Insulation Materials and How Each Degrades
Why this matters
Insulation is the one material on most jobs that nobody inspects until something else has already failed, and it is also the material most likely to be quietly destroying what it is wrapped around. A wet section of pipe insulation is not merely a section that has stopped working. It is a damp jacket held against bare metal, which is a better corrosion cell than open air. Knowing which door a given class goes out through tells you where to look and whether you are dealing with a material at the end of its life or a material that was wrong for the location from the day it went on.
A sibling article covers the two broad families of insulation failure at concept level. This one is the per-material card: which mechanism takes each class first, and the tell.
Before you touch insulation on an older building
Thermal system insulation on pipe, boilers, ducts and vessels in buildings constructed no later than 1980 is presumed to contain asbestos under 29 CFR 1926.1101, the construction asbestos standard, unless it has been sampled and found otherwise; the general industry counterpart is 29 CFR 1910.1001. On such a building:
- Do not cut, scrape, sand, break, or sweep it. All of those release fibers.
- Do not pull a section to look under it. That is the exact action the presumption exists to prevent.
- Stop and have it sampled by a qualified person before any work that would disturb it, and treat damaged lagging as a hazard to report rather than something to tidy up.
Chalky white or grey lagging, wrapped fabric finish, corrugated air-cell paper, and hard-set plaster fittings on old pipework are all shapes worth stopping for.
The four doors
Every insulation, thermal or electrical, degrades primarily through one of four mechanisms, and each entry in the tables below names the one that takes it first.
- Water. Liquid water displaces trapped air, which is the only thing doing the insulating, and it stays against the substrate.
- Mechanical. Compression, vibration, foot traffic, sagging. Thickness is R-value, so lost thickness is lost performance in direct proportion.
- Time and chemistry. Blowing agents diffusing out, binders breaking down, plasticizers migrating away, ultraviolet attacking a surface.
- Heat beyond rating, which ages a material far faster than its calendar life suggests.
Thermal insulation, class by class
| Class | First door | What it looks like when it goes | Worth knowing |
|---|---|---|---|
| Fiberglass batt or board | Water | Heavy, matted, sagging; binder browns and the fibers lose loft | Wet fiberglass loses most of its insulating value until dried, and it will not recover its loft once matted |
| Mineral wool | Mechanical | Compressed, thinner than spec, especially where anything was set on it | Sheds water better than fiberglass and takes far more heat, but thickness is still the whole product |
| Expanded polystyrene (EPS) | Time and chemistry | Yellowed, friable surface outdoors; softened or dissolved where a solvent touched it | Attacked by many solvents and some petroleum-based adhesives and tapes, so the adhesive is a compatibility decision |
| Extruded polystyrene (XPS) | Time and chemistry | Surface chalks and erodes under ultraviolet; R declines slowly over years | Blowing agent diffuses out over time, so the aged value is the design value, not the fresh one |
| Polyisocyanurate | Time and chemistry | Facer delaminates or gets punctured, board goes soft and crumbly at wet edges | Two separate effects: R drifts down as the cell gas exchanges with air, and R falls off at low temperature, which matters for cold-side use |
| Closed-cell elastomeric (rubber pipe insulation) | Time and chemistry | Surface hardens, cracks and chalks outdoors within a few seasons unless coated or jacketed | It is only closed-cell if the seams are actually sealed; an open seam is a hole straight to the pipe |
| Cellular glass | Mechanical | Cracked at supports and anywhere a point load or vibration bears | Absorbs no water at all, which is why it is specified where wetting is unavoidable, but it is brittle |
| Calcium silicate | Water | Soft, crumbling, holds water against the pipe like a sponge | High-temperature capability, poor behavior once wet, and the wet failure attacks the pipe |
| Spray polyurethane foam | Time and chemistry | Shrunk away from substrates at edges, friable where mixed off-ratio | Ultraviolet-sensitive, so any exposed foam needs a coating, and an uncoated roof application is a countdown |
| Reflective or radiant barrier | Time and chemistry | Dulled, dusty, or installed tight against a surface | It works by low surface emissivity across an air space, so dust raises emissivity and a lost air gap removes the effect entirely |
Electrical insulation, class by class
| Class | First door | What it looks like when it goes | Worth knowing |
|---|---|---|---|
| Varnish or enamel on windings | Heat | Darkened, brittle, a burnt smell, insulation resistance falling over successive tests | The widely used rule of thumb is that thermal life roughly halves for each 10 degrees Celsius of continuous operation above the insulation's rated temperature |
| PVC cable insulation and jacket | Time and chemistry | Stiff, then cracked at every bend and clamp point; sticky where plasticizer has migrated | Plasticizer loss is what stiffens it, and heat and ultraviolet both accelerate it |
| Cross-linked polyethylene (XLPE) | Water | Nothing visible; failure is internal | Long-term wet exposure on buried or submerged runs drives internal degradation that a visual gives no warning of |
| Rubber and EPR | Time and chemistry | Surface cracking where the cable is flexed or stretched | Ozone and ultraviolet attack, worst under tension, so cracks appear at bends first |
| Paper and oil systems | Water | Falling insulation resistance, degraded oil test results | Moisture enters through breathing and through poor seals |
Contamination sits across all of these: oil, coolant, conductive dust and salt on a winding surface will drag insulation resistance down without any of the insulation itself having degraded, which is why a bad reading gets a cleaning and a re-test before it gets a rewind quote.
