Five window types, and how each one seals.

A window type is mostly a decision about how the sash meets the frame. Some sashes swing shut against a gasket and are pulled tight by locks. Others slide past their seals. That one difference explains much of what shows up in the air, water and structural ratings. The rest is practical: what the open sash gets in the way of, and how someone cleans the outside face.

Two ways to seal.

• Compression seal. The sash closes against a continuous gasket, like a refrigerator door. Locking points around the edge pull it tight. Casement, hopper and awning windows seal this way.
• Sliding seal. The sash has to travel along the frame, so nothing can clamp it. It runs past brush or fin seals, and the two sashes hook together where they meet. Hung and sliding windows seal this way.

A sliding seal needs a little clearance so the sash can move. A compression seal needs none. That is why swing types reach high performance classes more easily. It is a tendency, not a rule. The test standard lets any type be tested to any class, and there are hung windows with high ratings. If the letters are new to you, read what a window's class tells you first.

The five types, side by side.

Type	How it opens	Seal	Cleaning the outside face
Fixed	It does not	Glass sealed into the frame. No moving joint	From outside, or by reaching from an operable sash next to it
Casement	Hinged at the side. Swings in or out	Compression	In-swing: from the room. Out-swing: from outside, or by reaching around the sash
Hopper	Hinged at the sill. Tips in at the head	Compression	From the room, through a smaller opening
Hung	One or two sashes slide up and down	Sliding	Many have sashes that tilt in, so from the room
Slider	A sash slides sideways past a fixed light	Sliding	Some sashes lift out. Otherwise from outside

Notes on each.

Fixed. No hardware and no moving joint, so it is the simplest window to make tight. Most of the glass on a mid-rise facade is fixed. Operable sashes go where a room needs air or the code asks for an opening.

Casement. The whole opening is clear when the sash is open, so it ventilates well for its size. An in-swing sash needs clear space inside. An out-swing sash needs clear space outside, which matters over walkways and balconies.

Hopper. A small sash that tips inward. It suits bathrooms, high positions on a wall, and a vent light over a fixed pane.

Hung. The familiar North American window. Nothing projects into the room or out over the sidewalk. Blinds and insect screens are simple. It matches the look that historic districts often require. Only half of the opening can be open at one time, and the seals are the sliding kind.

Slider. A hung window turned on its side, with the same strengths and the same kind of seal. The lower track has to drain, so its weep holes must be kept clear.

What Vistaza makes, and what it does not.

Vistaza makes fixed, in-swing casement and hopper windows, in-swing and out-swing terrace doors, and a sliding door. Every operable window in the range is a compression-seal type.

Vistaza does not make hung windows or sliding windows. Where a project needs them, for example a replacement job in a historic district that must keep its hung windows, that is a different product from a different maker. It can be the right choice.

The certified structural classes show how much the type matters, even inside one series:

Product and type	Class	Air leakage
Vi6 casement	AW-PG100	0.06 cfm/ft²
Vi6 fixed	AW-PG75	<0.01 cfm/ft²
Vi6 hopper	CW-PG65	<0.01 cfm/ft²
Vi5 fixed	AW-PG70	<0.01 cfm/ft²
Vi5 casement	CW-PG50	<0.01 cfm/ft²
Sliding door	CW-PG50	0.07 cfm/ft²

The figures do not line up neatly by type, and that is the point. Each class belongs to one type, at the size that was tested. A series name alone tells you little.

Compare the class of the type you are buying, not the series it belongs to.

Choosing, room by room.

1. Start with fixed glass. Add an operable sash where the room needs air, or where the code asks for an opening.
2. Check what the open sash will hit. Inside: blinds, furniture, radiators, deep sills. Outside: walkways, balconies, neighboring sashes.
3. Decide how the outside face gets cleaned on the upper floors. If the answer is "from the room", that points to in-swing sashes or tilt-in hung sashes.
4. Look at sill heights. Operable windows with low sills on residential floors usually need opening limiters.
5. Mark any window that is also an emergency escape opening before the types are fixed.
6. Ask for the test report for each type on the schedule, and check the tested size against yours.

The Vistaza types and their ratings are on the windows page. To go through the options one at a time, choose yours.