PEB Insulation and Thermal Performance: Stopping Heat and Condensation

Two problems, not one

Insulating a steel building addresses heat transfer and condensation, and the second is frequently the more urgent.

A metal envelope conducts readily. Under direct sun a roof sheet gets very hot; in a cooled or humid building the inner face can fall below dew point and water forms on it. That water drips onto whatever is below.

A specification that considers only thermal performance will often still produce a building with a condensation problem.

What goes into the build-up

Layer	Function	Failure if wrong
External sheet	Weather barrier	Leaks at fasteners and laps
Insulation	Thermal resistance	Compressed or gapped, underperforms
Spacer or thermal break	Prevents compression at purlins	Thermal bridge at every purlin
Vapour barrier	Stops moisture reaching cold surfaces	Interstitial condensation
Liner sheet or membrane	Internal face, protects insulation	Insulation sags, gets damaged

Thermal bridging is the biggest practical loss

Insulation laid over purlins and compressed by the roof sheet loses most of its thickness at every purlin line.

Over a large roof, those lines add up to a significant share of the area, and the assembly's real performance falls well short of the insulation's declared value. Assembly calculation methods are set out in ISO 6946, and it is the assembly figure that ECBC 2017 assesses for commercial buildings.

The remedies — spacer systems, thermal blocks over purlins, or a continuous over-purlin layer — belong in the specification. Left unspecified, the installer will lay the insulation the quickest way, which is the compressed way.

Vapour control is direction-dependent

A vapour barrier goes on the warm side of the insulation.

In a cooled building in a hot humid climate, that is the outside. In a heated building in a cold climate, the inside. India spans several climate zones, and many industrial buildings are neither consistently heated nor cooled, so this cannot be answered generically — it must be determined for the specific building and its internal conditions by the project's consultant.

What is universal is that the barrier must be continuous: laps sealed, penetrations sealed, and not torn during installation. A vapour barrier with gaps is close to no vapour barrier at all.

Materials

Mineral wool is the common choice for industrial envelopes, covered by IS 8183 for bonded mineral wool, with rock wool suiting higher temperatures and higher densities and glass wool generally lighter and lower cost for a given thermal performance.

Whichever is used, specify by property — density, thickness, declared thermal conductivity, facing, required fire classification — rather than by material name. Fire classification is reported under EN 13501-1 for the complete faced product, and a faced product does not automatically share the classification of its bare core.

Installation is most of the outcome

• Fill the space without gaps; do not stretch material to cover
• Do not compress below design thickness, except where the build-up is designed for it
• Keep the vapour barrier continuous and sealed at every lap and penetration
• Protect the insulation from damage during and after installation
• Keep it dry — wet insulation does not recover, and wet insulation in a roof is a water-ingress problem that will recur until the leak is fixed

Standards referenced

Envelope requirements in ECBC 2017; bonded mineral wool in IS 8183; thermal resistance and transmittance calculation in ISO 6946; reaction-to-fire classification in EN 13501-1. Build-up design, vapour barrier position, declared thermal values and fire classification requirements for a specific building must be established by the project's consultants and the authority having jurisdiction.