Roofing with Wood
Some readers will notice a theme in this piece, one that they may recognise, and that’s because there is one and it is one that keeps on recurring. The theme is the moderating effect of using wood and wood fibre in roof construction on the internal environment of a building; and it’s worth revisiting a previously used example to illustrate the point and begin the story.
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.
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.
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 it’s not surprising that these spaces are often the hottest areas within a building.
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 traditional 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
Decrement delay – what is it and why is it important?
Different materials allow the passage of heat at different rates. The time it takes the peak temperature on the outside of a material, such as a wall or a roof, to make its way to a peak temperature on the inside face, is called 'time lag' or, more commonly, 'decrement delay'.
The effects of decrement delay are of concern where the outside temperature fluctuates significantly higher and lower than the inside temperature.
Decrement delay is an important factor in the design of lightweight buildings, typically roofs or steel or timber frame, and particularly in warm or warming climates.
By controlling decrement delay it is often possible to control and prevent the overheating of a building. A period of between 8 and 12 hours might be considered optimum.
A stable indoor temperature is an aspect of thermal comfort. In conditions where the outside temperature fluctuates relatively widely, a constant indoor temperature is desirable. For example an outside temperature might vary between 10ºC and 30ºC while internally it might vary just 1º above or below 20ºC. The fabric of the building envelope has effectively dampened the degree of oscillation.
The ability to attenuate the amplitude of the outside temperature to that of the inside is known as the 'decrement factor'.
Hence the closer the decrement factor approaches zero, the greater the effect the construction element has on attenuation. i.e. the smaller the decrement factor, the more effective the wall / roof at suppressing temperature swings.
The decrement factor is determined by the type and thickness of the materials that make up the wall or roof that the heat passes through.
The importance of decrement delay
Decrement delay will nearly always be more important in hotter climes than in the UK. An exception though, is in timber / steel frame construction. One of the more common criticisms directed at lightweight construction is the lack of thermal mass - which can lead to the familiar 'caravan effect' above. Whereas masonry construction has the obvious benefit of 'heavy' materials such as brick and block, framed structures are typified by combinations of cavity and lightweight insulation - leading to higher thermal diffusivity and so little in the way of decrement delay.
In more conventional roof construction types, heat is transferred rapidly from the absorbent roof finish through to the interior. By using wood fibre boards, the transfer of heat can be significantly delayed until the hours of darkness when overheating is less of a problem.
Perhaps the optimum way of utilising wood fibre insulation is shown in the example below. It shows wood fibre sarking as the windtight layer applied immediately over the tops of the rafters. Allowing for a substantial depth of insulation over the tops of the rafters will effectively raise the temperature at that location above the dew point and so reduce the risk of condensation.
Flexible wood fibre bats are friction mounted as infill between the rafters to provide the bulk of the insulation. Vapour control and airtightness is achieved through the use of membranes and or OSB (with additional taping of joints) protected by a service zone behind the plasterboard finish.
Compared with petrochemical insulation, the sections can be quite substantial - particularly when looking at achieving low u-values. The effect is to raise the ridge height - which might be a point of concern in urban situations.
So why is this not a common roofing solution in the UK?
Basically there are four reasons
- We live in the UK and don’t think that we get hot weather so we don’t consider it 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
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 low levels of diffusivity, designers can realise thermal performance more closely mapped to traditional masonry construction and begin to moderate internal temperature through the materials. .
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. You can see by the illustration below that a phase shift (measured in hours) of over 12 hours is possible even with roof joists of only 220mm
It is particularly important to consider Amplitude Dampening and Phase Shift in roof areas (see comments above). 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.
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. This is considered 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 Flex wood fibre insulation the situation changes dramatically. The 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, 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 you see the potential wood fibre insulation can play in making buildings more efficient, more comfortable and better to live in the why not talk to us at Ecomerchant about using wood fibre in your build or come to one of our training days at the National Self Build Centre in Swindon. To learn more call 01793 847 444
Thanks to martin Twamley from Steico and Sandy Patience from Greenspec for their help in compiling this article