Warm Roof or Cold Roof – Which would you choose?

  • Building Design Expert
  • 6 years ago

The insulation of our building stock should be relatively simple. Only of course if you understand the basic principles that operate in the design / installation of the insulation materials.

First of all the design is all about what type of the many insulation products available is best suited to any given form of construction, and second, where in that construction do we put the insulation to best effect?

Should be straight forward shouldn’t it? as long as you follow a ‘golden’ check list:

Lets look at roofs, because potentially this where the greatest choice of solutions lies.
The golden check list for roofs might comprise –

  •  Warm roof or cold roof, which would you choose if you have a choice?
  • What is the design insulation level? e.g do we simply need to achieve a Building Regulations minimum standard, or we looking to improve upon that?
  • What type of insulation material is best suited to achieve the design insulation level in conjunction with the construction type?
  • If insulation and construction are incompatible, is it possible to alter the construction and, or insulation so that they are?
  • If it is not possible to amend the compatibility of construction or insulation can an adequate compromise be achieved?
  • Is a vapour barrier required, or put another way – How effective does the vapour barrier have to be?
  • Where should the vapour barrier be installed in order to perform it’s task?
  • Is the chosen design solution ‘build-able’? – Are you, as a building designer asking your contractor to achieve the impossible?

When you put it that way it’s a daunting list. Isn’t it? Well it may be daunting to anyone who cannot fully answer each item. New build or retro-fit of insulation, the list is pretty much the same. The main difference being with Retro-fit the most desirable design solution may be more difficult to achieve.

The following are pencasts – Please ensure you have sound switched on – Click on the ‘Play’ button to start.

Pencast – Cold Roof Construction

 Pencast – Warm Roof Construction

It is perhaps surprising, and certainly unfortunate that some building professionals, and contractors are under a misapprehension as to the precise requirements in order to achieve warm roof, and, or cold roof construction.

Cold Roof Construction – requires that a suitable ventilated air space exists above the insulation layer, within which the air is constantly mobile, being replenished by fresh incoming air, whilst  the air resident within the void space is being expelled via a natural vent action at another point. On a flat roof this could be ‘air in’ at one ventilated facia soffit, and ‘air out’ via an opposite ventilated facia soffit. The resultant action of which does not allow condensation to occur to any significant effect.

Warm Roof Construction – requires that the entire roof construction is encapsulated within the insulation envelope of the building to be kept ‘warm’. The object being to design out the possibility of warm moisture laden air coming into contact with a cold surface and condensing.

The theory of ventilation is all well and good, but the ever increasing demands of the Building Regulations to reduce our carbon footprints and keep heat within our buildings for as long as possible, leads to the inevitable consequence that we retain more and more moisture that is just bursting to break free and condense.

Remember ‘schoolboy’ physics that says the warmer an environment is, the more moisture it will support as vapour before it condenses – Building designers must address this at detail design, and contractors must understand the consequences of not following that design to the letter.

So warm roof or cold roof, which would you choose?

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