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    • CommentAuthortimmywo
    • CommentTimeDec 5th 2017
     
    Hello Everyone,

    I am based in South Poland where the weather can be +30c in summer and -30c in winter. We are in the process of renovating a house and rebuilding the main log cabin section.

    We have foundations in and now using the Winter to plan, current topic is wall insulation and space heating requirements.

    Please see the SketchUp screen grab of an example wall section, the layers are as follows...

    Top OSB
    A: Cladding with air gap
    B: House wrap, Vapor Permeability OSB > Cladding
    C: OSB
    D: 25cm ESP insulation, approx R36
    C: OSB
    G: Internal wrap, Vapor Permeability OSB > Inside
    H: Wall finish

    Bottom Horizontal Logs (width approx 20cm)
    A: Exposed log face with chinking
    B: House wrap, Vapor Permeability direction undecided
    F: 20cm Mineral Wool Insulation, approx R32
    G: Internal wrap, Vapor Permeability Mineral Wool > Inside
    H: Wall finish

    Roof
    I: Metal roofing with air gap
    B: House wrap, Vapor Permeability OSB > Cladding
    C: OSB
    D: 25cm ESP insulation, approx R36
    C: OSB
    G: Internal wrap, Vapor Permeability OSB > Inside
    H: Ceiling finish


    My questions for the community...
    1. I am undecided on the Log cabin wall if the House Wrap should let vapor out (possible wet logs) or in (thought the Mineral Wool into the house)
    2. Now that I know the approx R-value of walls, ceiling, etc - how do I start to calculate the heat requirements for the space for underfloor heating, log burner and HRMV?
    3. Any other feedback on the design?

    Many thanks in advance,
    Tim
      wall section.PNG
    • CommentAuthorgravelld
    • CommentTime7 days ago
     
    The gap inbound of the OSB at the eaves needs filling?
    • CommentAuthortimmywo
    • CommentTime7 days ago
     
    Yes, sorry should have said that gap will be filled - did not want to mess around in SketchUp to fill the gap that's all!

    How on earth do you subscribe to thread updates on here!?
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: timmywoHow on earth do you subscribe to thread updates on here!?

    You don't, in the sense you're thinking of. There is an RSS feed or the really low tech way that I use is just to open the All Discussions page - http://www.greenbuildingforum.co.uk/newforum/ - and that shows you what is new provided you are logged in.


    I found the diagram and labelling too confusing to comment on, sorry.

    You will certainly need to let moisture out at the outside because of the cold winters.

    You may also need to let moisture out at the inside if you have air-conditioning and keep the place significantly cooler inside than outside during the summer. If you don't have air-conditioning then you should be OK with a VCL inside if you wish. Or breathable designs can be used whatever the circumstances, but need careful design.
    • CommentAuthortimmywo
    • CommentTime6 days ago
     
    Thanks djh - guess ill keep this thread open and refresh the page now and then.

    If you wish you can ignore the diagram, the sections I have listed are from outside in, for example...

    Bottom Horizontal Logs (width approx 20cm)
    Layer 1: Exposed log face with chinking
    Layer 2: House wrap, Vapor Permeability direction undecided
    Layer 3: 20cm Mineral Wool Insulation, approx R32
    Layer 4: Internal wrap, Vapor Permeability Mineral Wool > Inside
    Layer 5: Wall finish

    For this, Layer 2 is where I am confused in regards to what direction I should allow Vapor to move
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Layer two must be permeable, so vapour can escape in winter and perhaps also in summer. You must have at least one permeable side so any water that does get into the wall somehow has a means to escape. If you have cold winters, that needs to be the outside.

    Layer four is often impermeable (a VCL) in temperate climates but that can cause problems if air-conditioning is used. Because then vapour can enter through the permeable layer on the outside and condense on the cooler VCL inside.

    A 'breathable' design avoids those problems but usually needs careful modelling to be sure that water doesn't accumulate within the wall. So-called 'intelligent' (variable permeability) VCLs can also help resolve such problems.
    • CommentAuthortimmywo
    • CommentTime4 days ago
     
    Many thanks djh,

    There will be no air-conditioning, only MVHR.

    If Layer 2 allows vapor to escape out, would I need an air gap between Layer 1 (Exposed log face with chinking) and Layer 2 (House wrap)?

    I was hoping to avoid an air gap here so that the Mineral Wool would be right next to the logs and I could benefit from the R value of the logs. Also keeps the wall design simple.

    I was thinking to be sure to keep the log wall dry on the inside that I would allow vapor to escape into the building, ie... House wrap > Mineral Wool > Internal wrap
    Or, is that just asking for trouble?


    Thank you for your assistance, this is all very helpful, there appears to be little information I can find in regards to insulating a log cabin.
    •  
      CommentAuthordjh
    • CommentTime3 days ago
     
    Posted By: timmywoThere will be no air-conditioning, only MVHR.

    That's good; it simplifies things.

    If Layer 2 allows vapor to escape out, would I need an air gap between Layer 1 (Exposed log face with chinking) and Layer 2 (House wrap)?

    I can't answer that question myself. The answer may depend on local regulations (UK regs mandate air gaps in many cases, for example). Otherwise I think you would need to do (or have somebody else do) a condensation analysis using software such as WUFI. Insulation suppliers are sometimes willing to do them for customers and some consultancies will do them for a fee. Make sure they use dynamic simulation programs (BS EN 15026:2007) and not 'traditional' calculations, which are not very accurate.

    I was hoping to avoid an air gap here so that the Mineral Wool would be right next to the logs and I could benefit from the R value of the logs. Also keeps the wall design simple.

    You definitely need a drainage plane - that can be two sheets of membrane next to one another. But you may need ventilation as well depending how much water gets to the drainage plane and whether it is all year or there are drying periods etc. That's what the condensation risk analysis will show. As you say, ventilation would greatly reduce the effective insulation value of the logs. I don't know anything about log construction myself and there isn't a lot of it in the UK, so you might get more informed opinions from Americans or Scandinavians as well as people local to you.

    I was thinking to be sure to keep the log wall dry on the inside that I would allow vapor to escape into the building, ie... House wrap > Mineral Wool > Internal wrap
    Or, is that just asking for trouble?

    Again, that is something the condensation risk analysis (CRA) would show. It depends on the various temperatures and the water content of the timber and other materials and how they all change over the course of a few years. That is what the CRA models.

    There's a bit of discussion of some of the issues at https://buildingscience.com/documents/digests/bsd-105-understanding-drainage-planes
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