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    • CommentAuthorgravelld
    • CommentTimeJul 4th 2017 edited
     
    I don't get it.

    Every house I have lived in has cracks in the plaster, internal and external.

    So why is plastering or rendering judged to be good enough for air tightness?
    • CommentAuthorowlman
    • CommentTimeJul 4th 2017
     
    In a similar vein, we chase airtightness at the time of build yet who will guarantee that, 5,10,20 years down the line what ever the structure;- Buildings move, sticky tapes degrade, and humans, worst of all disrespect everything, open windows, fail to service MVHR, drill holes etc. etc..
    • CommentAuthorgravelld
    • CommentTimeJul 4th 2017 edited
     
    Posted By: owlmanIn a similar vein, we chase airtightness at the time of build yet who will guarantee that, 5,10,20 years down the line what ever the structure;- Buildings move, sticky tapes degrade, and humans, worst of all disrespect everything, open windows, fail to service MVHR, drill holes etc. etc..
    Yes, I was going to add that, but I wanted a tony-style succinct question to discuss.

    Is comparing with tapes valid? - a tape only really seals interfaces, whereas plaster is expected to seal a face - maybe comparison with a membrane is better?
    • CommentAuthortony
    • CommentTimeJul 4th 2017
     
    One thing to consider is that at least the cracks in plaster are visible and can be filled and redecorated recovering the loss of air tightness. The sane cracks behind typical linings cannot be repaired.
    • CommentAuthorGreenfish
    • CommentTimeJul 4th 2017
     
    And do the cracks go all the way through to somewhere air can flow?

    I have some cracked plaster in my masonry with cavity new build, plastering finished Feb 2014. Mostly on the single skin internal walls which is irrelevent, just a cosmetic niggle I will fix with the first re-decorating run now we are really dry. But some are on external walls, mostly where there are steels over window revels (materials meeting and expanding/contracting at different rates). I guess all of these are called "settlement cracks", although with the over specified foundation we are sitting on I am confident nothing is "settling".

    Likewise the external render coat has fine cracks in places too. But in different places. Have the concrete block beneath cracked too? Do all these fine cracks make an air pathway? I have no idea. But in a 60mph gale (better than any blower test) nothing seems to come from these cracks. Anyway like Tony said at least I can see and repair them.

    All normal but they are a niggle aesthetically, and not what I imagined my "perfect" new house to look like. What causes it? I blame the extremely hard concrete base coat used both inside and out, hard because of the high granite content of the local sand. Once dry there is no flex, only thing it can do is crack.

    The other question, for those experienced with newer masonry builds, is will they keep reappearing? Or is this just an early days thing that will abate over the first decade?
    • CommentAuthorgravelld
    • CommentTimeJul 4th 2017
     
    What type of render did you use?
    • CommentAuthortony
    • CommentTimeJul 4th 2017
     
    What causes it? Aircrete blocks. They move very easily, then thermal movements, omission of movement joints,
    • CommentAuthorGreenfish
    • CommentTimeJul 4th 2017
     
    Block = standard density "breeze" block, local supply so also have a lot of granite based sand along with the crap.
    Have some better thermal blocks (aircrete of some kind) in a row at the base, below internal floor level but not noticed any issues near there from outside.

    Render = traditional sand+cement applied in two coats
    Movement joints on the outside render, but cracks nearby instead of at the joint.

    Note the cracks are very fine, it is not a structural issue in any way
    • CommentAuthorgravelld
    • CommentTimeJul 4th 2017
     
    Did you wonder about using a more modern render?
    • CommentAuthorGreenfish
    • CommentTimeJul 5th 2017
     
    To be honest no, my brain was so tied up in the eco-tech that the builder didn't know about e.g. optimum insulation and air tightness details, MVHR, PV and ST etc., and the logistics of first fix etc. that the question of "what render?" didn't come up.

    In case someone else reads this before building what render would you have used?

    Actually the house is mostly cedar clad (part of the planning permission we inherited), but one part is render. Silly really, along with the complex layout that is not eco-friendly either. But it took the previoius owner 8 years of wrangling to get planning permission at all, we were told quite clearly by the planners that no variations in shape or finish would be allowed.

    We went with local materials that had a proven history of working (being robust) in the location.

    But to stay on topic: yes both my wet plaster finish internally, and render externally has some fine cracks, but they don't leak air.
    • CommentAuthorgravelld
    • CommentTimeJul 5th 2017
     
    I'm less experienced than you. But after putting up with the cracks all over our house it won't be S+C.

    This is what is motivating this post. I am looking to retrofit and it seems one strategy is to use the external render as an airtightness layer, insulating on top of that, then wind tightness from either rendered EPS blocks or membrane on trusses. But given there are a good number of cracks all over the existing render I wonder how well this would work...

