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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorKenny_M
    • CommentTimeMay 30th 2021
     
    Apologies if this is a stupid question.

    I'm using eps, which understand is reasonably vapour permeable, in the old part of the house, where I don't believe I have any chance of continous vapour control layer. My thinking is that if I can't be sure to control vapour getting in, I should use something that will also allow it out.

    I've run out of eps, but I have loads of foil faced celotex left from the work on the newer extension.

    Would I be right in saying that it's the foil that makes celotex/pir impermeable, and that if I ripped the foil off it would have a similar permeability to the eps?

    I did try to find an answer to this online, and although one site seemed to confirm, there was some contradictory info floating around!
    • CommentAuthorjms452
    • CommentTimeMay 31st 2021
     
    Sounds like a bad idea - PIR gets some of its insulating properties because it keeps the blowing gas inside (for years) so I wouldn't count on it being breathable.

    There are lots of different forms of polyurethane so you will find lots of contradictory info out there.
    •  
      CommentAuthordjh
    • CommentTimeMay 31st 2021
     
    Posted By: Kenny_MWould I be right in saying that it's the foil that makes celotex/pir impermeable, and that if I ripped the foil off it would have a similar permeability to the eps?

    I don't believe so. PIR is an expanded foam. EPS is a load of separate beads expanded so they kind of stick together. It's easy to visualise why EPS would be more permeable. PIR is more similar to XPS in this respect. Plus as jms says, lots of different products to confuse matters.
  1.  
    Agree with jms452. Allegedly the foil is part of the 'keeping the blowing agent in' strategy (don't ask about the edges!) IIRC. It is also, demonstrably, part of the 'dimensional stability strategy'. I have never completely 'peeled' PIR, but I have occasionally cut it down in thickness (same as removing one foil sheet). If, after that, you have a large banana that you want to insulate, then it's fine....
    • CommentAuthorKenny_M
    • CommentTimeMay 31st 2021 edited
     
    But couldn't something be impermeable to air, or the blowing gas in this case, but somewhat permeable to water vapour?

    At this site here https://italy.ediltec.com/en/technical-notes/-3 the table suggests that PIR with "permeable coatings" has a similar vapour resistance to EPS, but the next entry in the table shows an infinite resistance to vapour for PIR with "water proof coatings". That would seem to suggest that its the foil, rather than the PI that is vapour resistant.

    I read this morning on a forum thread, someone suggesting that the foil is there to keep in the pentane, which aligns with what you are saying about keeping the blowing gas in, so maybe the PIR without foil has a permeable coating that keeps the pentane in but allows vapour to pass? If that's the case then ripping the foil off would presumably make the PIR vapour permeable, but let the pentane escape!

    The only thing that doesn't add up about this, is that if it is the foil that keeps the pentane in, what's to stop the pentane leaking out of the unsealed edges while the PIR is in storage??? Confused, as usual! :)

    P.S. What I mean by contradictory info is that you go to seemingly technical sites like https://www.greenspec.co.uk/building-design/insulation-materials-thermal-properties/ and find it lumping EPS and XPS together as being vapour impermeable, when I know that these have very different resistances to vapour. On the other hand I found a forum where someone had posted an email from celotex advising against using their product in a motor home because it was permeable, yet as far as I am aware Celotex always has foil! Minefield!

    Edit: Nick. Just finished writing this before noticing your "don't ask about the edges" - too late!! :)
  2.  
    Posted By: Kenny_MI've run out of eps, but I have loads of foil faced celotex left from the work on the newer extension.

