Any retained layers need to work with the proposed construction.

Inspect the fabric and open it up where necessary before deciding what to retain. Problems in an old roof may be hidden beneath a sound-looking surface. Trapping moisture or an unstable layer beneath new insulation can mean doing the work again soon afterwards. Early inspection helps avoid that waste.

Buildings lose heat in different places. One may have an uninsulated upper floor ceiling, another leaky junctions or an inefficient services installation. An energy assessment can quantify the losses and compare options. This helps direct spending to work that produces a meaningful change in energy use.

Allow for changes in use after renovation. Occupants may heat previously cold rooms to a more comfortable temperature. Bills may then show a smaller saving than a simple model predicts, even though the work has improved the building. Part of the benefit has been used to improve indoor conditions.

Low thermal conductivity allows high thermal resistance in a thin layer. This can be useful above rafters, beneath a terrace over a heated room and below a refurbished floor. It may allow an existing threshold or clear opening height to be retained. Calculate the thickness for the particular alternatives being compared. A reduction of one half cannot be assumed for every application.

Other performance requirements may govern the choice, such as fire resistance, sound insulation or exposure to water. In those locations a different product or a combination of materials may be appropriate. Specify the insulation for the required function over its service life, taking thermal conductivity into account alongside these other properties.

Draw up the board layout before ordering. Allow for cut-outs, corners, installation sequence and the effective coverage of rebated boards. A layout reduces unnecessary cutting. Very small pieces can be difficult to fix securely and join tightly, so only use offcuts where a sound detail is possible.

Store boards so they stay dry and intact. Damaged or wet boards can become waste before they are installed. Sort construction waste according to the arrangements for collection. Composite components may require a different recovery or recycling process from clean sheet metal.

An environmental product declaration covers a particular product, reference unit and set of life-cycle stages. Comparing a cubic metre of each insulation material can be misleading when different thicknesses are needed for the same thermal resistance. Compare quantities providing equivalent performance and check that the declarations cover the same stages.

Durability and repairability matter as well. Replacing an assembly early because of an installation fault adds to its environmental burden. Testing seals, providing drainage and retaining access for maintenance all contribute to performance over time. The environmental merits of the whole retrofit depend on the combined design and execution.

Compare energy use before and after the work, allowing for the weather, indoor temperatures and the number of occupants. Monitor fuel or energy quantities and system performance under similar conditions. A bill in monetary terms can change simply because energy prices have changed.

Inspect the roof, keep outlets clear and repair minor leaks to preserve the benefit of the work. After improving the envelope, check the settings of the heating and ventilation systems. Coordinating the construction layers with the way the building is operated allows the insulation to perform as intended.