Insulation Materials

Insulation materials can typically be divided into types, depending on how they manage moisture transport in both liquid and vapour form:

• impermeable - preventing moisture, both vapour and liquid, from moving through the material
• moisture permeable - allowing moisture, both vapour and liquid, to move through the material
• vapour permeable - allowing only moisture in vapour form to move through the material

For more information on moisture transport mechanisms in buildings of traditional construction, see Properties of Traditional Building Construction.

A material can have good water vapour permeability, but poor hygroscopic and/or capillary qualities (for liquid water transport). Therefore, different types of insulation material and their system assemblies have unique properties that make them more or less suitable for a particular building's construction, its environment, and its flood risk.

In the flooding sector, 'moisture closed' is used to refer to impermeable materials. 'Moisture open', can refer to moisture or vapour permeable materials. For traditional construction it is important that 'moisture open' insulation is recognised as being moisture permeable.

When choosing insulation, it is important to consult the manufacturer's technical data and instructions for use and installation, as well as any guarantees and warranties. This information will confirm where and how the material has been tested and where it is deemed suitable for application. It will also confirm whether the system has only been tested on modern construction, and/or traditional construction. If insulation is not installed in accordance with the manufacturer’'s recommendations, they will not honour any guarantees or warranties and will not be liable if failure or deterioration occurs.

It is important to understand that the thermal performance of insulation materials decreases as they become wet. In addition, materials may not always return to their original state and different materials will perform uniquely after flood events.

Moisture permeable insulation

Moisture permeable insulation allows moisture, both in liquid and vapour, to move through the construction. Moisture permeable insulation materials may absorb moisture, but they will also allow it to readily evaporate, until moisture balance is reached. Their ability to continue to do this will depend upon the hygroscopicity, porosity and capillarity of the materials used, and the volume of moisture present. The shape and size of pores, how they connect with each other, and their surface finish will affect the speed at which moisture travels through a permeable material. The concentration of existing moisture in the building material will also impact moisture transport. Products such as lime, earthen materials and timber are moisture permeable products.

It is important that the term 'permeable' is not confused with 'breathable'. The latter is applied to numerous products, and in many instances, it is used to refer only to vapour permeability.

Presently there is not enough evidence on natural and organic insulation in terms of its moisture permeability and its integrity after wetting; it is likely these materials would need to be removed and replaced. In addition, although natural and organic materials can be environmentally friendly, they can also create the perfect environment for mould growth and fungal decay when exposed to moisture and high humidity for long periods.

Installing a moisture permeable system is likely to increase post-flood drying times, because unlike impermeable or vapour permeable materials, moisture will be able to more through the construction, increasing the quantity of liquid moisture to be removed from within the building.

Most manufacturers of vapour permeable insulation materials state they are suitable for areas where there is vapour present, but not a high presence of liquid moisture. This is because the integrity of the insulation is impacted by water, which may cause the material to expand and then shrink. There is no guarantee that the insulation will return to its pre-incident condition.

Moisture permeable insulation materials, might therefore, need to be removed before a flood event or disposed of after (unless the manufacturer can provide evidence that the insulation will return to its pre-incident state).

Impermeable insulation

Impermeable insulation products are not affected by water. They keep their integrity, and drying contractors can guarantee they return to their pre-incident condition. Impermeable insulation can be both rigid or loose fill; these include foamed glass, expanded or extruded polystyrene, rigid urethane foam, rigid phenolic foam and mineral wool. Some products are marketed as being water resistant but not waterproof. Water resistant materials can resist the penetration of water to some degree while waterproof materials do not absorb water at all. Understanding how your chosen material will respond once wet is important and needs to be factored into how the building will be dried, whether the insulation needs to be removed prior to a flood, or removed and replaced after a flood.

More commonly used in modern construction, insulation materials will either be impermeable or vapour permeable. Where these products are being considered it is important to assess whether they will reduce the levels of moisture movement through a building's envelope, which has the potential to result in moisture accumulation, decay and reduced thermal performance.

Loose fill insulation materials (cellulose, vermiculite, mineral wool, blown fibreglass) are generally impermeable or vapour permeable, they have air pockets that collapse under the weight of sodden material. This may render the insulation permanently ineffective, and it will likely need to be removed after a flood. Where the insulation does not fully collapse, it may have to be removed temporarily if it is preventing water from evaporating from the main structure. It can then be reinstated once the building has dried out after cleaning.

Many understand cork board insulation to be wholly appropriate for traditional construction because it is a natural and sustainable product. However, this is not the case for manufactured cork insulation products. To create these panels a number of hydrophobic constituents (which have the ability to repel liquid moisture) are used to prevent the material from absorbing water, which prevents the passage of liquid moisture meaning they are vapour permeable only.

Several manufacturers have tested cork board insulation and found that if it remains wet for a prolonged time, its insulating properties will be reduced to the same extent as its moisture content is increased. To make sure the guarantee remains valid, the manufacturer's installation requirements must be followed. These may include the use of glues, vapour barriers and polyethylene film. At the time of writing this guidance, many manufacturers of cork board insulation have only tested their product on modern constructions. They cannot, therefore, confirm whether it is compatible with a traditional building.

Selecting insulation

Careful consideration is therefore required when selecting appropriate approaches and insulation materials for improving the energy efficiency of historic buildings or those of traditional construction within a flood zone.

No two systems will be manufactured in the same way; even if they are made using the same basic materials, they will have different properties.

It is important to understand how the chosen material will respond to flooding.

For instance:

• how readily does it absorb water?
• will it remain dimensionally stable (i.e. will it warp)?
• how quickly will it dry?
• can it be dried in situ or will it need to be removed completely after a flood?
• does it have the potential to trap moisture and hide or increase the risk of mould, decay or condensation in the property?

For example, wood fibre insulation panels vary in additives, density and manufacturing process. Academics found that of six wood fibre insulation panels tested, all absorbed moisture quickly. However, panels that underwent a wet manufacturing process absorbed less moisture than those with a dry manufacturing process. Upon drying, five panels showed signs of shrinkage, and the sixth panel collapsed. Although five panels did not disintegrate, it is perceived that continuous cycles of wetting and drying will have an impact on their insulating capacity.

If reducing carbon emissions through energy efficiency and embodied carbon is the goal, then it is likely that insulation is not an appropriate option unless the insulation can be removed prior to a flood. This is because the savings made by insulating will never reach the carbon footprint associated with the manufacturing and transportation of the insulation material.

Within current understanding, site-mixed insulated lime plasters, where all constituents and their effect on the performance of the plaster are known, may be more suitable for use in areas at risk of flooding than other permeable materials. Historic England continues to gather information, evidence and data on this hypothesis.

Insulating additives (such as hemp, cork or aerated glass beads) can be combined with traditional wet finishes such as plasters and renders. Provided that the mix retains the moisture permeable properties of the traditional construction, these materials may be more suitable for use in areas at risk of flooding than other vapour permeable insulation materials. However, the permeability of proprietary, ready-mix insulated plasters and renders is currently not fully understood and further research is needed to establish whether they are an appropriate measure for traditionally constructed buildings.

Ensuring that both the building fabric and the insulation material can recover after a flood event is key if a building is to be resilient, energy efficient and carbon responsible. Recoverability, as defined by insurers and drying contractors, is the ability to guarantee that the material has returned to its pre-incident state. This includes maintaining its structural integrity and returning its moisture content to below the threshold for mould or decay organisms to germinate.