Adding Attic Insulation This Fall? Check for Knob-and-Tube Wiring First

October is when a lot of homeowners in Milwaukee start thinking about the attic. The first cold nights show you where the heat is going, and topping up the insulation is one of the most common fall projects. If your house was built in the 1930s or earlier, though, there's one thing to check before anyone blows in a single bag: the wiring.

Many older homes across Milwaukee County, from the bungalows on the north and south sides to the duplexes near downtown, were originally wired with knob-and-tube. Some of it is still in service today, often hidden in attics and walls where nobody has looked in decades.

What knob-and-tube wiring looks like

Knob-and-tube was the standard way to wire a house in the early 1900s. It's easy to recognize once you know what to look for:

• White porcelain knobs nailed to the joists or rafters, with single wires wrapped around them.
• Porcelain tubes where the wires pass through wood framing.
• Two separate wires running a few inches apart, instead of one cable with several wires bundled together.
• Cloth or rubber covering on the wire, often dark and brittle with age.

If you see any of this in your attic, whether it looks connected or not, treat it as live until an electrician tells you otherwise.

Why insulation and knob-and-tube don't mix

Knob-and-tube was designed to shed heat into open air. The wires are spaced apart and strung across open framing so they can stay cool. Burying them in insulation takes that away.

ENERGY STAR, the federal energy-efficiency program, lists knob-and-tube wiring in homes from the 1930s and earlier among the attic problems that should be corrected by a professional before insulating, because it “can be a fire hazard when in contact with insulation.” It also suggests telling your insulation contractor up front how old your house is and whether there could be knob-and-tube in the attic.

Age makes things worse. After 80 or 90 years, the original insulation on the wire can crack and flake off, leaving bare copper. Over the years, many of these systems also picked up splices and add-ons that were never done right, like a new light fixture tied in with tape, or an extension run to a room that never had enough outlets. You won't see most of that until someone goes looking.

Signs to look for before you insulate

Take a flashlight up to the attic, stay on the joists, and don't touch any wiring. Look for:

• Porcelain knobs or tubes anywhere along the framing.
• Loose wires lying on or under the existing insulation.
• Taped splices sitting out in the open instead of inside a covered junction box.
• Scorch marks or darkened wood near any wiring.
• Old insulation already piled over wires from an earlier project.

Inside the house, a few other clues point to older wiring: two-prong outlets throughout, very few outlets per room, fuses instead of breakers, or lights that dim when something kicks on.

What to do if you find it

Don't pull it, cut it, or cover it. The safe order is:

1. Have an electrician look first. A licensed electrician can tell you whether the knob-and-tube is still live, what it feeds, and what shape it's in. Our home electrical inspections are a good starting point if you're not sure what's in your walls.
2. Decide on the wiring before the insulation. In many cases, the practical answer is to replace the old circuits running through the attic before it gets insulated. That's far easier to do while the attic is open than after a foot of insulation is in the way. See our rewiring service for how that work is done in older homes.
3. Tell your insulation contractor. Let them know what the electrician found and what was changed, so they can plan the job around it.

Doing it in this order also means you only open up the attic once.

A good time to check the rest of the house

If you're already looking at the attic, it's worth asking a few more questions about an older home: Is the panel big enough for how you live now? Are the outlets near sinks GFCI protected? Are there working smoke and CO alarms on every level? Our electrical safety checklist walks through the basics.

Serving older homes across Milwaukee County

Couillard Electric works on homes of every age throughout Milwaukee County and the surrounding area. If you've found porcelain knobs in your attic, or you're planning an insulation project in an older house and aren't sure what's up there, contact us and we'll take a look before the cold weather sets in.