How to install window guards: you are sizing it to the opening a child could pass through, not to the window

The two numbers that decide everything

The opening width. Measure jamb to jamb inside the frame, at the height the guard will sit. Guards are adjustable within a range - 23 to 35 inches and 35 to 58 inches in the models here - and you want your number inside the range rather than at either extreme, because a guard extended to its last notch has the least overlap in the adjustment.

The opening height. This is the one people get wrong. Open the window all the way and measure the clear vertical gap. A double-hung sash that travels 24 inches has a 24-inch opening; a casement that swings clear has an opening the height of the whole light. The guard's end posts come in fixed heights - 13, 17.375 and 21.75 inches on the Guardian Angel, covering maximum openings of 17.25, 21.5 and 26 inches - and the post you order has to cover your gap. If your opening is taller than the tallest post, the manufacturer's own guidance is more than one guard, stacked.

Write both numbers down with the room name next to them. If you are doing a whole upstairs, the windows will not all be the same, and ordering four identical guards for four windows is the second most common mistake here.

What your frame is made of

Wood or clad wood. The easy case. The supplied screws bite into solid timber; pre-drill a pilot hole if the wood is old and dry so you do not split the jamb.

Vinyl. A vinyl jamb is a hollow extrusion with a reinforcing channel somewhere inside it. A screw driven into bare vinyl holds a guard until the day it matters. Find the reinforcement by probing with a small pilot hole, or fix into the wood framing of the rough opening just outside the vinyl - which often means mounting the guard slightly further into the room.

Aluminum. Similar to vinyl but less forgiving: drill slowly, use the correct bit, and expect to need self-tapping screws rather than the wood screws in the box.

Masonry reveal. If there is no timber at all, you are into plugs and masonry screws, and at that point it is worth getting someone who does this for a living, because a guard is only as good as its weakest fixing.

The install, step by step

Dry-fit first, every time. Set the guard in the opening at the height the instructions give, extend it until both end posts touch the jambs, hold a small level across the top bar, and mark each screw hole in pencil. Five minutes here saves an hour of filled holes.

Then drive the outermost screws on each post first so the guard supports itself, and fit the rest afterwards. Tighten the width adjustment and its locking screws before you fit the final fixings, not after, because the adjustment moves as you tighten.

Finally, go round the whole perimeter looking for gaps. The bars themselves are spaced under four inches by design, but the gap between the end post and the jamb, and the gap under the bottom bar, are both created by your install rather than by the manufacturer. Those are the two places a guard fails.

What to buy before you start

For any bedroom above the ground floor, buy a guard that states compliance with ASTM F2090 and has an emergency escape release. The pressure-fit alternative and the sash stop below both have a place, and neither is a substitute for a guard on a window a child could fall from.

Testing it, and the four-inch rule

The CPSC's window safety guidance is built around a single number: an opening of four inches or less will not pass a small child. Test your finished install against it with something four inches across rather than by eye, and check every gap, not just between the bars.

Then test the release. Operate it from inside the room, one-handed, with no tools, and do it twice so you know it was not a fluke. Then make every adult in the house do it once, including anyone who babysits. A guard whose release only you can find is a guard that becomes a hazard in a fire, which is precisely why the ASTM standard exists in the form it does. The difference between a guard and a stop is set out on window guards vs window stops.

If you rent

Ask first, and ask in writing, because window guards are one of the few childproofing products a landlord may be obliged to fit - several large cities require them in units with young children, and many leases allow them outright. If screws are genuinely not possible, the pressure-fit guard goes in a window that is not an egress window, and a sash stop handles the rest.

Do not compromise on the window a child sleeps next to. If the only acceptable fix there needs screws and the landlord says no, the answer is to move the furniture a child climbs away from the window and keep that sash locked shut, not to accept a weaker guard. The rest of the rental picture is on babyproofing an apartment or rental, and the whole-window walkthrough is on how to childproof windows. Crank windows need a different approach entirely, on how to childproof casement windows.