Weathertightness versus Condensation


Definition and Principles of Condensation

Condensation is the function of interior collection of water vapour to reach surfaces having a temperature below that of the dew point of the vapour and by that condensing and turning back into liquid. Condensation in building construction is an unwanted phenomenon as it may cause dampness, mould, health issues, wood rot, metal corrosion, weakening of mortar and masonry walls, and energy penalties due to increased heat transfer.

To alleviate these issues, the indoor air humidity needs to be lowered, or air ventilation in the building needs to be improved. This can be done in a few ways, for example opening windows (not in winter in Australia and NZ), turning on extractor fans, using internal dehumidifiers that are correctly scaled to perform the task demanded to bring down, considerably, the R.H., drying clothes outside and covering pots and pans whilst cooking.  Air conditioning or ventilation systems can be installed that help remove moisture from the air and move air throughout a building. This is already being done in the designs of office buildings where lesser internal humidities are being created than in a house with no proper winter heating and/or refrigeration to remove moisture.  The amount of water vapour that can be stored in the air can be increased simply by increasing the temperature inside the building, however, this can be a double-edged sword as most condensation in the home occurs when warm and moisture heavy air meets a cool surface. This is, in southern parts of Australia and in the whole of NZ, commonly in the winter period and with high exterior RH being almost always current.

It also takes place well within materials and cavities within the wall or roof itself and particularly, in NZ, on the cold surfaces of outside applied building wraps and wall underlays, and which are furthermore exposed to the drained and fully ventilated NZ type of batten cavity. The NZ batten cavity likely to take on the exterior conditions due to high ventilation rates. Claddings in the form of brick veneer in housing is a differing subject to the NZ batten cavity. The air volume behind the brick or cement block cladding is one single space all around the four sides of a house and the differing wind pressures create a forced draft that can crate air changes per hour of up to 200. Result being the wall underlay (or as called “sarking” in AU) is exposed to the temperatures of the outside or some small amount of 1 to 2 degrees C above it. In places like Canberra (AU) or Queenstown (NZ) with minus 6°C for long periods, this becomes serious.

Condensation first turned up in a big way in 1964 with the construction by the Victorian Housing Commission of their many 20 storey blocks of apartments all around the city and their use of six inch thick precast concrete panels with no proper insulation or vapour barrier in the walls.  CSIRO investigated the matter.  Such walls were just R 0.24. (In 2016 Melbourne demands R 2.8)

We also were aware of it from the condensation, in winter, on single glass windows and aluminium frames.

Ric Bonaldi was a Government appointee to the technical advisory committee of the Building Division of CSIRO from 1964 to 1988.

Whereas “Weathertightness” is to prevent rain and wind to enter or even pass through a wall or roof assembly. The subject is well covered in our three publications, see our web-page.

We show the following computer outputs on the temperature gradients of exterior walls as experienced in both Australia and New Zealand: -

FIG. 01 (below) This climatic diagram of an Australian brick veneer house is an extract from the Government ABCB handbook 2014 (page 90) on “Condensation in Buildings” Edition 2.

It assumes outside conditions in winter at early mornings with 6°C at 85% RH. It also assumes inside conditions as 20°C at 50% RH. This latter can only be assumed if the building is correctly designed for A/C, which would hardly apply to a single storey BV house. Further to this it assumes dry brickwork and an airspace cavity of 50mm that is fully sealed to reach the quality of insulation shown in the graph. Cavities in BV houses never are designed nor built to such criteria. In fact, the whole of the cavity all around a house is a single volume cavity, wide open by weep holes along the bottom of the wall and over the top of windows. With differing wind conditions around a free standing building this type of cavity is like a wind tunnel with air being blown in on the positive sides and extracted on the lee sides of the building. When that occurs, the outer bricks together with the cavity air no longer form part of the overall R rating of the wall.

In the case of Australia our building code results in around 50 to 100 air changes per hour for such a ventilated cavity. In the case of New Zealand with their outdated E2/AS1 and their Fig. 73C their ventilation for the identical construction is double that of ours and up to 200 air changes per hour.

This results in the air space having no real insulation value and in fact brings the space temperature and conditions to be equal to that of the outside air.

The ABCB figure 01 also clearly shows that when the outside temperatures go below the 6°C, condensation will occur in the wall. This would be typical for Melbourne and Hobart. In the case of NZ, the whole of that land mass will show condensation in winter. This design is not suitable for any parts of NZ and fully contradicts the rules of “place the vapour barrier onto the warm side of the insulation”. Here it is on the cold side of the wall insulation. The ABCB shown sarking (called wall underlay in NZ) is in fact a vapour barrier whilst the wall underlay used in NZ is a breathable plastic film allowing the escape of the water vapour from inside the building into the cavity. However, it hardly changes the outcome, see Fig. B_171, however with lots of condensation in the wall.

FIG. 01 as per ABCB publication (the single graph in a 130-page book)

False assumptions are included in this graph: for wet bricks, the cavity air and the foil reflectivity.

