation”
especially in systems like skillion roofs and curved roofs. NZMRM and underlay manufacturers believe the answer lies in increasing what is currently the non-existent ventilation of roof spaces. We have approached DBH and BRANZ to request an urgent investigation into this issue, so far without result.

Here Stuart Thomson provides some background and his own inimitable take on the situation.

This article is the second in the series on Underlay/Ventilation/ Insulation which discusses the designer’s role and responsibility for ventilation of roof spaces. There is no doubt that the last few decades have been marked by an accelerated speed of change. While it is difficult to keep up with the latest iPhone technology it is equally hard to keep up with the changing regulations and technology of the Building Industry.

One of the advantages of ‘maturing’ is that one can look back and remember how and why things were done years ago. Buildings were simpler in those days but the level of skill required to be a trades person was much more onerous than at the present time. From the 1st March 2012, although an LBP is required to design or supervise most building work, he does not have to have undergone any formal trade education or have any trade qualification. Sure he needs referees and to have been working in a trade for some time
but there is no guarantee that this person has been taught by anyone with trade or design qualifications.
Registered Architects are assumed to have studied and have an understanding of the science of moisture in building. Recent experience does not bear this out.

The irony is that as building has got more complicated the numbers of indentured tradespersons with ‘inherent skills’ is now less as they come to retiring age. But this is not simply the fault of teachers and students; it is part of the social and educational changes in the trade system and is something that is not going to revert. What this means is that although we have a performance based Building Code we now require prescriptive solutions that do not require ‘inherent skills’ but have recipe book designs which must be followed for both successful design and installation.

We do have a serious problem of excess moisture within buildings and the Roofing Industry believe that this is not addressed in the Building Code nor understood by designers.


So how has this come about and what has changed in residential building in the last forty years? Houses were quite draughty (air-leaky), often with slatted foundations, with weatherboards, wooden windows, doors and barges covered by wide facings boards. The fireplace provided an automatic seven air changes an hour, and by the fireside in the winter, your front was toasted and your back cold and draughty. Some windows remained permanently open for ‘fresh air’, clothes were ‘aired’ on the clothesline, and a shower was a one minute affair over the bath. We put on more clothes in the winter and went to bed early with a ‘hottie’. Condensation was seen occasionally on the windows, but in the roof space it was taken care of
by the bituminous underlay under metal roof cladding.

A moisture problem? What moisture problem?

Until insulation was made compulsory it was common practice to ventilate the wall space because timber at  hat time was likely to be very wet and a common phrase heard was that ‘the birds were still singing in it’.
Dwangs were either turned on their side or drilled with a 1” hole to provide ventilation. A hole was included the top plate to allow moisture laden air to escape into the attic space which in turn was vented with either gable vents or soffit vents as recommended in NZS 3602: 1975 which said: cavities including
cavities beneath flat roofs should be ventilated by such means as:

a) grilles in eaves;
b) louvre frames in gables;
c) a continuous gap in the roof soffit:
d) ventilating ridging: or
e) other suitable means

Up to this time was also common practice for the builder to leave the job for up to a month or more after the external cladding and roof had been completed to allow the drying process to continue. It was not until the late 1970s that the use of kilndried timber became into general use that the problem of wet timber was considered to have disappeared. A major change came in 1977 when by law all houses were required to be insulated because this was the time of the energy crisis (1973, 1979) and everyone’s attention was drawn to saving the cost of electricity or gas for heating.

However what started out to be an energy conservation initiative, ended up to be the opposite (due to vigorous and dubious advertising) because the owner expectation of ‘warmth’ jumped 5 degrees.

At this time it seemed logical (to some) to gum up the holes and it was decided that ventilation of the
wall was not an issue any more and that installation could be stuffed in the cavity willy-nilly. Coincidentally at this time the chimney disappeared, weather- stripped aluminium windows and concrete floors became the norm so that draughts were reduced to ziltch.

This was when we lost the plot. What everybody (almost) had forgotten was the science of moisture and this included BRANZ who still continue to maintain that ventilation of the attic space is not necessary. Moisture from wet timber is one thing but moisture generated within a house is another Considerable moisture is generated within the house by the cooking, bathing washing and drying and breathing – as you do.

This water vapour must be vented to the outside at its source and never into the roof cavity. If it is not then this moisture has to go somewhere, and usually because warm air is lighter than cold air, it ends up at ceiling level and when this is permeable it ends up in the attic space.

