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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    Hi folks,

    Been busy renovating our 1960s bungalow and making the floor and walls airtight as we do it. We’ve now completed nearly half the house in terms of living area and need to consider sorting the roof space so we can get some insulation up there. As we’re aware that the airtight tight membrane is to go on the warm side, we have been waiting to be in a position to install that before insulating, but this will require better weather as we’ll need to be doing some of the work from the outside by removing roof tiles (shallow roof space).

    As we will be installing MVHR when we’ve finished all the building work, we’ve decided to install recycled-plastic insulation as the first layer (110mm) as we’ll need to be messing about with it to install vents.

    The question is, can we put the airtight barrier over this layer of insulation, across the ceiling joists and then pile on other insulation, probably a deep layer of fibreglass or rockwool, the MVHR ducts will be within this layer - we’re going to create a ‘warm area’ around the loft hatch where the MVHR equipment will be housed.

    It’s not the end of the world if we have to move the insulation out of the way to install the membrane if this is a complete no-no, but wanted to get a view if this would be ok as it will save time and it will be easier to install over the joists than around them.

    Many thanks in advance for any advice
    •  
      CommentAuthordjh
    • CommentTimeJan 11th 2024
     
    The airtight membrane can theoretically go anywhere in the wall or roof. What matters is that it is continuous. Also, if it is near the outside of the insulation then it should be breathable, whilst if it's near the inside it might need to be a vapour control layer as well. Details depend on the actual construction.

    Why not put your airtight membrane over the roof joists and ceiling first, and then put your insulation over the top? Or use a paint-on airtightness product under the ceiling?
  2.  
    Hi,
    We have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation. Due to the amount and type of work involved in that, it will have to wait until warmer weather. The house is very cold so the thinking is to chuck some insulation up there now which will be quick and will help retain some of our heat. Laying the membrane over that will be simpler than removing this insulation and dipping the membrane between the joists before putting the insulation back.
    Thanks.
    • CommentAuthortony
    • CommentTimeJan 11th 2024
     
    I would like to see a vapour barrier on the warm side to protect the joists from moisture, this could double as your airtightness layer. In the walls it could be the plaster or paint but needs to be hermetically sealed to it. The floor might need more thought.
    •  
      CommentAuthordjh
    • CommentTimeJan 12th 2024
     
    Posted By: GreenApprenticeHi,
    We have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation. Due to the amount and type of work involved in that, it will have to wait until warmer weather. The house is very cold so the thinking is to chuck some insulation up there now which will be quick and will help retain some of our heat. Laying the membrane over that will be simpler than removing this insulation and dipping the membrane between the joists before putting the insulation back.
    Thanks.
    I don't think I would compromise the long term plan for the sake of a smaller bill this winter. Either buy some warmer work clothes or turn the heating up. A fan heater in the loft would probably make it comfortable enough to work.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 12th 2024
     
    Might it be possible to lay the membrane with the insulation on top now, to the 90% that will be in the middle of the attic, and leave longer lengths of the membrane around the edges, to be tied in with the eaves detail later, when you get to that stage? It would be better on the warm side of the ceiling joist, as mentioned above, but you've got what you've got.
  3.  
    It's been mentioned on here a few times but I highly recommend this website to test construction build-ups (requires free registration to unlock all the features): https://www.ubakus.de/en/r-value-calculator/

    There's a function that advises on moisture risk. It's the much-criticised Glaser method, so very basic and nowhere near as good as WUFI, but as an initial check for peace of mind it's probably worth your time.
  4.  
    GreenApprentice - You said above that you will be installing MVHR later hence the recycled-plastic insulation. That implies ceiling ducts along with the ducting.
    Is the recycled-plastic insulation loose fill or blanket?
    If you put the air tight membrane over the joists etc. then when the MVHR is put in you will need to properly seal the vents. If the membrane is over the joists it can easily be lifted along with the blanket type insulation, the MVHR easily installed without the need for duct penetrations, then the membrane over the ducts and joists then the insulation over that. i.e. no insulation between the joists and then everything is below the membrane except the insulation which is above.
    Just a thought.
  5.  
    Thanks all! Some really great suggestions here, and enough to stop me deviating from the original plan.

