How to Insulate a Dormer Roof…

When making changes to a dwelling or when constructing a new dwelling it must comply with the current building regulations, when altering the insulation in a dormer part L of the building regulations must be adhered to.  Some key points of the building regulations are: the maximum elemental U-value of a pitched roof shall not exceed 0.16 W/M2 K.  That to avoid excessive heat loss, thermal bridging should be limited.  To limit thermal bridging reasonable care should be taken to ensure the continuity of the insulation and in the case where rigid insulation is being used a 50mm air gap should be maintained behind the insulation. 

 


To insulate an existing dormer, we outline three of the more common methods below:


The first method is to use rigid insulation.  This method requires the use of 150mm thick insulation between the rafters and a 50mm air gap to prevent cold bridging occurring.  If on the existing rafters there is not enough space for this a batten could be fixed to the underside of the rafters to increase the depth.  The thermal conductivity of this product is between 0.020 and 0.023 W/m K, which is the most thermally efficient product that is in common use. 

The second option is to use mineral wool insulation.  This method requires the use of 320-350mm of insulation in between the rafters.  As mineral wool insulation is breathable it does not require an air gap.  Although if it is possible to have an air gap, then it is advised that one should be left.  To achieve the 320-350mm thickness in mineral wool it has to be built up in 150mm and 100mm layers.  The thermal conductivity of mineral wool insulation is between 0.040 and 0.044 W/m K. 

The third option is to use spray foam insulation.  The main benefits of spray foam insulation are that it can be applied relatively thin and it is also breathable so it does not require an air gap.  It can also be applied to hard to reach areas where other forms of insulation may fail to cover.  The air tightness of this material is also superior.  The thermal conductivity of spray foam insulation is between 0.032 and 0.037 W/m K. 

In all of these methods, in cases where there is no sufficient space in the rafter to fit the required insulation, as stated above battens can be used to increase the depth/create an air vent alternatively insulated plasterboard ( using rigid insulation) can be used to reach the required U-value.  Ait tightness membrane is applied to all the above on the underside of the rafter.

To insulate a new dormer the same three methods can be used, however, there are changes to the construction method that can be used.  When using the rigid form of insulation it is possible to place counter battens underneath the battens used for the tiles or slates. 

This method provides the air gap that is required, which allows for more head room on the inside.  Although this is a popular method of construction, it is also possible to have the 50mm air gap as well as cross battening the roof.

The method shown below is widely view as the industry best practise as it is very effective in cutting out any thermal bridging, this involves placing 60mm of rigid “sarking” insulation above the rafters, with mineral wool insulation filling the rafter this is then battened on the underside of the rafter to which a plasterboard slab is fitted.

It is also important to note that regardless if it is a new or an existing dormer that the insulation is connected up with the insulation in the walls of the building.  Otherwise heat will be able to escape from this area.

When undertaking construction work in your home we recommend obtaining the services of a construction professional.

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