Wednesday, 26 January 2022

Which insulation should I use? Introduction

People often wonder what kind of insulation to use. This is a great question, which we will come to later. Two questions are more important: Should I use insulation? and Where should I put it?

Yes!

Whatever insulation you are using is almost certainly better than not using insulation. Any insulation will reduce your heating bills and cooling bills and make the building more comfortable. If you're comparing the environmental impact of different kinds of insulation, then you first need to compare that one-off impact with the on-going impact of your heating or cooling. 

Where?

Putting insulation in the right place is important, and this means insulating all around the building. Putting on an extra scarf in the winter will keep you warm, but it will only be really effective if you're wearing a warm coat and good shoes. And if you remembered to put on your trousers. In the same way, sticking some more insulation into a wall cavity will make a bigger difference if you're also insulating the rest of the walls, the roof and the floor, and making sure that your windows are insulating properly. More insulation is usually better, but going from zero to 10cm will have a much bigger effect than going from 10cm to 20cm. If there is an uninsulated part of your building, those extra 10cm would be much more effective there.

How does insulation work? 

Insulators slow down heat. Some materials, like metals, are good conductors and will quickly move heat. You can tell a good conductor because it feels cold when you touch it. Unless it has just been on the cooker, in which case it may feel very hot. Good conductors are bad insulators. 

There are many insulators around you. Wood and paper are insulators, the wool in your sweater is a good insulator and the polystyrene trays from the supermarket are very good insulators. The best insulator around you is probably air. And most insulation for building works by tapping air.

Foam of Fibre? 

Insulation can be broadly divided into two groups: foams and fibres. 

Fibres

Examples of fibres are glass wool, rock wool and cellulose fibre. Fibre insulation works by trapping air among the fibres. This is exactly the same as your woollen sweater or down jacket, and insulation made from textiles is also available for buildings, including recycled materials or new wool. Fibre insulation does not usually have any compressive strength, so while it is good to add to walls and ceilings, it is not a good idea to put under a wall or foundation. Also, while the insulation works by fibres trapping air, it will stop working if air is getting through the insulation, and bad things can happen if water gets in. Therefore, it is very important to add weather protection on the outside and an airtight barrier on the inside of fibre insulation. 

Foams

Examples of foam insulation are polystyrene, polyurethane, polyisocyanurate and  phenolic foam. Polystyrene is available both expanded, known as EPS and styrofoam, or extruded, which is known as XPS, E is for expanded, X is for extruded and C is for confused!! The foam may be made of bubbles of air, or another gas that could be a better insulator, so foam insulators can have better performance than fibre insulators. This means generally that a thinner layer of foam will keep the house as warm as a thicker layer of fibre insulation. Compared with fibre insulation, especially cellulose fibre, the manufacture of foam insulators can use a lot of energy and fossil fuels, and can release some toxic chemicals. However, a thinner layer of insulation may mean thinner walls or roof and less need for other building products, so calculating their impact may not be simple. Or given the same wall thickness the higher performance will mean less heating over the lifetime of the building, which will usually save much more energy than the manufacturing of the insulation. 

Foam insulation can be strong, since the foam structure is rigid, so it can be used under floors, under foundations, or anywhere else where there is a load. 

While foam insulation does not usually let any air or moisture through, there can be gaps between the foam and the structure, for example where expanded polystyrene has been put in the space between pillars and beams of a wooden frame. If there is an earthquake and the building moves, this can leave a permanent gap. There may also be gaps between panels from the beginning if they are not installed carefully.

Depending on air flow, a gap of one millimetre in insulation can lead to a drop of performance by 50%, as well as increasing risk of condensation.

Form

Insulation may be available in four different forms: flexible blankets, rigid panels (batts), loose fill or sprayed foam. The construction technique will usually dictate what form the insulation must be in. Most insulation materials are available in different forms. For example cellulose fibre is available as flexible blankets, rigid panels or loose particles that can be blown into a cavity.

Shape and Size

Blankets and batts often need to fit spaces and they are available in standard widths. Insulation can be cut on site but this takes time, may increase gaps and may lead to more waste. 

Safety

Most insulation is harmless. Commercially available insulation materials have been tested for toxicity and will be fireproof, as long as the manufacturers have not lied in their testing. Glass wool is not nice to install, and may be unpleasant when a building is demolished or renovated but once within a wall structure it should not harm inhabitants. 

Strength

If you need insulation under a building then it needs to be structurally strong. This usually limits your choice to foams such as XPS (Extruded polystyrene). Some cellulose-based structural insulation is available. 

Open or Closed?

Insulation usually works by trapping air. Wool-based insulation traps air between the fibres, which are open and will usually allow some air to pass through the insulation. Foam insulation will usually stop air from getting through. Cellulose insulation is not as open as wool-based insulation, but not as airtight as foam. Since insulation is only effective and safe from condensation when it is airtight, open insulation requires an airtight barrier on one or both sides. Attention will be needed for joints and boundaries of insulation that is more airtight. Closed insulation will not let air through, but careful installation is needed to stop an gaps that air may get through.

Moisture Content

Different materials hold different amounts of moisture. Insulation with a capacity to hold moisture may be helpful in stabilising humidity levels in the situation where the absolute or relative humidity outside is not comfortable inside. Alternatively, there may be a risk where humidity builds up levels of moisture within the structure that can cause building damage or health risks.

Moisture Permeability

As well as the amount of water a material can hold, it may be better or worse at letting moisture through. 

Temperature Range

With some insulators, performance changes with temperature, so they may be good inside your walls, or above a foundation slab, but not so good on the outside where it gets colder. 