    Posted By: Doubting_ThomasIt's been mentioned on here a few times but I highly recommend this website to test construction build-ups (requires free registration to unlock all the features):https://www.ubakus.de/en/r-value-calculator/" rel="nofollow" >https://www.ubakus.de/en/r-value-calculator/

    There's a function that advises on moisture risk. It's the much-criticised Glaser method, so very basic and nowhere near as good as WUFI, but as an initial check for peace of mind it's probably worth your time.


    I’ll take a look at that link :bigsmile:
  6.  
    Posted By: Peter_in_Hungary
    Is the recycled-plastic insulation loose fill or blanket?


    The one I was looking at is blanket on a roll, wide enough to fit between the joists. Loose fill may be a better option, I haven’t seen that when googling. Will take a look.

    I like the idea of putting the ducts under the membrane as it would save sealing ducts out and back into the thermal envelope.

    Thanks!
    • CommentAuthorMike1
    • CommentTimeJan 12th 2024
     
    Posted By: GreenApprenticeWe have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation.
    If you were to use a 'smart' membrane (Pro Clima Intello or similar), rather than the Airguard (which I don't think is), you could probably do this. It's one of the details proposed in a paper from Aarlborg University (but not by Pro Clima, AFAIK) - see p7 of this pdf: https://vbn.aau.dk/files/72606879/Installation_of_vapour_barriers_in_existing_buildings.pdf

    It's also possible that the plasterboard could act as the airtight layer, if crack-free and sealed at all junctions.
    •  
      CommentAuthordjh
    • CommentTimeJan 13th 2024 edited
     
    Posted By: Mike1
    Posted By: GreenApprenticeWe have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation.
    If you were to use a 'smart' membrane (Pro Clima Intello or similar), rather than the Airguard (which I don't think is), you could probably do this. It's one of the details proposed in a paper from Aarlborg University (but not by Pro Clima, AFAIK) - see p7 of this pdf
    You mean fig 10? I've been suggesting that GreenApprentice follows the bottom diagram with regards to the membrane but to continue the insulation over the top of the joists so the membrane is everywhere warm. I'm not sure exactly what his current plan is.

    edit to add: following my suggestion, it doesn't matter whether the membrane is intelligent because it's warm everywhere.
    • CommentAuthorMike1
    • CommentTimeJan 13th 2024
     
    Posted By: djhYou mean fig 10?
    Yes, the lower diagram in fig 10. Sounds like we're suggesting the same thing then.
  7.  
    the plan was to do the lower diagram, the difference is that the joists are only 7.5cm deep so there will be a lot of insulation above them. Unfortunately I bought the non-smart version of the airguard membrane, but djh is suggesting that so long as there is insulation above that, which there will be, then it will be ok

    We’ll also put a breathable membrane on top of the insulation as suggested by Tyvek


    Posted By: Peter_in_Hungary
    If you put the air tight membrane over the joists etc. then when the MVHR is put in you will need to properly seal the vents. If the membrane is over the joists it can easily be lifted along with the blanket type insulation, the MVHR easily installed without the need for duct penetrations, then the membrane over the ducts and joists then the insulation over that. i.e. no insulation between the joists and then everything is below the membrane except the insulation which is above.
    Just a thought.


    Thoughts on this - if we put something over the joists to support and protect the membrane (like cardboard?) there would be an air gap above the ceiling that the MVHR ducts sit in, with the airtight membrane above

    Would this work?
    • CommentAuthorMike1
    • CommentTimeJan 14th 2024 edited
     
    Posted By: GreenApprenticethe plan was to do the lower diagram, the difference is that the joists are only 7.5cm deep so there will be a lot of insulation above them. Unfortunately I bought the non-smart version of the airguard membrane, but djh is suggesting that so long as there is insulation above that, which there will be, then it will be ok
    Yes, with the insulation it will be fine.

