HOT AIR

All insulation depends, to some extent, on trapping air. Even multifoils won't work without a sealed air space on either side. Achieving this requires a greater level of attention to detail in the installation stage. If the desired thermal performance is to be met, the job of putting in insulation can no longer be left to unsupervised labour. Random tests carried out on dwellings almost invariably reveal that there is a big difference between theoretical and actual values achieved on site. A great deal of this is due to seemingly insignificant air gaps brought about by poor installation practice.

Similarly air leakage from the building envelope can carry heat away at an alarming rate. Air tightness is the latest headache for house builders. In some cases it is self inflicted because unrealistic levels of air tightness are set at the design stage. If the architect aims for an air permeability rating of less than ten cubic metres per hour per metre squared then the builder is going to have to make sure that every details is checked during the build. In particular joist ends into walls need to be well sealed, because this leakage is difficult to remedy at a late stage.

It is often a more comfortable passage, especially on a small development of one or two dwellings to adopt a figure of 15 in which case you don't have to test. The problem here is that the extra levels of insulation places its own financial and practical burden upon the builder. It remains to be seen how much effect a poor air tightness figure will have upon the selling value of a home. My personal guess is very little indeed particularly as the opening of a window or the introduction of a cat flap nullifies the whole process. To my mind it is also flawed thinking and I think we are in the middle of another very bad experiment.

STRUCTURAL INSULATED PANELS (SIPS)

Sandwiching insulation in between structural timber panels is becoming increasingly popular. The panels are quick to erect and guarantee the required thermal performance for walls roofs and floors. The warm roof construction allows usable roof space and because the panels are structural there is no cold bridging from joists, studs and rafters. The air tight seal reduces draughts and noise transmission. On a practical level keeping the lightweight insulation in between two heavier sheets means that is far easier to handle on windy days. Warm roofs built up on site from PU boards or even multifoils are particularly difficult to handle even in moderate breezes.