Query Over Application of Screwfix's Pitch Roof Insulation

Discussion in 'Building' started by purplechap, 9 Jan 2005.

  1. purplechap

    purplechap

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    I was after a warm pitched (lean-to) roof. But the cost of the insulation boards and breather membrane are just too expensive when all I need to cover is about 4 sqm of roof.

    Then I found screwfix's "Pitch Roofing Insulation" which are 105x400x1200mm insulation boards with integral water barrier membrane. The picture in the screwfix magazine appears to show these boards placed between the rafters, so not the warm roof I wanted after all (105mm boards are a bit deep to stick on top of the rafters to create a warm roof).

    But I am confused. If the all-in-one screwfix boards have the equivalent of roofing felt stuck to the insulation boards, in a typical cold roof application I understood that ventilation would be required between the felt and the insulation within the rafters. So how do you think you are meant to ventilate the roof space with this product to prevent condensation?

    Any clues, or anyone used these before?

    Cheers
     
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  3. masona

    masona

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    For the 100mm x 50mm roof rafter you need the 55mm thickness to allow a 50mm air-flow at the back.
    The one you're talking about 105mm is suitable for 150mm x 50mm roof rafter to allow 50mm air-flow at the back.
    The advert is a bit misleading not explaining you need 50mm air-flow at the back to prevent condensation.
     
  4. purplechap

    purplechap

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    Thanks masona ;) I didn't realise you could ventilate the warm side of the insulation. I thought the whole point was to ventilate the cold side. (Or have I misunderstood what you meant by "the back"?)
     
  5. masona

    masona

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    Timber need to breathe without the risk of condensation of rotting & mildew on the roof rafters. Ventilation is important everywhere in the roof section.
    [​IMG]
    A good ventilation start at the soffit vent and circulate out to the top for replacement of air. If you do put insulation between the roof rafters then must have 50mm air-flow gap at the back and a approximately 100mm at the top and bottom row.
     
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  7. purplechap

    purplechap

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    Thanks masona (and for taking the time to include the picture). Always good to have a reply from masona - I think most here would agree.

    For the interest of discussion - if you have the time - I came across instructions from insulation manufacturers (referenced at the end) for a cold, unventilated roof application. Here it is critical to use a breather membrane rather than (non-breathing) felt (or similar) directly above the insulation boards. This allows any moisture in the rafters out through the roof. It is also essential to have a vapour control barrier directly below the insulation, and for all insulation to be tightly abutted and sealed together and with the wall cavity insulation. This discourages interstitial condensation within the roof in the first place.

    This can keep the thickness of the roof to a minimum and is seemingly a good compromise for those who were after a warm pitched roof where the ceiling follows the rafters.

    Breather membrane is a lot more expensive than standard felt, but it does away with the need for special roof vents (eg in the flashing) where the lean-to roof joins the main wall at the top. So I was going to try this option for the first time on my current job which is only a small porch.

    Finally the porch is not covered by building regs anyway (I checked) due to its size, so an opportunity to experiment unless you know any reason not to use this method.

    References:-

    http://www.recticelinsulation.co.uk/files/eurosarking-roofs.pdf

    http://www.celotex.co.uk/appl/PDF/SOL_BUR.pdf

    Cheers
    Paul
     
  8. masona

    masona

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    Yes you're right but what they won't tell you is that the breatheable membrane felt only guarantee up to 15 years, I've seen some rotted one. I have done many loft extension in very old properties and the roof rafters I've cut still smell like fresh wood because of no roofing felts and good ventilation. I've been told by my friend who's a surveyer said if you are going to put a insulation in the roof rafters then you must remove the ceiling insulation because of the increase of moisture condensation but I have not yet seen this. The breatheable roofing felt are suppose to be watertight but are they ? This is just my opinion that if you have a broken tiles and the idea of the roofing felt is to collect the rainwater to the gutter, with the breathable felt and the prolong of rainwater leaking I somehow think the rain will go through the felt over a period of time like a sponge effect and do more damaged to the internal roof section and you won't know for a long time because the rainwater is being soaked up into the insulation. I maybe old fashion but I still think we are insulation our property too much thus doing more harm than good. I generally believe this heat saving is coming from the government heat saving policies and now new home must have 10" insulation in the loft ceiling which doesn't make any real saving at all and now the owner's are moaning because you cannot stores anything in the loft unless the extra ceiling joists is added on. As you says do away with tiles vent but what I do is vented from under the front row of tiles with this
    [​IMG]
    then at the top depending the ridge finish I use this here which mean I can still have the non-breatheable felts and still keep the air gap behind the roof rafters insualtion.
    [​IMG][​IMG]
    I don't like the idea of a full insulation to the width of the roof rafters because air moisture need to get out in a reasonable time and I don't see how air can travel through insulation because I have tried this test by using a compress air motor and see if I can feel the air coming through the insulation and it was very very tiny amount went out. Remember this is all my own opinion & from past experience and not has any problem doing it my way as to what I've seen. You cannot beat a good ventilation !
    Off my head, normally you only need planning permission if the porch is going to be more than 3 sq metres, higher than 3 metres and is less than 2 metres away from the boundary & public footpaths but some area do differ slightly.

    These 2 website maybe of interest to you.

    www.knaufinsulation.co.uk and www.marleyroofing.co.uk
     
  9. purplechap

    purplechap

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    Masona.. thank you very much for the detailed response. I bow to your experience as, unlike you, I have not ever worked with roofs before. I will take your advice and ventilate!

    What you say makes sense. My house is an old victorian house with no insulation and very good ventilaiton in the roof space. It is in very good nick as a result.

    While I can understand the push to make us a more energy saving society, I heartily agree that over-zealous insulation causes more harm to a building's fabric than good.

    Thanks again
     
  10. masona

    masona

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    You can still insulated without going over the top and still insulated between roof rafters with the air gap at the back.
     
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