Home Forums General Discussion Underfloor ventilation and moisture barrier

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    Kat Achterberg

    One of our clients approached us with the following problem:

    “I have a 1940-50’s brick veneer artdeco house, single story with dug out basement underneath. The basement has a dampness issue which is two-fold. One there is a spring which sometimes runs, and two, the method of construction back then didn’t have a step at the junction of the concrete basement/foundation wall and the brick veneer, to push water in the cavity back outside the building. So what happens now is that moisture passes thru the brick veneer, runs down the inside face of the brick work, then down the inside face of the basement wall.

    One I am thinking would be the best way to deal with this issue, is to install underfloor insulation (it is a T’n’G floor with exposed timber joists & bearers.) most likely a polyester type between the joists. Once this is in place, staple polythene sheeting to the underside of the joists. My thoughts being, if I was to put the polythene on the ground as is most commonly done, the water running down the inside of the wall would just collect in pools on the polythene. By putting the polythene against the joists I would prevent this. What I thought would be best would be to run the polythene to the basement wall & then turn it down the concrete wall say 500mm, this would allow the rain to run down the wall & pass between the wall & polythene to the ground, but should minimise the vapour returning up & into the floor framing/living space.”

    Does anybody have experience with what he is suggesting or maybe some other ideas how to approach his problem? I would appreciate any input.

    Ian McChesney

    Hi Kat
    Always tricky with moisture issues. But I’m not sure that your client’s suggestion will deal with the issue though, and may create another one. I’d be very wary about stapling polythene to the joists. The polythene will provide a largely impermeable water barrier that may cause a moisture problem within the flooring components between the polythene and the T&G. Water could get into this zone by spills from above, leaky pipes, and by movement of moisture from below. The result could be creation of a damp, high humidity zone. I think looking at reducing the sources of moisture should come first, then work from there.

    If water is ‘running’ down the inside face of the basement wall it suggests the source could be more than just normal infiltration through the bricks – does the pointing need repair, are some bricks damaged etc? Drilling some external drain holes at the junction of the foundation and the bricks to divert water from the cavity to the outside might also be considered, although the base of this cavity can often be filled with rubble and not easily drained.

    As for the spring, can any of the flow under the house be diverted by digging a drainage trench on the outside?

    Reducing sources may or may not work, but regardless I think that sticking with the conventional polythene on ground would be best, and the best way of creating a dry basement. If there is still a problem with damp foundation walls run the polythene from the ground up and secure the polythene say to the bottom plate to cover the walls.


    Ian Mayes

    I totally agree with Ian McChesney about trying to stem the flow or ingress in the first instance. And about fixing the polythene to the underside of the joists, there is a grave danger of condensation forming on this surface and leading to bigger problems. Unfortunately that’s about all I have to add.

    Kat Achterberg

    Thank you Ian and Ian.
    Your comments are along the lines of what we were thinking as well. It seems like fixing this issue/s could be quite a big project.


    Kat – totally agree with Ian and Ian. I think it is a waste of time and money to install insulation and vapour barrier until you have addressed the moisture.
    Two suggestions to check / act with respect to external moisture.
    1. If the brick wall has been laid by a bricklayer then there will be weep holes on the bottom course… about every six bricks the vertical mortar joint is left out / scrapped out, to allow moisture running down back of bricks to exit externally. You need to get them to check that the peepholes haven’t been blocked… or worse still filled up with soil where garden may have extended to wall… then all that happens is soil in weep holes act as a wick and allow moisture to be sucked from soil into warmer house.
    A brick wall is a wet wall system… hence the cavity. Even fired bricks will absorb moisture and more so if (as Ian suggests) the mortar has deteriorated.
    Source of moisture is more likely to be failed head flashings above doors / windows… metal flashings are installed at head / foot of windows to divert moisture away from interior of wall. So check above above doors / windows that these flashings re still in good condition.

    2. You need to find the source of moisture which is external to the wall / house…if there is soil touching floor / brick wall, the moisture will find path through the brickwork.
    Get the owners to dig a DEEP field drain below the level of the floor and ensure it runs down to a stormwater drain… free draining material at base of drain, nova flow pipe and then at least 100mm of free draining material… then weed mat and soil. Deeper the drain the better
    Hydraulic pressure can be enormous and water under pressure from outside will find a path into dwelling… you need to get the water away from the wall.

    Once drain has been installed… and wall has dried out… may need to wait till next summer, then you can do something with vapour barriers and insulation. Unless you address sources of moisture then insulation / vapour barriers will be temporary sticking plasters and may make the situation worse.

    Good luck… and happy to have phone conversation if you can send pictures.

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