I live in a 1960s bungalow which I have insulated as much as I am able to (proper thickness loft insulation, blown polystyrene wall cavity insulation and double-glazed doors and windows).
My problem is with the 1970s kitchen extension, which has a flat roof and 9in hollow block walls. The roof has been insulated and the windows and door replaced with double-glazed versions. The walls however, are another matter. They are devoid of insulation other than a small section which has plasterboard dry lining which ends half way up the wall for some unaccountable reason.
Unsurprisingly, there is a huge condensation problem on the bare walls in cold weather, particularly on the west-facing wall. I’m fighting a constant battle with black mildew or mould (not sure which it is) in the winter months, but no matter how often I remove it, it keeps coming back, obviously because I’m not addressing the cause.
I have considered having the internal walls dry lined using insulated plasterboard but a reputable builder has informed me that this will just cover up the condensation problem rather than remove it. This has me very confused as I assumed that the plasterboard would provide an effective barrier, preventing warm air indoors from reaching cold walls.
Is the builder correct? If he is what are my alternatives? I should point out that I’ve looked into the possibility of externally insulating the walls, but the proximity of mains water and sewerage pipes and a large boundary wall renders this option unfeasible.
Your builder is correct, in that dry lining historic walls can present problems. Foremost of these is the potential for condensation to form within the walls thickness or behind the new dry lining.
With uninsulated hollow blockwork walls such as you describe, there is indeed a likelihood that condensation will occur on the inner face of the solid blockwork with subsequent mould growth as you have experienced.
This happens because of the differential between internal and external temperature. Air has the ability to hold water vapour or steam. The amount of water vapour held in the air is directly proportional to the air temperature. The higher the temperature, the more water vapour will be contained in the air. The lower the temperature, the lower the ability of air to contain moisture. Therefore, as the temperature drops water is released and condenses when air reaches what is termed “due point”.
Mould
If you imagine a solid block wall without insulation and the outside temperature at 6° with internal temperature at 20°. The external temperature can be transferred through the concrete blockwork, with the result that the temperature on the inner face of the wall matches the external temperature. This means that due point will occur on the internal face of the external wall surface with subsequent condensation and the potential for mould growth.
The problem with dry lining this type of wall is that condensation may continue on the solid wall surface which is now concealed behind the dry lining. This can lead to an unhealthy internal environment.
In general, the method that is applied to control this issue is the use of a vapour control layer. This can be noted as the foil backing on plasterboard or in some cases builders use a thin polythene layer on the warm side of insulation. Where insulation is fitted to the plasterboard during manufacture, which is now common, the foil backing will generally be concealed.
In addition to prevention measures, such as vapour barriers, it is wise also to improve ventilation. Good ventilation removes moist air and replaces this with fresh air. In modern homes demand control ventilation activates when moist air is detected. Usually when cooking is undertaken or when showers are used.
Despite your builder’s reluctance, I would recommend that you proceed with dry lining as the situation as you describe it is unhealthy and unsustainable.
You should ensure that all mould is fully killed off with bleach before works commence. You should improve permanent ventilation. Ensure also that you have an extract fan over the cooker that discharges to the exterior of the building rather than recirculating moisture laden air. You should also consider providing a demand-activated ventilation system when upgrading your extension.
An inspection and advice from your local chartered building surveyor should help to put you and your builder’s minds at ease.
Noel Larkin a chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie
