A new look at the requirements for weathertightness

POSTED: 02/29/2024

Houses are our refuge. In stormy situations, our houses need to keep the wild weather out, while we remain safe and protected inside. In a country like New Zealand, it can get stormy any time of the year. Therefore, all houses need to be weather resilient. Most of NZ is subject to high winds and heavy rain.

In the technical world, this requirement is referred to as “weathertightness”.

What is weathertightness?
Weathertightness is simply a term used to say that the house (or building) is able to keep the bad weather out, ensuring we as users, stay safe and dry inside.

Today, house designers have a wealth of materials and new construction methods, to achieve this. Use of water-resistant boards, when sealed at their edges with modern weatherproof tapes, can be used to  create what is known as a “rigid air barrier”. These systems ensure both the air (from winds) and moisture (from driving rain) are kept at bay. The simplicity of such design ensures consistent and reliable performance, even when untrained persons are employed to install these products.

Rigid Air Barriers

Yet experience shows that some houses do leak – either from air or moisture permeability. Why is this? What have the designers, and presumably the suppliers of such products, overlooked? To find the answer to these questions, we first need to understand what houses need, especially when the designer has employed the use of a rigid air barrier, which is supposed to provide resistance to strong winds and rain.

Ignoring for the moments the protection afforded by roofs over a house, it is obvious that exterior walls need to both keep bad weather at bay, while at the same time let light in, along with fresh outside air, to ensure good indoor air quality. So how are these achieved? Let’s look for the answers.

The materials used to keep the weather out, including rigid air barriers, need to be both water resistant and durable. The material or product needs to also resist the forces of nature during a storm, namely high winds. In New Zealand, winds are generally of a low impact when a house is located on flat land. But as the placement of houses gets higher and higher, so do the forces from stormy winds. In fact, the forces on some hills, especially exposed locations such as in Wellington near the airport, the wind forces can become huge, needing extra resistance to all materials to keep bad weather out.

Weathertightness of Windows and Doors and Other Penetrations

The designer also needs to account for the use of windows and doors. In New Zealand there is now a wide range of window and door types, which means that their shape and profiles are not consistent. Therefore, each shape and profile need to be accounted for when designing an exterior wall to be resistant to stormy wind and rain. Then there is the need to ensure other types of openings in the wall are equally resistant to wind and rain, such as air vents, pipes that go through the wall and electrical conduits, typically needed for heat pumps, security lights and the like.

Testing Products for Weathertightness

Suppliers of products designed to provide weathertightness, need to test their products in all kinds of weather (using a lab-simulated stormy environment) to verify that the products are both easy to install and can perform in a wide range of stormy situations.

 Such testing requires the combined efforts of:

a) the supplier/manufacturer,

 b) the designer,

c) a structural engineer looking at the effects of strong wind forces,

 d) building technicians experienced in the testing of such products.

 So how does one assess weathertightness of a product? What are the expectations? Are there rules or Standards for these types of products? Let’s look at each of these.

Practical Requirements

If the criteria from a home-owner’s point of view is quite simple – that a product simply must keep bad weather out, then that becomes the requirement. It is relatively easy to assess, even in a lab induced stormy environment.

In essence, when an exterior wall product is subjected to a combination of wind forces and driving rain, with all typical openings and the like, the wind and rain can’t get past the wall product. Simple as that. In a lab test situation, this is easy to see, therefore the assessment process is quite straight forward. However, from a lab technician’s experience, most new exterior wall products either leak air and or water, particularly around window and door openings. And if the right products are not used to seal pipes and conduits as well, these will also leak.

In other words, there is a science behind the correct design and use of suitable materials to achieve a wind and rain resistant wall product. Interestingly, there are few international Standards or Guides on how one should design and test such products, which is why it is such an important subject for home-owners and other professionals here in New Zealand.

Elements of a Wall -Technical Aspects

When looking at a modern exterior wall product, we need to understand each of the “elements” that make up the installed product. Here is what to expect:
a) either a building paper or a rigid sheet material, both resistant to the passage of wind and rain
b) fixings, to hold the material to the underlying framing or structure
c) rubber gaskets to seal all pipes passing through the wall to the building paper or rigid air barrier
d) windows and doors, including the supplied flashing materials
e) air vents and the like – though these often come through the soffit rather than through the wall
f) sealing tapes – sometimes as many as 3 or 4 different tapes for different purposes and substrates.

At first, these would all look like the type of elements that one might expect to be part of the exterior wall design. If as the supplier states, the base building paper or rigid sheet material is resistant to both air and rain, then how can an exterior wall leak? The answer, from experience, lies in the way that each type of opening or penetration is sealed to the building paper or rigid sheet material.

Take for example, the penetration through an exterior wall from a water pipe or an electrical conduit. How is this sealed to the building paper or rigid sheet material? Today, use is made of special ‘gaskets’ which are purpose-designed to seal against wind and rain. Therefore, not using such gaskets, would allow possible entry of unwanted wind and rain. The same can be said for other small penetrations, such as screw fixings. Then there are air vents. Interestingly few if any air vent manufacturers provide a flashing kit to ensure that the small opening is properly sealed against wind and rain. However, by far the greatest risk, lies in the inability to properly seal the perimeter of all windows and doors to the building paper or rigid sheet material. Ignoring for the moment the performance of the windows and doors themselves, let’s look at what is required to seal these to the building paper or rigid sheet material and thus ensure proper and reliable weathertightness.

 Sealing of Windows and Door to Achieve Weathertightness

As suggested above, the sealing of windows and door openings is almost a science and without good design, something that is not reliably achieved.
From experience with testing many windows and doors, what is needed is effective sealing between the outside perimeter of the window or door, to the surrounding building paper or rigid sheet material. Internationally, and here in New Zealand, it is understood that special sealing tapes are required. These sealing tapes are specially designed to allow two or three different surfaces to be sealed, one at a time, to ensure each sealing step is effective and reliable. This is achieved by one or more slits in the backing liner of the tape, allowing one section of the sealing tape to be applied at a time. In some cases, these tapes allow for the sealing to masonry or timber or uPVC substrates, each ensuring a reliable seal.

In some cases, the shape or profile of the window or door may contribute to the difficulty of sealing the item to the surrounding building paper or rigid sheet material. In these cases, careful design, and advice from a supplier of sealing tape products, is essential to achieving a reliable performance – especially when it comes time to verify the performance in the lab simulated environment for certification purposes.


We all want to live in houses that are designed to keep the wild weather out, while we remain safe and protected inside. As suggested above, the way to achieve this, particularly for houses in many locations in New Zealand, is to select systems that can reliably provide this performance. Getting good advice from experienced designers is the first step. Using systems that have third-party certification is the second step. Employing trained tradespeople to properly install these systems is the third step. Some system suppliers may also require site checks and record-keeping as well. All of which can provide the assurance that everyone, especially Councils, are seeking. Clearly, the requirements for weathertightness have changed.

C Prouse – Director, BEAL Research Institute – January 2024

Keywords: E2, E3, Rigid Air Barriers, sealing of windows and doors