We have had a number of queries from householders who, following the installation of insulation, have noticed an increased damp or foosty smell in their loft.  This is related to temperature, moisture, and ventilation; these should be considered as a whole rather than as separate factors. The following pays particular attention to lofts but the principles of temperature, moisture and ventilation, are applicable to all parts of a dwelling.


If a loft is uninsulated or poorly insulated (and therefore cold) it will draw heated air from the rooms below; this results in it achieving a ‘semi-conditioned’ state. The air from below will contain a certain amount of moisture, but because the loft is semi-conditioned, the surfaces of objects in the loft are likely to be above the dew-point. This means the moisture in the air will come into contact with these surfaces but will remain a vapour so does not condense into liquid water.

If the loft is well insulated, it becomes completely ‘un-conditioned’ because so little heat is reaching it from the rooms below. Yet air (bearing water vapour) can still permeate into the loft.  Now that the loft is much colder than before (because of the insulation) the temperature does reach the dew-point and the water vapour condenses on surfaces, causing dampness and possible mould growth. Hence the foosty smell. One should check for gaps around pipes or electrical wiring that lead to the loft and fill them to prevent moist air from the rooms below entering the loft.

Dampness may also be caused by ice forming in the gutters after snow has been melted by the sun and from underneath by comparatively warm air in the loft.  As the ice gets thicker, melt water backs up and goes under and between the roof tiles, thereby infiltrating the loft. The problem is also exacerbated by the snow lying on the roof for weeks, restricting ventilation to the loft.

Ventilation in the loft can be increased with the installation of vents located in the gables, soffits, dormers, or along the ridge of the roof.  In order to maintain a healthy loft, it is vital that ventilation remains un-obstructed.

Soffit baffles, also known variously as air chutes, insulation trays, or rafter vents, are typically made of plastic or wood, are roughly u-shaped, and fit between the roof joists to provide unobstructed air flow between the soffits (or eaves) and the loft; a good idea, especially if you are using a blown material such as cellulose fibre as insulation. They can also be used as a buffer to enable rolled insulation to be pushed closer towards the underside of the roof.  This is also a good idea because if insulation is in contact with the underside of the roof it will cause thermal bridging where heat is conducted through the insulation (albeit slowly) and into the fabric of the roof.

Another alternative for ‘solid’ insulation is to chamfer the end of the insulation so that it mimics the slope of the roof. This allows the insulation to be laid a little closer towards the eaves – thereby slightly increasing the coverage of the insulation – while maintaining an adequate gap for air flow. This is also useful as it ensures that rolls of mineral wool do not touch the underside of the roof.  And it allows for sufficient insulation where your house ceiling meets the exterior wall, thus limiting air exchange between the living space and the loft as well as reducing cold bridging.

In conclusion, after your loft has been insulated, it will be at a lower temperature than before. So ventilation must be maintained or improved and moisture ingress kept to a minimum.