    For the new finish, probably silicone based. Too exposed for acrylic (AIUI). Ideally clad on the first floor, but depends on the budget.

    How have you tested whether the cracks leak air? I've got internal cracks too, you can never feel them leaking air, but pretty sure they must do. But again, I have no proof, so...
    •  
      CommentAuthordjh
    • CommentTimeJul 5th 2017
     
    Cracking has a lot to do with the relative stiffness of materials. And cracking in plaster has a lot to do with the skill of the plasterer both in applying the plaster but also in choosing the materials; not only the plaster but what additives and reinforcement etc.

    I don't have any cracks at all in my internal plaster that I am aware of where it is lime over straw bale. There are some hairline cracks at corners of windows and of the house but only on the parts that are timber framed and covered by Intello membrane plus woodwool boards as a plaster key. The internal plaster is my airtightness barrier except it is backed up by the Intello exactly at the places where there are some cracks. Maybe that's lucky or maybe it is the substrate that has caused the cracks?

    I do have some cracks in the external render, which again is lime, and some of these are on parts where the render is on the bales. But I have a lot less cracking than I was expecting with such a flexible substrate. I think the external render is probably more prone to cracks because of greater movement due to sunlight and rain. Most of the external cracks are on the south face. We use some lime putty or Beeck crack filler to fill them.

    I don't worry about the lifetime of the membrane and tapes I have used. They've been tested for '50 years equivalent' to date and should outlast me.

    You locate air leaks with an airtightness blower and a smoke pencil, or a cigarette.
    • CommentAuthorgyrogear
    • CommentTimeJul 5th 2017
     
    Posted By: tonyThe sane cracks behind typical linings cannot be repaired


    I've got some insane cracks in my staircase plaster...

    gg
    • CommentAuthorGreenfish
    • CommentTimeJul 7th 2017
     
    Posted By: gravelldHow have you tested whether the cracks leak air? I've got internal cracks too, you can never feel them leaking air, but pretty sure they must do. But again, I have no proof, so...


    Well I already had some of the cracks when the door blower air tightness test was done, and got a sub 1 (units I forget) result. The test was done poorly: MVHR outlets left unsealed, blower badly fitted to doorway, and an ill fitting door (later adjusted correctly). The tester couldn't get into his head that I didn't just want to pass, I wanted to know exactly how great the house was, and do some fine corrections if necessary. So in reality the house is even better than the results show. I guess I could have another test done (by someone that has an interest in eco houses).

    I also have the experiences that only come with good air tightness. For example if I open a window, not in direct wind, then nothing much happens. I can stick my hand outside and feel the breeze, but to let air into the house I have to open another window somewhere else. It is so odd to stand by an open window an feel like it is closed.

    But also I am on an exposed site, on a hill with a view of the sea, when the wind blows it blows. It exerts more pressure on the surfaces than reached in any blower test. It was how we found the ill-fitted door, and knew when we had correctly adjusted it, even the smallest of leaks are both audible (they whistle) and felt on the hand or by smoke. A gale makes a great test tool!

    Oh and blower test: yes the MVHR vents to the outside do need to be sealed during it. Again during a gale (towards the inlet) with MVHR off we do get air flowing from the room terminus. Maybe different MVHR units are impacted differently?

    So use the wind! :)
    • CommentAuthorgoodevans
    • CommentTimeJul 7th 2017
     
    Posted By: Greenfishduring a gale (towards the inlet) with MVHR off we do get air flowing from the room terminus


    Do you have the inlet and outlet pointing in different directions? if you do on windy days it would explain the unplanned air flow.
    • CommentAuthorGreenfish
    • CommentTimeJul 7th 2017
     
    The inlet and outlet of the MVHR are on the same house wall but ~2.5m apart. If I turn off the MVHR in a gale directly hitting that wall, then I do get a small amount of air flowing out of the room supply terminus.

    It is not an issue because the MVHR is on 24/7 (bar power outages), but it does confirm that the inlet and outlet should be blocked during an air tightness test. The BRE standards say to do that, my tester was just lazy.
    • CommentAuthorGreenfish
    • CommentTimeJul 7th 2017
     
    To keep on topic, even small leaks seem to be very obvious in an otherwise air tight build compared to an older generally leaky house. So while I would not have known if air was comming though any plaster/render cracks in my old Edwardian semi, too much was going up the chiminey and out the doors, you really can sense any tiny leaks in this well sealed new one.