01final ABCB SYD B-Vff.png (202 KiB)  Brick veneer house Sydney as per ABCB publication
FIG. 01 (above) from ABCB 2014 publication

FIG. B_172

B_172.png (94 KiB)  Australian brick veneer house, Government ABCB handbook 2014, page 90
FIG. B_172

This is the corrected version with the wet bricks applicable, the cavity ventilated to the outside and the foil sarking (in NZ called wall underlay) with reflectivity as per the manufacturers tests. It shows the start of condensation on the inside of the foil and its wall insulation.  Taken at the outside and inside conditions named by the ABCB authority.

FIG. B_171 (below) Shows the same assumptions, but with the NZ wall underlay replacing the foil AU sarking. Again, condensation is forming, shown in yellow.

b_171 (97 KiB)  As for fig. B_172 but with NZ wall underlay replacing the AU foil sarking
FIG. B_171

FIG. B_182 (below)

This figure allows for the corrections to the interior temperatures to be 17°C at 60% RH, a figure we have tested in many cases of normal housing, early morning in winter, here in Melbourne.

b_182 (93 KiB)  As for fig. B_172 but in Melbourne winter.
FIG. B_182

When going for even lower exterior temperatures e.g. Canberra (AU) or Queenstown (NZ) the condensation becomes much greater.

FIG. B_192 (below) shows the formation of condensation in brick veneer walls for places like Canberra AU and Queenstown NZ, at their winter conditions, using a foil vapour barrier in the wrong location. This is suggested by ABCB for their own home town, Canberra!! How much science in building construction does ABCB use and understand on this subject?

b_192 (95 KiB)  As for fig. B_172 but in Canberra AU or Queenstown NZ.
FIG. B_192

FIG. B_195 (as for B_192 but as built in NZ with their breathable wall underlay. (No vapour barrier shown anywhere) by the NZBC)

b_195 (93 KiB)  As for fig. B_192 but as built in NZ with their breathable wall underlay.
FIG. B_195

The NZBC nominate a Fig. 73C (brick veneer house) in their details that is completely contrary to anything ever done in Australia.  Rainwater is driving right into the open gap between the window frame and the brick opening.  Also weep holes under the opening bring rain back into the assembly.

Fig. 73C in the NZBC is explained as follows...

All NZ legislation needs to do is to upgrade their E2AS1 and E3 sections of the NZBC to bring the design into the 21st century.  Billions of NZ dollars have gone to waste over the last 20 years by not understanding the difference between “weathertightness” of a wall and the formation of &ldqho;Condensation in the wall” from internal high-water vapour conditions which is always expected to occur in housing and cheap apartments when no properly designed A/C is installed and maintained to extract the high inside water vapour.

The industry's idea of letting the wall to “breathe” by extracting the excessive room moisture to outside via the wall construction is all wrong.  Get rid of the moisture inside or reduce it or increase the inner winter room temperatures and revert to the proven science: – “place a vapour barrier on the warm side of insulations”. Take insulation batts out of within the wall frame and place the insulation onto the outside onto the frame, with a factory built in vapour barrier.  See Queenstown NZ application and graph below.

BRANZ in their publication 30 October 2015 state that E2/AS1 is an MBIE document, so if a building built to E2/AS1 fails, then the DBH (old name of that department) has some responsibility, meaning the Code with its terrible design construction drawing like Fig. 73C is somehow warranted by the NZ Government.

Solution for both Australia and New Zealand

The placing of an air cavity between a cladding and the frame on a house is not well understood in our industry and not understood at all in Canberra AU and Wellington NZ.  Air spaces are heavily influenced by what is known as convection.  Google describes it as:-

Convection.  When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection.  The fluid above a hot surface expands, becomes less dense, and rises.  A circulation is started.  In the case of air or gases the value of being an insulation diminishes, due to the circulation.”

There is little research available on air cavity performances.  In the case of fully sealed double-glazed window glass units the best value, as tested by manufacturers, is on a cavity that is between 10mm and 20mm.

In a 50mm air cavity on a brick veneer house that has the cavity extended right around the whole building, a considerable air flow is created by the wind and its variable forces from the pressure side to the lee side of the building.  The temperature in such an air cavity takes up, most likely, that of the outside air temperature.  Some variation is expected should the cladding be under the influence of sun radiation.  Such radiation will rise the temperature in the ventilated cavity.  When considering the formation of condensation within the wall, the cladding needs to be assessed as being wet and there be no sun influence, heating up the cladding materials.

In the case of the NZ application of their 20mm batten cavity, the cavity is fully ventilated as per the NZBC, therefore the above convection principles also apply to the majority of their eight different cladding materials the NZBC nominates and fully details in E2/AS1 (and with the wall insulation completely missing in the details).  The NZ section E3 applies as well and the overall rating is to be retained, allowing for the close to zero values of the cladding and its air cavity.

This brings us back to the basic troubles in all of NZ and parts of AU of having wall underlays that will be reaching body temperatures equal to that of the outside air (e.g. Queenstown NZ and Canberra AU at -6°C).  With the NZBC saying nothing about the needs for vapour barriers, nor the correct location needed (the NZ wall underlays are no vapour barriers), the wall automatically finishes up with condensation within the wall and results in timber decay, most of which is showing up after some 8-10 years after construction.  Read the Price Waterhouse Coopers report to the DBH of 2009.

