Ventilation for flat roofs
We had a call to our technical line earlier today from a customer asking about when or whether to ventilate a flat roof. The answer may be useful to some of you.
Firstly, there are two types of flat roof. A ‘Warm Roof’ and a ‘Cold Roof’.
A warm roof is defined as being when the all or the majority of the insulation is above the joists or deck. The roof itself is therefore kept warm. By ‘majority’, the rule of thumb is that in a domestic house (not flats), approximately a third of the total thermal performance of insulation can be placed below the deck with two thirds above the deck (sometimes this is called a ‘hybrid warm roof’). Ventilation is not needed in a warm roof.
A cold roof is where all of the insulation is placed either between and under or completely under the joists or deck. No insulation is on top. In this way the roof itself is kept cold. You only need ventilation in a cold roof. Cold roofs, by the way, are not recommended for high humidity areas – see the blog on ‘How to insulate roofs of swimming pools, saunas and high humidity areas’.
BS 5250: 2011 Code of Practice for Control of Condensation in Buildings stipulates that there should be a ventilation gap between the insulation and the bottom of the flat roof deck of at least 50mm. In a concrete deck, for example, this gap could be created by fixing 50x50mm timber battens at 600mm centres and attaching the insulation to underside of these battens. Where the battens abut the wall, there should be a ventilation gap created to allow air from the outside to be drawn into this space.
The amount of ventilation or air change needed can be calculated by using Approved Document C and Approved Document F (in England & Wales) and Section 3 of the Technical Handbook (Scotland). However, it should be noted that cold flat roofs are not encouraged under Scottish Building Regulations because of more extreme weather.
The majority of domestic cold flat roofs use insulation ‘between and under’ the joists. The rule of thumb for foil faced boards (like Thermapitch TP10 or Kooltherm K7) in this scenario is that you can have no more insulation under the joists than you have between the joists. It is preferred to have the bulk of the insulation between the joists to reduce any risk of condensation.
BR 262 – Thermal Insulation avoiding risks (Pages 17-21 cover warm roofs and pages 22-23 cover cold roofs, giving a summary of the regulatory requirements and general good practice recommendations for VCLs, ventilation etc).
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