    What about selling on the Celotex and buying the correct EPS that you want/need ?
    • CommentAuthorKenny_M
    • CommentTimeMay 31st 2021
     
    Thanks Peter, yes, I could do that. Its not really the cost, I just like to avoid waste and re-suse what I have first as I work through the house. I am sure I will use the celotex, probably in the attic room where vapour permeability won't matter one way or another.
  3.  
    50 times more moisture's transported into a wall by lack of air-tightness than breathability so don't get too hung up on it. Firstly you'd need a PVP (Partial Vapour Pressure) difference between the inside air and the wall, this happens for only 4-6 weeks per annum in Ireland and the Uk, then there's a PVP equilibrium period for 3 months, then 8 months when the wall is in drying mode. Secondly the first 10mm of plaster soaks in 90% of the water vapour and a coat of paint can restrict it by 50% so anything more than 10mm of breathable render has minimum benefit. Thirdly a humidity activated fan has a similar effect to breathable plaster and is probably a lot cheaper.
    So I'd make the old wall airtight with a coat of plaster internally and use the Celotex, foam them together as you're installing to restrict air movement and tape the joints with aluminium tape. If the old wall's visibly damp I'd vent the cavity behind the insulation to the outside.
    • CommentAuthorKenny_M
    • CommentTimeJun 1st 2021
     
    Thanks Viking, really interesting to read a different take on this. When you say the wall is in drying mode for 8 months, do you mean drying by passing vapour from interior to exterior, or the opposite?

    I'd always presumed that due to the fairly damp climate where I am in central Scotland, that the absolute humidity is greater on the outside than the inside for most of the year, but I imagined that would be a slightly different story in the south of England or Ireland.

    I wouldn't say I am too hung up on the idea of breathability, in terms of transmission of vapour from the interior into the walls, but I am also not convinced that I would have a snowballs chance in hell of getting a proper air tight seal in the old part of the house, without ripping the room, walls and wooden panels apart. I monitor the RH and its lower inside the house than out, for most of the year - I know this is an over simplification, and there is a temp differential at play, but wherever there is vapour generated we ventilate well, so I am not overly concerned about interior vapour entering the walls.

    I'm more concerned with the idea that if actual water (rather than vapour), gets into the wall, that it has a pathway out, because again, old house, which seems to find creative ways of allowing water to enter! The existing ventilated pathways around the windows and behind the walls/panels allow for this, but if I fill with insulation the EPS seems more likely to allow it to escape - even if it just works its way to the stonework it would probably just absorb into the sandstone. I am concerned that if water gets in behind impermeable insulation, that it wont have a pathway to escape.

    That's my thought process anyway, it may or may not hold water - I should hang my head in shame for that pun! :)
    • CommentAuthorEd Davies
    • CommentTimeJun 1st 2021
     
    Posted By: Kenny_MI'd always presumed that due to the fairly damp climate where I am in central Scotland, that the absolute humidity is greater on the outside than the inside for most of the year,
    That's rarely the case. Usually the outdoor absolute humidity is lower than the indoor absolute humidity because when outdoor air is brought in by whatever ventilation happens (deliberately or otherwise) its absolute humidity is preserved (water is not gained or lost) then it is warmed up to match indoor air (again, keeping the same absolute humidity though the relative humidity is reduced) then water is evaporated into it from people's lungs, dishes drying, shower water, etc, increasing both its absolute and relative humidity.

    I'm in the north of Scotland and have just done a plot of the indoor (in my study) and outdoor (at an airport about 30km away) specific humidities for the last year and, as expected, the specific humidities outdoors are pretty much always lower. Specific humidity is only slightly different from absolute humidity for these purposes.
    • CommentAuthorKenny_M
    • CommentTimeJun 2nd 2021
     
    Yes, that makes sense, the air outside is the starting point, and you are not 'drying' it by bringing it inside, just warming it and so reducing the RH. But having said that, when its raining the RH outside must be higher than inside, and it seems to rain a lot. I know that I did some RH tests here inside and out a couple of years back, then converted to absolute with an online calculator, and I didn't find much difference between the absolute humidity inside or out.

