The Environment and Insect Pests in Historic Houses

Failing to control environmental factors such as light levels, visible and Ultra Violet light, inappropriate levels of humidity and temperature and biological agents such as insect pests and mould can all lead to deterioration of collections. There are other causes of deterioration and loss, such as chemical agents, theft, and physical actions, but these are not examined here.

Light - damage

High light levels, particularly, direct sunlight cause damage to many types of collections: fading of woodwork and veneers, fading of watercolours and textiles. Bright light can also cause certain wood stains to darken, paper, plastics and textiles to become brittle and lacquers to deteriorate.

Light - Materials which can be damaged by light

Watercolours Natural history specimens
Basketry - coloured Photographs
Fur, feather, bone and ivory Textiles
Items made of lacquer Wooden furniture/items
Painted sculptures/ceramics/glass/metals Plastics
Leather and vellum Prints and paper
Paintings: oil, acrylics, panel paintings  

Actions to take to protect historic collections from excessive exposure to light.

  1. Monitor how the light moves across rooms during the day and at different times of the year.
  2. Move items which can be damaged by light away from windows and direct sunlight.
  3. Consider installing sun blinds, Ultra Violet blinds or sun curtains at sunny windows which can be pulled down/pulled across when necessary to reduce the amount of light coming in while still retaining views at other times of the day.
  4. If a carpet or items cannot be moved consider providing temporary protection/cover to protect at least from direct sunlight but be aware that this will not protect furniture from damage caused by high temperatures from direct sunlight.
  5. Consider installing Ultra Violet film on windows, where the glass is suitable. Take professional advice. It is not suitable for early historic glass.
  6. Try to avoid keeping watercolours and other light sensitive images on permanent display. Consider displaying items in turn, keeping them in the dark when not on display.
  7. Try to avoid keeping photographs on permanent display and consider getting copies made to replace the originals which can then be kept in the dark.
  8. If items are displayed in cases consider providing covers for the cases.
  9. Consider providing individual curtains for paintings, such as large water colours on permanent display.
  10. Avoid the use of fluorescent lights which emit high levels of Ultra Violet Light.
  11. Measure and monitor light levels using handheld lux meters and dosimeters.

Relative Humidity and Temperature

For collections in museums and historic houses percentage relative humidity is the unit which is used to measure the amount of water in the air. Relative humidity (RH) is defined as the amount of water in the air divided by the maximum amount of water the air could contain at that temperature, before condensing out. It is expressed as a percentage. 10%RH would be very dry and 90%RH very humid.

Relative humidity is directly related to temperature. In a sealed unit an increase in temperature will result in a decrease in relative humidity.

There is currently much debate about appropriate levels for relative humidity. In the past there have been tight guidelines for each material type but these can be expensive and difficult to achieve, unsustainable to maintain and are increasingly thought to be broadly speaking unnecessary except in specific circumstances. For example for certain types of unstable glass it is crucial to maintain a very tightly controlled environment. It is, however, important to continue to avoid the extremes and to avoid fluctuations.

Relative Humidity - damage

Excessively high and low relative humidity levels can damage some materials and fluctuating relative humidity can be particularly detrimental for collections. Above c.65% RH mould can start to grow in certain conditions and this can lead to further deterioration. Moulds can also be a health risk for humans. High levels of relative humidity can encourage insect pest activity, which further damage collection, and cause metals to corrode. Low levels of relative humidity can cause damage such as splitting of wood in furniture and panel paintings and drying out of adhesives.

Fluctuating relative humidity can cause veneers, joins and adhesives in pieces of furniture to contract and expand at different rates, causing deterioration; the paint/canvas/panel of a painting to contract and expand at different rates, leading to loss of paint and loss of surface and cracking when salts crystallise beneath the surface of some archaeological ceramics, stone and plaster.

Relative Humidity - Materials which can be damaged by high relative humidity

Watercolours Textiles
Other Organic materials Photographs
Furniture Paintings
Prints Metals

Relative Humidity - Materials which can be damaged by low relative humidity

Furniture Paintings
Other Organic materials  


Relative Humidity - Materials which can be damaged by fluctuating relative humidity

Furniture Paintings
Other Organic materials Archaeological ceramics - some
Stone sculpture Plaster
Unstable glass  

Actions to take to protect historic collections from extremes of relative humidity and fluctuating relative humidity.

