Much of the carpenter's work is renovation. Here is retrofit insulation of ceiling, sloping roof and external wall, the difference between internal and external insulation, the moisture trap internal retrofit insulation can create, and the tightened renovation U-values in BR18.
Introduction
Retrofit insulation is one of the most effective energy improvements in the existing building stock — and a large part of the carpenter's and post-initial training's daily work. But it is also a place where a well-intentioned improvement can create a moisture problem if the dew point is moved to the wrong place. Good workmanship here is as much about moisture as about insulation thickness.
Where is retroinsulation done — and in what order
Not all areas give equal value for money. The ceiling against a cold roof space is usually the cheapest and most effective place to start: it is easily accessible, and heat rises. Next come sloping roofs, external walls, and finally ground floors, which are most expensive to reach.
- 01Ceiling against unheated attic: often the easiest — mineral wool or spraying on top of the existing ceiling, with care for vapour barrier and walkway.
- 02Sloped roof/catslide: harder, because the ventilation gap above the insulation must not be closed (see roof article) — there must still be air between insulation and roof sheathing.
- 03External wall: either on the outside or on the inside — two very different solutions (see below).
- 04Ground floor: most expensive because the floor must be broken up; typically done with a major renovation.
External vs. internal external wall insulation
If an external wall is to be retrofitted with insulation, the big decision is: outside or inside. The two solutions are not equivalent — they move the wall's temperature and moisture conditions in different directions.
- 01External insulation: the best building physics. The existing wall is kept warm and dry, thermal bridges are wrapped, and the dew point is moved out into the new insulation. The downside is that the facade's appearance changes, and that eaves, plinth and windows must be adapted.
- 02Internal insulation: used when the facade must be preserved (e.g. heritage buildings). But it makes the existing external wall colder and wetter, because it now lies outside the insulation — and it requires a well-thought-out, tight vapour brake and a moisture technical assessment.
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