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Home clinic: My house is well-insulated so why is it so damp?

Question: I live in a two-storey detached 1970 property (210sqm) which is heated with oil radiators and a back boiler stove.

I have condensation in some of the external walls but a particular room that was converted from a garage is very bad. The walls were pumped about 13 years ago and the attic is insulated and there is double glazing throughout. Would a heat ventilation recovery system solve this problem and how much would it cost? And how do you make an older house airtight?

Answer: Unfortunately condensation is a very common problem in our damp Irish climate. All too often it will develop into a mould problem if not tackled. The source of the condensation is usually poor ventilation, poor insulation or insulation in the wrong places. It could also be made worse by high humidity levels in the house such as drying clothes indoors.

You don’t mention whether your problems started or got worse after your insulation works but it’s safe to assume that they did. Many people embark on insulation upgrades without fully understanding the potential pitfalls.

Firstly, when insulating a house, you need to do so as completely and evenly as possible. Heat moves from hot to cold areas. When a house is unevenly insulated, the weaker areas will actually have more intense heat loss than they did before. This often lowers the surface temperatures in these areas below the critical dew-point temperature, and condensation is the result.

If you pump your wall cavities, the consistency of insulation fill is unknown because of unseen obstructions in the cavity itself, and it will have weak points or ‘cold bridges’ around windows and doors.

External insulation is always a better solution as it has fewer cold bridges if done correctly, but it is always much more expensive. The good news is that you would have had to pump your cavities before externally insulating anyway so that option remains open to you in the future.

Your former garage may be more problematic for a number of reasons. Firstly, I expect that it has more external walls or roof area to lose heat from than other rooms – and this will be even worse if it faces north or east.

Finally, it is also likely to have a concrete flat roof – there is a common misconception that 1970s flat roofs leak. Of course, some do but most often the problem is condensation rather than rain. If your concrete roof is cold or insulated from below, then any humidity from inside the room will find a dew point and condense within the concrete only to drip back down into the room.

Concrete roofs are best insulated from above with a good vapour barrier on the warm (or room) side of the insulation – this requires a re-roofing. If it is a timber roof construction, then it’s possible that it is inadequate ventilated.

Adequate room ventilation, typically a 100mm (4 inch) wall vent, will often prevent condensation but may not always fully eliminate the problem. As your house dates from the 1970s, there is a good chance that not all the rooms have wall vents. It would be a good idea to check that you have them, and that they are not blocked with old socks or scrunched up newspapers!

Heat Recovery Ventilation (HRV) would probably solve your condensation problems, however, it would be costly to retrofit and expensive to run. A HRV works on the basis of extracting warm, stale air from humid areas such as kitchens and bathrooms, and replacing it with balanced fresh air delivered to bedrooms and living areas.

The heat from the stale air is used to warm the incoming air which reduces the amount of heating needed overall. However, the system is best suited to new build houses or deep retrofit projects where the level of air-tightness and insulation is extremely high. Otherwise, the system is sucking in external air through the gaps in the walls, floors or windows and then filtering and heating it at a significant strain to the fan and at a high energy cost.

A better solution for a retrofit with lower levels of insulation and airtightness is a Demand Controlled Ventilation (DCV) or Positive Input Ventilation (PIV) system. Both are much cheaper and easier to retrofit. They also ensure a good number of air changes that should keep condensation and mould issues under control.

Typically they run continuously in your attic space using a 13W motor which is relatively cost effective to run (less than €30 a year). I would estimate that on your house a DCH system would cost about €2,500 – €3,500 to install, including electrical work. It will depend on the number of fans you need – a typical unit can serve up to four wet rooms, for example, kitchen, utility or bathrooms).

A PIV system should be significantly cheaper than this at about €1,200 – €1,500 but, to my mind, would be less satisfactory as it pushes cooler attic air into the house and forces air changes through the gaps in the walls and floors and so on.

It is important to understand that no house should be entirely airtight. That would be unhealthy. Instead a house should have controlled levels of ‘planned ventilation’ instead of leaks and draughts throughout. This is difficult to achieve on an existing 1970s house unless undertaking a deep retrofit.

While hard plaster on walls is quite airtight, your windows will most likely be quite leaky. Your floors and attic may also be ventilated which often leads to draughts within heated spaces. To achieve decent levels of air-tightness usually involves replacing windows, using membranes on the floors and external ceilings which means re-plastering and re-wiring too.

I know that the above might not be exactly what you wanted to hear. However, once the source of the problem is clear the solution will follow more easily. As always I would highly recommend that specialist advice is sought from a local registered architect who has inspected your house. Often the problem may be from a number of interrelated issues which all need to be addressed for a successful outcome.

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