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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorgravelld
    • CommentTimeOct 4th 2018
     
    My cavity wall house has weep holes above some of the window/door apertures.

    What's the point? Don't they just over-ventilate the cavity and lead to (slightly greater) windwash of the CWI? Don't they contribute to air leakage?

    If they're needed, why are they removed when EWI is added? And why do only some of the apertures have them?

    Wondering if I can have them removed when I re-render. Seems kind of pointless to seal off the top of the cavity and tape/grommet penetrations if I'm allowing holes elsewhere.

    Will BC take a dim view of covering them?
    •  
      CommentAuthordjh
    • CommentTimeOct 4th 2018
     
    Posted By: gravelldWhat's the point?

    They allow drainage of the cavity above the lintel.
    • CommentAuthortony
    • CommentTimeOct 5th 2018
     
    They came about after a single stone clad house had problems with water penetrating via the lintels down the window reveals.

    They are completely unnecessary and in all my 29 years of building I never used any, I must have built in thousands of lintels into vanity walls.

    My own house now 9 years old does not have any weep holes nor does it have any problems from not having them.
    • CommentAuthorCerisy
    • CommentTimeOct 5th 2018
     
    Careful - single skins of brickwork aren't watertight. Many years ago there was a well known experiment where they built a half brick thick wall and played a hose on it. The water ran down the other side of the wall. While both bricks and the cement mortar are in themselves water proof, normally built walls allow water through the cracks between the bricks and mortar. Possibly a result of poor workmanship? Possibly just to be expected in that sort of construction?
  1.  
    Posted By: Cerisyboth bricks and the cement mortar are in themselves water proof


    I'm not convinced that's true. Bricks are absorbent and will readily permit water through if wetted for long enough. There's an industry in water repellent creams designed to upgrade brickwork to being less permeable and it should certainly be assumed that water will find it's way through a single skin as you say.

    Is wind-washing really that likely through something the size of a weep hole? I suspect you'll get some air movement but doubt this would be worth the risks of not allowing that air out again so the cavity can dry.
    • CommentAuthorgravelld
    • CommentTimeOct 5th 2018
     
    Posted By: CerisyCareful - single skins of brickwork aren't watertight.
    Rendered walls though?
    • CommentAuthorgravelld
    • CommentTimeOct 5th 2018
     
    Posted By: Doubting_ThomasIs wind-washing really that likely through something the size of a weep hole? I suspect you'll get some air movement but doubt this would be worth the risks of not allowing that air out again so the cavity can dry.
    Well I guess it's not a binary thing. But it would be worse with extra air movement.

    I'm also thinking from an AT point of view. Like it or not and without the luxury of a lot of capital and/or ability to disrupt in an incremental retrofit, achieving AT is going to be a gradual thing. In some cases I think it might mean trying to reduce air movement outbound of the notional AT layer in one part of a building.

    For example: it might be impractical to fix a plasterboard tent inbound of a cavity wall. But we can make it a lot better by sealing the outbound cavity. Obviously you have interaction with the rest of the building but AT, again, isn't binary and by reducing the penetrations we can still improve the AT.

    I guess that's desk theory though.
  2.  
    Posted By: gravelldwe can make it a lot better by sealing the outbound cavity.


    I can see where you're coming from, but I'm just not sure that's true. To quote djh who said it much better on another thread:


    "The primary purpose of air tightness is to stop infiltration of air through the thermal envelope.
    The secondary purpose, and in some ways more important, is to stop air carrying moisture to a cold place where it can condense.

    So the main air barrier must be on the inside in a cool climate, unless the design is a 'warm' one that keeps all decay-prone materials away from any cold regions.

    In addition, there is a phenomenon known as 'wind washing' whereby external air blows or convects through the cold side of the insulation and reduces its effectiveness, so if the construction is vulnerable to this, then an additional air barrier is needed on the outside. If and only if there is no condensation risk, then this can serve as the primary air barrier and an internal one can be dispensed with, although again there is a need to prevent convection through the warm side of the insulation so it is usual to have one anyway."


    Having re-read your original post properly I now see that you're intending to render the outside. This is in effect a super version of the water repellent creams and I guess will markedly improve the weathering of the surface. What I can't tell is if you're intending to also add insulation. If not then I think you're potentially making an airtight layer on the outside face. Without the 'plasterboard tent' inside that you mention above, what stops moisture laden warm air from making it's way out into the cavity and then condensing as it hits colder temps?
    • CommentAuthorgravelld
    • CommentTimeOct 5th 2018
     
    Correct, rendered, no extra insulation.

    I'm assuming you're suggesting rendering + AT works to the outside face make it airtight. You have a good point here.

    The render will be breathable but the concern is that before it even gets to the render the inner face of the external skin would be at dew point and cause the vapour to condense.

    The plasterboard tent could not be used to mitigate this; it will not stop air going from the warm internal space into the external space. It's inherently leaky and will escape from plug sockets, ceiling and floor junctions etc.

    Maybe, then, all air leakage works should be confined to the internal AT layer - e.g. plasterboard to ceiling junctions.

    Kind of disappointing, but maybe "safety first"...
    • CommentAuthorgravelld
    • CommentTimeOct 5th 2018
     
    Another thing about preparing for the future...

    It sounds like what you're saying is it's best to keep a level of ventilation in the cavity when the outer skin is cold.

    Maybe though all "non designed" ventilation, e.g. gaps around penetrations, should still be sealed (given they will be rendered over) and "designed" ventilation left in the form of an open top and/or weep holes which are far easier to rectify in the future?
    • CommentAuthorCWatters
    • CommentTimeOct 7th 2018 edited
     
    Posted By: CerisyCareful - single skins of brickwork aren't watertight.


    I agree, but apparently the house has been rendered which should make the wall more or less water tight.

    My belief is that weep holes are only needed on un-rendered bricks. My guess is the house wasn't originally rendered?
    • CommentAuthorCWatters
    • CommentTimeOct 7th 2018
     
    This from the NHBC. See bottom of page 6..

    http://www.nhbc.co.uk/NHBCPublications/LiteratureLibrary/Technical/StandardsExtra/filedownload,25275,en.pdf

    quote

    Where the wall is finished with a render and the render is complete over the whole face of the wall and is sealed round door and window openings there should be no path for water penetration into the wall cavity. For this reason weep holes may be omitted.

    However, if the render stops short of the top of the wall and has a fair faced masonry panel above it then weep holes should be provided over the cavity tray within both the fair faced and rendered section of the wall.
    • CommentAuthorgravelld
    • CommentTimeOct 9th 2018
     
    Strange, because where the weep holes are is where an extension was added - the walls were previously rendered and then rendered again over the extension to meet the old walls. Maybe the advice changes over time, this extension was from the early 2000s.

    Thanks for your help.
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