How to Insulate a Flat Roof from the Inside

Insulating a flat roof from the inside, also known as a cold roof insulation method, helps improve energy efficiency and prevent heat loss. Proper insulation not only keeps a property warm in winter and cool in summer but also reduces condensation and damp-related issues. While insulating from the inside is an option, it must be done correctly to prevent moisture buildup, which can damage the roof structure over time.

The first step in insulating a flat roof from the inside is selecting the right insulation material. Rigid foam boards such as PIR (polyisocyanurate) or phenolic insulation are commonly used due to their high thermal performance and ease of installation. Mineral wool or fibreglass can also be used, but these require an additional vapour control layer to prevent condensation. The chosen insulation should meet building regulations to ensure sufficient thermal efficiency.

Before installing insulation, the roof structure must be assessed for any signs of leaks or damage. Any existing dampness should be addressed, as trapping moisture inside can lead to mould growth and structural issues. Ventilation is a key factor in a cold roof system, so air gaps must be left between the insulation and the roof deck to allow airflow and prevent condensation buildup.

To install the insulation, the material is cut to fit between the roof joists, leaving a small gap above for ventilation. A vapour barrier is then placed beneath the insulation to stop warm air from reaching the cold roof deck, which could cause condensation. The vapour barrier should be properly sealed at the edges and around any openings to ensure it is airtight. Finally, plasterboard is fixed over the insulation to create a smooth internal finish.

Proper ventilation must be maintained to avoid long-term issues. Ventilation gaps or soffit vents should be installed to allow continuous airflow above the insulation. Without adequate ventilation, moisture can become trapped inside the roof space, leading to rot and damage to the timber structure.

In conclusion, insulating a flat roof from the inside requires careful planning to ensure effective thermal performance while preventing condensation problems. Using high-quality insulation, installing a vapour barrier, and maintaining proper ventilation are all crucial steps in achieving a well-insulated and long-lasting roof.