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Housing - New Build: over fascia vent

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  1.  
    • CommentAuthordebbiel
    • CommentTimeMay 26th 2017
     
    Help please, I'm feeling confused about insulation/condensation.

    My timber frame house was originally designed to be single storey with non habitable attic space. The roof insulation was going to be pir or sheeps wool.

    While the timber frame was being built, we were offered a good deal on attic trusses, so decided to make it habitable attic space.

    Downstairs in the walls we have a lovely mix of woodfibre insulation between studs, VCL, then woodfibre boads. Internal walls have thermafleece for acoustic insulation.

    At some point we were persuaded to use Actis Hybrid in the roof. Not sure in hindsight that this was the best option, but we went ahead with the roofing being done with Boost R Hybrid as the underlay. The roofers fitted over fascia vents and Marley dry ridge system for the clay tiles. Over fascia vents were shown on the building regs drawings, but that was for the original insulation. I was told the roofers had experience with Actis, so assumed they were correct in still fitting them.

    We are now about to do the between the rafter stage of insulation with Actis Hybris (there will then be a VCL on the face of the rafters). My dilemma is that Actis are now saying we shouldn't have the fascia vents as it will allow airflow in the gap between the underlay Boost R Hybrid and the Hybris,, which could cause condensation. They are recommending we cover the vents with mineral wool.

    I've read so many places that vents are important but now I've tried to research it and I'm even more confused. So please anyone who has an opinion share it here, because generally you guys explain things really well. (I'm a first time self builder, generally going well, just feeling overwhelmed with this bit!) Thanks!
    •  
      CommentAuthorfostertom
    • CommentTimeMay 26th 2017 edited
     
    Can you make a little drawing of how the various layers go at the eave? I'm sure the answer wil be clear.
  3.  
    Posted By: debbielMy dilemma is that Actis are now saying we shouldn't have the fascia vents as it will allow airflow in the gap between the underlay Boost R Hybrid and the Hybris,, which could cause condensation. They are recommending we cover the vents with mineral wool.

    ang on a mo !! mineral wool is not air tight so covering the vents with mineral wool will still allow a (reduced) air flow.
    But what FT said - a sketch would be helpful
    • CommentAuthordebbiel
    • CommentTimeMay 26th 2017
     
    Here's my Friday night attempt at recreating my roof. Hope it makes sense, but let me know if not. I should mention, in case it makes a difference, that this dilemma applies to the main roof which is habitable, and a small attached gable end which isn't habitable, but we're doing it all warm roof construction. It's the red arrows that show the air flow they're concerned about.
    • CommentAuthordebbiel
    • CommentTimeMay 26th 2017
     
    mineral wool is not air tight

    Ha Peter, yes, another reason why I don't have much confidence in their tech dept!
    •  
      CommentAuthorfostertom
    • CommentTimeMay 26th 2017
     
    So both the Hybris and the Boost'r Hybrid membrane contribute to insulation. So yes, fatal to have ourtside air flowing between - must be still air or no air gap. With that air flow, the Boost'r will contribute nothing to insulation - might as well use roofing felt.

    It's the space above the Boost'r, under the tiles, that needs to be ventilated. looks like the Hybris leaves a 25mm (?) gap above it, to rafter top surface? Oh dear, I was going to say lay the Boost'r to drape (sag down) into that space, so there's a ventilatable gap above the Boost'r - but I see it's too late. No point in ventilating above the Boost'r, as the under-tile space is blocked by every tiling batten.

    As the Boost'r is 'breather' and so presumably is the Hybris, left without a VCL under the rafters there would be plenty of water vapour passing up through, needing to be ventilated away. But no ventilation so you'll be relying entirely on the perfection of the VCL, to prevent water vapour passing upward from the habitable loft space. That will be your challenge - and making the Building Inspector happy.

    At any rate, make the Boost'r drain out under, not over the eaves vents (if you leave them in place) - you may have to lift the lower tiles and alter things. Or seal them airtight - as said above, Rockwool won't do that.
    •  
      CommentAuthordjh
    • CommentTimeMay 26th 2017
     
    OK I'll have a go. Thanks for the diagram Debbie, that gives us something definite to discuss.

    As I understand the data sheets at http://www.insulation-actis.com/produitsactis.php?p=3&l=3&rub=58&vert=2 both the solid blue Boost R Hybrid and the blue-crossed Hybris are airtight, and both require an airspace on either side to achieve their claimed insulation values.

    Looking at the drawing from the top:

    - tiles are fine

    - battens are fine

    - there seem to be some missing counterbattens - these are necessary to allow rainwater that gets behind the tiles to flow freely downhill to the eaves, and they also assist with the free flow of air up the roof (ventilation) to evaporate condensation and other liquid water. If they aren't there, water will collect above the battens and cause trouble.

    - the Boost R Hybrid has an air gap on top and below which is good.

