Warm and Cold Deck Roof Insulation


8 minute read:

The differences between warm and cold deck roofs:

The terms used to describe warm and cold deck roofs simply refer to the position of the roof insulation in the buildup. For instance, a warm deck roof is any roof system with insulation above rafter level, meaning the rafter space below is comparatively warm. Whereas, in a cold deck roof, insulation is situated beneath and in-between the rafters. You may ask yourself, why is a cold roof named as such if it represents a fully insulated system? Well, even though the space below is insulated, there is nothing over the top of the rafters to prevent cold air from getting into the roof space and conducting through the rafters themselves as a thermal bridge. 

Whether you’re working on a warm or cold roof construction, each require a very different type of roof insulation to insulate the building effectively and to prevent condensation. There are many different types of roof insulation on the market, and it is important to get the right material for your project.

The best Roof Insulation for warm and cold deck systems:

Cold deck roof insulation:

There are different grades of insulation material for use in-between the rafters, all varying in performance and price. Generally, the pricier it is, the better the thermal performance will be. PIR board or mineral wool are the most common and although they cannot reach building regulations on their own, they can be used as part of a hybrid system with a high quality multifoil insulation to achieve lower U values.  

A good vapour barrier is also required beneath the rafters to prevent any moisture entering the rafter space from below. But how to increase the thermal efficiency of the roof if you have shallow rafters and need to reach building regulations?

TLX Silver has the answers:

Our 2-in-1 insulating vapour barrier, TLX Silver, is the missing link, as it manages condensation risk whilst providing an excellent level of insulation. Its shiny reflective layers reflect infra-red radiation back into the house and prevent heat radiating out into the environment. Since TLX Silver is only 30mm thick, it can simply be rolled out underneath the rafters and will significantly reduce the amount of additional insulation required between the rafters to meet building regulations, saving headroom.

TLX Silver has the additional ability to limit the amount of solar radiation getting into the house in Summer, therefore helping to keep it cool. 

Checklist for cold deck roof insulation:

  • Ventilated beneath the timber deck
  • PIR board or mineral wool for use in-between the rafters
  • TLX Silver 2-in-1 insulating vapour barrier underneath the rafters, fitted with 38x38mm battens
  • A standard 12.5mm plasterboard underneath

Warm deck roof insulation:

A multifoil insulation such as TLX Gold 2-in-1 insulating breathable membrane can be installed directly over the top of the rafters, either pulled taut and counter-battened or simply draped over the top if you can’t raise the roof height. Like you would with a cold deck roof, insulation is then fitted in-between the rafters to give an added thermal benefit.

But don’t you still need a vapour barrier underneath the rafters to prevent condensation?

TLX Gold has the answers:

Since TLX Gold 2-in-1 insulating breathable membrane sits at the coldest level of the temperature profile, therefore keeping everything beneath it warm, there is no need for an additional vapour barrier. The beauty of TLX Gold lies in its Sd intelligence, meaning it adjusts its rate of breathability according to varying levels of humidity in the air. In this way, it controls the passage of water vapour through its breathable layers and allow it to escape before it has the chance to cause a condensation risk.

Checklist for warm deck roof insulation:

  • TLX Gold 2-in-1 insulating breathable membrane fitted over the top of the rafters
  • PIR board or mineral wool for use in-between the rafters
  • A standard 12.5mm plasterboard underneath