Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



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.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!


powered by Surfing Waves




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthorlineweight
    • CommentTimeMar 27th 2019
     
    Below is a detail shown in the TRADA book 'Timber Frame Construction".

    It shows a warm deck sandwich roof meeting a wall with a timber frame inner leaf.

    The roof joists are supported on top of the wall which means they bridge the wall's insulation layer.

    The note says: "Insulation between joists at perimeter of roof with vapour control layer to warm side of insulation"

    But how is that supposed to work? Unless I'm missing something, that implies that you are going to have to cut a rectangle of whatever your VCL membrane is, to fit in each section between the joists , and then fold and tape it along at least three sides to form a seal against the joist sides and the underside of the roof deck.

    Doesn't seem very practical to me - time consuming and introduces loads of points where the seal might not be made properly. Or am I misinterpreting?
      Screen Shot 2019-03-27 at 16.32.56.jpg
    •  
      CommentAuthordjh
    • CommentTimeMar 27th 2019
     
    I don't understand it either. The drawing appears to show the joists cut off where they meet the inner wall, with nothing supporting them. Then a load of insulation above the timber frame, and then a long piece of timber to which the overhanging joist stubs are fixed. In which case the VCL on the inside of the timber frame could just be run up inside the insulation. But I don't see how the roof would stay up!

    If the joists run all the way through then there's no point in a VCL between them, because the joists will pass vapour along themselves.

    Have you asked TRADA to clarify?
    • CommentAuthorlineweight
    • CommentTimeMar 27th 2019
     
    I didn't interpret it as cut off joists, as the note states insulation between joists. I think they have drawn that bit of insulation-between-joists in section, as if the section cut is just this side of the joist. Then the piece of timber on the cold side of that insulation is a kind of noggin.

    Either way it's not clear.

    Perhaps I should email TRADA with the question. I have the 5th edition of the book. I think that's the most recent?
    • CommentAuthorlineweight
    • CommentTimeMar 27th 2019
     
    Posted By: djh

    If the joists run all the way through then there's no point in a VCL between them, because the joists will pass vapour along themselves.


    Do you mean in a beneficial way (ie mosture in the insulation layer dissipate by passing along the joists) or non beneficial way (ie joists provide a route for moisture to move through to the cold side of the insulation layer)?
    •  
      CommentAuthordjh
    • CommentTimeMar 27th 2019
     
    Posted By: lineweightDo you mean in a beneficial way (ie mosture in the insulation layer dissipate by passing along the joists) or non beneficial way (ie joists provide a route for moisture to move through to the cold side of the insulation layer)?

    I wasn't particularly thinking of beneficial or non beneficial, just pointing out that timber is not a vapour barrier especially along the grain. Neither is it an air barrier along the grain either.
    •  
      CommentAuthordjh
    • CommentTimeMar 28th 2019
     
    Just to clarify what I was thinking more clearly. I didn't mean that timber is never airtight but that it can't be relied on. Any slight cracks - shakes - along the timber can allow the passage of air. And even if there are none when the timber is installed, they can appear later as it ages.
    • CommentAuthorlineweight
    • CommentTimeMar 28th 2019
     
    Posted By: djhJust to clarify what I was thinking more clearly. I didn't mean that timber is never airtight but that it can't be relied on. Any slight cracks - shakes - along the timber can allow the passage of air. And even if there are none when the timber is installed, they can appear later as it ages.


    Ok, yes I agree.
    • CommentAuthorlineweight
    • CommentTimeApr 11th 2019
     
    Looking at this again.

    Do they intend that the VCL is at the plasterboard ceiling layer?

    In which case it would be on the 'warm' side of that perimeter insulation.

    But that would be vulnerable to penetration with downlights etc.

    In any case I've just asked TRADA the question.
    • CommentAuthorlineweight
    • CommentTimeApr 16th 2019 edited
     
    So... for anyone that is interested:

    Someone at TRADA got back to me. Confirmed that the intention of the diagram is not to show the joists bearing on thin air - it's just the way it's drawn. They sit on the timber frame.

    As to the VCL placement though: basically, if I understood correctly, the message was not to really worry about VCL continuity through that zone of the joists. He agreed it would be impractical to try and cut a rectangle of membrane to go between each pair of joists. He pointed out that similar circumstances can arise at intermediate floors and other places. As he put it "there is no history of decay" at such details when the VCL is not continuous.

