L7: Building Envelope Flashcards
What is considered a low-slope roof?
Pitch below 25%
What are the basic components of low-slope roofs?
Deck Thermal insulation Vapour retarder Roof membrane Drainage
What is the minimum slope a roof deck can have and why?
2% so it can drain
What can happen if a roof is not drained properly?
Progressive structural collapse (puddles = structural deflection cycle)
What are the potential placements of thermal insulation and vapour retarder?
Below deck
Between deck and membrane (most common)
Above membrane
How is insulation below deck installed?
Insulation –> deck –> membrane –> ballast
Between underside floor framing members - exposes deck and membrane to wide temperature fluctuations and requires ventilation so vapour can escape
How is insulation between deck and membrane installed?
Deck –> insulation (2 layers) –> membrane –> ballast
Insulation supports membrane from below (but exposed to temp fluctuations) and protects deck from above from temp fluctuations
Must have topside vents
What are topside vents and why are they required when insulation is between deck and membrane?
To let out trapped vapour; without them, insulation decays and blisters/ruptures form in membrane
Located 1 every 100m2
How is insulation above membrane installed?
Deck –> membrane –> insulation –> ballast
Insulation protects membrane from temperature and no vapour because membrane is on warm side of insulation
What are examples of ballast?
Pre-applied thin concrete layer
Crushed stone
Interlocking concrete blocks (pavers)
What is a common material for insulation above membrane and why?
Polystyrene foam panels because it is exposed to water and must maintain qualities when wet
What are the categories of membranes for low-slope roofs?
Built-up roof membrane
Single-ply membranes
Fluid-applied membranes
What is built-up roof membrane?
2-4 layers of asphalt felt imbedded in bitumen; applied hot and merges into singe membrane
What are single-ply membranes and the two types?
Affixed to roof deck by adhesives, ballast or fasteners
Thermoplastic - softened by heat and welded at seams
Thermosetting - cannot be softened so joined by adhesive
What are fluid applied membranes and where are they used?
Liquid coats applied with roller/spray gun cure to form rubbery membrane
Used for complex shapes eg. domes
What is a potential problem with fluid applied membranes?
They create moisture barriers but if there is a failure and moisture gets in, it causes leaky building
What are the two ways ‘flat’ roofs can be drained?
Outer edge towards external gutters
Internal roof drains in roof
How can slopes be created in roofs?
Roof structure is sloped
Sloped topping screed on flat roof deck
Tapered insulation on flat deck
What is glass made of?
Sand mixed with other materials, heated into a liquid then cooled
What are the 8 types of glass?
Annealed Tempered Heat-strengthened Laminated Spandrel Tinted and reflective Insulating Low emissivity
What is annealed glass?
Cooled slowly in controlled conditions so breaks into large shards
What is tempered glass?
Edges cooled rapidly (compression) and core cooled slowly (tension); breaks into small square sections
How much stronger is tempered glass than annealed glass?
4 times
How does heat strengthened glass compare to annealed/tempered?
2 times stronger than annealed so used when higher stressed but do not require safety factor of tempered
What is laminated glass, what happens when it breaks and where can it be used?
Two sheets of glass with a layer of PVB in between not as strong as annealed
Broken shards hold together
Skylights, bullet proof glass
What is spandrel glass?
Opaque glass that covers spandrel areas, hiding features
What is tinted and reflective light?
Chemicals added to reduce glare and solar heat gain; heat absorbed and radiated inside
What is the visible light transmission in tinted glass and compared to clear glass?
Tinted - 14%-75%
Clear glass- 85%
How is heat reducing glass compared?
Glazing luminous efficacy (Ke) measures energy-conserving potential; how much heat is blocked compared to light
What is reflective coated glass?
A type of tinted and reflective glass with metal films attached to the surface which blocks heat and light (lowest Ke)
What is insulating glass?
Sheets of glass separated by air/gas to improve insulation
What is low-emissivity glass?
Glass with a metallic coating which reflects light and infrared
Why is triple glazing losing its market?
Because it is the same as double glazing with a coat of low-emissivity which is much less bulky
How is exterior cladding supported?
At each storey by the frame
What do curtain walls support?
Internal portion supports loads and external portion does not support any loads
What are the primary functions of cladding?
