Shelter ยท Gear Review
Best Window Security Locks. Matched to the window.
Factory window latches are built to hold against weather, not a pry bar. For the vinyl windows in most homes, the Prime-Line U 9809 closes that gap for a few dollars a window. Here is what fits the others.
At a glance: Prime-Line U 9809
Strengths
- Clamps onto the window track by hand, with no drilling and no tools.
- The adjustable jaw fits vinyl rails from 1/16 to 1/2 inch thick.
- Locks a window closed or partly open for ventilation.
- A flip lever releases it in seconds, with no key to find.
Limitations
- Overtightening the jaw makes the lever hard to close, so it takes some trial to set.
- Some vinyl tracks have a thin lip the clamp can crush.
- On a double slider it locks only the inner sash.
The verdict
The right lock depends on the window, not the brand.
Before the comparison: here is where we land, and why. Walk the house first and note what kind of window each ground-floor opening is.
Best overall: vinyl windows
Prime-Line U 9809
A diecast clamp with a flip lever for vinyl sliders and many single-hung vinyl windows. No tools, adjustable fit, quick release.
~$10 to $13 for two
Older aluminum sliders
Prime-Line U 9819
A small aluminum block with a thumbscrew for thin metal rails up to 1/8 inch. The cheapest way to secure every slider in the house.
~$5 to $8 for four
Old wood double-hung
Prime-Line U 9929 flip lock
A steel flap on the outer sash that blocks the lower sash from rising past it. Locks a window shut or open a few inches.
~$8 to $10 for two
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Why the pick wins
A latch keeps out weather. A lock keeps out a pry bar.
A police study of burglarized homes found that windows with simple stock latches were easily pried open, while windows with locks were not. The U 9809 adds that lock to the vinyl windows most homes now have.
01
Made for vinyl
The jaw adjusts to rails from 1/16 to 1/2 inch, a range built for vinyl frames rather than the thin aluminum rails of older windows.
02
No drilling
It clamps onto the track by hand, so renters can use it and nothing has to be patched at move-out.
03
Closed or vented
Clamp it at the sash for a closed window, or a few inches back to hold the window open for air while still blocking it.
04
Fast to release
Flip the lever and slide it off. There is no key or hex tool to find, which matters on a window that is also a way out.
The Chula Vista Police Department's study of 569 burglarized homes found the point of entry was a window in 44% of cases. The method used most often was prying a latched window open with a tool (24%), and in 17% of cases the burglar broke a window. Not one burglar in the study attempted to break a double-pane window. (Chula Vista Police Department)
That points to the job a secondary lock does: it makes a closed window hard to slide or pry. SafeWise's roundup of aftermarket window locks also favors simple clamp and flip locks of this kind for sliding and double-hung windows. (SafeWise) We found no independent lab test that compares these locks against each other, so we weigh fit, ease of release, and what thousands of owners report.
The picks
Three locks, three kinds of window.
All three open by hand from inside, with no key. Crank-out casement windows pull tight into their frames when closed and usually need only a working factory latch.
Best overall: vinyl windows
Prime-Line U 9809 Sliding Window Lock for Vinyl Windows
Prime-Line designs the U 9809 for horizontal sliding vinyl windows and many single-hung vinyl windows. You hand-tighten the diecast jaw onto the track, then flip a cam lever to clamp it. With it set against the sash, the window cannot slide; set a few inches back, it holds the window partly open.
The setup takes a little trial. Tighten the jaw too far and the lever will not close; leave it loose and the lock can be knocked off the track. Owners who are happy with it describe finding the point where the lever closes firmly by hand.
Check two things before you order. Some vinyl tracks have a thin lip along the top that the clamp can crush, and on a window where both panels slide, a clamp on the inner track does not stop the outer panel.
Specifications
Older aluminum sliders
Prime-Line U 9819 Sliding Window Security Lock
A small aluminum block that slips over a thin window rail and locks in place with a thumbscrew. Prime-Line sizes it for rails up to 1/8 inch, which suits the metal-framed sliding windows in many older homes and apartments. It goes on in about a minute with no tools.
It costs little enough to put on every slider in the house. The thumbscrew has a pointed tip that bites into the rail, so expect a small mark where it sits.
If your rails are thicker than 1/8 inch, it will not grip. Prime-Line's U 9802 is the same idea for metal rails up to 3/16 inch; on vinyl, use the U 9809 above.