First: –

To avoid condensations within exterior walls as well as within roofs you need to adopt the “place a vapour barrier on the warm side of the insulation”.  This is a common and internationally adopted recommendation.  If you keep on placing insulation materials within the framing of a wall you need a vapour barrier onto the inside of the framing, then attach another batten and the plasterboard lining.  This allows the electrical wiring to be done within that batten cavity.  A vapour barrier is NOT a breathable flexible wall underlay as used in NZ.

Alternative: –

Adopt 21st century materials that have even better insulation factors than the batt type of loose fill and has its own vapour barrier, and on both sides.

The following diagram shows the construction. It uses large sheets of insulation attached to the outer frame face and reduces costs of labour.  Double sided foil faced insulation also can be used in hot climates where condensation can occur with high outside temperatures and with high humidity and internal cool temperatures by A/C systems (e.g. Malaysia, Singapore etc).

B_200.png (97 KiB)  How to build a house in Queenstown NZ in the 21<sup data-recalc-dims=st century">
Above: –

How to build a house in Queenstown NZ or in Canberra AU in the 21st century and to prevent condensation in winter.

Explanation of the saying: – “place a vapour barrier on the warm side of the insulation”

This is well recognised all over the world and referred to in Australian literature back in 1977 as follows:

Vapour Barriers

One solution to the condensation problem is to include a vapour barrier to prevent water vapour reaching any surface which is cold enough to cause condensation.  The vapour barrier should be placed on the warm side of the structure and/or the insulation material part of the wall, roof or floor in order to keep its temperature above the “Dew Point”.

A vapour barrier is defined as any membrane which will sufficiently restrict the migration of water vapour from the warm moist interior of a building to the wall, roof or floor cavity where it may contact a cold surface.  Vapour barriers may be formed by such differing materials as a well-preserved and constantly maintained film of paint on the inner lining, a polyethylene film or an impervious metallic layer such as aluminum foil.  Common Australian vapour barrier is made from Aluminum foil, laminated to paper and reinforced with fiberglass and/or sisal fibres.  It is one of the most effective and widely used.  In the case of a metal curtain wall claddings it is the inner metal sheeting on the spandrel sections of the wall component.

(The above includes extracts from the 1977 research and publication by St Regis-ACI Pty Ltd Company in Australia manufacturing Sisalation sarking sheets and as drawn in the ABCB Fig. 01)

FIG. 07 (below) NZ Government design figures for the Auckland NZ winter

05Auckland temp_Gov data2.png (134 KiB)  Government data on winter conditions for Auckland NZ
FIG. 07

FIG. 08 (below) Where does condensation come from? Source from NZ BRANZ. BRANZ ought to tell the NZ Government all about it! It should have also shown the current trends to adopt re-circulating type of range hoods over kitchen stoves that retain all the water vapour inside the building in lieu of extracting it through the wall to the outside or to a ducted exhaust system!

06_people_a.jpg (213 KiB)  Government data on winter conditions for Auckland NZ
FIG. 08 Where does condensation come from?

NZ Solution

Retain the DBH idea of the 20mm batten cavity but without natural ventilation.  Adopt our Fig. B_200 with the taped outer foil insulation (insulation with foil to both sides) thereby avoiding passing all the interior water vapour out through the walls.  Cavity to be drained on the bottom and above any openings in the wall with simple drainage holes or slots.  Holes not greater than 6mm diameter or if slots 6mm high (vermin proofing).  Total drainage to be a cross sectional area of 200mm² per 1000mm of cavity length.  If other details are used in placing insulation, then place a vapour barrier on the warm side of the insulation plus a taped joint vapour barrier onto the outer face of the framed wall.

Note that the foil faced insulation panels become the NZ rigid wall underlay allowing stud spacings at 600mm c/c.  Flexible underlays demand studs at 450mm c/c.  All together the B_200 detail is likely to show cost savings.

The flexible wall underlay used in NZ is not a vapour barrier.  The material is a breathable plastic sheet and if lapped and taped will not pass liquid water but passes water vapour in both directions.  NZ has done no research to assure the taped faces stay watertight for the life span of a building.  Water vapour then passes from the high humidity batten cavity back into the wall frame as well as vapour from inside the building out into the cavity.  No logic in using such materials in cold climates like NZ.  Also no logic in ABCB in Canberra AU that has -6°C winter days for several days and suggest Fig. 01 that will show heavy wall condensation at that temperature when Fig.01 clearly shows condensation to start as soon as outside temperature is lower than +6°C!

Our way, the simply drained air cavity, and correctly applied insulation, together with the outer cladding material will then start to contribute to the total R rating of the wall.

One other point is the warning not to attach any flexible metal foil to metal wall or roof framing.  Use a thermal break foam tape between the two.  When using the foil faced rigid insulation boards, that is not a problem.