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Key points

  • Thermal insulation reduces heat transfer and waterproofing keeps water out; a good roof needs both in the right order.
  • In an inverted roof the membrane goes on the slab, XPS boards above it, then a protection layer; the insulation protects the membrane from sun, heat and damage.
  • Insulation above the membrane must tolerate water without losing performance, which is why closed-cell XPS is used.
  • The Saudi energy conservation code SBC 602 sets a maximum U-value for roofs by climate zone.
Written for:OwnerContractor

A hot top floor is a familiar problem, usually caused by a roof without enough insulation or with insulation installed wrongly. To compare insulation materials see thermal insulation.

1) The regulatory requirement

The residential energy conservation code SBC 602 divides Saudi Arabia into climate zones and sets for each a maximum heat transfer coefficient (U-value) for building envelope elements, including roofs1. The roof U-value is calculated from all its layers together by the designer.

2) The inverted roof: recommended order

In an inverted (protected membrane) roof, from the bottom up:

  1. The concrete slab.
  2. Screed to falls towards the drains.
  3. The waterproofing membrane (sheet or liquid-applied). See roof waterproofing.
  4. XPS insulation boards.
  5. A separation layer (fabric) depending on the system.
  6. Protection layer: pavers, gravel or ballast depending on the design.

In this order the insulation protects the membrane from mechanical damage during and after construction, from temperature extremes, and from UV that causes premature failure, and the manufacturer states XPS keeps its performance for the life of the roof in this use2.

Inverted roof layersFrom top to bottom: Protection layer: pavers or gravel, then Separation fabric, then XPS insulation boards, then Waterproofing membrane (sheets or liquid), then Screed sloped to the drains, then Concrete slabProtection layer: pavers or gravelSeparation fabricXPS insulation boardsWaterproofing membrane (sheets or liquid)Screed sloped to the drainsConcrete slab
Inverted roof: the insulation sits above the waterproofing and protects it from sun, heat and damage. A simplified section; details follow the design and the manufacturer's system.

Final layer order and details are set by the designer and the approved manufacturer system.

3) Mistakes that waste the insulation

Common installation mistakes seen on site.

  • Using water-absorbing insulation above the membrane in an inverted roof; it loses its value when wet.
  • Gaps between boards and unstaggered joints; heat passes through the gaps.
  • Puncturing the membrane after insulating to fix a tank or AC unit, without treatment.
  • Weak falls that let water pond.
  • Neglecting the upstands at walls, the most common leak point.
  • Crushing the insulation by walking on it or storing heavy materials before protection.

4) Additions that improve performance

  • A light-coloured finish reflects part of the sunlight.
  • Partial shading of the roof (canopies, solar panels). See rooftop solar.
  • Radiant barriers in roofs with a void. See radiant barriers.

5) Inverted or conventional?

Inverted roof

  • The membrane is protected from sun, heat and damage.
  • Suits roofs in use (seating areas, gardens).
  • Needs water-tolerant insulation (XPS) and protection weight.

Conventional roof (insulation under the membrane)

  • The membrane is directly exposed to the sun unless protected.
  • Allows more insulation types, since the insulation stays dry under the membrane.
  • Any hole in the membrane wets the insulation below.

Sources

The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.

  1. 1Residential Buildings Thermal Performance to Comply With the Energy Conservation Code of Saudi Arabia (SBC 602) — Engineering, Technology & Applied Science Research (ETASR)
  2. 25 Reasons to Choose an Inverted Roof (Protected Membrane Roof with XPS) — DuPont Performance Building Solutions
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This guide is for general information and planning, based on standards and technical guides. Standards are revised periodically; in Saudi Arabia the binding reference is the Saudi Building Code and the instructions of the competent authorities. For design and execution, use a licensed engineer or technician. Found an error? Tell us.