Q: I have what I believe to be a condensation problem in a bungalow bedroom semi outside wall. I have been offered free cavity wall insulation but a proper survey ( checking cavity snots) was not done. Would cavity wall insulation if done/surveyed correctly prevent condensation and who could I trust to do properly? Thanks .

A: Condensation can occur in poorly insulated properties where the surface temperature of the external walls is low and the temperature inside the property is warm. When the warm moist air in the property comes into contact with the cool external wall, the water vapour in the air condenses. If the cavities are filled, the surface temperature of the inside of the external wall will increase therefore reducing the risk of condensation on the surface.  However, there is still a risk of condensation if the warm air penetrates the brick work and hits a cold surface in between the cavity.  This form of condensation is a bit more dangerous because it cannot be seen and it is often only noticed when damage to the building fabric has already occurred.

If there’s a solid concrete lintel over the windows which isn’t improved by cavity wall insulation, and/or an area of the window reveal that’s next to the outside leaf of the brickwork, a thermal bridge can be created.  If the building is not adequately ventilated, cavity wall insulation in these circumstances can also cause a problem 

Cavity walls are usually insulated using blown fibre which allows any condensate to tickle down the wall to the damp-proof course.  This takes it away from the wall.  

Another way in which cavity wall insulation can cause damp is if there are lumps of mortar (snots) lying on the cavity tie.  If the cavity is filled with insulation that is not breathable, this will not allow ventilation which means that if any rainwater hits the external skin of the wall and penetrates to the snot, it will not evaporate, but will penetrate through to the inner skin of the wall and emerge as a damp spot.

The free insulation companies use breathable insulation which reduces the risk of condensation, which is why they will not have carried out the full survey.  They should check for adequate ventilation though.

It is worth checking that the installers are reputable and are using the material described above. If the free insulation has been offered by your energy company, you can phone them and ask for the credentials of your installer.  However, we would suggest that you commission someone who understands how buildings work (such as ourselves(!)) to check the source of your existing condensation problems before installing insulation.

By Akta Raja and Alex Rice

About the author: greentomatoenergy specialises in cost-effective renewable technologies and low carbon building.

If you have a question about anything in the above blog, please ask it in the comments section below.