Insulating a balcony or terrace

22 August 2018 Kingspan Insulation UK
Balcony Image

Often when constructing a balcony or terrace area for refurbishment or new build, there will be a requirement to put insulation in the deck of the balcony / terrace to ensure that the room below is kept warm.

If the intention is that the room below the terrace areas is to be a warm habitable space then there will be a requirement to meet the Building Regulations for a flat roof, as the terrace or balcony area will be acting as a roof for the room below. If the intention is for the room below not to be a warm space, rather it is a unheated room for a garage for example, we suggest that you read our blog posts on garage conversions instead, so that in a few years’ time if you want to convert your garage, you already know what work needs to be done.

We suggest that for a flat roof in a domestic building the following U-Values (in W/m2.K) are achieved:


  England Scotland Wales
New Build 0.11 0.10 0.11
Refurbishment 0.18 0.18 0.18
Extension 0.18 0.18 0.15

When designing and building the deck of the balcony or terrace you will need to ensure that water does not pool on the terrace and drains away. This can be done by adding falls (a slight slope) into the construction. These should be smooth and steep, a minimum 1:80 fall is recommended in BS 6229:2003 (flat roofs with continuously supported coverings. Code of practice). This helps to prevent rainwater ponds from forming. These falls are normally provided by the supporting structure being directed towards the rainwater outlets and can be created through screed, timber firrings or through the use of tapered insulation.

Key problems

Due to the need to meet building regulations there is a certain thickness of build up for the balcony / terrace which will be necessary in order to achieve the required U-value. The thickness of insulation will depend on the type of insulation used which then leads onto whether you have the space available to accommodate that insulation. There is also the issue of level access.

Level access

Ensuring that there is a smooth transition from room to terrace without a step is a key part of the design of a terrace, but can also be a legal requirement in some circumstances. Approved Document M governing access to and use of buildings states that: “People, regardless of disability, age or gender, should be able to gain access to buildings and to gain access within buildings and use their facilities, both as visitors and as people who live or work in them.”

Ceiling Height

Lowering the height in the room below and insulating below the balcony deck (using Kooltherm K107 pitched roof board between the joists and Kooltherm K118 Insulated Plasterboard below them) is the alternative to insulating above the deck. This may mean dropping the ceiling in the room below, lowering the head height which may not look so good aesthetically as well as the more practical implications of a loss of head room and feeling of space. This solution would need ventilation.


The solution would be to use insulation with high thermal performance, such as vacuum insulation panels, which achieve the U-Values required with a much thinner solution than more traditional materials. Using such thin insulation would help with the level access requirements to the balcony / terrace without impinging on the height of the ceiling in the room below. For more information about VIPs, please visit the OPTIM-R page on our website.