Specifications
Old wood double-hung
Prime-Line U 9929 Window Flip Lock
A stamped steel bracket with a hinged wedge, mounted on the window above the lower sash. Flip the wedge down and the lower sash cannot rise past it. Mount it a few inches up and you can keep the window open for air with the lower sash still blocked.
Prime-Line makes it for wood single- and double-hung windows and does not recommend it for dual-pane windows. On a narrow sash, the instructions say to shorten the nails first so they do not reach the glass. Some owners swap the nails for short screws.
The metal is thin and the edges are sharp, and a few owners found it flimsy. For older wood windows with worn latches, it is still the simplest secondary lock we found that does not involve drilling through both sashes.
Specifications
What owners actually say
Beyond the spec sheet, after the install.
These patterns come from owner reviews across more than 16,000 ratings. A theme appears here only when many owners raise it independently.
Prime-Line U 9809
Owners consistently describe it as sturdy for the price and easy to put on, and many use it to keep windows cracked open while away or asleep.
The mixed reviews center on the clamp. Owners who overtighten it report a lever that will not lock, owners with thin-lipped tracks report it does not fit, and owners of horizontal sliders recommend two side by side when the window is propped open.
Prime-Line U 9819
Owners consistently call it the simplest, cheapest upgrade on the list: a minute per window, no tools, and discreet once installed.
Fit is the recurring complaint. It is small, it does not grip thicker vinyl rails, and the pointed thumbscrew can scratch or dent the frame if it is cranked down hard.
Prime-Line U 9929
Owners of older homes like that it lets them leave old wood windows open a few inches for air, and that it looks at home on period trim.
The complaints are about build: thin stamped metal, sharp edges that can gouge a sash, and nails instead of screws.
The one thing to know
Every lock on a way out has to open in seconds.
Before you add a lock to any window, decide whether that window is someone's second way out of a room. NFPA's escape planning guidance asks households to mark two ways out of each room, including windows, and to check that doors and windows can be opened easily. (NFPA)
All three locks here open by hand, with no key. Show everyone in the house how they release, including children old enough to follow the escape plan, and skip keyed window locks on any window that is a way out. Our locks and door reinforcement handbook covers which lock suits which window.
How they fail, what they cost
Small parts, small costs.
None of these locks uses batteries or anything else that runs out. Securing every ground-floor window in a typical house costs about what a good deadbolt does.
A clamp that works loose
Clamp locks can creep as a window is opened and closed against them. Check them when you check the smoke alarms, and retighten by hand.
Marks on the frame
Thumbscrews and clamps leave small marks on a rail. Renters should expect that and mention it to the landlord before installing.
A worn factory latch
A secondary lock is not a reason to ignore a broken latch. Replacement sash locks for most windows are inexpensive and widely available; match the screw hole spacing on the old one.
What it costs over time
Nothing on a regular schedule. Expect to replace a lost or broken lock now and then, at a few dollars each.
What we didn't pick
And who should skip these.
Keyed window locks and bars
They stop entry and can also stop escape. NFPA asks that security bars have emergency release devices inside so they can be opened immediately. We leave keyed locks off any window that is a way out.
Windobully
An adjustable lock sold for both sliding and double-hung windows. It has far fewer reviews than our picks, little published detail on materials or fit, and some owners report it breaking within weeks.
Unbranded multipacks
Marketplaces carry many lookalike aluminum stoppers in packs of four to ten. Most have few reviews and no stated rail range, so fit is a guess. The Prime-Line versions cost about the same and state what they fit.
Who should skip these picks
Casement windows usually need only a working factory latch. If glass breakage worries you more than prying, look at the glass instead. Sliding glass doors need a sturdier lock than these window clamps.
Where it fits in your readiness
A baseline, window by window.
Secondary window locks belong in the Survive tier alongside the deadbolt: the everyday baseline in place before any hazard arrives.
Hurricane, wildfire, and flood evacuations
An evacuation can leave a home empty for days. Locking every ground-floor window is part of leaving it ready.
Heat waves and power outages
When the air conditioning is off, a lock set a few inches back lets you keep windows open for air while they stay blocked.
Civil disruption
During unrest nearby, windows that will not slide or pry open make a home a poor target, the same as on any other day.
Maintenance
Five minutes, twice a year.
- Try to open each locked window by hand. It should not move past the lock.
- Retighten any clamp or thumbscrew that has crept loose.
- Check that the factory latch on each window still closes fully.
- Walk the fire escape plan and have everyone release the lock on their bedroom window.
- Keep the spare locks from each pack in one place for windows you add later.