Before you answer that, can I ask you to look at the two pictures below?
Now I hope you already know where this is going…………… 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.
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 or amplitude dampening (confusingly there are other terms to describe this mechanism too) 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’
It doesn’t take much to rapidly heat a building through a lightweight structure such as a roof, as we make better use of our living spaces turning attic rooms into bedrooms and creating top floor living spaces with vaulted ceilings its not surprising that these spaces are often the hottest areas within a building. The obvious fact is that a roof is a very large solar collector so during warmer and sunnier weather a building will absorb heat (solar gain) through windows, walls and roofs this heat moves up to the top of the building as hot air rises. If you are designing a loft living space or vaulted ceilings then it is important to remember a timber roof is a lightweight material with low thermal storage capacity, the heat from the sun will quickly heat it up and this heat will transfer into the attic space, just like in the caravan.
In addition a pitched roof area is quite large compared to the internal volume of the space and so this increases the risk of heat transfer which can be exacerbated by the type of cladding material, we all know how hot a black slate roof tile will get, if left in direct sunlight it may well be possible to fry an egg on a slate roof even on a sunny afternoon in June.
So we end up with bedrooms and living spaces that get too hot and create uncomfortable sleeping conditions but by the time we find this out it’s normally too late. The best time to decide to fix this problem is before you build, design in the moderating effect of wood fibre into the roof and walls if you are using a lightweight structure like timber or steel frame if that is not possible there are a number of options to use wood fibre as a retro fit to achieve the same results. To give you a ‘heads up’ on what a difference material choice can make see the table below.
So where do we go wrong?
Basically there are four reasons
- We live in the UK and don’t think that we get hot weather so we don’t consider that overheating is 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 insulation choices on U value
When we build, and in most cases, insulation products are chosen mostly on the basis of a combination of their u-value and their thickness since the most commonly used insulation materials such as polystyrene, polyurethane and mineral wool have broadly similar densities and heat capacities, their decrement capabilities are relatively insignificant so we end up with thinner sections that sit on top of the building (the roof) with little thermal mass and no capacity to buffer or moderate heat transfer… which inevitably leads us back to the caravan effect. However using wood fibreboard insulation materials which boast, comparatively high levels of diffusivity, designers can realise thermal performance more closely mapped to traditional masonry construction and begin to moderate internal temperature through the materials. The effect of material choice is illustrated in the graphs below.
Compared with conventional insulation materials like glass wool, Steico wood fibre insulation products come with especially high density. This density is the key to summer heat protection as mass acts as a heat buffer. This leads to a phase shift, which is the time span between the highest external temperature and the highest internal temperature. The aim of summer heat protection is to delay the heat transfer through the fabric of the building so that the high midday temperatures would only reach the internal side when it is already cooler outside.
It is particularly important to consider amplitude dampening (decrement delay)and phase shift (sometimes called decrement factor) in roof areas. The ratio of external surface to room volume is very high, so attic rooms have a high area for potential temperature transfer. The areas directly under the roof coverings can get very hot in summer (under slate or aluminium for example temperatures could easily rise up to 80 °C) and this leads to increased heat in the rooms below. As many roof constructions have a very low thermal mass, they are particularly suitable for the installation of wood fibre insulation.
With the exception of the roof cladding and the internal plasterboard the thermal mass of the roof construction is entirely reliant on the insulation. It is therefore vital for amplitude dampening and phase shift that insulation with a low thermal diffusivity is used. Ideally an amplitude dampening value of 10 (TAV 10 %) and a minimum phase shift value of 10hrs is what should be achieved. With an external temperature of 35 °C and a possible temperature under the roof covering of 80 °C, it is important to ensure that the influence this temperature has on the internal climate is delayed through amplitude dampening and phase shifting.
In hot summer conditions you can get very different results using various insulation materials. If you compare 2 roofs, which both have a U value of 0.18 W / (m2*K), utilising mineral wool with a thermal conductivity of 0.035 and a density 20 kg / m3 you achieve an Amplitude Dampening of 6 and a Phase Shift of 6.8 hrs. This results in an internal temperature under the roof of 29 °C at 20:00 hrs. a temperature that is far too hot for comfortable sleeping conditions. The external temperature at this time is also 29 °C so there is no respite if you try to cool the room by opening the windows. If you swap the mineral fibre for Steico Flexwood fibre insulation the situation changes dramatically. The two insulations have the same thermal conductivity but the wood fibre has a density of 50 kg/m3 which increases the thermal storage mass by 5 times, due to its improved thermal storage capacity. In this scenario the amplitude dampening rises to 12 and the phase shift to 11:00 hrs. This results in an internal temperature of only 21 °C at 01:00 hrs. If this is still too uncomfortable then opening the windows will cool the building as the external temperature at that time is only 15 °C.
