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  1.  
    Just noticed the neighbours having their walls insulated with cavity wall - their old air bricks (near roof level and ground level) have been concreted over. None of the other houses around here (all similar age/build) have had this done - but we all have a problem with damp on the inside walls near these vents. To date I had always assumed that the additional ventilation is a good idea to reduce the damp, but it did make me think - should I be blocking up my vents too?
  2.  
    Am I right to assume the air bricks ventilate the living space & not the structure?

    Air bricks can lead to cold spots on walls, which ironically can lead to damp problems. This is especially true where a clay duct is used. Insulation should increase the general temperature of the wall's inner leaf & thereby reduce condensation, but this doesn't remove the need for ventilation of the living space.

    Have they had some other form of ventilation fitted? Do they have double glazing with integrated trickle vents?

    David
    • CommentAuthortony
    • CommentTimeJun 29th 2010
     
    Block the up, insulate them the same as the wall having removed any cavity liner.
    • CommentAuthorCWatters
    • CommentTimeJun 29th 2010
     
    What David said. The air bricks may well be preventing condensation generally in the house but causing it locally. If you block up the air bricks you might cure the local problem but cause or make condensation worse elsewhere. It depends if you have enough other sources of ventilation either deliberate or accidental.
  3.  
    Originally there would have been an external air brick to the cavity (about 5 courses up from dpc and 10 down from roof line) and a corresponding metal vent on the inside. Some of these metal vents have been plastered over, most of the others have rusted shut. All the metal vents and the surrounding areas suffer from condensation damp.

    All the double glazing has vents and all suffer from condensation around the upvc elements in the deepest of winter.

    and we have a multi-fuel boiler, so we have a huge hole in the wall.

    What's the standard practice though - block these air bricks or leave them open?
    • CommentAuthortony
    • CommentTimeJun 30th 2010
     
    Block them up but first remove the cavity liner (a cold bridge) then insulate the cavity then make good.
  4.  
    Posted By: adrian hollisterAll the double glazing has vents and all suffer from condensation around the upvc elements in the deepest of winter.

    and we have a multi-fuel boiler, so we have a huge hole in the wall.

    What's the standard practice though - block these air bricks or leave them open?
    As Tony says, insulate the cavity & block them up. However, make sure you have large enough trickle vents in the windows of each room & consider discontinuous extract fans in kitchen, bathroom, etc. If condensation is still an issue then consider positive input ventilation or centralised continuous extract.

    David
    •  
      CommentAuthorfostertom
    • CommentTimeJun 30th 2010
     
    On a related note, I'm currently worrying about EWIing over airbricks that through-ventilate underfloor space under a timber ground floor. Or used to through-ventilate, till a previous extension blocked off one side of it.

    The EWI is being carried down to bottom of found level all round, as downstand perimeter insulation. Part of the house has solid flagstone floor so that will do a fair job of reducing its heat loss, making its surface temp comfortable, and adding it to the building's heat storage capacity, without disturbing it, as the subsoil warms up, within a year or so. The other part has suspended timber floor over nominally ventilated void and we don't want to disturb that either, but the thermal situation's not so obvious. Wondering whether the void space and possibly damp surface of the void's soil floor will warm up similarly, if no longer vented to outside. The soil floor surface is of course quite low down, relative to the bottom of the perimeter downstand insulation.

    To replace or improve on the existing through ventilation, and to stand a chance of satisfying the Bldg Insp, the thought is to incorporate that subfloor into the house's MHRV system. It would be supplied, as a 'room', with warmed-up external air, which wd well reduce its RH as delivered, but wd still contain all the external air's moisture, which cd condense if the void's surfaces remain cool - but no more so than it wd anyway with direct through ventilation. This cd be seen as a waste of reclaimed heat but on the other hand wd facilitate the long term warming-up of the underfloor soil generally, as part of the building's heat storage.

    The supplied MHVR air cd be left to filter up thro the floorboards, for return - but wd that introduce cooler/damper air, even with mould spores etc? Or there could be a MHVR extract from the void - but wd that be returning similartly cooler/damper/mouldier air to dilute the temp of the exhaust stream from which heat recovery in happening? Perhaps the underfloor extract shd have its own small extract fan exhausting direct to outside?

    Any thoughts welcomed. It cd be a gd solution to the old problem of insulating suspended ground floors without tearing the place apart.
    • CommentAuthortony
    • CommentTimeJun 30th 2010
     
    How about including the underfloor void into the heated and insulated envelope of the house with its own air input duct(s) block all holes to outside -- then the floor will be protected and the whole house warm.

    Unless there is a positive water pressure everything will warm up and dry out and it will all be nice and warm and more efficient.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 30th 2010
     
    Yes maybe one day but that wd mean major disruption for access to do work in the void - wd also detach the subsoil underneath the void from any use as deep storage - but that's dubious anyway because it's so little contained by the downstand perimeter insulation. Unfortunately it is liable to occasional flooding, tho not quite up to Grd Fl level for a long time now.

