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SOLIMIDE, Aerogel, and the Moisture Layer: Choosing Airframe Insulation Materials
A fuselage insulation package looks like a thermal problem and behaves like a moisture problem. The thermal math is easy to get right; what quietly kills a package is water, finding a torn film, an untaped seam, or a frame that runs cold. The families on the H-O airframe insulation page sort along both axes at once, and this guide walks each one by the decision it wins, with methods named by designation and per-grade values kept on the maker TDS.
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Airframe insulation fails by water before it fails by heat
The thermal math in a fuselage package is the easy part. What quietly kills an airframe insulation package is the moisture layer: a torn film, an untaped seam, a bay with no drain path, a frame that compresses the blanket to nothing and runs cold. So the families on the H-O fuselage and ECS page sort along two axes at once, the thermal-and-acoustic job and the moisture-and-flammability job, and a good spec names both.
This guide walks the families by the decision each one wins, with methods named by designation: ASTM C177 and ASTM C518 for thermal transmission, ASTM E96 for water-vapor behavior of the encapsulation film, ASTM D1056 for compression-deflection and set of the sponge gaskets and spacers, UL 94 for flammability classes, and FAR 25.856 as the transport-category framing for thermal-acoustic insulation.
Per-grade values are the maker-TDS figures cited on the parent page.
The thermal core: mass against thickness, again
Polyimide foam: the broad-area workhorse
Sidewall and crown blanket bays, underfloor runs, and ECS plenum insulation want the lightest documented core, which is open-cell polyimide foam: SOLIMIDE aerospace grades are the airframe insulation benchmark, ultra-low density per grade with flame, smoke, and toxicity data on the maker's TDS commonly evaluated against FAR 25.856(a), thermal conductivity per ASTM C177 and ASTM C518.
The foam is the broad-area thermal and acoustic workhorse of the package; its performance in service depends on the converted geometry that wraps frames and carries drainage features rather than on the headline conductivity number.
Aerogel blanket: when the bay is shallow
Where a bay is too shallow or a duct clearance too tight for a conventional core to hit its thermal target, ArmaGel aerogel blanket delivers the target in millimeters, conductivity per ASTM C177 and surface-burning data per ASTM E84 on the TDS. Aerogel buys thickness margin where the airframe will not give up depth, and the trade is the same mechanical delicacy that the encapsulation and facing answer. Specify it where clearance is the binding constraint, not as a default.
Thin polymer film: where there is no room at all
For electronics-adjacent layers and tight covers where even aerogel is too thick, AeroZero thin polymer aerogel film insulates at film thickness. It is a local-layer answer, not a broad-area core, and it earns its place where millimeters decide.
The moisture layer: specify it like structure
The difference between a package that stays light and one that comes back waterlogged is the moisture strategy, and it is a converted feature of the part.
Encapsulation film carries the vapor behavior, screened per ASTM E96; the seam closure method keeps vapor from reaching a cold skin from inside the wrap; die-cut drain holes and scallops give condensate a path out; and standoffs keep the package off the skin. Kapton and Apical polyimide film serves as a durable encapsulation and facing film where its temperature and dielectric behavior suit the location, and PTFE-coated fiberglass faces the runs that take handling and abrasion.
The drainage geometry is part of the drawing, not an installer's improvisation.
The acoustic and sealing layers
Cabin quiet is two different jobs: absorption, which the porous polyimide core provides, and mass, which a limp barrier ply provides. Where the package needs a mass term, BISCO A2 sound barrier adds the limp-mass layer rather than asking the foam to do a job it is poor at.
At the pack-bay and access-panel interfaces, the seals sort on cycle life: BISCO 7xxx FR silicone sponge and kSil V-0 carry the cycling interfaces with compression-set data per ASTM D1056 and the flammability listing the call-out demands, while BISCO HT and BF silicone foam and die-cut silicone-foam thermal spacer blocks handle the contact points where frames would otherwise bridge the insulation.
Airframe Families by the Job They Win
| Family | Job in the package | Moisture role | Method position | Where it lives |
|---|---|---|---|---|
| SOLIMIDE polyimide foam | Broad-area core | Needs encapsulation | ASTM C177 / C518 | Sidewall, crown, underfloor, ECS plenum |
| ArmaGel / AeroZero | Thin core / film | Needs encapsulation | ASTM C177 per TDS | Shallow bays, tight ducts, local layers |
| Kapton / Apical / PTFE-coated fiberglass | Encapsulation / facing | The vapor barrier | ASTM E96 vapor | Films, facings, seams, drainage features |
| BISCO A2 sound barrier | Acoustic mass | n/a | Limp-mass ply | Where the package needs a mass term |
| BISCO 7xxx FR / kSil V-0 | Cycling seal | Closes the bay | ASTM D1056 set | Pack-bay and access-panel interfaces |
Illustrative and relative only; confirm each grade on its current technical data sheet. Thermal cores per ASTM C177 / C518; encapsulation vapor behavior per ASTM E96; sponge set per ASTM D1056; insulation compliance findings under FAR 25.856 belong to the airframer.
