# What is the difference between knob and tube and cloth wiring?

> Knob and tube vs cloth wiring, BX and modern cable: how each type is built, how to tell them apart in a Boston house, and why the difference changes the work.

URL: https://deckerbostonknobandtube.com/guide/knob-and-tube-vs-cloth-wiring/

Knob and tube and cloth wiring are not two names for the same thing. **Knob and tube** runs each conductor on its own path, held off the wood by porcelain; **cloth-sheathed cable** wraps both conductors in a single woven jacket fastened along the framing. The two share rubber-and-cloth conductor insulation, and neither had a ground wire in its earliest form. [Decker Boston Knob and Tube](https://deckerbostonknobandtube.com/) arranges inspection and replacement of every old wiring type across Boston and nearby cities, carried out by licensed and insured electricians.

In practice a Boston-area house built before 1940 rarely has just one. Original circuits, mid-century additions and a remodel or two leave several methods side by side, and the work plan depends on knowing which is which.

Key takeaways

- “Cloth wiring” describes the insulation; “knob and tube” describes a layout. Knob and tube is one kind of cloth-insulated wiring, not the only kind.
- Knob and tube keeps hot and neutral apart and relies on open air to cool. Cloth-sheathed cable bundles them and doesn’t.
- Old armored cable, often called BX, adds a metal jacket that sometimes served as the only ground path.
- Modern NM cable carries a dedicated ground wire, which none of the oldest methods has.
- Mass Save’s insulation rule names knob and tube specifically, so telling the two apart can decide whether an attic can be insulated.

## The four wiring types side by side

The quickest way to see the difference is to compare how each type carries its conductors and what protects them. This is a guide to the families, not a dating key: installation habits overlapped for years.

|  | Knob and tube | Cloth-sheathed cable | Old armored (BX) cable | Modern NM cable |
| --- | --- | --- | --- | --- |
| How conductors travel | Separately, often inches apart | Together in one fabric jacket | Together inside a spiral metal jacket | Together in a plastic jacket |
| What holds it | Porcelain knobs and tubes | Staples along framing | Straps and box connectors | Staples along framing |
| Conductor insulation | Rubber, then cloth | Rubber, then cloth | Rubber, then cloth or paper | Thermoplastic |
| Ground | None | None on early cable; a thin bare wire on some later cable | The metal jacket, if it is intact and connected | A dedicated ground wire |
| How it sheds heat | Open air around each wire | Through its jacket | Through its jacket | Through its jacket, rated for its use |

Modern NM cable is often called by the brand name Romex. The [guide to knob and tube mixed with newer cable](https://deckerbostonknobandtube.com/guide/knob-and-tube-mixed-with-romex/) covers what happens where it was spliced onto old runs.

## How to tell them apart in a Boston-area house

The layout gives knob and tube away; the jacket gives cloth cable away. The best places to look are unfinished spaces and the panel, and the [identification guide](https://deckerbostonknobandtube.com/guide/how-to-identify-knob-and-tube-wiring/) has the full checklist.

### In the basement and attic

- **Knob and tube:** two single wires following separate routes across joists, wrapped around porcelain knobs or passing through porcelain tubes, often a hand’s width or more apart.
- **Cloth-sheathed cable:** one flat or round cable, usually dark fabric, stapled to the side or underside of a joist and running straight into a box.
- **Armored cable:** a ribbed metal jacket, sometimes rusty, clamped to the framing and ending at a metal box.

### At a box or fixture

This is where the two get confused. Knob and tube conductors enter a box through flexible loom, a woven protective sleeve, so each wire arrives inside its own fabric tube. Cloth-sheathed cable arrives as one jacket holding two wires. Home inspectors have asked exactly this on their own forums, because a single box can look like either. An electrician settles it by following the run back toward the panel rather than judging by the box.

> Look, don't handle
>
> Rubber and cloth insulation of any kind can crumble when it’s bent. Photograph what you find and leave it in place; a run that looks disconnected may still be live.

## Why the difference changes the work

The two types fail in different ways, so they are judged and replaced in different ways. The insulation is the shared weak point; the layout decides most of the rest.

### Heat and insulation

Knob and tube was rated on the assumption of open air around each conductor. Bury it in blown-in insulation and that assumption fails. That is why Mass Save’s assessment standards treat any evidence of knob and tube as a barrier to insulation until a licensed electrician deals with it, and why the [insulation barrier service](https://deckerbostonknobandtube.com/knob-and-tube-insulation-barrier/) exists. Cloth-sheathed cable has a jacket of its own and isn’t named in that rule, so identifying the method correctly can change what an energy assessment allows.

### Grounding

None of the oldest methods has a separate ground wire. Armored cable is the exception that complicates things: its metal armor sometimes did double duty as the fault path, a path that depends on every joint along the run staying tight. The [ungrounded outlet upgrade](https://deckerbostonknobandtube.com/ungrounded-outlet-upgrade/) page explains how each case is handled.

