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  1. #1
    Join Date
    Nov 2011

    Default Insulation and the fear of doing the wrong thing

    This is my first message on this site but i have been reading the Forums for the past few weeks, I was all set to start insulating my house internally with rigid phenolic insulation until i started to read about all that can go wrong,
    Our house is 130 years old with high ceilings and can be quite cold in the winter and expensive to heat,
    My plan for insulation was to place insulation in between a timber frame and then add insulated plasterboards to eliminate cold bridging, is this a bad idea / are there too many risks involved? Also why is this the most popular form of internal insulation if it is so dangerous?

    External Insulation is an option on the gable end wall and also the back walls but not on the front wall as it is a brick finish but it is very expensive and i have doubts about the long term effects,

    Ijust want to do the right thing and forget about it i really don't want problems down the line,

    Thanks in advance for any advice,

  2. #2
    Join Date
    Jul 2010



    Hi, hyperlink is linked to your question - particularly on outside work.

    As for the studs you did not give the center distance assume 16" or 24" centers x 1.5" width? each has an R4 value whereas your insulation is likely around 19R.


  3. #3


    All the comments suggestions raised over the years in this forum have a few points in common.
    1. Increased levels of insulation raise the interior temperature of the building
    2. Human activity within the building - breathing, cooking, washing, perspiring, gives rise to water vapour. Reduced levels of ventilation can / will increase the moisture content in the interior. High interior temperature can hold a significant amount of water vapour, but if is above the dewpoint then the interior air is comfortable and not clammy.
    3. Irrespective of the amount of insulation, unless there is ventilation, the relative humidity will rise and condensation will occur This arises whenever the moisture content in the air exceeds the dewpoint temperature for that moisture content. Conversely, if the RH remains below the dewpoint temperature there will never be condensation.
    4. When / if the water vapour - moisture in the air hits a free surface which is at a temperature below the dew point temperature for that level of moisture, then condensation will occur and water pooling can occur.
    5. If water is allowed to collect, then mould and fungal growth is possible . Moulds and Fungi are always unpleasant and in some cases serious health hazards. The warmer the temperature at which this occurs the greater the potential for growth. Whether there will be mould growth depends on the environment , these are living organisms and need organic matter on which to live – Clean concrete blocks will not provide much sustenance, whereas wet timbers are an excellent food.
    6. If the dew point temperature is within a solid body ( not on a free surface,) then liquid water cannot form and there is no fungal growth. This solid body can be within the insulation or concrete wall or whatever.
    It therefore follows that with increased insulation there needs to be either better ventilation or at least better water vapour permeability, so as to reduce the buildup of moisture. One of the advantages of natural fibres – Wool cellulose Timber compared to synthetics e.g glass fibre is that these can absorb more water vapour and can literally provide more “breathing space” before the air becomes saturated with water vapour. Normal old houses have plenty of ventilation – either through badly fitting windows and doors , so there are plenty of places for the excess water vapour to go. It is when new windows and insulation is added that the potential problem can arise.
    Consider that it could be 20 degrees inside and 0 degrees outside and assume that the dewpoint for a specific relative humidity is about 10 degrees. Assume a solid wall of say concrete. 20 cm thick - In the absence of any internal insulation , the temperature within the wall will fall from 20 to 0 degrees uniformly. At about 10cm in the wall thickness the dewpoint temperature is reached, but this causes no problem as it is in the middle of a solid structure. Now assume that insulation is added internally so that the insulation value of the total wall is doubled. Then the dewpoint of 10 degrees now occurs exactly at the junction of the insulation and the concrete. If there is any gap here then it is possible for water to form.
    However there is another condition - water cannot form at the dewpoint unless moisture has had the opportunity of travelling through the insulation at a greater rate than it is removed by passing through the wall..
    Constructions aim to prevent vapour penetrating the surface of the wall or to use the materials more permeable to water vapour the deeper one moves into the wall so as to reduce the prospects of pools forming
    Attic insulation works well because water vapour flow through painted plasterboard is low and the fibreglass insulation is more permeable so there is no tendency for liquid water to pool at the dewpoint. -
    A structure which has plaster / phenolic insulation board close to the interior will only have a small amount of permability, particularly if the plaster is painted. Ceramic Tiles or varnished timber cladding will likewise reduce the flow of water vapour into the walls. If this backs onto rough unpainted concrete blocks, the permability of the concrete is probably less than the plaster and no pooling forms.
    Of course external insulation is a win win situation as the insulation will move the dewpoint closer to the external surface maybe even into the insulation.

    So in essence, there is nothing wrong in principle with internal drylining, provided the other things are taken into account . ... and ventilation is one of these things. In summary - It would not be a good idea to mount insulated plasterboard drylining using dabs of adhesive onto a previously sealed plastered surface covered in based wallpaper , as this would give excellent conditions mould growth.

  4. #4
    Join Date
    Nov 2011


    Thanks for your help folks, it has given me alot to think about,

    I am a carpenter and would like to do some of the work myself could you recommend some text books for me to digest i want to be as well informed as possible,

    I think i will have to leave external insulation for the moment as it is out of my budget, this recession hasn't been too good to my type, i would be going with 24' centre's 3' x 2's with 50mm insulation,

    Would you recommend getting a specialist in before i undertake the work?


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