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General: Airflow advice for cavity wall

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  1.  
    • CommentAuthorjezza22
    • CommentTimeDec 1st 2008
     
    Hello, hoping to get some advice on the next stage of my house insulation project...

    I have a standard brick inner and outer leaf constructed detached house with 50mm (currently) empty cavity. The ground floor is suspended timber with 1m deep void below the ground floor and lots of double brick height air bricks set in the OUTER brick leaf of the walls.

    After reading on this forum it seems like a good idea to full fill the 50mm cavity which I will do once I've replaced all the indows etc. but in the meantime I have cold air entering the sub-ground floor void via the air bricks and then entering the 50mm cavity and blowing all round the house right up to the loft!

    This effectivly means we're living in a single skin property from an insualtion point of view.

    To stop this I was wondering should I expanding foam fill or hand fill with mineral wool type insulation the cavity gap around the perimeter of each air brick? so the airbricks will still allow air into the under floor void but not allow it into the cavity.

    I guess something like this would need to be done anyway before full filling the cavity to stop the fill material dropping out in to the void at every air brick?

    Any help much appreciated.
    • CommentAuthortony
    • CommentTimeDec 1st 2008
     
    Good idea --- but it will not stop air blowing arround in your cavity as there are lots of other points of entry but it will reduce it considerably..

    Dont feel to bad as there are millions of others with the same problem -- at least you now know and ate doing something about it.
  3.  
    Hi Jezza,

    You should be aware, perhaps, that the draughts are currently doing you a service, ventilating the house. Our experience is that when you fill the cavity and do other draught sealing, you may have a humidity/condensation/mould problem.

    I still think you should go ahead, but take into account that you will need to do something about ventilating the house as well.

    Peter
    • CommentAuthorjezza22
    • CommentTimeDec 1st 2008
     
    Thanks Tony - OK, would you use expanding foam or stuff mineral wool in?

    What would a cavity fill company do to stop poly beads escaping at these points - the same thing as me?
    • CommentAuthorjezza22
    • CommentTimeDec 1st 2008
     
    Thanks Peter - the ground floor void will still be well ventilated as I will not block the air flow from the air bricks just the air flow into the cavity at each air brick.

    Or are you saying I should have air flow in the cavity?
  6.  
    Hi Jezza,
    I am talking about the interior of the house. At present all the draughts that you want, rightly, to eliminate, are ventilating the house interior and dealing with the water from washing, cooking, breathing etc. At least, it is clear that was the case in our house. When you get rid of those draughts, the humidity in the house will rise. In our case that has meant condensation on windows, mould on windows and walls. We urgently need to do something to address that, it seems possible you will have a similar issue.

    I am pleased that we have managed to make our house less draughty, but not so pleased that we need to open all the doors and windows twice daily, as well as wipe condensation, and black mould off walls and window frames.

    Peter
  7.  
    Can you get new air bricks that span the cavity into the inner leaf thereby isolating the cavity from the air entering through the current air brick? Perhaps have a look at the builders merchants and ask around. This will help when they do the cavity fill else I expect they might just block them up with insulation...

    As for what to put in the cavity, I'd avoid mineral wool as I recon there is a condensation risk in fully filled cavities and foam or polybeads seem less likely to be adversely affected in terms of their insulation values. I'd be interested to know what others think about this for future reference.
    • CommentAuthortony
    • CommentTimeDec 1st 2008
     
    Are you going to take out the air bricks? Then you could use second hand slate or tile to make a cavity liner ( a tube through the wall )

    What ever you do will be better than whatever the insulation contractor would have done. They sometimes use a springy brush thing!

    Polystyrene beads are best preferably coated ones.
    • CommentAuthorjezza22
    • CommentTimeDec 1st 2008
     
    Peter, I get your point now but think I have a while to go before I'm 'airtight'!

    As a side note, I've recently installed an air-to-air ASHP to heat a large, very cold area of my house. It was -7 deg here this morning and it was happilly blowing hot air out - amazing. Anyway, my point is that the air-con unit can also dry the air (and cools) so it will clean the air and DEHUMIDIFY which should help with the condensation if needed. A bit like climate controll in your car demists the windows on a rainy day.

    Chris / Tony - I'll see if you can buy 'longer' air bricks and swap them as that sounds like a great idea. When I fill the cavity I'll use the coated poly beads - thanks Tony.

    Thanks everyone.
    • CommentAuthorchuckey
    • CommentTimeDec 1st 2008 edited
     
    Stuff the top of your cavities(in the loft) with a good tight,woodge of glass fibre, it will slow down the upward flow of hot air. Do like wise around your air bricks, but not so easy to do, unless you lift floorboards or remove them. If you go for foam beads (highly recommended), the fitters will just smash out the air bricks and replace them after stuffing around them with foam, likewise they do the tops of the cavities in the loft. Only disadvantage with poly beads is that, they don't use enough glue with them, so they migrate out of tiny holes around pipes and such like flaws in the outer skin.
    Frank
    On edit:- Airbricks are U/S unless they go right through both skins of the wall.
    • CommentAuthorPeter Clark
    • CommentTimeDec 2nd 2008 edited
     
    Posted By: jezza22I get your point now but think I have a while to go before I'm 'airtight'!


    Sounds like the ASHP has you sorted. I was just suggesting that we need to plan for ventilation at the same time as sealing, we didn't and are a bit unhappy.

    As far as being 'airtight', I know we are not, because some draughts are worse after cavity filling!

    I assume that is because, whatever is driving the airflow through our house, stack effect, wind etc, will probably be less 'forceful' now, BUT, instead of having lots of tiny holes, it has a few bigger holes, such as kitchen extract etc. As a result the draught from those bigger holes is now more noticeable. So we have more draught unpleasantness and less adequate ventilation!

    Seems like airtightness should be an all or nothing thing, I am not convinced that our present arrangement is more energy efficient than before we had the CWI, it is certainly less pleasant.

    The ASHP sounds like a goodun', what is the noise like? is the compressor inside the house envelope or outside?

    Peter
    • CommentAuthorjezza22
    • CommentTimeDec 2nd 2008
     
    Peter, main unit with compressor is outside and runs at about 50dbl. I was concerned about it before the install but was told I wouldnt hear it and thankfully when it is running I cant hear it at all unless I go and stand right next to it. It makes a low drone kind of noise. It is on an outside wall mounted on brackets and my office is directly next to it. Cant hear a thing when working.

    The internal unit is a bit noisy when it first starts up to get the room up to tempurature then the fan slows down and again I cant hear it unless standing right below it - I have the fan set to auto but you could set it to low rather than auto so it is never noisy. There is an ultra quiet mode as well if you are sleeping in the same room.

    So noise either inside or outside no problem and I will certainly use more of these devices (I have not got mains gas).
  13.  
    Jezza,

    Thanks, I will have a nose around the web.

    Peter

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