    If its normally more humid inside than out, then vikings point about walls being in drying mode 8 months of the year must be drying towards the exterior. If this is the case, doesn't it make sense to use permeable insulation and direct excess vapour into the walls which are 'drying' most of the year? Or am I missing something.
  4.  
    Windows'd be open more during the 8 months so you'd expect the Water Vapour PVP to be similar on both sides of the wall with a few daily spikes. Clothes on a clothes-line in a shed dry most days, even when it rains.
    1m3 of 20 degree air @ 100% RH holds 19.6 grammes of water.
    1m3 of 10 degree air @ 100% RH holds 9.6 grammes of water.
    1m3 of 0 degree air @ 100% RH holds 3.0 grammes of water.
    When it's coldest outside for 6 weeks the PVP vapour drive into the wall is greatest.
    It's probably better to use breathable insulation but you wanted to use up the Celotex.

    This discussion should probably be about air-tightness because if any moisture gets into the wall it will most-definitely be from rain, a water-leak or an air-tightness leak and really unlikely to be from water vapour.
    Instead of thinking about how to make it breathable 99% of the time, think about how to make it air-tight 99% of the time. A French Drain would dry the wall and stop rising damp.
    • CommentAuthorKenny_M
    • CommentTimeJun 3rd 2021
     
    Posted By: Viking HouseThis discussion should probably be about air-tightness because if any moisture gets into the wall it will most-definitely be from rain, a water-leak or an air-tightness leak and really unlikely to be from water vapour.
    Instead of thinking about how to make it breathable 99% of the time, think about how to make it air-tight 99% of the time. A French Drain would dry the wall and stop rising damp.


    You say that I should make it air tight, which unless I have misunderstood, in this context is about preventing water vapour getting into the wall, which you said before that is unlikely.

    I'm not all that concerned about water vapour from the interior. Where I am coming from is that I have an old house which currently has ventilated spaces around the windows, and behind the walls. When I get leaks right now, it runs down through these cavities and under the floor into the earth. I want to insulate these spaces, but if I get leaks, there is no where for it to go. Large amounts of water are going to come out somewhere, like through the ceiling plaster, but smaller amounts will sit there, probably unknown. I am presuming, that something like EPS, and open cell foams are more likely to allow the water to find a pathway out, than something that is impermeable, such as foil faced insulation.

    I know the easy answer is to ensure the house is water tight, and that's the plan, but Scottish weather has a habit of disagreeing with me on that point.

    I'm just a DIY'er in this game so I may have this all wrong, but I am attracted to a combination of EPS, EPS beads and open cell foams because I think this is more likely to allow water to work its way through.
  5.  
    That's great Kenny, have you seen water flowing through the cavities? Could there be a roof leak? If you're plastering the walls leave the windows open for a few days or get a de-humidifier. EPS Beads would be good in the cavity and allow water to drain. What's the wall build-up?
    • CommentAuthorKenny_M
    • CommentTime7 days ago
     
    Yes, I have seen water coming through, but only once. As I mentioned in the earlier post, this was a bit of a freak event last winter when rain froze in the gutter and the ice expanded through into the roof space, but its the sort of thing that could happen every couple of years or so. Its not a permanent leak so difficult to diagnose for sure where it came in.

    I intend to get up into the area where I think it came in, but at the moment that area above the bay is closed, like they have built and boarded the main roof, then built the bay roof over it, so I will have to find the right location, and cut through to get to it. Hopefully I can fit something there to prevent this happening again.

    I am not plastering any walls, just filling behind what used to be shutters and sash box, trims around windows and other timber trim. Its quite a large area, and in some parts I am getting 100mm of insulation in, by taking the shutters out to get access behind. Because this is a single skin in the bay it means that stone pillar, then insulation, then timber trims/panels. That's where my caution lies, because there will be no cavity. EPS seems to work well in modern cavity walls, so think it might be best here too.

    Later, I may try and pour EPS beads down behind the existing lath and plaster walls, but not started on that yet, and there may not be much of a gap to make it worth it.
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