Short answer: none of these three automatically kill a deal, but all three can kill your ability to get a homeowners insurance policy — which, practically speaking, kills the deal anyway, since most lenders won't close without proof of insurance. Here's what each one actually is, how worried to be, and what it costs to make an insurer stop side-eyeing your new house.

Knob-and-tube wiring

What it is: An electrical system standard from roughly the 1880s through the 1940s, where individual wires run through ceramic knobs and tubes instead of the sheathed cable used today. If your inspector points at ceramic knobs in the attic or basement holding up bare wires, that's it.

Is it dangerous? Not inherently, if it's original, untouched, and in good condition — the wire itself doesn't degrade just by sitting there. The real problems are structural: it has no ground wire, its rubber-cloth insulation gets brittle with age, it wasn't built for the amperage a modern household pulls (space heaters, multiple chargers, a home office setup), and decades of amateur "additions" tend to bypass the knobs and tubes that made it safe in the first place. Insurers treat it as a red flag mostly because of what's been done to it over 80-plus years, not the original engineering.

How to identify it: Age is your first clue — a home built before roughly 1950 is a candidate, especially if it hasn't been fully rewired. But the only way to know for sure is a licensed electrician physically checking the attic, basement, and any accessible wall cavities.

Insurance and financing angle: This is the one that actually stalls closings. Per Insure.com, most mainstream insurers decline to write a policy on a home with active knob-and-tube wiring at all, and insurers who will write one charge 50–100% above standard premiums, route you into a stripped-down HO-8 policy, or send you to a specialty/excess carrier or your state's FAIR plan as a last resort. Some carriers will approve coverage conditionally if the wiring gets replaced within about 30 days of closing — which is a real timeline pressure if you're relying on that policy to close your loan.

Cost to fix: Full replacement is generally what insurers want; partial patch jobs usually don't satisfy underwriters. Estimates vary a lot by home size — Honeycomb Insurance cites $3,500–$9,000, while Insure.com cites a wider $12,000–$36,600+ range once drywall repair is factored in. Get a quote for your specific square footage before you assume either number applies to you.

Aluminum wiring

What it is: Solid aluminum branch-circuit wiring installed in place of copper, most commonly in homes built roughly between the mid-1960s and mid-1970s, when copper prices spiked and aluminum was the cheaper substitute.

Is it dangerous? This is the one with an actual, measurable fire risk attached, not just an insurance technicality. Per the Consumer Product Safety Commission, pre-1972 homes with aluminum wiring are 55 times more likely to develop a "fire hazard condition" at a wire connection than copper-wired homes. The mechanism: aluminum expands and contracts with heat more than copper, and it oxidizes at connection points in a way that increases electrical resistance over time — connections slowly loosen and heat up, sometimes with no warning before something fails at an outlet, switch, or junction box.

How to identify it: Check exposed cable in an unfinished attic or basement — aluminum wiring is typically stamped "AL" or "ALUMINUM" along the jacket every few feet. (Cable marked "CU-CLAD" or "Copper-clad" is copper-coated and isn't the same issue.) Home age is a starting clue, but confirming it means having an electrician actually look at the cable, not just guessing from the build year.

Cost to fix — and the good news: Unlike knob-and-tube, full rewiring usually isn't necessary. The CPSC's approved fix is repairing the connections specifically, not the wire itself. The gold-standard method is a COPALUM crimp — a specialized copper-sleeve splice requiring a manufacturer-trained electrician — with AlumiConn connectors as a newer, less field-proven alternative. (Twist-on wire nuts and older "CO/ALR" rated devices are explicitly not considered reliable long-term fixes.) This connection-by-connection approach is often called "pigtailing," and it's dramatically cheaper than replacement: per TCA Electric, full rewiring of a single-family home can run $50,000+, versus roughly $2,000–$3,000 to pigtail a comparable unit — about a tenth of the cost, hitting the connection points where the actual risk lives.

Insurance and financing angle: Same playbook as knob-and-tube — some insurers require an inspection and remediation before they'll bind a policy, and will drop or deny coverage if you don't comply, according to Hippo. Even insurers who do write a policy often charge more for a home with unremediated aluminum wiring. Budget the pigtailing cost and time into your closing timeline if your inspection turns this up.

Polybutylene piping ("Poly-B")

What it is: A flexible plastic water supply pipe used widely from about 1978 to the mid-1990s as a cheap alternative to copper — an estimated 6 to 10 million U.S. homes have it.

Is it dangerous? Not in the fire sense — this one's about sudden water damage. Research cited by InterNACHI found that chlorine and other disinfectants commonly used in municipal water supplies react with the material over years, causing micro-fractures that flake off from the inside of the pipe and eventually penetrate the wall, often at joints and fittings. Failures can happen with no warning — you find out when water is running down your ceiling, not before.

How to identify it: Look at exposed supply lines near the water heater, under sinks, in the basement, or at the main shutoff — polybutylene is usually gray (sometimes white, silver, black, or blue), flexible, ½"–1" in diameter, and often stamped "PB2110" along the pipe, per Modern Castle. It was never used for drain lines, only supply. Again: a build date in that late-1970s-to-mid-1990s window is a clue to look closer, not a diagnosis — confirm with a licensed plumber or your inspector.

The class-action history: This isn't a fringe concern — it was the subject of one of the largest class-action settlements in U.S. history. The 1995 Cox v. Shell Oil settlement paid out $950 million, and a related case, Spencer v. Shell Oil, added $120 million more for affected pipe and fittings. Both settlement windows closed decades ago, and a 2017 attempt to revive claims (Hurt v. Shell Oil) was dismissed, so there's no compensation path left for a current buyer — per Modern Castle and InterNACHI. If you're buying a home with Poly-B today, you're buying the pipe as-is, lawsuit or not.

Cost to fix: Full replacement with PEX or PVC typically runs up to about $10,000; upgrading to copper instead pushes that to $15,000+, per Modern Castle. Cost varies with how much wall-cutting access is needed, so get more than one quote if this shows up in an inspection.

Insurance and financing angle: Many inspectors flag Poly-B specifically because insurers do — some carriers won't write or renew a policy on a home with known polybutylene plumbing without repiping, similar to the wiring issues above. Given the well-documented failure mode and the closed lawsuit window, insurers treat this less like an "old pipe" issue and more like a known-defect issue.

The bottom line for your offer

All three of these show up more often the older the home is, but "how old is it" is only ever a hint — never treat a build year as a diagnosis. If a listing was built anywhere from the 1940s through the mid-1990s, ask your inspector to specifically check for all three, and budget time in your closing timeline: getting an insurance quote can take days, remediation quotes take longer, and some lenders won't clear to close without an active policy in hand. None of these are automatic walk-away red flags — but they're all negotiable line items. Use the inspection findings to ask the seller for a credit or a price adjustment rather than just eating the remediation cost yourself.

Sources

Costs above are general national estimates as of August 2026 gathered from industry sources and vary significantly by region, contractor, and home size — get local quotes before budgeting a specific number into an offer.