What wet insulation does to the metal under it
Corrosion under insulation, usually shortened to CUI, is corrosion of the pipe or vessel wall driven by water trapped in or under the insulation. It is common, hidden by definition, and worst in the temperature range where the surface is wet but not hot enough to dry itself: commonly cited as roughly 10 to 350 degrees Fahrenheit for carbon steel, with API RP 583 as the reference practice. Below that range the water is frozen, above it the wall dries.
Two consequences a shop should carry:
- Closed-cell insulation resists wetting far better than fibrous insulation and gives you no protection at all from CUI once water gets past it. The closed cells keep water out of the bulk of the material, which is why its thermal performance holds up. Water that reaches the pipe through an open seam or a penetration is still against the pipe, and now it is under a jacket that will not let it dry.
- On austenitic stainless steel, wet insulation plus chlorides is a cracking risk, not just a corrosion risk. External chloride stress corrosion cracking is why insulation used in contact with austenitic stainless is specified against leachable-chloride limits, which is what ASTM C795 covers.
Worked: two open seams on a chilled water line
A carbon steel chilled water supply line in a mechanical room. Supply temperature 45 degrees Fahrenheit. Room conditions 78 degrees Fahrenheit at 60 percent relative humidity, which puts the dew point at about 63 degrees Fahrenheit. One inch of closed-cell elastomeric insulation, glued seams, four years in service.
The physics at an open seam. The pipe surface is at roughly 45 degrees Fahrenheit and the room's dew point is about 63, so the pipe surface is about 18 degrees Fahrenheit below dew point. Wherever room air can reach that surface, condensation forms continuously, not occasionally. That is the whole mechanism: this is not a leak and there is no water source other than the air in the room.
What four years of it produced. Two seams had opened, both on the underside of horizontal runs where the adhesive line had been stressed at installation. At each open seam the insulation was noticeably heavier than the sound sections and water ran out when it was lifted. The steel under both was pitted and scaled. Every other section of the same line, on the same pipe with the same fluid in the same room, was dry and sound.
Why the material class did not save it. The elastomeric insulation performed exactly as its class predicts: the bulk material was not waterlogged, and the sections either side of each open seam still measured near their design thickness and were dry inside. The damage was entirely at the seams, which is the mechanical continuity of the installation rather than a property of the material.
That distinction decides the repair. If the material had failed, the answer is a different material. Here the material is right for the duty and the seams are the defect, so the answer is re-sealing seams and re-checking the rest of the run for the same shape of defect, plus assessing the pitted wall on its own merits, because a pitted pressure boundary is a replace rather than a re-wrap.
The failure mode to avoid. Re-insulating over a wet, pitted pipe. The new insulation looks correct on the day, the pipe surface is still about 18 degrees Fahrenheit below dew point, the moisture already present has nowhere to go, and the wall keeps thinning under a jacket nobody will open again for years. The pipe gets dried and assessed before anything goes back on it, every time.
The vapor retarder is part of the material, not an accessory
On below-ambient service, cold pipe and cold duct, the moisture drive is inward, from warm humid room air toward the cold surface. The vapor retarder therefore belongs on the warm, outer side of the insulation, and it has to be continuous: every hanger, support, fitting and screw through it is a path for vapor to reach the cold surface and condense inside the assembly.
That is why cold-service failures cluster at penetrations and supports rather than in the middle of a straight run, and why a support that clamps directly through insulation to a cold pipe is a designed-in wet spot. On above-ambient service the drive is outward and a vapor retarder is not the governing concern, which is exactly why a detail copied from a hot line onto a cold line fails.
Where the choice is actually constrained
Most insulation decisions have more freedom than people use, but four constraints are real:
- Temperature limits are per material and per manufacturer, both the upper limit and, for some closed-cell products, a lower one. Take the figure from the product's own data sheet rather than a general table.
- Contact with austenitic stainless steel constrains you to insulation qualified against leachable-chloride limits.
- Exposed outdoor service constrains you to a jacketed or coated assembly regardless of core material, because ultraviolet takes the surface of nearly every organic insulation within a few seasons.
- Fire and smoke requirements for the space constrain the assembly including its jacket and adhesive, and they are set by the building code and the authority having jurisdiction, not by the insulation catalogue.
Everything else, including thickness, is an engineering choice you can make on performance and cost.
References
- 29 CFR 1926.1101 (construction) and 29 CFR 1910.1001 (general industry), asbestos, for the presumption applying to thermal system insulation in buildings constructed no later than 1980
- API RP 583, corrosion under insulation and fireproofing, for the susceptibility temperature range
- ASTM C795, thermal insulation for use in contact with austenitic stainless steel, for leachable-chloride limits
- Manufacturer product data sheets for per-product temperature limits, aged thermal values, and jacketing requirements
- See related: How Insulation Fails (Generic); How to Inspect Insulation Without Destroying It