The door that usually prompts the call
A typical request starts with a front door that has a tall glass sidelight a few inches from the deadbolt. The owner has noticed that anyone willing to break one pane could reach in and turn the thumb turn, and the lock would open as if they had a key.
A double cylinder deadbolt replaces that thumb turn with a second keyed cylinder. With no turn piece on the inside, broken glass stops being a way to work the bolt. The trade is that everyone inside now needs a key to get out, including in a fire.
That trade is why our double cylinder deadbolt installation starts with questions about the door and the people behind it, not with the lock. If you are still weighing the two designs, our single vs double cylinder comparison covers the pros and cons household by household. This page is about doing the job properly once a keyed interior looks like the right answer.
Text version of this diagram
Panel 1: seen from inside, a sidelight sits right next to the door's deadbolt. An arm through a broken pane can reach the thumb turn and open the lock.
Panel 2: the same door with a double cylinder deadbolt. There is only a keyhole on the inside, so reaching through the glass achieves nothing. The inside key hangs on a hook on the hinge side of the door, well away from the glass.
Doors where a keyed interior earns its place
A double cylinder lock is a fix for one specific weakness. It is worth considering when most of these are true:
- Glass is within reach of the lock. That can be a glass panel in the door itself, a sidelight, or a window next to the frame.
- The door is not the home's required exit. Most homes have another exterior door that can stay on a thumb turn (see the code section below).
- Everyone who sleeps in the house can find and use the inside key quickly, in the dark, without help.
- The household will keep the key in one spot, every time the bolt is thrown from inside.
If the glass is well away from the lock, or this is the only way out, skip the second cylinder. A single cylinder deadbolt paired with a reinforced strike plate and long screws answers the more common attack, a kick at the jamb. Consumer Reports found that swapping a stock strike for a box strike with 3-inch screws significantly improved kick-in resistance for most locks it tested.
Egress rules that shape double cylinder deadbolt installation
Residential codes based on the International Residential Code cover this in section R311.2. The 2009 model IRC wording, as widely quoted, reads: "Egress doors shall be readily openable from inside the dwelling without the use of a key or special knowledge or effort."
States and cities adopt the code, often with their own amendments, and the wording that matters is the version adopted where you live. North Carolina's adopted residential code, for example, requires "all interior egress doors and a minimum of one exterior egress door" to be readily openable from the egress side without a key. A 2023 interpretation from the NC State Fire Marshal's office confirms only one code-compliant exterior egress door is required.
Under wording like that, a keyed interior on the required exit door is a problem, while a secondary exterior door may be treated differently. Elsewhere the text may read more broadly. Minnesota, as another example, adopts the 2018 IRC with amendments of its own.
Storefronts and offices play by commercial rules
Businesses fall under different provisions. The 2024 International Building Code, section 1010.2.4, allows key-operated locks on the egress side of the main door in certain occupancies only if it is the main exterior or tenant door, it is readily distinguishable as locked, and a sign reading "THIS DOOR TO REMAIN UNLOCKED WHEN THIS SPACE IS OCCUPIED" in 1-inch letters is posted on or next to it.
The building official can revoke this for due cause.
NFPA 101 sets similar conditions and adds that a key must be immediately available to anyone inside while the door is locked. Our page on deadbolt locks for commercial doors covers business hardware in more depth.
How we run a double cylinder deadbolt installation
Every double cylinder deadbolt installation we take on follows the same order. The code questions come before any tools are unpacked.
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Identify the door's role
We count the exterior doors and work out which one serves as the required exit. A keyed interior on that door is where most code problems start.
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Check what your area has adopted
We look at the code edition and amendments in force locally. If the wording leaves room for doubt, we tell you exactly what to ask the building department before install day.
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Map the glass
We note which panes put the lock within reach. Sometimes the honest answer is that a single cylinder lock plus a glass upgrade does the job better.
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Agree the key storage spot
The hook or holder goes in before the lock does, so it is in place the first night.
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Fit and key the lock
We confirm door thickness and backset, fit the deadbolt, and key both cylinders to the same key, or to your existing key if the keyway matches.
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Test from both sides
We operate the bolt with the door open and closed, from inside and out, and hand over the agreed number of keys.
Hardware, door prep and keying
Most residential double cylinder deadbolts are made for the same door prep as single cylinder models. Schlage's B-series instructions, for example, call for a 2-1/8" cross bore, a 1" edge bore, a 2-3/8" or 2-3/4" backset and a door 1-3/8" to 1-3/4" thick.
That means converting a door that already has a deadbolt is normally a swap in the existing holes, much like any other deadbolt replacement on a prepped door. We still check the box against your door before we start, because some doors are thicker than the standard range and some models use their own prep.
Heavier hardware exists if you want it. Schlage's commercial B600 series is rated ANSI/BHMA A156.36 Grade 1 and includes a double cylinder version, the B662. On the keying side, both cylinders normally take one key, and if your other locks share a keyway we can match them so the inside key is simply your house key. Our page on keying every door to one key explains when that works.