    He did mention that the same issue arises with any airtight layer, of course.

    If you look in the Accredited details (see extract below), the suggestion seems to be that the airtight layer has to be at ceiling level (and then you have the issue of penetrations.

    All seems a bit of a fudge.

    I'm interested in anyone's thoughts on this. Is there a better way to do it?
      Screen Shot 2019-04-16 at 14.53.22.jpg
    •  
      CommentAuthordjh
    • CommentTimeApr 16th 2019
     
    Posted By: lineweightAs to the VCL placement though: basically, if I understood correctly, the message was not to really worry about VCL continuity through that zone of the joists. He agreed it would be impractical to try and cut a rectangle of membrane to go between each pair of joists. He pointed out that similar circumstances can arise at intermediate floors and other places. As he put it "there is no history of decay" at such details when the VCL is not continuous.

    If he's not worried about VCL continuity then why bother with a VCL at all? It isn't impracticable to cut rectangles of membrane - there are people on this forum who have done it, IIRC. But it is easier to use a Tony tray or a ledger beam in new construction and avoid the problem altogether.

    I wouldn't expect to find decay at the detail especially. Vapour has this wonderful ability to travel and condense at the coldest point

    If you look in the Accredited details (see extract below), the suggestion seems to be that the airtight layer has to be at ceiling level (and then you have the issue of penetrations.

    The classical solution is to add some battens and put the surface finish (plasterboard etc) below that, creating a service cavity. Or simply don't have any light fittings in the ceiling, as we have done.
  1.  
    Posted By: djh
    If he's not worried about VCL continuity then why bother with a VCL at all?


    I guess because there's a "history of decay" in general constructions where there isn't? But I agree it doesn't seem logical.

    Posted By: djh It isn't impracticable to cut rectangles of membrane - there are people on this forum who have done it, IIRC. But it is easier to use a Tony tray or a ledger beam in new construction and avoid the problem altogether.


    I am always thinking about builder-proof details. Cutting rectangles would only work with a conscientious builder.

    Had to look up what a 'Tony Tray' is. Wouldn't work so well with timber frame like in the TRADA detail. But I can see that a ledger beam could work. VCL running behind it, and only punctured at the points where the ledger beam is bolted back to the wall framing. It could then get as far as the underside of the ply deck.

    Posted By: djhI wouldn't expect to find decay at the detail especially. Vapour has this wonderful ability to travel and condense at the coldest point

    The classical solution is to add some battens and put the surface finish (plasterboard etc) below that, creating a service cavity. Or simply don't have any light fittings in the ceiling, as we have done.


    That always seems inefficient to me - you have two separate ceiling voids wasting ceiling height, unless you move to a flat roof buildup that has some insulation between the joists. And not having any ceiling fittings still leaves it prone to being compromised in the future when someone decides they want to put some in.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: lineweightAnd not having any ceiling fittings still leaves it prone to being compromised in the future when someone decides they want to put some in.

    It's well-nigh impossible to idiot-proof anything. The manual for the house should describe the air/vapour barrier and explain why its integrity is important. And perhaps for good measure emphasise that making holes in it compromises its integrity? :bigsmile:

    But everybody has their own idea of what's acceptable and what the best compromise is. It's very rare that we all agree on something, even on here where we supposedly all share similar intentions about building. So preserving a building is all about individual responsibility, IMHO, and that involves taking the time to understand what the issues are.
  2.  
    Almost no-one expects there to be a 'house manual' or to read it.

    Instead they'll make minor alterations themselves where they seem 'normal', or perhaps consult a builder for other stuff who is likely also to make assumptions about what's conventional.

    Most people would consider making a hole in a plasterboard ceiling with airspace above it as non-controversial, whereas almost no-one would make a hole in a plywood deck. Therefore while it's not possible to fully idiot-proof anything, having the VCL at deck level vs ceiling level seems a lot *more* idiot proof.

    Of course, the appropriate solution is very different depending on whether you're designing/building your own home vs. designing something to be built someone else and occupied by someone else again.
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    As I said, everybody has their own idea of what's acceptable and what the best compromise is. :bigsmile:
Add your comments

    Username Password
  • Format comments as
 
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press