Two rules that sort the page
First, decide the moisture strategy before the material: a thermally perfect core in a bay with no drain path becomes a waterlogged, acoustically dead package by the next heavy check, so the encapsulation film, seam method, drainage features, and standoffs are specified with the core, not after it.
Second, name the flammability framing the part actually answers: FAR 25.853 governs interiors and FAR 25.856 governs thermal-acoustic insulation, and an interiors-credentialed material does not automatically carry the insulation data, so the drawing should name the rule and the material data package should be assembled against it.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We take polyimide foam, aerogel blanket, thin polymer film, polyimide and PTFE-coated facing films, sound-barrier ply, and silicone sponge stock from the material manufacturers and die-cut, knife-cut, laminate, and kit it to your drawing, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system in Winsted, Connecticut.
We recommend a family and grade direction from your thermal target and bay depth, acoustic duty, moisture strategy, flammability framing, and attachment method, and grade-level values are confirmed against the manufacturer's current TDS. Insulation compliance findings under FAR 25.856 and airworthiness sign-off stay with the airframer.
Frequently asked questions
What insulation is used in aircraft fuselage walls?
A thermal-acoustic blanket package, not a single material. The broad-area core is typically SOLIMIDE polyimide foam, an ultra-light open-cell foam whose flame, smoke, and toxicity data is documented on the maker's TDS and commonly evaluated against FAR 25.856(a).
Around it sit the moisture layer (an encapsulation film screened for vapor behavior per ASTM E96), an acoustic mass ply where the noise target needs one, and silicone-foam thermal spacers at the frames. Aerogel blanket substitutes for the core where a bay is too shallow for conventional thickness.
Why does aircraft insulation get wet, and how is that prevented?
Because condensation finds a torn film, an untaped seam, or a bay with no drain path, and the core then holds water for the rest of its life, gaining weight and losing acoustic performance. The prevention is to specify the moisture layer explicitly: encapsulation film with vapor behavior per ASTM E96, a defined seam-closure method, die-cut drain holes and scallops, and standoffs that keep the package off the skin. The drainage geometry is a converted feature of the part, not an installer's improvisation.
When is aerogel blanket worth it in a fuselage package?
When clearance is the binding constraint. Aerogel delivers a given thermal resistance in a fraction of conventional thickness, so it earns its place in shallow bays and tight duct clearances where a conventional core cannot hit its target in the available depth. Where depth is free, the lighter polyimide core usually wins on mass and cost. Specify aerogel by the clearance it solves, per the ASTM C177 conductivity on the TDS, and encapsulate it like any other core.
What is the difference between FAR 25.853 and FAR 25.856 for insulation?
They are different rules for different jobs. FAR 25.853 governs compartment-interior flammability; FAR 25.856 governs thermal-acoustic insulation, with a flame-propagation paragraph (a) and a burnthrough paragraph (b) and dedicated test methods described in the FAA's AC 25.856 guidance. A material with interiors credentials does not automatically carry the insulation data, so name the applicable framing on the drawing and assemble the material data package against it.
Does H-O certify insulation installations or hold airworthiness approvals?
No. H-O converts the documented materials to the qualified drawing, with material traceability and lot-code TDS records that support your certification file. Showing FAR 25.856 compliance for an installation, and airworthiness sign-off, belong to the airframer and its test houses. Our role is bounded and precise: die-cut, knife-cut, laminate, and kit the cores, films, spacers, and seals to your drawing, repeatably.
Get an airframe insulation recommendation
Tell us the thermal target and bay depth, the acoustic duty, the moisture condition of the bay, the flammability framing, and the attachment method, and we will recommend a family and grade direction and quote die-cut parts to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- FAR 25.856(a) and (b) (thermal-acoustic insulation flame propagation and burnthrough); FAR 25.853 (compartment-interior flammability); FAA AC 25.856 (guidance and test methods).
- ASTM C177 and ASTM C518 (steady-state thermal transmission, the core methods).
- ASTM E96 (water-vapor transmission of the encapsulation film, the moisture-layer screen).
- ASTM D1056 (compression-deflection and set for sponge seals and spacers); UL 94 (flammability classes, per grade).
- Manufacturer technical data sheets for the polyimide foam, aerogel blanket, thin polymer film, polyimide and PTFE-coated facing films, sound-barrier ply, and silicone sponge grades named on the parent application page.
This article summarizes insulation material-selection principles for aircraft fuselage and ECS systems using the families and published test methods cited on the parent application page. Comparisons are qualitative; per-grade values quoted are maker-TDS figures and must be confirmed on the manufacturer's current technical data sheet at your thickness, temperature, and installed condition. Standards (FAR, ASTM, UL) are cited by designation for context, not as a claim of qualification.
H-O is a precision die-cut converter, not a raw-material producer, and does not perform thermal design, certify materials, or hold airworthiness approvals; those belong to the airframer and its test houses.