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Ask the Expert: What Do I Need To Know About External Roof Insulation?

Niall Crosson

By Niall Crosson

Wednesday 31st May 2023

Externally insulating a roof can be the most effective option to upgrade thermal performance. This is especially pertinent if there is a living space in the roof that requires a comfortable indoor environment throughout the year, regardless of the weather conditions outside. In this blog we will look at some common questions asked when it comes to using wood fibre insulation to thermally upgrade a roof.

Ask the Expert: What Do I Need To Know About External Roof Insulation? Ask the Expert: What Do I Need To Know About External Roof Insulation?

Firstly, why should I consider external insulation on a roof?

Applying external woodfibre insulation on a roof can provide many benefits, including:

  • Increased insulation performance, particularly when combined with insulation between the rafters. This dual-layer approach minimizes heat transfer and ensures better energy efficiency within the building.
  • Eliminates repeating thermal bridges at structural members. Repeating thermal bridges follow a regular repeating pattern and are spaced evenly within the building envelope. By addressing these thermal bridges, the insulation system maintains a consistent and effective barrier against heat transfer.
  • Increases the thermal mass of the roof which is key to reducing overheating in summer months. The added thermal mass helps to absorb and distribute heat, preventing the interior from becoming excessively hot.
  • Acts as a weather resistant layer on the outside of the roof, especially at the building phase, protecting it from the elements such as rain, snow, and UV radiation. This added protection extends the lifespan of the roof and enhances its durability.
  • Improves the windtightness of the roof, as the insulation system creates a tight seal against air infiltration, reducing drafts and enhancing the overall energy efficiency of the building.
  • Improves acoustic performance of the building by reducing sound transmission through the roof, creating a quieter and more comfortable indoor environment.
  • Gutex woodfibre is exceptionally vapour open and breathable, offsetting condensation and moulds risk.
3D graphic of a roof insulated with Gutex woodfibre
The ridge of a roof insulated with Gutex Multitherm woodfibre.
Gutex Multitherm woodfibre boards being fitted over roof rafters. Half completed.
Solitex Plus roofing membrane being attached to The ridge of a roof insulated with Gutex Multitherm woodfibre boards on a roof.

What is the importance of breathability in external roof insulation?

If moisture were to penetrate the roof during the construction stage, or by other means over the life of a building, it should be able to dry out as quickly as possible so as to avoid long term structural, durability and health issues. When a roof is covered on the outside with an impervious membrane or insulation with a high vapour resistance (e.g. foil-faced rigid insulation), this restricts the passage of vapour to the outside and significantly increases the length of time required for the roof to dry out. Breathable external roof insulation, therefore, provides greater levels of security against moisture accumulation over the life time of the building.

Living spaces in attics can be too cold in winter and too hot in summer. How can I increase comfort levels all year round?

As the roof within an attic space has a low thermal mass, coupled with the dark tiles predominantly used on the Irish and British market, these spaces can become both oppressively hot in summer and extremely cold in winter. Woodfibre insulation has the optimum combination of high thermal mass and high thermal resistance, which helps to offset these major fluctuations in temperature inside the living space. The gradual release of stored heat energy from the insulation during cooler periods helps to balance the indoor temperature, while wood fibre’s high thermal resistance, or R-value, inhibits the flow of solar heat through the insulation. This results in optimum comfort levels throughout the year.

Looking to purchase external roof insulation? 

Take a look at the insulation and materials we stock at Ecological Building Systems here.

Blog author

Niall Crosson

Niall Crosson

Technical Director (BTech, MEngSc, MIEI, CEPHC)

Niall is the Technical Director for Ecological Building Systems and was also the company’s first employee. Niall holds a first class honours degree as a Bachelor of Technology and a Masters in Eng.Sc. Niall is a certified Passivhaus Consultant and has provided technical guidance on many award winning low energy projects in Ireland and the UK, and has authored or co-authored several chapters in both the Passivhaus Handbook and the Passivhaus Designers Manual. Niall is also on the board of directors of the Irish Green Building Council and provides guidance on several national standards committees.  Along with this, Niall has contributed to several construction publications including The Journal of the RIAI.

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