Insulating A Pitched Roof – When Does It Ventilating

The importance of roofing insulation for energy efficiency

First of all, let’s consider the importance of the roof for ensuring energy efficiency. Warm air has a natural tendency to rise upwards and is lost through the fabric of the roof if it is un-insulated; therefore this element alone can account for as much as 25% of a buildings heat loss.

Insulating along the sloping rafters forms a warm roof and is an option for both new builds and refurbishment projects. The insulation can be installed between the rafters and underneath the rafters to extend the thermal envelope up along the pitch of the roof, allowing for a new heated room within the loft space. This popular application requires some careful design considerations to minimise the risk of interstitial condensation and guidance on how to meet target U-values as set out in the current Building Regulations.

In fact, at the Celotex Technical Centre when it comes to pitched roofs, the most popular questions asked are ‘How much Celotex do I need to meet current Building Regulations? Which Celotex board is the best one to use? Do I need a vapour control layer?’

Well to answer these highly popular questions…

….the thickness of Celotex required to meet a U-value in line with current Building Regulations depends entirely on the depth of the rafters and if there is a requirement to fully ventilate or not.

When does a pitched roof need ventilating…?
….The ventilation requirements of a pitched roof depends on the type of roof membrane used under the tiles or slates and battens to keep moisture or rainwater coming in from outside. If the roof covering or roof membrane is a material of high moisture resistance or impermeable, then the building regulations require a 50mm wide ventilation gap beneath the roofing felt and tiles. This is typical practice when an old fashioned black sarking felt is in place and commonly found in existing buildings.


Ventilation is to reduce the risk of interstitial condensation forming on the underside of the impermeable felt membrane and settling on the timber rafters. It is generally formed by using eaves ventilators and ridge or abutment ventilators. This allows for air to enter in one opening and exit the other forming a cross flow of ventilation. This is known as a ‘fully ventilated’ airspace as opposed to an ‘unventilated’ air space.

Alternatively, an unventilated pitched roof may be designed. The principle applied here is warm moist air rising from below is allowed to permeate through the roof membrane removing the need to fully ventilate the roof. This means the impervious roofing felt with a high moisture resistance is replaced with a more permeable membrane or a breathable membrane. The 50mm ventilation gap, as well as the ridge ventilators, is no longer required but instead the airspace may be left unventilated.

This simplifies the design and leaves more space between the rafters for insulation. The actual airspace required below the membrane is better confirmed by a third party certificate as provided by the manufacturer, but it’s commonly accepted that the airspace can be 25mm or less.

And so….
…the thickness of Celotex FR5000 placed between the rafters is the depth of the rafters less the depth of the airspace above. For example, where rafters are 150mm deep and a black sarking membrane is used, then 100mm of FR5000 can be installed which still leaves a 50mm gap for a fully ventilated airspace. Another example, where rafters are 175mm deep and a breathable membrane is used then 150mm FR5000 can be installed while maintaining 25mm unventilated airspace above.

One thing both a ventilated and unventilated roof space have in common is the rafters are not usually deep enough to fit the thickness of insulation required to achieve a target U-value, plus allow for either the ventilated or unventilated airspace above. Celotex GD5000 can be installed as a second layer of insulation to the underside of the rafters. It provides a continuous layer of Celotex to meet the target U-value and because it is has a 12.5mm plasterboard finish it also forms the internal ceiling of the new room.

This prevents increasing the depth of the rafters by fixing timber battens to the bottom of them between which the extra thickness of Celotex can be installed. The use of Celotex GD5000 as a continuous line of insulation also limits the unfavourable impact of repeat thermal bridging on the U-value allowing for a thinner solution and so saves on valuable headroom.

Why do I need a vapour control layer when the roof is ventilated or a breathable membrane is used?
A vapour control layer in conjunction with the correct use of ventilation and roof membranes will effectively eliminate the risk of interstitial condensation. It’s the two design principles working together that minimise the damaging effects of condensation on the timber structure.

Condensation is formed when warm moist air rises and condenses into a liquid on contact with the colder surfaces above the insulation. The idea behind a vapour control layer is to install it on the room side of the insulation so it blocks the passage of warm moist air rising into the pitched roof structure. However, warm moist air by its very nature will always find the passage of least resistance so the use of roof membranes and ventilation will effectively manage the small amount of condensation that will inevitably form.

Celotex GS5000 has an integral vapour control layer built in. It is positioned between the plasterboard and Celotex foam insulation. When the boards are tightly butted together, the tapered edges of the plasterboard are sealed with scrim tape and jointing compound to form an effective vapour control layer with a high vapour resistance.

…beyond the U-value

Insulation is effective within the pitched roof but a strong fabric performance means the design and installation extends beyond the U-value and thicknesses of Celotex. The construction materials used to form the fabric of the roof each have different thermal and moisture properties; how they come together impacts the building physics and the way a building uses energy.

To ensure the roof is built to a high standard the design shows how junctions and openings are insulated and finished to maintain a continuous thermal envelope and air tightness. This plays an important factor when meeting the required energy targets of the current Building Regulations.