Keeping water out
Preventing air leakage
Controlling light, heat radiation, heat conduction and sound
What are the conditions required for water to penetrate cladding?
Water present at outer face
Opening for water to pass through
Force to move water through opening
How are walls made watertight?
Keeping water away from the wall - deep overhangs but unavoidable in high rise
Barrier wall approach to eliminate openings - covering gaps but requires secondary defences eg. drainage
Eliminate or neutralise forces
What are the 5 forces that can move water through an opening and how are they overcome?
Gravity - do not slope inwards
Momentum - cover over joints or labyrinth
Surface tension - provide a drip
Capillary action - opening wider than a water drop so it cannot bridge it
Air pressure differences - rainscreen
What is surface tension?
Water can move horizontally by holding on to a surface
What is capillary action?
Water travelling horizontally through small openings by ‘holding on’ to both sides
How does air pressure differences move water?
If pressure is different, wide travels through and can carry water
What is a rainscreen and how does it work?
Air barrier against back up wall which equalises pressure between inside and outside with small air currents that pass in and out
Why must air pressure systems be compartmentalised?
To accommodate for different air pressures at different parts of the building cladding
What are the secondary functions of cladding?
Resisting wind forces Controlling water vapour Adjusting to movement (thermal, moisture, structural) Resisting fire Weathering gracefully Ease of installation
What are 5 cladding systems?
Masonry veneer Stone panels mounted on steel subframe Stone cladding on steel trusses Precast concrete curtain walls Metal (aluminium) and glass
How does masonry veneer cladding work?
Supported on a steel shelf angle (each storey) and tied to a structural back up wall with brick ties
What are two types of structural backup wall used in masonry veneer cladding?
Concrete masonry unit (CMU) - preferred, stronger
Light-gauge steel frame - lightweight, deflection
Where are soft joints placed in a masonry veneer cladding system and what do they do?
At the bottom of shelf angle and vertically at regular intervals
Allows independent movement of veneer and frame
What must be put in masonry veneer cladding to allow moisture to escape and from where?
Weep holes
The air cavity between the veneer and the backup wall
What makes up a stone panels mounted on steel subframe cladding system?
Vertical steel members transfer vertical and horizontal loads from panels to frame
Horizontal aluminium fits in slots on panels
Gaps filled with backer rods and sealant
Non-structural backup wall provides air barrier for thermal insulation, wiring, interior finish
What makes up a stone cladding on steel trusses cladding system?
Prefabricated sheets of stone mounted on structural steel trusses (transfers load to frame)
Sealant, backer rods, non-structural backup wall
What makes up a precast concrete curtain wall cladding system?
Precast panels (rebar or prestressing) typically full floor height and adds considerable load to building frame
Why is aluminium the metal of choice in a metal and glass cladding system?
Protects against corrosion and can have a variety of surface finishes and shapes
What are thermal breaks?
Put in aluminium to reduce rapid conduction of heat and avoid condensation inside
What are the modes of assembly for a metal and glass cladding system?
Stick system
Unit system
Panel system
Column-cover-and-spandrel system
What is a stick system and what are the components?
Shipped in low bulk pieces and assembled on site
Vertical mullions Horizontal rails Spandrel panel Glass Interior mullen trim
What is a unit system?
Factory assembled large shipping bulk erected by crane
What is a panel system?
Homogeneous units formed from metal sheet with custom-made die/mould; economical for large number of identical panels and has large shipping bulk
What is a column-cover-and-spandrel system and what are the components?
Custom design each time
Column cover section
Spandrel panel
Glazing infill
How can glazing be installed?
Inside or outside (inside for taller buildings - bottom half of mullion installed after glass)
How does the rainscreen principle apply to a metal and glass cladding system?
Outer and inner gaskets in mullions and horizontal rails to stop water penetration; inner gasket if outer fails, acts as an air barrier
Water drained through weep holes
Why must a metal and glass cladding system provide for thermal expansion and how is it done?
Aluminium expands twice as much as glass during thermal expansion
Gaskets provide flexibility between glass and aluminium
Vertical - telescoping joints
Horizontal - rails cut shorter (interrupted by mullions; mullions cannot be cut because they are continuous over the building)
