a03 - Roof ventilated construction

Introduction

Traditional roofs with tiles are ventilated roofs. This means that the watertight layer has a ventilated cavity under it. Not all roofs have this construction. From the 1970’s on, the asphalt shingle roof without the ventilated cavity became popular.

Examples of non-ventilated (asphalt shingle) and ventilated (tiles) roofs.

Roof details of the demo house in Mons (B).

Ventilated roof of the demo house in Mons (B).

 

Building physics

The most important difference between both types are the physic aspects. A well insulated roof (like most roofs since 2000) has a risk of internal condensation. The ventilation cavity will transport water-vapour from the construction to the outside. The construction of the cavity needs enough space at the lower and the upper part of the roof.

 

Roof layers

The construction is built up in different layers:

  • 10 mm gypsum board
  • vapour barrier
  • 400 mm cellulose insulation
  • 35 mm Homatherm fiberboard / 22 mm Celit 4D
  • 65 mm cavity
  • solar modules

This construction is open for vapour and a vapour barrier is not needed in the Belgian climate.

 

Relation with PV-Module

The efficiency of PV cells is influenced by the temperature of the cells. In general an air-cavity of around 50 mm will give an air flow that is enough to ventilate the heat away. Also the openings on both sides of the cavity need to be big enough otherwise they will hinder the airflow or even stop it.