News update July 2015

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Time to put a lid on overheating


How heat is transferred through a building is critical to creating a healthy internal environment

Imagine two scenarios a caravan and a stone built house, imagine them both on a warm summer’s day. In the caravan, as soon as the outside cladding starts to heat up, heat output is recorded within minutes on the inside face as some heat quickly transfers through the aluminium / lightweight insulation composite; whereas as the face of the stone wall heats up, the heat is absorbed by the stone as it progresses slowly from the outside to be delivered from the inside surface several hours later.

The interesting, and often baffling, aspect of this phenomena is that the two materials can have very similar u-values - so that in steady-state conditions where heat applied at a constant rate over a period of time to the external face of both materials, there is an equally constant flow from the inside surfaces of diminished heat.

Light structures with little thermal mass overheat rapidly this is due to the use of thin lightweight insulation 
Stone and other dense materials absorb and store heat and release slowly limiting internal fluctuations.

What is different about the caravan and stone house examples is that the heating is not steady-state: real life heating from the sun varies throughout the day. The variability is known as 'periodic heat flow', and fortunately, for the purposes of building design, it is almost entirely predictable.

This all relates to a massively overlooked principle - decrement delay (sometimes called phase shift) which is defined as 'the time it takes for heat generated by the sun, to transfer from the outside to the inside of the building envelope and affect the internal conditions'

The effects of decrement delay are of concern where the outside temperature often fluctuates significantly higher and lower than the inside temperature as in a vaulted roof space for example.

Decrement delay is an important factor in the design of lightweight buildings, typically roofs or steel or timber frame,for this reason, and as our climate warms we too will need to pay attention to overheating in buildings.

So why do we end up with hot rooms in our houses?

Basically there are four reasons

  • We live in the UK and don’t think that we don't get hot weather so we don’t consider overheating a problem
  • The industry norm is to default to synthetic insulation such as polyurethane, polystyrene or mineral wool.
  • We don’t know that there is a solution to the problem
  • We base our insulation choices on U value
Call us to find out more about using wood fibre building materials to control overheating in your building, it's the natural sustainable choice.

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