    Before building I thought "how will I know", but my experience is that it is pretty obvious providing that generally the air tightness is good.
    •  
      CommentAuthordjh
    • CommentTimeJul 7th 2017
     
    Posted By: GreenfishThe inlet and outlet of the MVHR are on the same house wall but ~2.5m apart. If I turn off the MVHR in a gale directly hitting that wall, then I do get a small amount of air flowing out of the room supply terminus.

    Interesting. I suppose it could be due to greater wind speed at the intake than the exhaust, which would suggest your exhaust is above your intake, like mine. Or alternatively it might suggest greater resistance in the exhaust ductwork, I suppose.

    it does confirm that the inlet and outlet should be blocked during an air tightness test. The BRE standards say to do that, my tester was just lazy.

    Fortunately IMHO, its not BRE that set the standard, it's ATTMA. I'd be asking for a free retest to the standard if I got that kind of service from a tester, and personally I'd ask for the retest to PH standard as well as SAP. You're at a level where it would be more accurate.
    • CommentAuthorgoodevans
    • CommentTimeJul 7th 2017
     
    presumably painting/rollering some thin coat acrylic/silicon render the the inside of block work would act as an easy to apply long lasting and effective air barrier. In particular I'm thinking of the bit of wall on gable ends between the ground floor and the second floor which the plasterer does not get to (or for that matter the bit of wall just under the wall plate).

    Would the regular plaster have a problem with adhesion at the overlap with the render below? are their better pain on air barriers?
    •  
      CommentAuthordjh
    • CommentTimeJul 8th 2017
     
    Posted By: goodevanspresumably painting/rollering some thin coat acrylic/silicon render the the inside of block work would act as an easy to apply long lasting and effective air barrier. In particular I'm thinking of the bit of wall on gable ends between the ground floor and the second floor which the plasterer does not get to (or for that matter the bit of wall just under the wall plate).

    Would the regular plaster have a problem with adhesion at the overlap with the render below? are their better pain on air barriers?

    Dunno. In my case I used some bits of DPM to cover the gap between ground floor and first floor and linked it to the plaster with Contega tape. The wall to roof junction linked the plaster to the Intello roof membrane with the same Contega tape.
    • CommentAuthorGreenfish
    • CommentTimeJul 8th 2017
     
    Posted By: djh
    Posted By: GreenfishThe inlet and outlet of the MVHR are on the same house wall but ~2.5m apart. If I turn off the MVHR in a gale directly hitting that wall, then I do get a small amount of air flowing out of the room supply terminus.

    Interesting. I suppose it could be due to greater wind speed at the intake than the exhaust, which would suggest your exhaust is above your intake, like mine. Or alternatively it might suggest greater resistance in the exhaust ductwork, I suppose.
    Intake and exhaust are at the same level on the wall, so that leaves ductwork resistence or maybe there is just something in the unit that limits reverse flow into the extracts. The flow is small (not a gale), but I happend to have my head by a supply terminus during a gale with the MVHR off. Maybe the extracts were doing the same and I just didn't notice?

    it does confirm that the inlet and outlet should be blocked during an air tightness test. The BRE standards say to do that, my tester was just lazy.

    Fortunately IMHO, its not BRE that set the standard, it's ATTMA. I'd be asking for a free retest to the standard if I got that kind of service from a tester, and personally I'd ask for the retest to PH standard as well as SAP. You're at a level where it would be more accurate.
    By fluke the company that provided air test and SAP forgot to invoice me, so I never paid. Natural justice in my book, so I decided not to chase it. But if I want to know exactly how good the house is then I need to get a proper test done. I think it will be interesting to do that after say a 5 year settling period (not there yet although time flies).
    • CommentAuthordelprado
    • CommentTimeJul 8th 2017
     
    ive come to the conclusion that everyone worries about air tightness too much. I am renovating our house and just plastered whole room, around joists, etc, and with door and window closed I get 2000ppm co2 within about 20 minutes! Not much air leaving the room, and not enough to worry about i dont think. I dont understand why people use these expensive membranes
    •  
      CommentAuthordjh
    • CommentTimeJul 9th 2017
     
    Posted By: GreenfishIntake and exhaust are at the same level on the wall, so that leaves ductwork resistence or maybe there is just something in the unit that limits reverse flow into the extracts. The flow is small (not a gale), but I happend to have my head by a supply terminus during a gale with the MVHR off. Maybe the extracts were doing the same and I just didn't notice?

    Could well be. I suppose another possibility is that one terminal is closer to a corner than the other. I was about to write that the one nearer the corner would experience higher pressure, but I realize I can't explain that, whereas if the intake is further from a corner I could pretend to explain that by hand waving about stagnation pressures etc.

    Natural justice in my book

    That's what it sounds like to me too!

    But if I want to know exactly how good the house is then I need to get a proper test done. I think it will be interesting to do that after say a 5 year settling period (not there yet although time flies).