    Posted By: GreenApprenticeWe’ll also put a breathable membrane on top of the insulation as suggested by Tyvek
    Presumably then you have a ventilated roof and the aim is to stop wind washing of the insulation. Which sounds fine, if Tyvek claim suitability for that (I've not read their datasheet).

    Posted By: GreenApprenticeThoughts on this - if we put something over the joists to support and protect the membrane (like cardboard?) there would be an air gap above the ceiling that the MVHR ducts sit in, with the airtight membrane above
    Two basic options:

    1) Keep the airtight membrane against the ceiling plasterboard and cut (and seal) air vents through it, with the ducts running horizontally through (and surrounded by) the insulation. Or
    2) create a service void above the insulation to run the ducts through, with the airtight membrane above as you describe. Plywood fixed to the top of the ceiling joist would be more robust than cardboard - 75mm deep joists would just about work with 75mm external diameter MVHR semi-rigid ducts, though you could add strips of ply on the joists to gain extra height where you have ceiling diffusers.

    However, IMHO a service void between selected joists that only accommodates MVHR ducts isn't adding a great deal (compared to one that extends across the entire ceiling), so I'd choose the 1st of these options.
    •  
      CommentAuthordjh
    • CommentTimeJan 14th 2024
     
    A service void between joists is only so much help. You'll almost certainly want to run some ducts at right angles to the joists in places. So I'd go with option 1 as well.

    I don't know what duct system you're thinking of using, but I can generally recommend semi-rigid, which have a smaller dimension as Mike1 suggests. If you do choose such a system then do think carefully about how you'll manage the ducts around the plenums (a.k.a. distribution boxes). There's an awful lot of pipes to get into a small volume! But that's pretty much their only complication; in all other respects they're much better.
  8.  
    There is nothing to say that insulation has to be laid on a flat plane. You could lay the vent pipes where you need them, between the joists and across the joists as required then put the membrane down over the lot then the insulation over that. To stop wind wash you need something that stops the ait interchange between what is in the insulation and what is free in the loft space but will let water vapour through, thick(ish) paper will do that without the expense of tyvek

    Will the air tight membrane be sealed at the edges? (of the membrane strips and the edges of the loft) if so how?
  9.  
    Thanks for the comments.

    Posted By: Peter_in_Hungary
    Will the air tight membrane be sealed at the edges? (of the membrane strips and the edges of the loft) if so how?


    Yes, been using tescon elsewhere but happy to consider other products, been looking at the Tyvek alternatives. There’s a lot of ‘loft wood’ to wrap and seal around also especially at the eaves. I like the double-sided butyl tape idea for the gable block walls at each end, but not sure my husband will be happy with having to clamp it

    Posted By: Peter_in_Hungary
    thick(ish) paper will do that without the expense of tyvek



    Like lining paper?

    Posted By: djh
    I don't know what duct system you're thinking of using, but I can generally recommend semi-rigid, which have a smaller dimension as Mike1 suggests.


    That’s what we’re going to be using from BPC
  10.  
    Posted By: GreenApprentice
    Posted By: Peter_in_Hungary
    thick(ish) paper will do that without the expense of tyvek



    Like lining paper?

    Yes - but something a bit wider than the standard wallpaper roll might be better
    On one job I did with 3 x 100mm glass wool I put the paper between the 2nd and 3rd layers figuring that although the last layer would get wind wash it was held much better by the 3rd layer than having nothing to restrain it. (It is a 36M x 6M loft with vents both ends)
    •  
      CommentAuthordjh
    • CommentTimeJan 16th 2024
     
    Posted By: GreenApprenticeThat’s what we’re going to be using from BPC
    Which system? You might want to start another topic to continue this discussion.
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