Saunas, swimming pools and wet rooms are warm and very humid – typically 75% to 85% relative humidity – and you will have to be careful with how you insulate. Large volumes of warm, moist air are notorious for creating problems with condensation.
The advice from the BRE is that you should insulate a roof over these conditions by creating a fully cool roof. A cool roof is a roof where the insulation component is entirely over the rafters, joists or deck. No insulation should be in between or under the rafters, joists or deck.
This allows the internal side of the insulation to be at the same temperature as the room below and minimises the number of gaps that could allow moisture vapour into the construction. This moisture vapour, if it condenses, can cause mould.
Placing the roof coverings entirely outside of the structure allows for a continuous layer minimising detailing and the potential for inconsistency. This barrier protects the critical roof structure as it remains above dew point avoiding condensation and its damaging effects.
Let’s just examine for a moment what would happen if you didn’t create a cool roof. If your insulation was between and under rafters or joists or just under the deck, the effect would be that there is more opportunity for moisture to pass through the construction. As the construction is mostly warm, then there is a greater risk of this moisture condensing with all the associated problems this could cause.