Heat Capacity

Different insulation materials also hold different amounts of heat. Increasing the thermal mass of the building will make more stable temperatures. In most cases, the insulation performance, in other words the thermal conductivity, is much more important than the heat capacity. 

Thermal Conductivity or U value

Last, but very much not least important! Thermal conductivity,  gives a value of the material. U value is for a particular thickness. For both, lower values are better insulators. If the insulation is twice the thickness, the U value will halve. U value is measured in W/m2K and thermal conductivity is in W/mK.

Ecological Decisions

There are many different insulation materials available. They come in different forms and give different performance in terms of heat, airtightness, moisture and sound, and they have different price tags. There is no single "best insulation material". Architects and builders may have favourite materials, and manufactures usually promote their own materials and point out shortcomings in competing materials (for example this online article by the founder and managing partner of Havelock Wool).

There are also different impacts on environment during manufacture and disposal, but to quote Schmidt et al. (2003):

"Many people believe that the emerging insulation products based on biological resources (cellulose), such as flax and paper wool, are much more environmentally friendly than a product based on natural mineral resources such as stone wool. This belief may, however, be unfounded."

If you are worried about fossil-fuel based insulation materials, you need to first consider how much fossil-fuel energy the insulation will save. Sure, you may not be planning on burning gas or oil, but if you're heating with electricity, that electricity has been made with fossil fuels. It may be "green" energy, but the solar panels and wind farms have been made with fossil fuels, and the hydroelectric dams poured with concrete, so you're still not at zero carbon. And even if you did find a way of producing energy without any carbon input, you'd need to calculate whether it's better to use it for your house, or supply that energy somewhere else to offset other people's carbon use.

Schmidt et al. conclude: "The energy and environmental impacts saved during the use phase [of insulation] are more than 100 times larger than the impacts in the rest of the lifecycle".
This sentiment is repeated by the US National Park service for the Pacific Northwest:

"Do not substitute a "green" insulation material for a non-green material if doing so will result in lower overall energy performance. Even though the environmental impacts of the insulation material might be lower for the green product, the overall environmental impact of the building would likely be greater by lower insulating values."

So while important, before you worry about what happened during the manufacture of the insulation, and what will happen if someone burns the insulation at the end of its life, the first thing to worry about is burning less to heat the house while the insulation is doing its job. Building constraints may limit the thickness of insulation, in which case some insulators may not have sufficient performance. Even if thickness is not limited, the extra thickness required for lower performance insulators may require extra materials to keep it in place and more external wall finish to cover it.

It may also be that natural materials attract "natural" pests, molds and fungi. You may want a material that will decompose naturally after its lifetime, but you definitely do not want the material to decompose while it is part of your building. Until the 19th century, the only insulation materials were organic, and the levels of insulation ranged from poor to non-existent. With the new scientific understanding and product availability of the industrial revolution, man-made materials became available, and these were used because they were cheaper, performed better, and would last longer than natural materials. The higher energy costs in the long depression of 1873-1896 boosted use of man-made insulation materials in industrial sites, and a hundred years later the oil shock gave another incentive to insulate homes (see Bozsaky, 2010 for more detailed history). Many "natural" insulators are recent developments that package natural materials in familiar forms of man-made insulation products. They do not necessarily have longer and more reliably determined lifetimes, or lower toxicity than man-made materials, and they may depend on chemical treatment to make them resistant against fire and pests.

In terms of carbon emissions, wood-based products are sometimes considered carbon-negative, since carbon captured by wood is being stored in the building. This may be true, but if the wood were not used in the building would it still be a living tree? Read more about that in a previous post.

References

Bozsaky, David (2010). The historical development of thermal insulation materials. Periodica Polytechnica Architecture, 41. 49-56. doi:10.3311/pp.ar.2010-2.02.

Danielle Densley Tingley, Abigail Hathway, Buick Davison, & Dan Allwood (2017). The environmental impact of phenolic foam insulation boards. Proceedings of the Institution of Civil Engineers - Construction Materials, 170(2). https://doi.org/10.1680/coma.14.00022

Danielle Densley Tingley, Abigail Hathway, Buick Davison (2013) BIG Energy Upgrade: Environmental burden of insulation materials for whole building performance evaluation. University of Sheffield.

Department of Interior. (nd) Environmental Considerations of Building Insulation
National Park Service – Pacific West Region 

Passivhaus Plus (2016). Cellulose insulation improves airtightness by 30% — PYC Systems

Seyedeh Shiva Saadatian (2014). Integrated life-cycle analysis of six insulation materials applied to a reference building in Portugal.

Schmidt, A., Clausen, A. U., Kamstrup, O., & Jensen, A. A. (2003). Comparative Life Cycle
Assessment of three insulation materials—stone wool, flax and paper wool. 

Anders Schmidt, Allan Astrup Jensen, Anders U. Clausen & Ole Kamstrup (2004). A Comparative Life Cycle Assessment of Building Insulation Products made of Stone Wool, Paper Wool and Flax: Part 1: Background, Goal and Scope, Life Cycle Inventory, Impact Assessment and Interpretation. The International Journal of Life Cycle Assessment 9(1): 53-66 https://www.researchgate.net/publication/282754710_A_Comparative_Life_Cycle_Assessment_of_Building_Insulation_Products_made_of_Stone_Wool_Paper_Wool_and_Flax_Part_1_Background_Goal_and_Scope_Life_Cycle_Inventory_Impact_Assessment_and_Interpretation
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