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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthortonya
    • CommentTimeApr 30th 2021
     
    I want to insulate a garage roof that I had covered with fibreglass onto board, attached to rafters I put up. I want to
    add insulation between rafters 500 mm with gap between it and the roof. Currnetly the roof does not condensate but slightly worried if I add foli backed insulation it may. Any thoughts very welcome.
    • CommentAuthoran02ew
    • CommentTimeApr 30th 2021
     
    Try to create an airflow between the top of the insulation and the underside of the roof decking board, a ventilated 50mm gap. This could be done with soffit vents drilled either end? Or remove the fascia board cut it down by 25mm and prefix over fascia vent on top before reducing.and existing noggins will need to be drilled or reduced in height to allow air flow over.
    • CommentAuthoran02ew
    • CommentTimeApr 30th 2021
     
    Also use foil back plaster board or other vcl before plaster boarding.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    What Andrew says. Another alternative might be to insulate on top of the fibreglass (an inverted roof) depending on exactly how it's arranged.
    • CommentAuthortony
    • CommentTime7 days ago
     
    If you insulate the roof will be slightly colder and so more likely to gather condensation this can be drawn from indoor or outdoor air and ventilation is there so that it can disperse. A vapour barrier will reduce but not eliminate for cold roofs. Consider doing warm roof on top.

    I live the idea of 500mm insulation - that is what I have in my loft
    • CommentAuthortonya
    • CommentTime7 days ago
     
    Thank you for your comments and apologies for not responding before. The fibreglass has been fitted (not be me-i Traded it for doing his website!). So I can't remove it. It overlaps the facia board and hangs into or just above gutter so I can't really modify that side with an air vent.
    Tony, I appreciate what you are saying re the temperature of 'void' will increase with insulation so maybe the re will be condensation...
    Since I wanted toi insulate inbetween all the joists as opposed to running across joists (merely for aesthetics as I want to see the joists) its a lot of work and undoing if it condensates badly!
    Maybe to be safe I could just insulate one area between two joists and wait and observe-just screw the plasterboard and take down and have a look. Lots of other work to do.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: tonyaThe fibreglass has been fitted (not be me-i Traded it for doing his website!). So I can't remove it. It overlaps the facia board and hangs into or just above gutter so I can't really modify that side with an air vent.

    Not sure who you're replying to, but I don't think anybody has been suggesting removing the fibreglass. I certainly wasn't and I don't think Tony was either. Insulation on top of the waterproof layer is called an 'inverted roof' and is what I was talking about. Yes, the insulation gets wet, but it doesn't care.

    Since I wanted toi insulate inbetween all the joists as opposed to running across joists (merely for aesthetics as I want to see the joists)

    Doesn't sound like a good reason to me. If the particular aesthetics are of such a concern to you, then do the insulation first and add extra 'joists' afterwards where they will be visible.

    Maybe to be safe I could just insulate one area between two joists and wait and observe-just screw the plasterboard and take down and have a look.

    I'm not sure that will tell you much as it may be difficult to guarantee that the conditions inside your sample are the same as will be if you did the whole job.
    • CommentAuthortonya
    • CommentTime7 days ago
     
    I appreciate what you are saying in all of your comments djh. Not quite certain about the inverted roof? Do you mean that you create a sealed unit under the fibreglass roof so no water can get into the room? And that you ensure that the gap inbetween the fibrglass roof and the inner roof is airrated properly?
  1.  
    Tonya,

    you write: ''I want to
    add insulation between rafters 500 mm with gap between it and the roof. ''

    Unless you have among the fattest rafters I have ever seen, do you perhaps mean 50mm? 500 + an air gap would be *two feet*. Is that really what you mean?

    If you have a 'warm roof' you don't have to worry about ventilation. ventilation is to whisk away water vapour before it can condense on the underside of the wooden deck. If you have a 'warm roof' - the insulation above, rather than below, the deck - the deck is in the warm zone and 'safe'. You don't need a gap. A true (not 'hybrid') 'warm roof' has no insulation under the deck. Does that make sense?
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: tonyaNot quite certain about the inverted roof?

    I mean something quite specific. My hope was that you would search for inverted roof and understand it. Maybe you could start at https://www.designingbuildings.co.uk/wiki/Inverted_roof
    • CommentAuthortonya
    • CommentTime5 days ago
     
    <blockquote><cite>Posted By: Nick Parsons</cite>Tonya,

    you write: ''I want to
    add insulation between rafters 500 mm with gap between it and the roof. ''

    Unless you have among the fattest rafters I have ever seen, do you perhaps mean 50mm? 500 + an air gap would be *two feet*. Is that really what you mean?

    If you have a 'warm roof' you don't have to worry about ventilation. ventilation is to whisk away water vapour before it can condense on the underside of the wooden deck. If you have a 'warm roof' - the insulation above, rather than below, the deck - the deck is in the warm zone and 'safe'. You don't need a gap. A true (not 'hybrid') 'warm roof' has no insulation under the deck. Does that make sense?</blockquote>

    Yes this makes sense entirely and yes 50mm.
    Its too late adding insulation above 'the deck' as the fibreglass has been added.
    My feeling is that if I insulate below the rafters- I have some rolls of multi foil that I could use-and then plasterboard over the fil it will not condensate. I used similar method in a annex in an old building in Dorset I owned and there was no problem with condensation. I think the fibregalass together with the substarte boarding won't get so cold as to cause an issue. My experience is that concrete block work causes issues of condensation especially around window reveals. Given that I have the foil more than enough- it won't cost ma naything to do the work and will only take a day max. I can then observe the outcome.
      garage roof.jpg
    • CommentAuthortony
    • CommentTime5 days ago
     
    No, you can add insulation on top of the waterproof layer, roof joists look rather slender, what is their span?

    A green roof may be a possibility
  2.  
    Is this garage heated, any source of moisture inside?

    If not, then no problems with condensation on the OSB deck even on starry nights, it won't mind.

    If so, 50mm of multifoil will not be enough insulation value. Try laying insulation boards on top of the fibreglass, and clad over them with something just to protect from wind and rain.

    How were you thinking of ventilating the gap between the rafters?
    • CommentAuthoran02ew
    • CommentTime5 days ago
     
    Posted By: WillInAberdeento protect from wind and rain.


    And birds
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