    I'd really appreciate help in clarifying what wd happen to heat flows and condensation in the scenario I described - and how or whether to plumb it into the MHVR.
  5.  
    Posted By: fostertomI'd really appreciate help in clarifying what wd happen to heat flows and condensation in the scenario I described - and how or whether to plumb it into the MHVR.
    I don't think it makes sense to put the underfloor void on the house side of the MVHR unit. If you're not planning to insulate & heat the void to the same level as the rest of the house then it will either act as a sink for heat from the supply side or dilute the temperature on the extract side. Have you thought about leaving one or two vents in corners of the void & taking the MVHR inlet from below the floor? This will help to ensure consistent ventilation while any heat lost into the void will in effect pre-heat the MVHR inlet.

    David
    • CommentAuthorsavagehk
    • CommentTimeFeb 7th 2016
     
    With apologies for resurrecting an old thread, what did you decide to do in the end? We need to improve ventilation to our subfloor and the thought of connecting it somehow to the MVHR did cross our minds to ensure airflow.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 7th 2016
     
    In some seasons of the year, the ventilation actually imports condensation, by bringing warmish damp air into a colder space.

    Provided you can guarantee the underfloor space bone-dry, e.g. as per attached, then you can try negotiating with the Building Inspector to seal the void airtight (no through ventilation).
      Downstand med.jpg
  6.  
    Apologies for resurrecting this again!

    Tom, this looks like a brilliant solution that I could adopt. How essential is it that I get down to the bottom of the founds / alongside subfloor dirt? I can get180mm beneath joist level outside (I'm reluctant to take up the concrete paths), and 250mm beneath the lowest timbers (purlins) internally (I'll have periodic access to the subfloor in each room as I renovate). An internal and external solution, but I doubt it would warm the subsoil all that much to allow me to block the vents and benefit from a warmer floor?
    • CommentAuthordelprado
    • CommentTimeApr 13th 2017
     
    Simple question - if you block up airbricks under suspended timber floor - even if you then fill the void with insulation, the moisture level in that area will be very high (presumably) because the ground is wet. Where does that moisture go and what stops it going into the timber floor?

    Thanks
    • CommentAuthortony
    • CommentTimeApr 13th 2017
     
    Sorry that is not how things happen assuming polystyrene the temperature at any point nearer the floor will be warmer than one further away. Therefore the partial vapour pressure of moisture in the air there will be higher than it is further away. So moisture moves away from the inside of the house not towards it.

    Assumes no flooding, assumes house warmer than surroundings.

    So partial vapour pressures keep the timbers dry:bigsmile:
  7.  
    @Tony

    Sorry that is not how things happen assuming polystyrene the temperature at any point nearer the floor will be warmer than one further away. Therefore the partial vapour pressure of moisture in the air there will be higher than it is further away. So moisture moves away from the inside of the house not towards it.

    Assumes no flooding, assumes house warmer than surroundings.

    Does that mean it is OK provided those conditions are met to a) Block up air vents, b) Fill sapce beneath a suspended wooden floor with EPS beads then c) Put vapour barrier and floating floor on top of suspended wooden floor?

    Or am I missing something?

    Cheers

    Ferdinand
    • CommentAuthortony
    • CommentTimeApr 15th 2017
     
    Yes, exactly, and that is what I would like the whole country to do .
    •  
      CommentAuthordjh
    • CommentTimeApr 15th 2017
     
    The one fly in the ointment is that building regs and NHBC standards say there should be sub-floor cross ventilation. So you will have to argue your case with the building inspector if you wish to close up the vents.
    • CommentAuthortony
    • CommentTimeApr 15th 2017
     
    There is a philosophically tricky point here in that yes the regs do require a void to be ventilated but say nothing about where ther is no void :cool:

    For example no ventilation is needed under a ground bearing slab
    • CommentAuthorEd Davies
    • CommentTimeApr 15th 2017 edited
     
    Ventilation isn't needed under a passiveslab. So what's the definition of when you do or don't need ventilation? Does filling the void with placky beads cross that line?

    (ETA: cross-posted 56 seconds after Tony.)
    •  
      CommentAuthordjh
    • CommentTimeApr 15th 2017
     
    The regs don't talk about the void, they talk about the suspended floor.
    • CommentAuthorgoodevans
    • CommentTimeApr 16th 2017
     
    I'll have a stab here:

    A suspended floor is where some (or usually most) of a floor area spans discrete elements such as beams/piles etc.

    A floor is not suspended if it is designed to be supported over it's entire area by the ground below - however provision may be made in the floor to span small local patches of the ground where it may be of poorer bearing strength (hence mesh reinforcement in thin ground slabs).

    Unless the void below a suspended floor is filled with load bearing material - it is still suspended.

    The failure of a suspended floor could be more dramatic (due to the drop) when compared to a unsuspended floor. This may be why suspended floors have specific ventilation requirements in the UK guidences. (however for rc concrete with the correct exposure characteristics I do not see why).

    Suspended floors are often required to allow the earth (clay) below to expand and contract without moving the floor - filling a void created for this reason may not be a good idea!
    • CommentAuthortony
    • CommentTimeApr 16th 2017
     
    Sometime there are issues with smells and or radon to be considered when talking about ventilation.
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