So the moral of the story, if that doesn’t sound too trite, is that buildings in the UK do overheat, partially because we are having warmer weather but equally we are making better use of building space refurbishing and utilising lost space such as attics and roof spaces, and we are also designing differently, larger open spaces, more glazing and lighter construction systems. In all of this natural wood fibre insulation has a key role to play and is gradually becoming recognised for the considerable benefits it brings to the thermal performance of a building .
If hot sticky nights are not your thing and you see the potential wood fibre insulation can play in making your home more efficient, more comfortable and better to live in the why not talk to us at Ecomerchant about using wood fibre in your build.
Download the Steico Summer Heat Protection Brochure
Thanks to Sandy Patience from Greenspec for content and glossary and to Steico UK for images and technical examples.
Confused by the terminology, check out the mini glossary below
Decrement delay also called amplitude dampening
Refers to 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. Materials affording higher rates of decrement delay will have a low lambda (thermal conductivity or k-value) value, high specific heat capacity and high density.
Insulation materials offering a high decrement ‘factor’ include cellulose fibre (7.3 hr), wood fibre insulation board (11.3 hr); whereas materials with a low decrement factor would include low-density mineral fibre (3.7 hr) and polyurethane/polystyrene.
Decrement delay can be useful in the design of timber frame buildings. Insulation with a high decrement factor can be used to limit solar over-heating in particularly warm climate conditions. For example, installing wood fibre as a roofing insulation will likely slow-down the heat transfer from a sun-heated roof surface, through to the inside by around 11 hours – or until the wee morning hours when the external temperature will have fallen considerably..
Decrement factor also called phase shift
The time taken (measured in hours) for heat to transfer between opposite surfaces of a material. (see above)
k-value (or λ lambda value)
The k- value, otherwise known as the thermal conductivity or lambda value, of a material to lead or to resist heat transfer. When used in reference to insulation, the lower the k-value, the better the insulation.
λ(lambda) – value
Otherwise known as ‘thermal conductivity’ or ‘k-value’, the lambda value is a measure of a material to lead or to resist heat transfer. Units are W/mK. When used in reference to insulation, the lower the lambda, the better the insulation.
The lambda value is used to calculate the thermal resistance of a particular material, or ‘R value’ by combining the lambda value and the actual thickness of the material. Hence R=t/λ
The capacity of a material to resist the transmission of heat. The R-value is calculated by combining the lambda value (thermal conductivity) and the thickness of the material. Hence R=t/λ, where ‘t’ is the thickness. Units are measured in m2K/W. Used in connection with insulation, the higher the R-value, the more effective the insulation. The R-value is also used to calculate the U-value (thermal transmittance) where U = 1/R
Heat absorbed through direct transmission through glazing, roof or wall sections (primary transmittance). Energy is also absorbed by the glazing, roof or wall and subsequently transferred inwards by convection and radiation (secondary transmittance).
Thermal conductivity (K-value)
A measure of the rate at which heat is conducted through a particular material under specified conditions.
The ability of construction materials to absorb, store and release heat. Thermal mass can be used effectively to absorb daytime heat gains (reducing cooling load) and release the heat during the night (reducing heat load), thereby maintaining a constant level of comfort through stable temperature. Materials of high thermal mass include water, stone, earth, brick and concrete. More recent innovations include ‘phase change’ materials that store energy whilst maintaining constant temperatures. The quality of thermal mass is usually described in terms of ‘admittance’. Admittance is the ability of a material or construction such as a wall to exchange heat with the environment when subjected to a simple cyclic variation in temperature. For buildings, this is 24 hours. Admittance is measured in W/m2K, where temperature (K) is the difference between the mean daily value and actual value within the space at a specific point in time. Key variables that determine admittance are thermal capacity, conductivity, density and surface resistance. (Note that ‘K’ is used in a slightly different way from that involved in the calculation of u-value)
Thermal resistance (R-value)
Thermal resistance is the measure of a component’s ability to restrict the passage of heat across its thickness. The R-value is calculated by combining the lambda value (thermal conductivity, or ‘k-value’) and the thickness of the material. Hence R=t/λ, where ‘t’ is the thickness. Units are measured in m2W/K. Used in connection with insulation, the higher the R-value, the more effective the insulation. The R-value is also used to calculate the U-value (see below)
Thermal transmittance (U-value)
Thermal transmittance is a measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction. This measure takes into account the thickness of each material involved and is calculated from R-values of each material as well as constants accounting for surface transmittance (Rsi and Rso, inner and outer surfaces respectively) and also for a small standard air gap (Rso). Thermal transmittance is measured in W/m2K
U-value (thermal transmittance)
Thermal transmittance is a measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction. This measure takes into account the thickness of each material involved and is calculated from R-values (where U=1/R) of each material as well as constants accounting for surface transmittance (Rsi and Rso, inner and outer surfaces respectively) and also for a small standard air gap (Rso). Thermal transmittance is measured in W/m2K