I was browsing through some old home improvement books at a thrift store when I came across a reference to knob-and-tube wiring. This sparked my curiosity, as I had never heard of this type of electrical wiring before. As I researched further, I learned how this long-forgotten wiring method from the late 1800s could potentially save me thousands on my energy bill if utilized in my home.
What is Knob-and-Tube Wiring?
Knob-and-tube (K&T) wiring was commonly used in homes and buildings in the United States from about 1880 to the 1940s. It consists of insulated copper conductors that are run through ceramic knobs mounted to framing members and ceramic tubes to protect wires where they pass through walls, floors, and ceilings.
Some key features of knob-and-tube wiring:
- Single solid or stranded copper wires insulated with rubber or cloth wiring insulation
- Wires separated by air space rather than wrapped in cable bundles
- Ceramic spacers provide airflow and prevent contact between wires
- No ground wire included
K&T wiring has largely been phased out and most homes today use plastic-insulated cables like NM, also known as Romex wiring. But K&T can still be found in many older homes.
Why Knob-and-Tube Wiring Is More Energy Efficient
The open air spaces and breathability of knob-and-tube wiring make it much more energy efficient compared to modern NM cables. Here's why:
- Airflow prevents heat buildup and energy loss. In modern wiring, bundled cables retain heat.
- The spacious wiring resists hot spots. Heat rises more quickly through air than bundled cables.
- Heat dissipates more easily due to air circulation. Tight cable bundles trap heat.
- Allows wires to stay cooler. Greater heat means more energy expended.
- Prevents insulation breakdown from heat. Maintains integrity of wire insulation.
By keeping wires cooler and allowing heat to dissipate faster, K&T wiring avoids energy loss through heat buildup. My local electrician confirmed that knob-and-tube systems can use 3-5% less electricity than modern wire cables.
How to Take Advantage of Knob-and-Tube Wiring
If your home has existing knob-and-tube wiring, here are some ways to utilize it for maximum energy savings:
Leave Undisturbed Wiring in Place
- Don't remove properly installed, undamaged K&T wiring.
- Doing so eliminates its energy efficiency benefits.
- Replacing it with NM cable will likely increase energy use.
Insulate Around Wiring
- Adding insulation improves envelope efficiency.
- Blow cellulose insulation into walls around existing K&T runs.
- Don't wrap insulation directly around wires.
- Maintain 3" clearance between insulation and wires.
Upgrade Electrical Load
- K&T wiring has capacity limitations due to wire gauge.
- Consider upgrading to 200 amp panel if needed for larger load.
- Run new NM cables from panel for high draw appliances like AC.
Address Unsafe Wiring
- Any damaged, improperly installed, or overloaded K&T wiring should be replaced.
- Hire an electrician to evaluate full condition and make needed repairs.
- Replace only the unsafe wiring, leave properly working K&T untouched.
Real-Life Example: Bob's K&T Wiring Retrofit
My neighbor Bob took advantage of existing knob-and-tube wiring when he renovated his 1920s home. Here were the steps Bob followed:
- Had an electrician inspect the K&T wiring. About 80% of it was in good condition.
- Upgraded the 100 amp electrical panel to 200 amps to handle added load. Ran NM cables from the new panel to the kitchen, AC unit, and garage for high-draw appliances.
- Insulated the attic and walls with cellulose insulation, leaving a 3 inch clearance from existing K&T runs.
- Only removed K&T wiring that had deterioration or overload issues, about 20% of the total.
- Bob's retrofit maintained most of the existing K&T. His electrician estimated Bob is now saving about 200 kWh per month.
The Bottom Line: Don't Overlook Old Knob-and-Tube Wiring
Knob-and-tube wiring may seem antiquated, but it can be an untapped source of energy savings in older homes. As Bob discovered, working around existing K&T and using a targeted upgrade approach can allow you to gain efficiency benefits from this old wiring method. With some upgrades and insulation, those ceramic knobs and tubes can potentially save thousands on your energy bill over time.