  1. Avoid central heating systems set at high temperatures for the comfort of residents or guests.
  2. Avoid hanging paintings,  textiles  and other organic materials over radiators.
  3. Avoid storing items in damp areas, such as basements
  4. Reduce the impact of localised heating on objects exposed to direct sunlight by using blinds or sun curtains.
  5. Avoid excessive use of gas fires.
  6. Measure and monitor levels of relative humidity using for example hygrometers.
  7. Consider installing humidistats on heaters/radiators.
  8. Review efficiency and effectiveness of heating system.

Actions to take on discovering mould

  1. Obtain health and safety advice if the mould is extensive. Some people are particularly sensitive to exposure to mould spores and should not be exposed to an environment where they are present.
  2. Increase ventilation to the area to encourage air movement and reduce the growth of mould.
  3. Ventilation can be provided by electric fans and by opening internal doors. External doors and windows can be opened as long as the external relative humidity is not greater than the internal RH.
  4. Determine whether there is a specific cause for increased levels of RH leading to the growth of mould. For example a cracked drain pipe may cause damp to penetrate an external wall, leading to the growth of mould on tapestries hanging on the wall.
  5. Repair any structural defects.
  6. Where mould is found on the back of paintings hung on external walls increase the potential for air movement behind by placing spacers between the frame and the wall.
  7. Check other items in the room carefully for mould, including books on shelves and the backs of furniture.
  8. Mould can develop very quickly following a flood or water ingress, certainly within 48 hours in the right conditions. Increase ventilation, introduce electric  fans if possible and monitor collections carefully for mould.


Insect Pests

With the changing climate in the UK and elsewhere insect pests are on the increase. They reproduce more frequently over the year and are becoming an increasing problem in homes, museums and historic houses.

Insect Pests - damage

The principal insect pests for collections are: certain moths, such as the larvae of webbing clothes moth, certain borers, such as the larvae of the furniture beetle, the larvae of certain beetles such as the carpet beetle and  insects such as silverfish and booklice.

Insect Pests - Materials which can be damaged

Wooden Items Furniture
Basketry Textiles e.g. wool
Paper Wallpaper
Unstable glass Natural history collections
Leather Fur, feathers
Books Prints
Watercolours Card
Horn Seeds in collages etc

Actions to take to protect collections from insect pests

  1. Maintain a good housekeeping regime, cleaning carpets and floors to the edges, cleaning underneath rugs and carpets, and keeping areas clean.
  2. Inspect collections periodically for pest activity and inspect items as they are brought into the house..
  3. If items are found to have an active pest infestation isolate them until they can be treated, to stop the infestation spreading.
  4. Identify the insect pests which are found.
  5. Try to find the source of infestation.  For example look under loose covers over upholstered furniture which may be hiding an infestation or a particularly badly infested item.
  6. Ensure that chimneys are swept and kept free of debris. Bird's nests in chimneys are a common source of infestation.
  7. If the environment is damp determine whether there are any structural causes such as broken downpipes and undertake structural repairs.
  8. Set up a pest monitoring programme, using insect traps to monitor the extent of pest activity. Marking the location of traps on a plan, dating them and checking them regularly.
  9. Try to discourage cluster flies from entering the building, for example by installing blackout blinds at windows.
  10. Remove cluster flies as soon as they are found. The carcasses are a prime source of food for carpet beetles, a major insect pest for collections.
  11. Seal up cracks and crevices and where possible discourage plant growth adjacent to buildings.
  12. Contact a conservator-restorer who is experienced in dealing with insect pests in collections.

Further Information

"Energy Efficiency and Historic Buildings" English Heritage November 2010

"Insect Pests found in Historic Houses". English Heritage Poster - available to download via the website, via Publications

Prevention and treatment of mould outbreaks in collections (PDF format) 86KB

The relevant chapters in The National Trust Manual of Housekeeping Butterworth-Heinemann, (2005) give comprehensive guidance and advice.

Bullock, L.  “Light.” In The National Trust Manual of Housekeeping, Butterworth-Heinemann, (2005), 93-101.

Staniforth, S. "Relative Humidity" In  The National Trust Manual of Housekeeping, Butterworth-Heinemann, (2005), 103-113.

Child, R. "Biological agents of deterioration". In The National Trust Manual of Housekeeping, Butterworth-Heinemann, (2005), 81-89.

This article offers general guidance and is not intended to be a substitute for the professional advice of an accredited conservator-restorer. The views expressed are those of the author or authors, and do not necessarily represent the views of the Institute of Conservation. The Institute of Conservation and its partners accept no liability for any loss or damage which may arise if this guidance is followed