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Housing - Renovation: Are eaves vents or soffit vents needed after MVHR and vapour layer

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  1.  
    • CommentAuthorstonecold
    • CommentTimeNov 25th 2024
     
    Currently doing a retrofit - lots of insulation, cold roof in attic, MVHR will need to ventilating attic, external solid wall insulation goes up to roof line, and we will be fitting a vapour barrier between upstairs / bedrooms and attic - which will prevent damp from the house getting into the attic. EWI is vapour open but wind tight.

    We are getting re-roofing and a breathable felt put in fairly soon. Obviously loft insulation will be on loft floor, with some rafter insulation only where there are sloped bedroom or bathroom ceilings below (room in roof).

    Will we be able to block eaves ventilation then? What about soffit intake? Are ventilated roof tiles / slates a possible alternative?

    We want to avoid attic condensation. What do EnerPHit standard houses do?
    • CommentAuthorbhommels
    • CommentTimeNov 25th 2024 edited
     
    What kind of roof coverings do you have? Slates, interlocking tiles, traditional tiles?
    From what you describe you have a 'ventilated roof' which means there are deliberate openings (often due to the tile structure) to have a lot of air swirling in your roof space. When adding insulation at ceiling level, there is no need to change anything, assuming that the roof covering itself (tiles) will stay the same or get replaced by something similar.
    When insulating sloping ceilings, make sure you retain a >25mm air gap between the membrane and the insulation.

    Unventilated roof spaces need a plan for air circulation, often involving soffit or fascia vents and a 'dry ridge' or some other means to expel (rising) air at ridge level. [edit: soffit -> soffit or fascia]
  3.  
    bhommels said: "When insulating sloping ceilings, make sure you retain a >25mm air gap between the membrane and the insulation."

    25mm had been common practice since the 2010 Building Regs brought in the requirement for insulation upon re-roofing. It's worth noting, though, that the BEIS Best Practice for rooms-in-the-roof states (on p.21):

    ''Figure 4: Typical through ventilation of rafters and roof void
    Retrofit designer: A minimum of 50mm ventilation gap should
    be retained between insulation and membrane...''

    https://assets.publishing.service.gov.uk/media/61d727d18fa8f50594b59305/retrofit-room-in-roof-insulation-best-practice.pdf

    (First published Jan 2022)

    I have seen a number of examples where roofs on properties with a 25mm vent path only have had mould growth and moisture issues, though to be fair I have seen many which haven't, and in some of those that had, poor detailing may have been an issue.
  4.  
    ''Will we be able to block eaves ventilation then? What about soffit intake? Are ventilated roof tiles / slates a possible alternative?''

    Don't block the cold roof ventilation!

    ''We want to avoid attic condensation. What do EnerPHit standard houses do?''

    If the ventilation paths and insulation are detailed correctly there is no serious risk of thermal by-pass. That is, the air you want to ventilate the void will not get into the 'warm bits' unless you allow it to.

    Wherever possible I use eaves vents - approx 15-25mm clear opening - over the whole width of the property at front and rear. Good practice is to staple some wind-tight membrane up the underside of the rafters at low level so that you form a 'fabric tube' for the incoming and outgoing air and that air cannot get between the layers of insulation.

    The resultant 'howling gale' should ensure that you have little or no condensation, even in the coldest parts of the year.

    Some suggest ridge vents too but I have never found them essential.

    I am not sure what you mean by ''MVHR will need to ventilating attic,''.

    The MVHR will be ventilating the habitable space, not the cold roof void, hence the ventilation strategy you (and I) described.

    P.S: Have read your OP better, and note that you do not have rooms in the roof, so ignore my first post, though others may find it useful if they do have rooms in the roof or even small areas of sloping ceiling.

    I have also never found tile/slate vents particularly successful, not least since too few are often put in.
  5.  
    Keep noting other bits! While you re-roof are you taking insulation across the wall-plate so that EWI touches roof insulation, mitigating, though not wholly avoiding, the thermal bridge?
    •  
      CommentAuthordjh
    • CommentTimeNov 26th 2024
     
    My guess at ''MVHR will need to ventilating attic'' is that he means the MVHR will need to be placed in the cold attic. If that's what it does mean, then it needs an insulated, airtight box to surround it with access and enough space for servicing it (filters every 3 months with ours). The insulation needs to be as good as the rest of the house. Ditto for any ducts run in the attic.
  7.  
    djh said: '' If that's what it does mean, then it needs an insulated, airtight box to surround it with access and enough space for servicing it (filters every 3 months with ours). The insulation needs to be as good as the rest of the house".

    Exactly: The "MVHR Shed" - a highly insulated box for the unit and for all the duct runs.
    • CommentAuthorphiledge
    • CommentTimeNov 26th 2024
     
    Definitely keep the eaves vents as they'll help deal with any moisture ingress through the roof covering or if someone breaches the vapour barrier in the ceiling below
    • CommentAuthortony
    • CommentTimeNov 27th 2024
     
    Also as warm air in a roof void cools at night condensation can occur and this needs to be ventilated away.

    One thing about roof ventilation that thr vast majority don’t understand is that water vapour can move through them without any air movement happening!
    • CommentAuthorgravelld
    • CommentTimeDec 1st 2024 edited
     
    If EWI is installed and it's taken over the wall plate, filling out where the soffit would be in the eaves, how much space does that leave for soffit vents?

    Or, if a minimum 25mm gap has to be maintained does that imply at least a 25mm soffit vent is required... is this just a continual mesh rather than a solid material with vents in it?
  11.  
    Gravelld wrote:

    ''If EWI is installed and it's taken over the wall plate, filling out where the soffit would be in the eaves, how much space does that leave for soffit vents?

    Or, if a minimum 25mm gap has to be maintained does that imply at least a 25mm sofft is required... is this just a continual mesh rather than a solid material with vents in it?''

    It depends on the individual circumstances. If the existing soffit is not sufficient to accommodate the depth of EWI you require you'd have to extend the roof-line, but yes, if you have, say, a 200 soffit and 150 EWI you'd install some sort of vent. In terms of 'free area' of ventilation if I remember correctly the actual 'holes' should equal the free area of a 15mm 'slot' across the whole roof-line.

    If amendment of the roofline is required and the house is detached one could put 'sprockets' on to give a 'kick' to the roof-line (i.e. flatten it out slightly), potentially to accommodate a 'fatter' slice of insulation over the wall-plate.
    • CommentAuthorgravelld
    • CommentTimeDec 4th 2024
     
    Thanks Nick. Nice tip with the 'sprockets'.

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