    - the air gap below the Boost R Hybrid must NOT be ventilated via vents whether blocked with rockwool or not. The air temperature below the Boost R Hybrid is intended to be and hopefully is warmer than outside air.

    - the Hybris has an air gap above it, which is good, but seems to have something solid below it - coloured brown - which is not good.

    - there's a VCL on the inside of the Hybris, which appears to be unnecessary, since the Hybris has an Sd of over 90 m, contrary to Tom's assumption.

    So there seems to be quite a lot to redesign. If the drawing is correct, the roof will need to come off for starters.
  8.  
    Posted By: djhSo there seems to be quite a lot to redesign. If the drawing is correct, the roof will need to come off for starters.

    Aghh ! But probably right.
    The cheap option would be to leave things 'as is' and loose the benefit of the Boost R Hybrid breathable membrane EXCEPT that with no counter battens as said above any water /debris on the membrane gets stuck, (unless the membrane has a decent sag to allow drainage.

    IMO the roof should never have been installed with out counter battens and again just IMO you would have a claim against the designers or roofers, who ever took the decision not to use counter battens.

    If the counter batten issue can be resolved then as FT said put the Boost R Hybrid under the vents. But even then you are relying on the fascia and soffit being air tight and remaining so - perhaps a big ask!
    • CommentAuthordebbiel
    • CommentTimeMay 27th 2017
     
    Yikes!

    You're all correct, there were no counter battens used. The reason for that is in the installation guidelines

    http://www.insulation-actis.com/docspdf/BOOSTR-HYBRID-INSTALLATION-GUIDELINES.pdf

    which states that where it is a non boarded roof, the membrane can be draped over the trusses with no need for counter battens. So that it is why it was done - whether that realistically leaves enough drainage space I don't know.

    Fostertom, we have been advised by Actis to leave a gap between the Boost R and the Hybris of around 25mm, and then another gap between the hybris and VCL (although we were also told it 'doesn't really matter if they touch, it just reduces the thermal values a bit'). If sealing the air vents is the way we go, could it be done with tape, or plastic strips taped in place?

    djh, I think it's my drawing that's confusing re the solid brown bit! It's supposed to be the side view of the timber truss, with the hybris where it will be attached with the gap above and below - so you're seeing the timber behind it. Hope that makes sense. I don't understand 'the Hybris has an Sd of over 90 m' - what does Sd mean? Fostertom seems sure I need a VCL, djh thinks otherwise - any chance of a rematch from you both!

    The drainage from the Boost R goes over the vents via underlay support tray into the gutter. I'll have to have a look to see how tricky it would be to change this.

    So, it seems worse case scenario is that this all leads to condensation, but what is the actual result of that - uncomfortable living conditions, rotting timber frame, or anything else I haven't thought of?

    Thank you all for your comments, I really do appreciate them, even if I'm now wishing I'd never met these Actis guys...
    • CommentAuthordebbiel
    • CommentTimeMay 27th 2017
     
    I forgot to mention that one idea Actis came up with to stop the airflow was to staple the Hybris up to the Boost R at the bottom to 'seal' the gap. But I wasn't convinced by that one.
    • CommentAuthordebbiel
    • CommentTimeMay 27th 2017
     
    I've had an idea. It's possibly crazy, but let me run it past you all.

    How about I staple a shortish length of Boost R onto the top of the wall (roof side) then take it across the eaves cavity and tape it to the underside of the Boost R that is already installed? Thus cutting off the eaves cavity with its eaves vents and any other weaknesses. There would be room on the wall cos it's 140mm and the Hybris is 120mm.

    It would have to be done in sections of 600mm widths between the trusses.

    I've shown it in purple on the drawing.

    What do you all think?
  12.  
    Posted By: debbielThe reason for that is in the installation guidelines

    http://www.insulation-actis.com/docspdf/BOOSTR-HYBRID-INSTALLATION-GUIDELINES.pdf" rel="nofollow" >http://www.insulation-actis.com/docspdf/BOOSTR-HYBRID-INSTALLATION-GUIDELINES.pdf

    which states that where it is a non boarded roof, the membrane can be draped over the trusses with no need for counter battens.

    So does the membrane 'drape' over the trusses? i.e. IMO this would mean at least 25mm sag (clear space) mid way between the trusses. If not then the membrane was not installed as per the guidelines. If you do have the necessary sag then it should be OK.

    Posted By: debbielHow about I staple a shortish length of Boost R onto the top of the wall (roof side) then take it across the eaves cavity and tape it to the underside of the Boost R that is already installed? Thus cutting off the eaves cavity with its eaves vents and any other weaknesses. There would be room on the wall cos it's 140mm and the Hybris is 120mm.
    It would have to be done in sections of 600mm widths between the trusses.

    The problem with that idea is that you are expecting the tape to stick to wood and other things to make an air tight joint - It doesn't! It looks OK when you do it but after 6 months or 2-3 years (depending upon the tape) the adhesion dries out, especially with the temperatures under tiles, and the stick fails. Oh and staples won't make an airtight joint.

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