You've figured out the look and feel. You've got the massing and aesthetics sorted, chosen textures and colours, and brought the client on board. Now it's time to get into the detailing.
The seemingly easy move is to grab the building code-prescriptive details and the supplier's typical drawings, pull them into the drawing set, and try to get them to fit together nicely, while still achieving the desired look. But many times, that's easier said than done. Sometimes, it is impossible to do so with the information available.
Before diving into the specifics, here's a high-level checklist of what to consider when detailing a high-performing, durable building envelope:
- The right assemblies: How to select the right assemblies for structure, aesthetics and sequencing
- The 4 Ds: How to apply deflection, drainage, drying and durability to every detail
- Transitions and terminations: How to create sealed, redundant and failsafe junctions
- Material compatibility: What to watch for beyond dissimilar metals
- Movement accommodation: How to allow for structural, thermal and moisture-related movement
- Constructability and maintainability: How to design for efficient buildability and long-term upkeep
The Right Assemblies
Before getting into specific products and details, you'll need to choose the overall assembly, such as timber-framed, precast concrete, steel stud, SIPS, etc.. That decision should align with the structure, desired look, and how the building will ultimately come together. You'll need to take into consideration:
- Structural movement: Short, square buildings tend to be stiff; tall, narrow buildings tend to move more. Mass timber is more flexible than concrete. Base-isolated buildings move less than buildings fixed to the ground. Select a cladding system that aligns with the movement profile of the building. Metal claddings and stick-framed curtain wall tend to accommodate more movement than masonry claddings and unitised curtain wall. If you're trying to specify a stiff cladding (e.g. brick or stone) on a flexible building (e.g. mid/high rise mass timber), then you will need to do a lot of work to accommodate movement without damage, whereas using a flexible cladding (e.g. weatherboards) over a stiff building (base isolated concrete frame) will work very easily. Select the appropriate assembly for the specific structure.
- Construction sequencing: A constrained site or expensive scaffold can drive the need for unitised curtain wall or prefabricated panels. For mass timber structures, quick-close roof or wall assemblies reduce moisture absorption during construction. Choose assemblies that support the construction programme, not those that hinder it.
- Energy efficiency and comfort expectations: Different building types have different thermal and acoustic needs. For instance, floor-to-ceiling windows look great in a residential tower but are likely inappropriate in hospitals. A house with an uninsulated attic and metal roof is typical, but if that space is used for HVAC or storage, a warm roof would be better. Think of the expectations for each space and use assemblies that meet those expectations easily.
- Aesthetics: This one is listed last because if the first three are not considered, the aesthetics and perception of the building will deteriorate much faster than intended. Ideally, choose a look and massing that work well with the structure of the building and complement the strengths of the exterior envelope assemblies.
The 4 Ds: Deflection, Drainage, Drying, Durability
These are the essential performance layers of every well-designed detail:
- Deflection: Includes claddings, flashings, caps, saddles, canopies, and sealant joints. The job of the deflection layer is to block water at the exterior by shedding water, stopping wind-driven rain, mitigating capillary action and protecting the other seals within the assembly.
- Drainage: A second line of defence that lets water escape if it gets past the deflection layer. This includes drained cavities behind cladding, roofing underlays, or mats under tanking. Properly designed, it provides redundancy - when cladding or sealants fail over time, water is still kept out.
- Drying: The ventilation pathways, brick vents, open joints, and gaps around fire stops help dry any water that doesn't drain immediately. This layer is designed to promote airflow and evaporate drips, while also promoting long-term moisture removal from the full assembly.
- Durability: This includes consideration of all the products, components, and materials in the assembly, as well as their interaction and compatibility with each other, and how each one will withstand the exposure it will encounter over its lifetime. Every component should be suited to its environment. UV, wind, rain, and salt; all will wear materials over time. Elements that are hard to replace should be the most durable. If something is buried in a wall or under a deck, don't skimp on it.
Sealed, Redundant and Failsafe Transitions and Terminations
Most systems are weathertight on their own, but the weak points are where one system meets another. Apply the 4 Ds at every transition and termination to ensure continuity and consistency:
- Start with line tests. Can you draw a continuous line through the detail for airtightness and weathertightness? Do the insulation layers align with minimal thermal bridging? Is the deflection layer at the exterior mostly continuous with some laps over ventilation openings?
