Any insulation work carried out on an existing building is a decision that affects the remaining lifespan of the house. It should be planned and carried out with a view to lasting for at least 50 years. Whilst it is possible to improve insulation retrospectively, the fixed costs would be incurred twice in that case, making any further retrospective insulation work uneconomical. The financial outlay for an insulation project comprises fixed costs – that is, costs that would be incurred in any case – and variable costs. The latter depend on the thickness and quality of the insulation. 

As the fixed costs (for professional fitters) or the time required (for DIY) are very high for all insulation methods, one should

• either install particularly thick insulation (using inexpensive insulation materials)
• Or, if the insulation thickness is limited, the insulation should be of particularly high quality, using high-performance insulation materials such as polyurethane, phenolic resin or aerogel mats at the greatest possible thickness.

The guiding principles are: don’t do things by halves – if you’re going to do it, do it properly.

A detailed breakdown and weighting of the costs for insulation measures can be found in the attached document.