Price depends mainly on the hardware grade you choose, whether the door needs any new prep, and how many locks are keyed together at the same visit. The deadbolt installation cost guide explains how those factors show up in a quote.
A key storage plan that works half asleep
The inside key is the part of the job people underestimate. A double cylinder lock is only as safe as the household's ability to find that key without thinking.
Key storage plan we agree before leaving
- One fixed spot for the inside key, on the hinge side of the door or another place no arm through the glass can reach
- A spare on every adult's and older teen's key ring, so a missing hook key is never a dead end
- Everyone who stays over, including sitters and visiting relatives, shown the spot on their first visit
- The key goes back on the hook every time, not into a pocket or a drawer
- Never leave the key sitting in the inside cylinder
That last point matters more than it sounds. A key left in the inside cylinder turns the lock back into a thumb turn, the exact thing the double cylinder was meant to remove.
Alternatives when a second cylinder is the wrong fit
If the code check or the household rules out a keyed interior, the glass is still the weak point. These are the options we talk through instead.
| Option | What it addresses | The catch |
|---|---|---|
| Single cylinder deadbolt with a reinforced strike | Kick-ins at the jamb, with an easy exit kept | The thumb turn is still reachable through the glass |
| Security film on the glass | Slows someone trying to break through the pane | Results depend on the film and the quality of the install |
| Stronger glazing or a solid panel | Removes or toughens the weak point itself | A glazier or door job, not a lock job |
| Removable (captive) thumb turn deadbolt | Turn can be taken off when the home is empty | Ask your AHJ whether removing it counts as special knowledge |
| Keypad or smart deadbolt | Keyless entry from outside | Still has a thumb turn inside, so the glass issue remains |
If keyless entry is the real goal, our smart deadbolt installation service covers the options, but pair it with one of the glass fixes above.
Field tip: run a lights-off drill before you commit
Before booking, try this at night. Turn off the lights, have each person in the home start from their bed, and ask them to walk to the door and open it using the key from where you plan to hang it.
If anyone hesitates, fumbles or forgets where the key lives, solve that first: move the hook, add a night light, or choose one of the alternatives. Tell us how the drill went when you book your double cylinder deadbolt installation, and we will place the hook and plan the keys around what you learned.
Next step
Talk to a Deadbolt Fixers specialist
Tell us about the door, the lock and what is going wrong. We will tell you what the job involves before any work starts.
Frequently asked questions
Will you install a double cylinder deadbolt on my only exterior door?
Usually not. Codes based on the IRC expect at least one exterior egress door to open from inside without a key, so on a home's only exit door we recommend a single cylinder lock plus other upgrades unless your AHJ confirms otherwise.
Can my existing single cylinder deadbolt be converted to a double cylinder?
The thumb turn half cannot simply take a second cylinder, so the whole deadbolt is swapped. Most residential double cylinder models fit the same 2-1/8" face bore and 1" edge bore, so new drilling is rarely needed.
Where should I keep the inside key for a double cylinder lock?
On a fixed hook or holder near the door that everyone in the home knows, but out of reach from any glass. The key should be in that spot every time the bolt is locked with people inside.
Can the inside and outside cylinders use the same key?
Yes, and they normally do. If your other locks share the same keyway, both cylinders can also be keyed to the key you already carry.
What does your code check involve before a double cylinder install?
We identify whether the door is the home's required exit door, look at the code edition and amendments your area has adopted, and flag anything unclear for you to confirm with the local building department before install day.
Can I put a double cylinder deadbolt on a back door or side door?
Sometimes. Under wording like North Carolina's adopted code, only one exterior egress door must open without a key, so a secondary door may qualify, but other jurisdictions word the rule differently and the AHJ decides.
What can I do about a sidelight if a double cylinder lock is not allowed?
Common options are security film on the glass, a stronger glazing or solid panel, and a reinforced strike on a single cylinder lock. We go through which ones fit your door during the visit.
Does security film on the glass do the same job as a double cylinder deadbolt?
It targets the same weak point, the glass, without putting a key between you and the exit. How much it slows a break-in depends on the film and how well it is installed, so treat it as a delay rather than a barrier.
What is a captive or removable thumb turn deadbolt?
It is a deadbolt whose inside turn piece can be taken off when the home is empty and put back when people are inside. Ask your AHJ whether removing it counts as special knowledge where you live, since that decides whether it is acceptable.
What happens if the inside key gets lost?
Use a spare from someone's key ring and put it on the hook straight away, then cut a replacement. If the key may have left the house, rekeying both cylinders is the safer move.
Does a keypad smart lock solve the glass problem?
No. Keypad and smart deadbolts still have a thumb turn on the inside, so glass within reach of the lock is just as much of a weak point as it is with a basic single cylinder model.
What should I ask the building department before installing a double cylinder lock?
Ask which residential code edition and local amendments are in force, and how they treat your required exit door versus secondary doors. Describe the exact door, including any glass, and note the answer and the name of the person you spoke to.