- Next, ask: if water gets past the exterior, does it easily drain down a cavity and out via a flashing? Is there ventilation in that cavity to dry out the items? Is the cavity free and clear from floor to floor? Once water drains out of a cavity or off of a flashing, does it drip away from the building to the ground, or does it run down vertical surfaces, causing premature staining?
- Lastly, re-examine the sealant joints. Are they just for aesthetics and deflection, or are they the only line of defence against water and wind pressure? Are they replaceable and maintainable long-term? If they crack and fail before being replaced or maintained, is there a redundant drain pathway behind them to safely remove the water?
We explore this in more depth in our article Building With Science 007.
Material Compatibility
Most people know about avoiding direct contact between dissimilar metals, but there's more to watch for:
- Sealant to membrane compatibility: Some sealants can be applied directly to bitumen-based (torch) membranes, while others need a separation layer. This is common around doors.
- Water runoff reactions: Metal runoff or chemicals in treated timber can stain or corrode nearby surfaces.
- Adhesion: Some surfaces and membranes can be pretty smooth/slippery and may need primers, transitions, or surface treatments to allow tapes and sealants to adhere properly
- Flexibility: Are the two materials equally flexible and will move together, or will movement of one material move or damage the other without a slip layer (e.g. adhering tiles to membrane)
Movement Accommodation
Regardless of the building or material, everything moves, and that movement should be accommodated without causing damage. Some examples of movement include:
- Building movement: Account for interstorey drift (side-to-side), which on average is around 10mm SLS or 40mm ULS, and live load deflection (up and down) at displacements usually less than the interstorey drift. To accommodate both wall framing, RAB, and wrap, a horizontal movement joint would likely be needed just below the floor structure, and vertical movement joints would be required at specific corners. Rigid, brittle heavyweight cladding, such as brick, also requires vertical and horizontal movement joints and movement-accommodating brick ties to allow for building movement without damage.
- Thermal expansion: Solid materials grow when warm and shrink when cold. This is most severe for metals, especially aluminium, so long lengths of cladding, roofing, metal battens, flashings, etc., should have accommodation for thermal movement.
- Moisture movement: Timber and masonry expand when wet and contract when dry. Protect mass timber during construction and detail brickwork with movement joints every 7 metres minimum. For mass timber, this means protecting the timber during construction to prevent growth and shrinkage cycles, which can cause cracking and increase water entry.
- Freeze/thaw: This is a common consideration in cold climates. Since water expands when it freezes into ice, any water entering an external envelope assembly could freeze and damage components if there is no space to accommodate the expansion. Most times, this is accommodated with a cladding or roofing drainage cavity; however, it is a significant concern when concrete is used as the exterior cladding, as cracks can form and grow over repeated freeze-thaw cycles, causing deeper damage. Therefore, designing to minimise cracking, applying coatings that shed water, and conducting annual inspections and repairs are crucial to maintaining durability.
Constructability and Maintainability
It's easy to overlook this in the rush to complete documentation, but it's crucial for maintaining construction flow and reducing long-term maintenance costs.
- Fixings, sealants, tapes, and coatings: Can they be installed within the expected sequence of events? Sometimes, flashings, angles, window flanges, and other elements are in the way. Sometimes, the tape detail does not consider which side has the adhesive, and sometimes, certain coatings cannot be applied to specific substrates.
- Insulation install: Which side of the wall or roof does it go in from, and does it need to be protected from rain?
- Warm roof assembly: Can the vapour barrier go down all at once to protect the interior from rain? Can the assembly be temporarily sealed at night to avoid rain, or does the building need a tent?
- Access: Can the roof, decks and other critical locations be accessed to inspect and maintain? Do panels or hatches need to be installed to allow access?
- Transition details: Can one assembly be maintained or replaced without damaging or affecting adjacent assemblies? (e.g. can you replace the roof membrane and upturns without removing the cladding and doors?)
There are always more specific considerations to explore when showing code compliance, but asking these questions upfront will make detailing faster, more robust and ultimately more buildable.
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