The NZMRM Code of Practice is quite clear on the subject of ventilation and in section 4.6 it details the requirements to comply with that document.

Air (like water) is a conveyor and can hold various amount of water vapour dependent on its temperature; the warmer the air the greater the amount. When warm air containing water vapour hits a cold surface at its dew point, then water vapour changes phase to water resulting in condensation.

Designers were then caught between a rock and the other place. They were required to design in opening windows (15% of floor area) but there was (and still is) no legislation to make anyone open them.
The NZBC E3 and E3/AS1 only exacerbate the problem. E3.1 The objective of this provision is to-
Safeguard people against illness, injury, or loss of amenity that could result from the accumulation of internal moisture; E3.2 Buildings must be constructed to avoid the likelihood of

(a) Fungal growth or the accumulation of contaminants on linings and other building elements; and
(c) Damage to building elements being caused by the presence of moisture. But E3/AS1 says: 1.1.4. b) Insulated cavities shall be enclosed with no ventilation N.B. The MRM and RANZ have met with the DBH and asked them to justify the E3/AS1requirement and explain how it sits with E3.

New Zealand is out of kilter with the rest of the world. Overseas it is common practice to ventilate the attic space and a British standard BS 5250.2002 is a Code of Practice for the control of condensation in buildings. Also the UK building regulations 2004 have a separate section in their approved document C/2 which considers condensation in roofs which details soffit and ridge ventilation as a requirement for both attic and skillion roofs. NZ used to agree.

In the USA there is a prescriptive requirement for the amount of ventilation based on the ceiling area of the building. For every hundred square metres two thirds of a square metre of free open area is required and is divided between the soffit and ridge.

Most metal roof claddings have effective chimneys in the corrugations or ribs. They were not designed for that purpose but because they slope upwards (to let the rain flow downwards), the corrugations allow warmer air to rise, creating natural ventilation. This is described as the stack effect. Also because the corrugations or rib ends are at the highest point (except on curved roofs) it is just about impossible to stop the hot air getting out entirely. This ventilation can be inhibited by filler blocks and flashings at either end of the corrugation, but given the normal changes of temperatures during the day and night there will be a small constant leakage of air, including water vapour.

This ventilation chimney is not only necessary on the top side of the underlay but also on the underside as well since the underlay needs air movement on both sides to dissipate the condensate absorbed by it. The COP requires a 20mm gap but most times that amount is unnecessary.

For ventilation to occur a bottom inlet and a top outlet are required which allows natural convection to do the rest.

The laws of physics do not change. Water finds its own level. Water Vapour pressures will equalise. Hot air rises – (actually cold air sinks).

When moisture accumulates at a rate greater than its removal and the capacity of the structural materials
to store it, then condensation is inevitable.

Leaky homes were not caused by untreated timber but by the lack of ventilation to dry it out. It is not often recognised that tongue and groove timber ceilings allow free moisture entry while ceilings perforated like a Swiss cheese with downlights are really small chimneys conveying moisture laden warm air into the attic space.

Industry experience from the investigation of many attic moisture complaints has shown that the occupants of the house and their lifestyle habits have a major bearing on the severity of the condensation problems experienced. However this should be irrelevant as the building design should be able to cope with the variations in peoples’ habits that are part of today’s society. The idea that people are prepared to open their windows at night to provide ventilation has gone out the same window that was supposed to be open. In many cases windows have been sealed up by tenants to cut down on heating costs. An analysis of these complaints has shown that the majority could have been avoided had positive ventilation been included in the design. This however goes head-to-head with the Acceptable Solution to Clause E3 of the building code (E3/AS1) which has been interpreted as prohibiting the ventilation of insulated cavities. Also designers have either intentionally or unintentionally forgotten about ridge vents, gable vents and soffit vents.

The simple solution is to blame the underlay for roof moisture problems when most people do not  recognise that this is only the symptom and not the cause. Roofers also field their share of the blame  because the assumption that such a large amount of water from the roof must be a leak. Almost invariably the dew point is on the underside of the underlay and without sufficient means of ventilation the underlay
becomes overloaded which results in wetting of the insulation leading to mould.

