Windows, thermal mass and the forgotten facade

Houses are drawn from the room that looks out. The elevation that faces the water or the garden gets the glass, the money and the argument. The other three elevations become leftover wall. Climate does not honour that hierarchy. The forgotten facade — the one that is not the view — is often the one that decides whether the view is still pleasant in February.

This is a materials essay about glass, mass and the wall as a system. The plan argument and the verandah as climate equipment sit in designing for climate, not just for the view. The brief that should contain the glazing ratio sits in concept design. What follows is what the envelope is made of, and what it will do at 3 p.m.

Glazing ratio is a heat budget

Glass is not a wall. It is a product with a frame, a solar heat gain coefficient, a U-value, a BAL rating and a leak path. Count it. A north wall that is half glass can be a winter strategy if the eave is sized. A west wall that is half glass is often a cooling system with furniture in it. An east wall of glass is a summer morning you will not sleep through. A south wall of glass is a heat leak in July, except in the tropics where south can be the kinder face.

Write the glazed area on each elevation as a percentage of that wall, and as a square-metre total. Say which panels open, which are fixed, and which the occupant will actually use. A six-panel stack that stays shut because the track fills with grit is not ventilation. It is a very large IGU.

The National Construction Code, NatHERS and, in New South Wales, BASIX will force a number onto this glass. They are happier if you gave them a number first. Product is expensive when it is asked to correct a ratio. It is reasonable when it finishes a ratio that was already adult. The climate work of an opening is the opening, not the hardware catalogue.

SHGC is the number under the view

Solar heat gain coefficient is the fraction of sun that becomes heat in the room. Architects still talk about "good glass" as if glass were a single virtue. It is at least two: how much sun you want, and how much heat you will tolerate when you cannot have the sun. A low SHGC on a shaded north window in temperate Australia can steal the winter you were counting on. A high SHGC on an unshaded west window is a radiator you specified.

Match the glass to the orientation and the shade, not to a single house-wide product:

• North, with a working eave: you may want winter sun. A moderate SHGC can be the point. The eave is the summer switch, not the coating.
• East: morning sun is useful in winter and a nuisance in a bedroom in January. External shade or a lower SHGC, and a room that can be closed.
• West: assume you want less sun, not more. External shade first. Then a lower SHGC. Then less glass.
• South: gain is small. U-value and comfort near the glass matter more than a solar story.
• Skylights and roof glass: these are west windows that cannot be eaved in the ordinary way. Treat them as a load, or make them operable and shadeable.

U-value, coatings and the frame

U-value is the overnight and winter companion. A beautiful winter sun on a single-glazed north wall is a beautiful loss after dark. Frames matter. A good IGU in a poor frame, or a frame that is a thermal break in name only, is a brochure. Specify the system — glass, spacer, frame, installation — as the thing that was tested.

Low-e coatings are not interchangeable stickers. Some cut gain, some keep heat in. Read the numbers for the climate you are in. Put WERS ratings on the window schedule next to the size and the opening. Do not write "aluminium sliding door" and leave the SHGC to a substitute.

Eaves are the cheaper coating

A north eave, sized for latitude, can do in timber or metal what a coating does in a laboratory: cut the high summer sun and admit the low winter one. It has no moving parts if you do not give it any. It fails when it is drawn as a style — a token 300 millimetres because the elevation looked heavy — or when it is asked to shade a west wall that needed a vertical screen.

Check the eave on a section at midsummer and midwinter, at the hour the room is actually used. If the summer sun still hits the glass at 2 p.m., the eave is decoration. If the winter sun never enters, you have built a cave. The verandah, the hood, the pergola and the deciduous vine are the same device in different clothes.

West and east glass want vertical shade: a fin, a blade, an external blind the occupant will use, a screen the BAL will accept. Internal blinds are a glare control. They are not a solar control. The heat is already in the room. Bushfire and cyclone change the shade you are allowed. A timber screen in an ember path is a different product from a timber screen on a BAL-LOW courtyard. The forgotten facade often holds the services and the neighbour. It can also hold the shade the glass needed.

Night purge is a schedule for mass

Thermal mass — a slab, a masonry wall, rammed earth — stores the day's temperature and gives it back. That sentence is true and incomplete. Mass stores whatever you give it. Give it an unshaded west window and it will give you the punishment at 10 p.m. Give it a shaded north sun and a night that can take the heat away, and it will give you a milder room tomorrow.

Night purge is the missing half of most mass conversations. Openings high and low, a stair allowed to be a chimney, a security and insect-screen arrangement the occupant will leave open, and a night that is actually cooler than the day. A heatwave in which the night stays warm is a week when mass without a machine is a liability. Design for the ordinary summer night, and for the week when the house must close. Operable is the word. A fixed highlight is a photograph. A skylight that cannot open is a cooker.

Brick and concrete mass versus a lightweight skin

The Australian suburban default is a light frame with a brick skin outside the insulation. That brick is weather and a look. It is not the mass the energy model wished for. Reverse brick veneer, a block wall inside the insulation line, a slab with an insulated edge — these are mass used on purpose.

Choose mass when the climate has a useful night and the mass is inside the insulation line. Choose lightweight when the climate is humid and the night is shy, or when the house is raised.

• Mass, used on purpose: reverse brick veneer, a block wall inside the insulation, a slab with an insulated edge
• Mass as a look: a 110-millimetre skin outside the batts, a polished slab under an unshaded west slider
• Lightweight, used on purpose: a pale, vented skin, shade, a breeze, the option to close
• Lightweight as a failure: a dark zinc box with no eave

A Queenslander is a shade-and-breeze machine. Putting a concrete bunker on a wet tropics lot because mass is fashionable is how you store the wet season. Alpine houses want mass with insulation outside it. Mass and light are both assemblies. Neither is a moral position.

The forgotten facade is the one that works

The view elevation will always be over-designed. The others will always be under-considered unless you refuse that habit. The south wall can hold the services, the rain and a modest window that does not leak heat. The west can be almost blind, or a small window with a screen. The north can be the glass you can afford because you did not spend it on the other three.

Insulation, colour and airtightness live on the forgotten facades more than they live on the view. A dark uninsulated west wall behind a feature tree is still a west wall. A garage on the west is a useful sacrifice if the living rooms are elsewhere.

The view should be a window you can shut. The forgotten facade is the wall that lets you occupy the house after you shut it.

Write the four elevations as a single heat budget. Put the SHGC and the eave on the same section as the slab edge. Say whether the mass is inside. Say which openings are the night. The NCC will then be asked to ratify a climate, not to rescue a photograph.