Roof & Wall Envelope
Insulation Blankets for Metal Building Roofs and Walls
Definition: An insulation blanket is a flexible roll of glass wool or mineral wool, usually laminated to a facing that acts as a vapor retarder, installed over the purlins and on the girts of a metal building. It reduces heat transfer, controls condensation and improves acoustic comfort under steel cladding.
Key facts
- Usually glass wool or mineral wool, supplied in rolls cut to length.
- Facings include reinforced polypropylene, foil or vinyl laminates.
- The facing is placed on the warm side to act as a vapor retarder.
- Insulation is compressed where it crosses purlins and girts, reducing performance there.
- Spacer blocks or thermal breaks are used to reduce compression losses.
Role in the structure
Insulation blankets do not carry structural load. They are draped over purlins or held against girts and clamped in place by the cladding. The fibrous layer slows heat flow, while the facing on the warm side limits moisture vapor reaching the cold steel sheet, where it could condense and drip or cause corrosion.
Why insulate a metal building?
A single sheet of steel offers almost no resistance to heat flow, so an uninsulated building gets hot in sun and cold at night. Insulation stabilizes interior temperatures, reduces heating and cooling energy and improves comfort for people and stored goods.
Insulation also controls condensation. When warm, moist inside air touches a cold roof sheet, water forms and drips. A continuous facing with sealed joints greatly reduces this risk.
Facings and vapor retarders
The facing gives the blanket its interior appearance and acts as the vapor retarder. Light, reinforced facings brighten the space and resist tearing during installation. The right facing permeance depends on the climate and the humidity inside the building.
Side tabs on the facing are overlapped and sealed by stapling, taping or folding, so the vapor retarder is continuous. Torn facings and open joints should be repaired with matching tape before the cladding goes on.
Installation methods
In the simplest method, the blanket is rolled over the purlins and the roof sheet is screwed down on top, compressing the insulation at each purlin. A banding or support mesh may be used underneath to carry the blanket between purlins.
Higher-performance systems hold the insulation in a deeper cavity using spacer bars, long clips or a liner, so the full thickness is achieved and thermal bridging at the purlins is reduced. The approach is chosen to meet the project energy target.
Acoustic and fire aspects
Fibrous insulation absorbs sound and reduces rain noise on metal roofs, which can be significant in offices, schools and sports halls. Thicker insulation and perforated liners further improve acoustics.
Glass and mineral wool are generally non-combustible, but facings may affect the surface spread of flame classification. Confirm that the complete insulation and facing product meets the fire requirements of the governing code.
Materials & coatings
- Glass wool or mineral wool blankets in rolls.
- Reinforced polypropylene, foil or vinyl laminate facings.
- Facing tapes and sealants for vapor retarder continuity.
- Support banding, mesh and thermal spacer blocks where required.
Design considerations
- Thermal performance target and climate zone of the site.
- Interior humidity and need for a high-performance vapor retarder.
- Compression losses at purlins and girts and use of spacers.
- Fire classification of the facing and insulation.
- Acoustic needs for rain noise and reverberation.
- Fastener length compatible with the compressed insulation thickness.
Fabrication
- Blankets are cut to roof slope or wall height lengths with facing tabs.
- Rolls are labeled by area and packed to stay dry.
- Facing color and type are confirmed before production.
Erection
- Install insulation only as far ahead as cladding can be fixed the same day.
- Keep rolls dry; wet insulation loses performance and can damage facings.
- Pull blankets taut and seal facing tabs continuously.
- Repair any tears in the facing with matching tape.
Inspection & quality
- Insulation is continuous with no gaps at eaves, ridges or openings.
- Facing joints are sealed and free of tears.
- Blankets are dry and not over-compressed between purlins.
- Interior appearance is neat and facing is taut.
What to include in your RFQ
- Insulation type and thermal performance target for roof and walls.
- Facing type, color and vapor retarder performance.
- Fire performance requirements of the facing and core.
- Whether spacer or liner systems are needed to limit compression.
- Interior use and humidity conditions.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcCovering (INSULATION). Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Which side does the vapor retarder go on?
In most cases it goes on the warm side of the insulation, which is usually the interior. In some hot humid climates the strategy differs, so it should be confirmed for the project.
Why does my metal roof drip inside?
Usually because moist interior air reaches the cold roof sheet and condenses. A continuous, sealed vapor retarder and adequate ventilation are the main remedies.
Does compressed insulation still work?
It works, but less effectively where it is compressed over purlins. Spacer systems or a liner cavity help retain full thickness.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
