More natural light from replacement windows: headers, egress and tempered glass

People ask for daylight and get quoted glass. Glass is the last decision on the list. What decides how much bigger an opening can get is the framing above it, the life-safety rule inside it and the glazing-safety rule around it. Until those three are answered, choosing between a picture unit and a casement is choosing the colour of a car nobody has costed.

The header is the first constraint

Everything above a window has to be carried around it. Widening the opening means a longer span, which usually means a deeper header, more jack studs at each end and a longer bearing length under it. On a non-bearing gable-end wall that can be modest. On a bearing wall under a second storey, or where a girder or a post lands near the opening, it is not. Ordinary cases are covered by the span tables in the residential code your jurisdiction has adopted. We size from those tables rather than from memory. Anything outside them (a long clear span, a point load close to the opening) goes to an engineer for a letter.

The structural item most people never hear about is bracing. Wall segments beside corners and between openings hold the building square against wind and seismic load, and those segments have a minimum length. Widening a window toward a corner can shorten one below what the bracing method allows. That is why a plan reviewer sometimes asks for engineering on what looked like a small change. Finding that out at plan review is inconvenient; finding it out after the wall is open is expensive.

The escape opening: what a bedroom window still has to do

Sleeping rooms need an emergency escape and rescue opening, and the numbers are specific. The opening needs 5.7 square feet of net clear opening, or 5.0 square feet where it is at grade-floor level. It also needs a minimum clear height of 24 inches and a minimum clear width of 20 inches, with a sill no higher than 44 inches above the finished floor.

The word that matters is net. That area is measured through the sash as it actually opens, after the frame and the sash have taken their thickness out of it. That produces three consequences worth knowing before you shop:

• A fixed picture unit cannot serve an escape opening at all, whatever its size
• A horizontal slider can miss the number where a casement in the same rough opening clears it, because only half a slider opens
• Raising the head does nothing for the sill height rule; where the sill is too high, the sill is what has to move

A larger opening can also create the obligation rather than satisfy it, if the room becomes a sleeping room in the process. That is a conversation to have with the department at permit stage, not with a tape measure afterwards.

Safety glazing: where the code wants tempered

The residential code does not list window types that must be tempered, it lists hazardous locations, and glazing in one of them has to be safety glazing. The categories cover glass in doors and immediately beside a door edge, and large panes with a low bottom edge where somebody could walk into them. They also cover glazing at stairs, landings and ramps, and glazing near tubs, showers and pools. The dimensions attached to each category differ between code editions and are subject to local amendment. So rather than reciting numbers with a shelf life, we mark every opening that plausibly falls into one and confirm it with the department issuing the permit.

Three practical consequences follow. Tempered costs more. It adds lead time because it is a separate production step. It cannot be cut or drilled afterwards, so a measurement error is a reorder rather than a five-minute adjustment. On delivery the proof is the permanent etch in a corner of the lite (the pane of glass). If that mark is not there, the glass is not tempered, whatever the paperwork says.

What happens to the outside of the wall

Once the header, the cripples and the new rough sill are in, the exterior gets rebuilt around them. The weather-resistive barrier is cut and re-lapped so it drains over the new head flashing rather than behind it. A sill pan goes under the unit, and the cladding is patched back. The honest part is the patch. New vinyl against panels that have been on the wall fifteen years reads as new even in a correctly matched colour. Fiber cement and engineered wood can be primed and painted, so a patch blends. Stucco needs a return and a texture match that is rarely invisible. On a front elevation this is often why a project turns into re-cladding one whole wall.

Then, and only then, pick the unit

With the opening fixed, the useful comparison is daylight opening rather than rough opening. Frame sightlines differ enough between lines that two units in the same hole deliver noticeably different amounts of visible glass. That figure is on the manufacturer's data, not the price list.

After that the trade-offs are familiar. A fixed picture unit gives the most glass per opening and cannot ventilate or serve escape. Casements open the full width and seal tightest on a wind-facing wall. Transoms add daylight high on a wall without giving up privacy. Orientation still governs the glass package, and those numbers come off the certified rating label for the exact unit. The replacement window page goes through them, and the Reno page covers why one house gets two specifications on two walls.

What the measure visit produces

• The existing rough opening, the daylight opening, and what the room is used for
• What is above the window: storeys, roof framing, any point load near the opening
• The length of the wall segments either side, so a bracing problem is found on paper
• Every opening in a hazardous location, marked for safety glazing
• The escape-opening arithmetic for any sleeping room, done against the actual sash
• Two options priced side by side wherever the header makes the wider version a real decision