# PEB Insulation and Thermal Performance: Stopping Heat and Condensation

> Insulating a steel building means controlling both heat transfer and condensation. A metal envelope is a good conductor and its inner surface can fall below dew point, so the build-up has to address thermal bridging at purlins and girts and include vapour control on the warm side, or the insulation gets wet and stops working.

## 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. Which side that is
depends on the building:

| Building | Warm side, where the barrier goes |
| --- | --- |
| Cooled, in a hot humid climate | The outside |
| Heated, in a cold climate | The inside |
| Neither consistently heated nor cooled | Determined for the specific building by the project's consultant |

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.

## Frequently asked questions

### Why does condensation happen in metal buildings?

Because sheet metal conducts heat readily, so the inner face can drop below the dew point of the air inside. Moisture then condenses on it and drips. In industrial buildings the internal moisture load can be high — from processes, washing, vehicles or simply many people — so it is not only a cold-climate problem. The control is a continuous vapour barrier on the warm side plus enough insulation to keep the inner surface above dew point.

### What is thermal bridging at purlins and why does it matter?

Where insulation is compressed between the roof sheet and a purlin, its thickness — and therefore its performance — collapses at every purlin line. Across a large roof that is a substantial proportion of the area, and the whole-assembly figure can be far worse than the insulation's declared value suggests. Spacer systems, thermal blocks or an over-purlin layer are the usual ways of addressing it, and they should be part of the specification rather than left to the installer.

### Which side does the vapour barrier go?

The warm side of the insulation, which in a cooled building in a hot humid climate is the outside, and in a heated building in a cold climate is the inside. Getting this wrong traps moisture within the build-up. Because India spans several climate zones and many industrial buildings are neither fully heated nor fully cooled, the correct arrangement must be determined for the specific building and its internal conditions by the project's consultant.

### Does insulation affect the fire picture?

It can. Insulation products and their facings are classified by test, commonly to EN 13501-1, and the classification belongs to the complete faced product rather than the bare core. What is permitted in a given building depends on its occupancy, size and use, determined under the applicable fire requirements by the fire consultant and the authority having jurisdiction. Any substitution must have its classification re-checked.

### Is insulation worth it on an unconditioned warehouse?

Often yes, for two reasons beyond energy. It reduces the extreme temperature swing under a metal roof in direct sun, which affects working conditions and stored goods. And it addresses condensation, which is the more immediate problem — water dripping onto stored product or a floor is a real operational issue regardless of whether the building is air conditioned.

### What is the most common installation failure?

Gaps and compression. Insulation that does not fill its space, is squashed at laps and fixings, or is torn during installation performs well below its declared value, and the shortfall is invisible once the sheeting is on. A discontinuous vapour barrier — unsealed laps, penetrations left open — fails in the same way, and both are workmanship matters that need supervision rather than specification.

## Sources

- [ECBC 2017](https://beeindia.gov.in/) — Building envelope
- [IS 8183](https://standards.bis.gov.in/website/published-standards/department-wise) — Bonded mineral wool
- [ISO 6946](https://www.iso.org/standards.html) — Building components — thermal resistance and transmittance
- [EN 13501-1](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Reaction to fire classification of construction products

---

Source: https://hagerstone.com/services/peb/peb-insulation-and-thermal-performance