Should you build a Warm roof or Cold roof?

When it comes to insulating the roof of your Garden Room, there are two main options to choose from. Warm roof or cold roof.

If you’re unsure as to the difference between warm roof insulation and cold roof insulation, keep reading.

Cold Roof

What is a Cold roof?

A cold roof is insulated by installing insulation between the rafters and since the only thing between the rafters is empty space, this is a good way to insulate your roof without using any extra head height.

why is it called a cold roof? The void above the insulation is uninsulated and vented with cold air. Although the majority of the ceiling is well insulated, the roof structure (rafters) is uninsulated, meaning on a winter’s day the cold will conduct through the rafters and into the room below. On a thermal imaging camera, you would easily be able to spot where the rafters are situated by looking at the ceiling. This is because wooden rafters conduct heat much better than insulation, this is called thermal bridging.


Warm Roof

What is a Warm roof?

A warm roof can be easy to install and offers higher thermal performance at the cost of a higher roof profile. In the construction of a warm roof, this is usually a sandwich of 2 sheets of plywood with insulation between which sits on top of the rafters.

As opposed to insulating between rafters, the warm roof insulation is simply installed on top of the rafters.

Warm roof insulation ensures the entire roof structure is evenly insulated, meaning a warm deck roof is more energy efficient than a cold roof.


Warm roof or cold roof comparison

Roof depth:

  • Cold roof: The insulation is between the rafters, so the overall height of the roof is less.
  • Warm roof: The insulation is laid above the rafters so the overall roof height is higher.

Vapor Control Barrier:

  • Cold roof: A vapor barrier is required where the internal surfaces are fixed meaning that if it gets punctured, there may be a greater possibility of moisture vapor permeating through from the internal environment.
  • Warm roof: If a vapor barrier is used, it is bonded on top of the substrate layer. Where the insulation and waterproofing layer is bonded it preserves the integrity of the vapor barrier.


  • Cold roof: Cold roofs should be ventilated with a minimum of 50mm clearance above the insulation for unrestricted air flow.
  • Warm roof: Warm roofs are unventilated.
garden room cold roof ventilation

Thermal Performance:

  • Cold roof: The insulation is within the structure which means the structural elements can become a source of thermal bridging. Some insulation may be required internally to mitigate this risk.
  • Warm roof: The structure is on the warm side of the insulation meaning that there is less chance of thermal bridging.

Moisture and condensation in the roof construction:

  • Cold roof: The vapor barrier is often penetrated when fixing the internal finishes. Also, the dew point is inside the structure. This means cold roofs should be ventilated to avoid the build-up of moisture and condensation.
  • Warm roof: Theoretically there should not be any moisture / interstitial condensation in a warm roof, hence warm roof constructions are not ventilated.


Choosing between a warm roof or a cold roof really depends on your own circumstances and the main choices below.

  • Cold roof: This should be used where the height of the building is restricted, usually due to planning restrictions, Thermal performance is compromised to gain a lower roof height.
  • Warm roof: This is the best-performing option and should be used when height restrictions do not apply.

If you are looking for more information on insulating other parts of your Garden Room, see our extensive Garden room insulation guide below

Frequently asked questions

What is a cold roof

A cold roof is a roof in which the thermal insulation layer is located immediately above or between the ceiling joists.

What is a warm roof

A warm roof is a roof in which the insulation layer is laid on top of the roof structure.

What is the difference between a warm roof and a cold roof

Cold roof: As the majority of the insulation is between the rafters, the overall build-up of the roof is often less.
Warm roof: The insulation is laid above the substrate layer so the overall roof construction is generally deeper.

Is a warm roof better than a cold roof

A warm roof is the easiest and most cost-effective type of insulation and is an ideal choice.
On some flat roofs, a warm roof would increase the height and then you should choose cold roof insulation.

How do you ventilate a cold roof?

A cold roof requires 50mm of continuous ventilation above the insulation, this needs to be vented at each end to provide airflow.

Heating and Insulation


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