Windows by the number: sill, head and where they go
A sill height decides what the window does. At 900 mm it lights a room, at 300 it ventilates one, at 1,500 it does neither.
Why copied window sizes trap heat and moisture
Most window schedules on house drawings get copied from another house down the road. You see a standard glass window with a low frame, copy it into every bedroom, and assume the room will breathe.
When windows sit at arbitrary levels, heat builds up against the ceiling. Moisture stays trapped inside the masonry. Fixing a misplaced lintel or cutting down an already-built brick opening after casting a slab will break your budget and your walls.
A window is not a decoration. It is an opening sized to handle daylight, shed rain, and move air across your skin. Getting the levels wrong means you live in a dark, humid box, or you spend decades running mechanical fans to do the work a properly placed opening does for free.
The standard levels and their sources
Every sill height and window opening must respond to a specific mechanical need.
The National Building Code Part 8 states that every habitable room requires openings on two different faces to achieve real ventilation. One single opening in a room is not ventilation.
Sun arithmetic for 8 to 16 degrees North latitude shows that north light carries no direct sun. This makes the north face the correct side to open for daylight without heat gain. At these latitudes, the sun geometry dictates that external shading must project at least 600 mm outward when paired with a 900 mm sill.
| Window type and location | Dimension or rule | Source |
|---|---|---|
| Shading projection over sill | 600 mm projection over a 900 mm sill | Sun geometry at 8–16°N |
| Habitable room ventilation | Openings on two different faces | National Building Code Part 8 |
| Wet compartment ventilation | High-level louvre touching an external wall | Professional practice; National Building Code Part 8 |
| Window reveal curtain clearance | 250 mm curtain overrun past reveal | Professional practice |
| Bed position relative to window | No bed placed under a window | Professional practice |
Professional practice requires that every wet compartment must touch an external wall and carry its own high-level louvre. An exhaust fan alone is not ventilation.
How to apply sills and shading to a layout
In bedrooms and living rooms, set your window sill to 900 mm above finished floor level. Pair this 900 mm sill with a minimum 600 mm external shading projection. This combination blocks high-angle sun from hitting the glass while allowing light into the middle of the room.
Because north light carries no direct sun at 8 to 16 degrees North, open your north walls with clear glass. Blank your west walls. If you must place a window on the west, keep the opening small, because the west wall takes low-angle afternoon sun when ambient air temperatures are already at their peak.
To ventilate any room, plan two openings on different faces, as required by the National Building Code Part 8. Set the inlet low on the windward side and the outlet high on the leeward side. The warm air rises, leaves through the high opening, and pulls cooler air in through the low opening.
In bathrooms, do not rely on an exhaust fan. An exhaust fan is a mechanical aid, not a substitute for fresh air. Put the bathroom on an external wall and provide a dedicated high-level louvre. This louvre exhausts steam directly outside at ceiling level before damp air can settle into your bedroom walls.
Always check where the curtains will sit. In our practice, we allocate a 250 mm curtain overrun past the reveal on each side of the opening. If you tuck a window tight into a room corner, the curtain stack blocks daylight and jams against the adjacent wall.
The layout mistakes this prevents
The most common mistake is placing a bed directly under a window. Our design rules forbid putting a bed under a window. Sleeping under an open sill leaves you exposed to drafts, rain spray, and exterior noise.
Another frequent failure is planning a single, large picture window on only one wall. A single window does not create air movement across a room. Air cannot enter if it has no path to exit. Adding an opening on a second face, as required by the National Building Code Part 8, creates cross-ventilation.
Never put an unshaded window on an east or west wall. Morning sun on an east wall lasts only an hour or two. Afternoon sun on a west wall strikes at an 18-degree angle when the masonry is already saturated with heat. A poorly shaded west-facing opening turns that bedroom into an oven for the night.
What this means for your plan
Check your concept sketch to ensure every habitable room shows window openings on at least two separate external walls. Verify that bathrooms sit along the perimeter with their own high-level louvres, and ensure no bed is drawn directly beneath a window opening. The tool applies these orientation, clearance, and ventilation rules automatically when it drafts your house layout. Take this checked brief to your licensed architect or engineer so they can detail the lintel heights and structural reinforcement for your build.
- PRACTICE: external shading at least 600 mm projection with a 900 mm sill
- NBC Part 8: openings on two different faces required for ventilation
- GEOMETRY: at 8-16N eaves want 900 mm and north light carries no direct sun
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