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Roof rafter insulation

Discussion in 'Roofing and Guttering' started by paulp51d, 7 Jul 2010.

  1. paulp51d

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    Hi,

    Can anybody advise me on insulation between the roof rafters?

    My son has just bought a small house and I took a look in the loft at the weekend and noticed that it looks a mess. There is too much insulating material piled up between the joists and there is no felting under the roof tiles.

    The excessive insulation has caused some condensation and mould to appear in one part of the ceiling, where the roof is closest to the ceiling.

    I plan to get rid of some of the fibre, sort out the unbelievably bad wiring (if I survive being up there amongst it for long enough), then I would like to get some insulation between the rafters.

    Is the spray stuff good, or should I just be packing in some rolls of insulation between the rafters? If you recommend the spray-on stuff, can I do it myself or should I use professionals? Can't believe that would be a cheap option.

    If you recommend just packing the rafters out with insulating material, what should I use?

    I have tried looking at a few web sites on this subject and they all want to waffle on and explain the science behind it. I, on the other hand, just want to know what to do.

    Any suggestions much appreciated.

    Thanks.

    Paul
     
  2. geraldthehamster

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    I don't think there's such a thing as "excessive" insulation. The current standard for fibreglass insulation above a ceiling is, I believe, 300mm.

    Certainly the insulation above the ceiling isn't responsible for condensation on the ceiling below, which is likely down either to insufficient room ventilation, or a *lack* of insulation above the edge of the ceiling. Condensation occurs when moist air hits a cold surface. A classic case would be a house with well-sealed windows kept shut, and people using baths and showers with inadequate bathroom ventilation, and the door open.

    Older houses often have no roofing felt under the tiles, and as long as the tiles themselves are in good condition, this shouldn't be a problem. In fact it will help with ventilation of the loft, to avoid any condensation there.

    If you have insulation between the joists, above the ceiling, then you don't want it between the roof rafters as well. Unless you intend to convert the loft to a habitable space.

    Hope this helps.

    Cheers
    Richard
     
  3. paulp51d

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    Hi, Thanks for a good reply.

    The reason I mentioned too much insulation was that I saw one web site that was suggesting that condensation could occur if insulation was not just between the joists, but starting to go up the roof aswell - closing off any venting between the loft floor and the roof, I assumed.

    I am not sure where the warm air was hitting the cold surface in that scenario.

    In this house the upstairs rooms are curved downwards at the window sides, where the ceiling follows the shape of the bottom of the roof.

    Where the ceiling slopes down with the roof, there is very little gap between the ceiling and the unfelted rook tiles. Something has made the ceiling damp/mouldy in this area (just a small area).

    The tiles themselves look pretty good, with no signs of damage. So, I was assuming that it is this small gap, possibly coupled with all the fibre that was causing the problem.

    Do you have a better guess?

    Paul
     
  4. Deluks

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    Pull the insulation away from the eaves, you should leave a gap here, and ensure that air can flow between the rafters and over the insulation. What eaves ventilation do you have? (Soffit/fascia vents?)
     
  5. paulp51d

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    Thanks, I will pull the insulation away and open up a vent. I don't know what kind of ventilation is there, I have only visited twice and I didn't notice. does it have a bearing?
     
  6. alastairreid

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    Yes! the loft space should be ventilated..soffit, tile, ridge vents..you should feel a draught on entering the loft space.
     
  7. paulp51d

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    OK, I will check it out. Thanks again.
     
  8. noseall

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    Your second sentence answers your first.

    It sounds to me like you have raked ceilings. This particular detail is one of the most overlooked in earlier buildings and one of the most vulnerable to condensation related mould.

    There is little room for protection as the ceiling is only inches away from the the surface of the roof tiles.

    To bring it up to spec' you will need to add celotex to the underside of the rafters.
     
  9. paulp51d

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    Thanks, I appreciate all the help. I will look into Celotex. Not sure what it is as I write, but I'm on it. Cheers

    Paul
     
  10. alittlerespect

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    Hi

    Don't forget to use a quality face mask when moving the insulation material ('drager' come to mind!)

    As fibreglass will likely replace asbestos in the 'whats not good for you and its too late for some people charts'.

    On a more positive note its likely that the damp is due to a bit of condensation, although the insulation material has high thermal resistance it does have the drawback of having minimal vapour resistance - the water vapour will pass through the insulation material and condensate either within the top of the insulation material or on the underside of the roof, which is why you need plenty of ventilation above the insulation, as mentioned already by Deluks keep the eaves clear and if necessary introduce some eaves ventilation.

