How to insulate roofs of swimming pools, saunas and high humidity areas

condensation and insulating in high humidity areas

You can tell we’re starting to move out of the recession when the number of enquiries from architects and designers about how to insulate saunas, swimming pools and wet rooms increases. 

These areas 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 warm roof. A warm 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, rot any timber components of the roof and reduce the thermal efficiency of the insulation.

Placing the roof coverings entirely outside of the structure allows for a continuous layer minimising detailing and the potential for inconsistency.  This warm 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 warm 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 cold, then there is a greater risk of this moisture condensing with all the associated problems this could cause.

More information:

BS 5250: 2011 Code of Practice for Control of Condensation in Buildings

BS 6229: 2003 Flat roofs with continuously supported coverings – code of practice

BRE Digest 336 Swimming pool roofs: minimising the risk of condensation using warm-deck roofing, 1988

BRE Information Paper 2/05 Modelling and controlling interstitial condensation in buildings

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “How to insulate roofs of swimming pools, saunas and high humidity areas
  1. If it is an existing pool hall and for various reasons a “warm roof” on a single storey pool hall can’t be created is it possible to successfully insulate directly behind finished ceiling height and have a cold but well ventilated roof space above same?

    • Hi Jim,
      I wish this was an easy answer, but due to the unique and such high humidity in swimming pools, this is one application where you really can’t cut corners. Soley relying on ventilation in this area would also not be recommended due to the high moisture present.


  2. Thanks kingspan for this article.
    Time and time again we see poorly specified indoor pools. It is worth noting that the target relative humidity with a good air handling system is 60% rh. All the ducting etc will need deigning within the warm deck, This is paramount to protecting the build fabric / steels / timber etc.
    Kind regards
    Alex Kemsley
    Compass Ceramic Pools (UK)

  3. Hi what is the solution if half the pool is inside a two story property and half is out side with a single storey flat roof ? Warm roof over the flat of course but what about inside Thanks Jim

  4. A bit confused about what you are saying about the roof insulation we are building a wooden garden room to house an exercise pool and hot tub, this is a flat roofed building it will be double skinned on the walls with red cedar with a breathable membrane and insulation in between the flat roof would be plywood and felted and have wooden rafters which we planned to insulated in between before cladding inde with cedar but reading your quotes not sure if this is correct please can you advise to enable us to make the correct decision thanks

    • Hello
      If this is a roof above a high humidity space then a warm roof insulation would be recommended. This is where the insulation is placed above the timber rafters / joists. A small amount could be placed between the rafters / joists however our Technical department will need to run a condensation risk analysis for you. In order to do this they will require a listed build up, a full project address as well as the internal space of the garden room so that they can take the correct humidity class. They can be contacted at

      Hope this helps,


  5. We are constructing a sauna as part of a new build against an external wall solid wall insulated externally with K15 as part of the rainscreen cladding. What type of insulation would you recommend internally for the sauna (wall, floor and ceiling)?

    • Stephen,

      Due to the high humidity levels, then condensation would be a concern especially for wall and ceiling. The floor would be less of a concern. We’d recommend a warm structure, but we would need to know the construction build-ups to advise you correctly. Please call our technical team on 01544 387 382 so we can talk it through with you.

  6. Hello, I am thinking of building a pool room, but not fully indoor as it would have a roof but would be exposed from one side, open to the garden (only with an insect net). The pool would have an automatic cover. Is the fact that the pool is not entirely indoor sufficient to prevent condensation problems, or should I add a ventilation / extractor system ?

    Thank you,

    • Hi James

      We only ever recommend external insulation for roofs and walls for a building area that surrounds a swimming pool. This is so as to reduce the risk of condensation.

      However, as yours isn’t really an insulation query, it’s a difficult one for us to answer. It may be beneficial to contact a local building control officer or SPATA – The Swimming Pool & Allied Trade Association – for their advice.

      Hope this helps.

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