We bought our house in 2005. It was originally built in 1942 with a range of extensions added on in the intervening years.
We refurbished and laid underfloor heating downstairs in a new side and rear extension, and kept the original flooring in our hall and the front sittingroom. Our issue is there has been a smell of damp in the hall over the last few years.
There are two ventilation bricks in the external wall of the sittingroom. I think the problem may be that the partner vents on the opposite side of the house were blocked up when the concrete was poured, so there is likely inadequate air circulation in the spaces underneath the floorboards in the hall and sittingroom. Can you please advise as to what is the best way to remedy this situation?
The hall does not have an external wall to the left as the garage was converted and has underfloor heating now. If we inserted a floor grille into one of the wooden floorboards in the hall, would that help air circulation or is a more significant intervention required?
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Ah, the entrance hall – the most trafficked room in any house, but one which one tends to rush through. Even building surveyors with “trained eyes” can miss things in a space one spends little time in. We all like to create a good first impression for visitors to our home and a damp smell in the entrance hall is not that.
There are three possible issues here. The first is a lack of crossflow ventilation in the underfloor void, as you’ve identified; secondly, it could be a defective damp proof course (DPC) in the 1940s wall; and thirdly, it could be an ill-placed damp proof membrane (DPM) in the adjoining garage floor – or a combination of all three.
The traditional suspended timber floor would have relied on underfloor void ventilation to prevent moisture build-up by way of vents to the front, rear and side walls to ensure a crossflow of air. The redevelopment and extensions to your property over the years may have resulted in the blocking of permanent vents on rear or side walls. The builder probably did not provide pipe ducts through the new concrete floors to deliver airflow to the void of the remaining suspended hall floor.
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It is difficult to address the damp course issue without a site inspection and input from a specialist. A DPC put down in 1942 is likely to have degraded over time, or it may have been compromised during the renovations mentioned above. The new concrete floor poured during the garage conversion may have bridged the existing DPC, allowing moisture to rise into the walls or subfloor. The builder at the time may not have considered the position of the existing DPM or DPC in the floor or wall. There may be a build-up of groundwater around the perimeter of the floor, causing dampness to rise up the separating wall.
The current problem, however, most likely relates to a lack of ventilation. You will need to ensure crossflow ventilation to the underfloor void and the solution could be quite expensive. Before undertaking any remedial work, do a thorough inspection of the house and eliminate some obvious issues.
Inspect the external ventilation bricks for blockages caused by debris, vegetation or alterations to external landscaping and confirm whether partner vents exist on the opposite side of the house. The builder may have ducted in air and the vents are simply blocked. If these are blocked by concrete or insulation, they need to be cleaned or replaced.
It is also important to ensure gutters, downpipes and drainage systems are directing rainwater away from the house. Check external ground levels to ensure they are at least 150mm below the DPC level to avoid moisture ingress. This is an incredibly common source of dampness to older walls.
Rule out a leaking plumbing appliance in a kitchen, bathroom or en suite adjoining the hall wall. This may be remedied by regrouting wall tiles or renewing a seal in a sink, shower tray or bathtub in the adjoining room.
Once the above inspections have been carried out and no obvious cause has been found, adequate natural cross-ventilation can be provided by piping air from an external vent fitted to the front wall at plinth level, directing air to the back of the hall floor to generate some form of airflow. The opportunity to do this when pouring the side garage and rear room floors has been missed.
If natural cross-ventilation is not feasible due to layout constraints, a mechanical ventilation solution, such as a positive input ventilation unit, could help circulate air under the floor and reduce moisture levels.
To your suggestion of inserting a floor grille, this will help the ventilation issue but is likely to cause draughts and is not, in my opinion, the best solution.
Pat McGovern is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland
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