### Routes for the new cable

Cloth-sheathed cable already runs like modern cable, stapled along framing into boxes, so a replacement can often follow the same path through the same holes. Knob and tube took separate routes for each wire and sometimes reached rooms by paths no cable can follow now, so its replacement needs its own route plan. That route plan is a large part of what [cloth wiring replacement](https://deckerbostonknobandtube.com/cloth-wiring-replacement/) and knob and tube replacement each cost.

## How each type ages

All three old types share rubber insulation that hardens and cracks with heat and time, and cloth that frays. Where they differ is where the damage tends to show.

| Type | Where trouble tends to start |
| --- | --- |
| Knob and tube | Later splices made in walls without boxes, insulation packed around runs, conductors sagging off knobs onto wood |
| Cloth-sheathed cable | Brittle insulation where the cable bends into boxes or sits above hot light fixtures |
| Armored cable | Sharp jacket ends cutting into insulation at boxes, and loose or rusted jackets that no longer carry fault current |

None of these is proof of a problem in a given house. Intact, unmodified runs of any type can still be in service. The [signs guide](https://deckerbostonknobandtube.com/guide/signs-your-house-needs-rewiring/) covers which symptoms are worth a prompt call.

## What it means for insurers, buyers and the scope

Insurers and inspectors use the terms loosely, so get the type confirmed in writing before deciding anything. Homeowners report carriers asking about knob and tube and fuse panels at purchase or renewal, and some report short deadlines that reached cloth wiring too. None of that is a published Massachusetts rule.

### A circuit map settles the mix

The useful document is a circuit-by-circuit list: which runs are knob and tube, which are cloth-sheathed or armored cable, which are modern, and where any of them join. That list decides whether the job is a few circuits, one floor, or a [whole house rewire](https://deckerbostonknobandtube.com/whole-house-rewiring/) that replaces every old type at once so no one returns to the same walls twice. The [partial vs full guide](https://deckerbostonknobandtube.com/guide/partial-vs-full-knob-and-tube-replacement/) works through that choice, and the [knob and tube inspection](https://deckerbostonknobandtube.com/knob-and-tube-inspection/) produces the map.

### Budget

A whole-house replacement falls in a typical Boston-area budgeting range of $8,000 to $25,000 or more, and the share of each wiring type is one of the things that moves a house within it. The [cost guide](https://deckerbostonknobandtube.com/guide/knob-and-tube-replacement-cost-boston/) breaks that down.

If you’re not sure what your house has, call Decker Boston Knob and Tube at [(617) 405-3599](tel:+16174053599) or [ask for an estimate](https://deckerbostonknobandtube.com/contact/). Tracing the circuits answers it for every room, not only the ones you can see.

## Questions about this guide

### Is cloth-sheathed cable safer than knob and tube?

Not by definition. Cloth-sheathed cable keeps both conductors in one jacket, so it doesn't depend on open air in the way knob and tube does, but its rubber insulation ages in the same way and early versions have no ground. The condition of each run, the splices and the protection on the circuit matter more than which of the two a house has.

### Does old armored BX cable count as a ground?

Sometimes the metal jacket was the only path back to the panel, and sometimes it was never a reliable one. Old armor can be loose at the box connectors, rusted or broken along its length, so an electrician tests the path at each box before relying on it. Where the jacket can't be shown to carry fault current, the circuit is treated as ungrounded.

### Can an electrician tell what kind of wiring is in a wall without opening it?

Often, yes, by reading the evidence at the ends. The panel, the boxes behind outlet covers, and runs in the attic and basement usually show which method feeds a room. What can't be seen is a concealed splice where one type joins another inside a wall, which is why circuit tracing matters more than any single box.

Want this answered for your house?

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## Related service

### Cloth Wiring Replacement →

Rubber-and-cloth insulated cable from the decades after knob and tube, assessed run by run and replaced with grounded, arc-fault protected circuits.

## More guides

Learn

What is knob and tube wiring, and what does it mean for a Boston house?

How the oldest wiring in Boston-area houses was built, what it still does well, where it goes wrong, and how Massachusetts rules, insurers and Mass Save shape what you do next.

Learn

How do you identify knob and tube wiring?

A field guide for owners and buyers: where to look in a Boston-area house, what each part of the system looks like, what gets mistaken for it, and where looking stops being enough.

Learn

What happens when knob and tube is mixed with Romex?

Decades of kitchen remodels and added outlets can leave an old house with new cable spliced onto its original circuits. The cable isn't the weak point. The places where it meets the old wiring are.

Decision Guide

What are the signs your house needs rewiring?

Old houses rarely announce bad wiring with one clear symptom. Sorting what you notice into urgent, worth planning for and not a symptom at all tells you what to do next.

Decision Guide

Should you replace part of your knob and tube or all of it?

Repair, partial replacement and a whole-house rewire answer different problems. The circuit map, the reason you're acting now and your plans for the walls decide which one fits.