Insulating an Old House
Improving a building’s thermal performance is now about a lot more than unrolling a layer of fibreglass insulation in the loft. Installing insulation means considering the entire building envelope: roof, floors, walls and even the panels of external doors. What’s more there is an enormous palette of insulation materials to chooses from so specifying the most appropriate product for each task is vital. Materials that allow the building’s fabric to breathe and have hygroscopic properties which assist in buffering changes in relative humidity are especially important in older buildings.
Insulation material can be divided into four main groups: natural, mineral, petrochemical and composite. Some products are ‘greener’ than others and generally it is natural and certain mineral insulants that are better suited to old buildings. There is no one size fits all solution so often materials are mixed and matched to achieve the required thermal performance across a building.
Among the most commonly used natural materials are cellulose (recycled newspaper), cork, hemp, jute, sheep’s wool and wood fibre. Suitable mineral insulants include aerogel, calcium silicate, foamed glass and expand clay.
Some products come in a variety of forms. This is the case with wood fibre which is available as batts, loose fill, blown or board products. Mineralised wood chips are another variation which may be used to fill cavities or voids. Foamed glass and expanded clay are both commonly used during the construction of limecrete floors.
Aerogel has leapt to prominence because it offers the best thermal performance available. Composed of 90 per cent air, its slim profile makes it ideal where space is at a premium such as within window reveals or door panels. The downside is its cost so its use is generally limited to situations where no other material can be used.
Direct comparisons between the thermal performance of different products is relatively easy as each material has its own k-value - the lower the value the better. Sheep’s wool insulation typically has a k-value of 0.037 W/m.K, this compares with 80 W/m.K for iron. Aerogel has a thermal conductivity of just 0.015W/mK.
In some cases decrement delay (the time it takes for heat to pass through a material) will be important as insulation materials with good decrement capabilities - like wood fibreboard - will play a part in preventing overheating.
As with any material, ease of handling and cutting should be borne in mind. Fixing methods must not be overlooked. Mechanical fixings that penetrate the material should be thermally broken to avoid thermal bridging. It’s also important to avoid squashing insulation as this will compromise its effectiveness. Similarly, board products must be cut to achieve an exact fit that avoids gaps around the edges.
Most natural and mineral insulation products are relatively pleasant to handle - although masks and protective clothing should be worn as directed by the manufacturer - and many can be recycled or composted at end of life. Cost and availability can discourage the use of these materials but such concerns must be offset against problems in the future relating to trapped moisture, damage to the building's fabric and the health of the occupants.
Text and images by Roger Hunt, co-author (with Marianne Suhr) of the Old House Eco Handbook