Home Forums General Discussion Dampness in a brick house, ventilation systems

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    I’m assessing an old brick house in Dunedin. It was apparently built in 1867, originally built/owned by a brick merchant. It is solid brick (300mm – 500mm+ thick walls, unfortunately I didn’t think to measure them, no cavity, interior and exterior). Concrete foundations, a few air bricks but there is no sub-floor access so I couldn’t inspect underneath.

    They have a problem with dampness in the house.

    There are drainage problems; in rain water pools on a paved area adjacent to the house on the uphill side. There are also some sources of internal moisture (no extraction in kitchen and bathroom).

    I supect there is no DPC or maybe whatever was there has deteriorated, and the house is subject to risiing damp. It looks like there is black polyethene in the soil around at least some of the house, on the outside of the foundations. I’m not sure how far or where it goes, or what its origin is.

    Does anyone know whether such a house was likely to have had a DPC installed when it was built? What was the practice back then?

    I will recommend drainage and to tackle the indoor sources, and also to open the windows daily, and secondary glazing for the windows with condensation.

    Is this a case where, if there is a rising damp problem, a positive pressure ventilation system might actually do some good? If there is still a problem after improving drainiage and it’s not possible to directly tackle the rising damp, can a ventilation system help to minimise the symptopms?

    The homeowner knows that a positive pressure ventilation system won’t do anything for heating, but they are hugely concerned about the moisture.

    Currently there is an open fire in the living room.

    Which would tackle the damp problem the best: A positive pressure system, a balanced system, or a woodburner with heat ducted from the lounge to other rooms, perhaps in combination with some type of ventilation?

    Is opening windows for 30 minutes a day (ie manual ventilation, if done consistently) as effective as a mechanical whole-house ventilation system?


    Ian McChesney

    Hi Alex
    Very interesting…and challenging. Re the house location – you mention an ‘uphill side’ – is the house on a hill, or largely on the flat butting against a hill? You mention there is no underfloor access, but is there space to get underfloor if there was access?

    I ask this because it might be that the dampness is also caused by evaporation from the damp underfloor area – if polythene groundsheet and underfloor insulation were able to be installed this could resolve some of the issue. Regardless, getting a hole cut in the floor to access the underfloor area (e.g. through the bottom of a wardrobe), even if it is just for inspection, in the overall scheme of things is a relatively low cost action to give a better understanding of the situation.

    Re the solid brick wall, rising damp and DPC. One sign to look out for re rising damp is whether there are any watermark lines around 600-900mm from the ground, which is around the limit of capillary rise. It’s probably fair to say that even if DPC was installed it has deteriorated, or other building practices may have compromised the damp course (e.g. internal plaster walls that bridge the DPC). I’ve reverted to my UK Energy Advice Handbook, since this is a more common situation over there – they suggest injected damp proof courses. The UK practice appears to be drilling a series of holes through the mortar and injecting a silicon based material that laterally spreads, thus providing a damp-proof layer. There is a useful video on YouTube showing how it works:

    Whether this is available in NZ/Dunedin I don’t know.

    Anyway, I think your systematic approach around drainage, and extract ventilation is good. These things should be done first – although coming into summer it might be hard to judge how effective they are. Regarding a log burner and/or ventilation system. It might come down to how the place is currently heated. But a log burner has the ability to provide great heat output, and a drying effect, so this could be good.

    In the UK positive pressure ventilation systems are being recommended for dampness situations (but not as a substitute for good heating, and not necessarily in preference to balanced pressure systems….the source I have is a few years old which is when PP systems were prevalent). They will blow some pretty cold air in the winter and evenings though (i.e. a negative heater!). Re whether PP ventilation or balanced pressure would be best – I suspect that it is the air change effect of the ventilation per se, not anything to do with the positive pressure, that provides the benefit. Balanced pressure systems provide the extra benefit of heat exchange.

    good luck

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