Should You Replace Knob and Tube Wiring? A St. Louis Guide

Short answer: yes, in most homes you should plan to replace knob and tube wiring. It has no ground wire, its old cloth and rubber cover gets brittle with age, and code says it can't be buried in insulation. If it is still live in your walls, have a licensed electrician check it and plan a whole-home rewire before you add insulation or new loads.

At Cain Electric in Gray Summit, owner Stanley Cain and our crew do residential rewiring in older homes across the St. Louis area. Here is what you need to know before you call anyone.

What is knob and tube wiring?

Knob and tube is the oldest kind of home wiring you will find in St. Louis houses. It uses single wires that run through porcelain tubes in the wood framing and are held up by porcelain knobs.

The American Society of Home Inspectors says it was used from the early 1900s to the 1930s, and in some homes as late as the early 1970s (ASHI). So if your house was built before about 1950, it is worth checking.

Is knob and tube wiring dangerous?

Knob and tube is not a fire by itself, but it has real limits that newer wiring does not. The wire was fine for what homes used 90 years ago. It was never made for today's loads, and most of it is now very old.

Here is why it's a concern:

• No ground wire. ASHI notes it "usually has no equipment grounding conductor." That means no 3-prong outlets and less shock protection.
• Brittle cover on the wire. The cloth and rubber wrap can crack and fall apart, leaving bare copper.
• Old splices and add-ons. Over the years, people tap new wires into it. Those splices are often not in a box.
• It needs air. The wire was built to cool off in open air inside walls and attics. Packing insulation around it traps heat.

Can you put insulation over knob and tube wiring?

No. The National Electrical Code, section 394.12, does not allow concealed knob and tube in walls, ceilings or attics where loose, rolled or foam insulation wraps around the wires (NFPA 70).

This matters right now. Fall is a common time to add attic insulation before winter. If your attic still has live knob and tube, deal with the wiring first. Then insulate.

How do you know if your house has it?

You can often spot knob and tube in 10 minutes with a flashlight. Look in places where the framing is open.

1. Go to the attic or basement. Look at the joists and beams.
2. Look for white porcelain knobs nailed to the wood with single wires wrapped around them.
3. Look for porcelain tubes where wires pass through the wood.
4. Check the outlets. Lots of 2-prong outlets in an old house is a clue, but not proof.
5. Check the panel. Old fuse boxes often go with old wiring.

Don't touch or pull on the wires. Finding knobs and tubes also does not tell you if the wire is still live. Some homes were rewired and the old wire was just left in place. An electrician can test it and tell you for sure.

Knob and tube vs modern wiring

The biggest differences are the ground wire and how the wire is wrapped.

Does knob and tube affect home insurance?

It can. ASHI tells homeowners who find it to call their insurance company, since some insurers want the house rewired before they will write a policy (ASHI). If you are buying or selling an older home, ask your agent and insurer early so it doesn't hold up closing.

Do you have to rewire the whole house?

Not always all at once, but the goal should be to get all the live knob and tube out. A good electrician will start with the circuits that matter most.

A common plan looks like this:

1. Test what's live. Find which circuits still run on knob and tube.
2. Fix the risky spots first. Attics with insulation, kitchens, baths, and laundry rooms come first.
3. Run new grounded circuits. New cable with a ground wire is the fix ASHI calls best.
4. Update the panel if needed. Many old homes also need a bigger or newer panel for today's loads.
5. Get the permit and inspection. Your electrician should pull any permit your city or county needs. Here's more on electrical permits in Missouri.

Electricians can often fish new wire through walls with small openings, so a rewire does not always mean tearing out plaster. Your electrician should walk you through the plan before work starts.

If your home also has aluminum wiring from the 1960s or 1970s, read our guide on whether aluminum wiring is dangerous.

Why winter makes it more urgent

Winter puts the most strain on old wiring. Space heaters, electric blankets and holiday lights all pull power through circuits that were built long before any of that existed.

If you still have knob and tube, keep space heaters plugged straight into the wall (never an extension cord), and don't run two on one circuit. For more cold-weather tips, see our post on winter electrical fire risks in Missouri homes.

FAQ

How old does a house need to be to have knob and tube? Most knob and tube was put in from the early 1900s to the 1930s, though some went in later. If your home was built before about 1950, check for it.

Can I just swap my 2-prong outlets for 3-prong ones? Not without a ground wire. A 3-prong outlet with no ground looks safe but isn't. An electrician can add a ground or use GFCI protection the right way.

Is it OK to leave dead knob and tube in the walls? Yes, if an electrician confirms it is fully cut off from power. Dead wire left in place is common in rewired homes.

Should I insulate my attic before or after rewiring? After. Code does not allow insulation to wrap around live knob and tube, so get the wiring handled first.

Get it checked before winter

If you think your home has knob and tube, the first step is a simple check. Cain Electric offers free estimates for residential rewiring and panel upgrades in St. Louis, Eureka, Pacific, Union, Washington and nearby towns.

Call us at (314) 607-8687 or request a free estimate.