    Sounds like a good plan. I can heartily recommend Paul Jennings.
    •  
      CommentAuthordjh
    • CommentTimeJul 9th 2017
     
    Posted By: delpradoive come to the conclusion that everyone worries about air tightness too much. I am renovating our house and just plastered whole room, around joists, etc, and with door and window closed I get 2000ppm co2 within about 20 minutes! Not much air leaving the room, and not enough to worry about i dont think. I dont understand why people use these expensive membranes

    In my case I worried about it because it's a consequence of going for PH certification. And I found that worthwhile because I'm sure it increased the quality of work of the tradesmen - it's a quick answer to 'why do we have to do it that way?' and by the end, they were pointing out details that needed changing to me.

    More generally, people 'worry' because it's to do with the cost of heating the air that in/exfiltrates.

    That's a fairly high CO2 reading. I suppose it depends on what ventilation there is, what CO2 sources there were and how big the room is. But I have MVHR and I'm guessing there was no ventilation in your room so I have very little idea of how fast CO2 would build up and how much airtightness would affect it.

    The choice between plaster and membranes as an airtightness layer is not one-size-fits-all. It depends on the likelihood of cracking, and on the number of penetrations (it's easier to put a membrane behind a service cavity than have to make a lot of electrical sockets airtight, especially if the electrician is not sympathetic). In my case I simply made a rule that there were to be no penetrations, so no sockets or switches in external walls, which made plaster an easy choice (there are actually a few penetrations for an external tap and external lighting). In the case of my roof, it was to do with the guarantee the membrane supplier offered with regard to condensation damage.
    • CommentAuthordelprado
    • CommentTimeJul 9th 2017
     
    "In my case I worried about it because it's a consequence of going for PH certification. And I found that worthwhile because I'm sure it increased the quality of work of the tradesmen - it's a quick answer to 'why do we have to do it that way?' and by the end, they were pointing out details that needed changing to me."

    DJH this is golden and I think I will borrow the phrase even though I am not going for the same! Workmen often just need a simple thing and they assume "that involves a book so I wont question it"...
    • CommentAuthorgravelld
    • CommentTimeJul 10th 2017
     
    Posted By: goodevanspresumably painting/rollering some thin coat acrylic/silicon render the the inside of block work would act as an easy to apply long lasting and effective air barrier.
    I'm also interested in this, although for a different application area. In the case of masonry cavity retrofit, where the air tightness layer becomes the outside face of the existing external leaf, with insulation to be installed outbound of that.

    In addition, in the case of existing cracked and possible blown S+C render, how the interface between modern render and S+C works - is this a weak point? I'm guessing so, but weaker than using S+C throughout?

    Posted By: djh
    In my case I used some bits of DPM to cover the gap between ground floor and first floor and linked it to the plaster with Contega tape. The wall to roof junction linked the plaster to the Intello roof membrane with the same Contega tape.
      Why didn't you just use Intello instead of DPM, cost? It's supposed to be for walls too isn't it?

      Posted By: djhI can heartily recommend Paul Jennings.
      The closest thing to an air-tightness celebrity there is :wink:
      •  
        CommentAuthordjh
      • CommentTimeJul 10th 2017
       
      Posted By: gravelldWhy didn't you just use Intello instead of DPM, cost? It's supposed to be for walls too isn't it?

      Yes, we had some DPM left over and I don't think we'd taken delivery of the Intello at that time but it would have been fine to use that instead.
      • CommentAuthorBenj
      • CommentTimeJul 15th 2017
       
      We left over a year between watertight stage and plastering. During this time the building moved due to the wood elements drying out and cracks appeared in the masonry. These cracks were meshed over and we have had no further problems.
      Internal plaster needed to be Gypsum based so that it would stick to the air tapes - still a lot of misinformation out there about sand and cement used in conjunction with these tapes.
      •  
        CommentAuthordjh
      • CommentTimeJul 15th 2017
       
      Posted By: BenjInternal plaster needed to be Gypsum based so that it would stick to the air tapes - still a lot of misinformation out there about sand and cement used in conjunction with these tapes.

      That's not something I've heard about. Do you have any more specific information about products and plaster types? Or links to docs?
      • CommentAuthorBenj
      • CommentTimeJul 15th 2017
       
      From https://proclima.com/media/downloads/brochure_constructions_interior_sealing_en.pdf

      "Gypsum plasters have very
      good adhesion on CONTEGA
      –eece. A bonding bridge – e.g.
      a reinforcing mortar – must
      be used for lime and cement
      plasters."

      We used bonding coat which is exceptionally sticky and so far crack free!
      The plasterer hated it though!
       
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