Air gap for rafter insulation

Discussion in 'Builders' Talk' started by Steve Stiffler, Jan 12, 2008.

  1. Steve Stiffler

    Steve Stiffler New Member

    I`m just converting my loft space and am starting to insulate the roof between the rafters,they are 75mm deep,i have a few 50mm kingspan boards so i cut a few lenghts and tried them in,obviously i only have a 25mm gap at best.
    Just had a look this morning and there is bad condensation on the underside of the felt,it is very wet and causing water to drip down.
    Would this be gone if the air gap was 50mm? also if it was to fix it would i just have to fix 1" battens to the existing rafters ?
  2. chip off the block

    chip off the block New Member

    you need 50mm but you needto have 25mm continuous eaves vents aswell as what use is an air gap with no air flow
  3. Steve Stiffler

    Steve Stiffler New Member

    I`ve only got a few pieces in and they are not down to the eaves,they start at the ridge and are about 4 feet down so there is plenty of airflow it must just need the full 50mm
  4. Steve Stiffler

    Steve Stiffler New Member

    Looks like the best idea for me is the foil type insulation,something like xfoil or tri iso 10,it`s a lot thinner about 25-30mm and equivilent to 200mm of rockwool.
    Does anybody know a good supplyer of these foils?
  5. kaintheo

    kaintheo New Member

    Do you have clear eaves vents? If so then you may find that the ridge will need to be vented too. It's very hard to achieve a good air flow from eaves to eaves on all but the smallest of roofs.

    Also remember that you haven't installed the plasterboard yet so you are bound to get more moisture coming into contact with the under felt. It's also worth remembering your roof is no different than a timber frame wall in a house except it's on a pitch. All timber frame wall's have a vapour barrier on the warm side of the insulation. If the moisture can't get to the cold surface, then it can't condensate.

    Hope this helps.

    P.S Try C.C.F or Sheffield insulations for you're materials.
  6. kaintheo

    kaintheo New Member

    Reference my above post.

    The roof is similer to a timber frame wall if you have a breathable membrane which needs an air flow above it to work effectivly. In any case, a vapour barrier should do the trick with ridge ventilation.
  7. J4M35

    J4M35 New Member

    You can get vents that can be installed from the inside and cant be seen from the out side.
    so if u decide to use vents at the top near the ridge then it might be worth doing a search and finding the post.

    saves u having to get up on the roof.
  8. footdeeper

    footdeeper New Member

    Water vapour is under pressure and it is condensing on the cold surface of the felt. To form a vapour barrier that works is almost impossible so although the advice about forming a ventilated space at the back of the insulation is good you should try to cut down the amount of water vapour in the roof space. Extract fans at the vapour source will help eg cooker hood extract, shower cubical extract etc. I'm afraid modern living creates lots of water vapour and you are in the same position as most other loft dwellers.
  9. kaintheo

    kaintheo New Member

    diesel diesel diesel,

    How have you worked out that the water vapour is under pressure?
  10. Joatmojo

    Joatmojo New Member

    The retrofitted roof vent you are after can be viewed at - saves a lot of bother, no need to scramble about messing the roof up on the outside.
  11. footdeeper

    footdeeper New Member

    The bad news Kaintheo is that I spent too much time at college - too much knowledge and all that! Water vapour does exert a pressure - it always tries to balance high and low humidity - same idea as Osmosis balancing weak and strong solutions in your biology days.
  12. ­

    ­ New Member

    Equilibrium vapor pressure, Saturation vapor pressure, also known as vapour pressure, is the pressure of a vapor in equilibrium with its non-vapor phases. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back into their original form (either liquid or solid). At any given temperature, for a particular substance, there is a pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. This is the vapor pressure of that substance at that temperature. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of molecules and atoms to escape from a liquid or a solid. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The Kelvin equation shows how equilibruim vapor pressure depends on droplet size.

    An example is water vapor when air is saturated with water vapor. It is the vapor pressure usually found over a flat surface of liquid water, [1] and is a dynamic equilibrium where the rate of condensation of water equals the rate of evaporation of water. In general, the higher the temperature, the higher the vapor pressure. When air is at the saturation vapor pressure, it is said to be at the dew point. Thus, at saturation vapor pressure, air has a relative humidity of 100% and condensation occurs with any increase of water vapor content or a reduction in temperature.

    So there! :^O
  13. kaintheo

    kaintheo New Member

    diesel diesel diesel,

    yes I,m aware that vapour exerts a pressure but you are suggesting that this is so great that vapour barriers do not work. If that is the case then all warm roofs and timber framed buildings are doomed.

    Without being over scientific, the solution is a simple combination of both mine and your suggestions.

    Extraction in kitchens and bathrooms.
    Vapour will not condense if it doesn't touch a surface that is a lower temperature, therefore a vapour barrier does work by keeping it on the warm side of the insulation. Any that does get through is dispersed by the ventilation, and this is why you must use a breathable/waterprrof membrane on timber framed walls or warm pitched roofs with a full fill insulation between the rafters.

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