Mould is a water problem. Without water there is no mould. Mould requires nutrients to grow and it gets it (cellulose) from dead plants - timber and paper. The cause is excess moisture which is not vented either from within the living space or from the attic space. The Medicos have got it wrong. The most important factor for asthma and allergies is humidity not temperature. If there is an excess of moisture in the air then mould will grow even at uncomfortable temperatures. Auckland’s Relative Humidity over the last 8 years
averaged 82%. Over 80% moulds grow and dust mites thrive (both are triggers) which is not good news
for those with asthma or allergies. New Zealand children have the second highest incidence of asthma
in the world at 15% while the ARC says that Auckland’s children rate is nearer 25%. Ventilation is needed
for the health of the occupants as well as the house.

There is a common myth that cold gives you colds. I worked at 13,000 feet up in the Himalayas putting on a few roofs, where the temperatures were well below zero. Nobody had colds up there and I did not get one either. Asthma was unheard of in the mountains.

David Schlim – a doctor who worked in Nepal for many years reports the following: Despite the fact that some people with asthma are provoked by cold and exercise, the vast majority of people with asthma do quite well at altitude. Asthma exacerbations are exceedingly rare at high altitude in Nepal, and no one has ever been evacuated from high altitude due to an asthma attack. There is also no evidence that people with a history of asthma are more prone to acute mountain sickness.

The history of ventilation is pretty interesting. The Victorians in the 19th century were obsessed with ventilation because they believed “the evil of bad air” (carbon dioxide) caused diseases like cholera, dysentery and typhoid. They polluted their own air with coal fires, candles, oil lamps, pipe smoking but really invented ventilation and we would now say they were paranoiac about fresh air. This attitude lasted
well into the 20th century as well because my grandparents and my schoolroom windows were open
summer and winter.

Air conditioned offices got people out of the habit of opening windows.

While ventilation is necessary to comply with E3 there are other very good reasons why ventilation is essential. Recently the investigation of closed stoves has shown that their failure to stay alight was due to lack of oxygen. Like the canary in the coal mine this is telling us that without a designed ventilation system the health of the occupants is at risk.

Recently reported - A terrible diet and a room with no ventilation are being blamed for the death of a man who was killed by his own gas. There was no mark on his body but an autopsy showed large amounts of methane gas in his system. His diet had consisted mostly of beans and cabbage and it was just the right combination of foods. It appears that the man, who was said to be a very big man with a huge capacity, died in his sleep from breathing his own poisonous gas. It was also said that had he had his windows opened it wouldn’t have been fatal.

There are three ways to prevent the condensation problems in roof spaces

1. to reduce the creation of moisture where it has generated such as the laundry bathroom, kitchen and
bedroom ;

2. to prevent such moisture entering the attic space by a vapour or air barrier;

3. to remove the moisture laden air from within the attic space to the outside by a passive but positive means of ventilation.

A number of buildings have produced condensation problems due to the installation of home ventilation systems. These systems are being grossly oversold because of the heat recovery claims made by their makers. A recent ECCA report prepared by the University of Otago confirms our view: Based on the modelling work detailed in this report, it is recommended that existing positive pressure mechanical ventilation systems should not be promoted and marketed on their heating and/ or cooling potential.
They do have one advantage but it is not the one they are being sold on – they do provide trickle ventilation
which can reduce condensation. But do you really want to recirculate stale air?

There are two main types – forced air and heat exchanger systems. Heat exchanger systems with the provision for additional heating are the only type that can be relied upon to reduce the likelihood of condensation through the seasons. Heat exchanger systems meet the G4 building code requirements
for ventilation with outside air, but forced air systems do not.

There is a lot more that can be said about ventilation: n What about the millions of dollars of damage that has been done to schools because of condensation and lack of ventilation?

-> What about the builders who create their own problem by not venting a building with a curing concrete slab?

-> What about skillion roofs and curved roofs that have no provision at all for attic space ventilation?( Not
surprisingly these roofs are most commonly affected because the volume of air is very limited)

-> What about the expectation of the underlay to cope with condensation from beneath when it has only been designed for condensation from above?

-> What about the added insulation now being squashed into roof cavities? –(this will be discussed
next time in the third article in this series)

There are a number of new innovations and products coming onto the market from overseas that claim to cure condensation. From adhered polyester fleece or polyurethane sprayed directly onto the underside of metal roof cladding, to intelligent membranes that are claimed to eliminate moisture buildup within the construction. While the MRM are aware of these new products there is much testing and New Zealand experience required before the added cost of such systems can be justified. In Europe the drive for an ’air-tight house’ for energy conservation is likely to become mandatory but is believed that there are much simpler and cheaper ways to solve these problems.