    Regards
     
  11. paulp51d

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    Thanks chaps. And with regards to the mask etc., when I stuck my head in the loft last week I was struck by how many particles were visibly floating around. I got itchy just from a couple of minutes staring in.

    I have already made a mental note that I am not going in there without boiler suit, mask, gloves, etc. I am not normally such a nancy, but this particular loft space looks especially grim and uber itchy.
     
  12. geraldthehamster

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    Ah, I see. Well you have two separate issues here.

    If ventilation is blocked at the eaves, then you could potentially get condensation in the roof space, through moist air rising from the house and not being able to escape. That's a different matter from getting condensation in the ceiling below.

    However, I doubt condensation in your roof space would be a problem, if you have no felt under the tiles. There should be plenty of air getting in round the tiles. This is a good thing, assuming your ceiling is properly insulated.

    As others have said, it sounds as if your problem below is due to a lack of sufficient insulation in the slopy bit (which you didn't mention). The moisture in the warm moist air in the room may be condensing out on the ceiling, which is colder because of said lack of insulation.

    How much space is there between the sloped part of the ceiling and the underside of the roof above? If you can't get 300mm of fibreglass in there, while still leaving an air gap above, then I'd use Celotex or Kingspan of whatever depth will fit (I suggest 100mm if there is room). However, you need to fit this directly above the ceiling, and not to the roof rafters above that, or you will still have a cold space immediately above the ceiling.

    If there isn't room for any space (ie the insualtion is in contact with both ceiling and underside of roof) then I wouldn't worry too much - coming back to my original point, the risk from blocking eaves ventilation is condensation in the roof space, but yours is probably well ventilated.

    You can reduce the risk of condensation in the roof space by adding a vapour barrier (plastic sheet) under the insulation, above the ceiling. The aim should be to keep moisture created in the house out of the roof space, and deal with it in the house by properly ventilating the house.

    Cheers
    Richard
     
  13. paulp51d

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    Thanks Richard.

    At present I don't know what the space is between the rafters and the ceiling in the area where the ceiling slopes with the roof, but I am pretty sure that it will be as small as is possible to build. It's a small house and I don't think there are any generous amounts of space anywhere.

    In fact, I suspect that getting into the loft and getting something like Celotex into the available space, whilst at the same time being able to see what I am doing might be a challenge.

    As I plan to board the loft properly, the first thing I intend to do is get the mountain of insulating material out as well as the thin layers of chipboard that are perched on top of it, waiting for somebody to fall through them. Then, when everything is out I will look at the cavity that is available between the ceiling and the sloped roof and make my mind up based on what you have advised.

    By the way, I looked at the Celotex web site and I see that they have a few different types of this material. Based on their technical info, I wasn't sure which to use. Would you know? I have e-mailed them and asked, but got no reply. Also, where do you buy Celotex, would it be from the likes of B&Q, or from a specialist merchant?
     
  14. geraldthehamster

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    B&Q sell Celotex (or possibly Kingspan), but I've found it cheaper at Builders' Merchants like Travis Perkins. I don't know the model name but there's a basic foil-backed variety that everyone stocks, which is used for this purpose.

    If there isn't enough space in your slopy bit, another option might be to insulate that portion of the ceiling from underneath - ie screw some 50mm Celotex through the existing ceiling, to the joists, then some plasterboard over that, and skim. or you can get insulated plasterboard.

    Generally speaking, I think that unless you're planning to do a full loft conversion to a habitable space, you'll be better off sticking with insulation above the ceiling, rather than between the roof rafters, which is a more involved job. To be honest I'm not sure of the best way to deal with your slopy issue, without seeing it, but as long as you understand the principles you'll be able to take an informed decision.

    Cheers
    Richard
     
  15. paulp51d

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    Richard,

    Thanks, I do understand the issues and I will save a print-out of the dialogue that we have had for reference.

    I went back to the house at the weekend and spent some time in the loft - more to do with a partition wall that I building for my son than to do with the damp in the ceiling.

    However, I made a couple of observations whilst I was there. Firstly, the house and all the nearby houses appear to have no ventilation to the roof at all. At least, no vents of grills in the soffit boards. They all look completely sealed at the bottom of the roofs. I looked at the gable ends as well, just in case they were vented that way, but they are not. I can't see any obvious ventilation. Ever heard of that?

    Secondly, I was wrong when I said that there was no felt under the tiles. I was correct that in some places there is nothing, but mostly there is a material that I don't recognise where the felt normally is. It looks like a brown oily paper stuffed with something that looks like horse hair.

    I didn't tear any of this stuff off to properly examine it, but does anybody have any ideas what this might be? It's clearly doing the same job that felt is supposed to do.

    Thanks again.
     

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