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.

Buy individually or both books together. Delivery is free!


widget @ surfing-waves.com




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.




    •  
      CommentAuthordjh
    • CommentTimeFeb 8th 2018
     
    Posted By: fostertomI don't know why you'd need OSB inboard of the framing other than it's the conventional place rather than external.

    Airtight layer can go anywhere in the sandwich - are you thinking of water-vapour-tight layer which yes should go inboard of the insulation? Two v different things tho often confused.

    Posted By: fostertomSmartply is just a brand of OSB

    Warmcel didn't say OSB - they said Smartply Propassiv i.e. a guaranteed vapour control layer. It also has racking strength, so there's no need for another racking board. Please read what is written.
    • CommentAuthoradam_w
    • CommentTimeFeb 8th 2018
     
    Tom,

    What I don't understand (among many other things) is timber frame houses and extensions have been built for years without any EWI and allowed to pass building regulations? You're suggesting 150mm of EPS on the walls which via my calculations is an R value of 4.84m2.K/W where as when I look at the 'new' Actis Timber frame solution, the outside is only covered in 35mm of their Hybrid Boost obtaining an R value of 1.35m2.K/W, surely this is insufficient based on your previous statements as with a total U Value of 0.15 its only 20% of the insulation outboard of the timber frame?

    Kind regards,

    Adam
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 8th 2018 edited
     
    I don't follow Actis/multifoil progress, having seem that promising line eviscerated to dubiousness by their 'conventional' competitors ganging-up (it's an old, and famous story here on GBF!) so I don't know what their technical schema would be, maybe VCLs etc - I don't know. I can tell you what I discovered in numerous comparative runs in WUFI - a guideline (AFAIC - others must check for themselves) for avoiding that condensation-on-timber danger point that Warmcel flagged up, however crudely.

    Your proposed construction has always been a little or a lot better than Bldg Regs requirement for insulation, and I and others have been suggesting 'do even beter, while you're at it'. What is safe, interstitial-condensation-wise, at Bldg Regs level of insulation, needs extra caution and measures as insulation improves further. Standard Bldg Regs construction rarely goes the breatheable route anyway. Smother it with VCLs and you'll prob tick the boxes. Do it breatheable, and I've recommended modest changes to your original design, to make it safe and best-bang-for-buck, AFAIC.

    Over to you now!

    PS Smartply PassivPlus is devil's spawn! Forget breatheability, with that.
    • CommentAuthoradam_w
    • CommentTimeFeb 8th 2018 edited
     
    Tom,

    Your opinions have always been greatly appreciated by myself, I hope that has come across in my replies. I'm much much happier with my build up since I first joined this forum a mere 3 weeks ago. I think the only remaining detail that im struggling with now is the vapour control layer as I got the impression that you think the product suggested by Warmcell was an overly expensive one?

    Thanks again,

    Adam
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 9th 2018 edited
     
    Have enjoyed this project adam! Trouble is, all I've said is towards a breathable solution - has little relevance if there's going to be a vapour control layer, whether in form of Smartply ProPassiv, a sheet of polythene, or Intelliwotsit.

    I guess there's a place, in some circumstances, for VCLs, and they do seem to fail safe in general use even tho people have the most half-baked understanding of how they work - but they seem like part of a brittle approach and I'm interested in buildings that are robust, tolerant of change, self-running, like the best (those that have stood the test of time) of old buildings.
    • CommentAuthoradam_w
    • CommentTimeFeb 9th 2018
     
    Tom,

    I sat up last night reading through all your replies and I realised that in order for full breathability, I shouldn't use a VCL, just regular plasterboard! I'm glad you've enjoyed heavily contributing to this thread, I'm much much happier with my proposed plans now and I can't wait to start! Thanks again for all your help,

    Adam
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 9th 2018
     
    Go fer it! Good student!
  1.  
    Posted By: fostertomSmartply PassivPlus is devil's spawn! Forget breatheability, with that.


    Tom, can you elaborate on this a little? Is it down to the glues used?

    I've been recommended ELKA strong board as a 'breathable' (diffusion open) alternative to OSB, but trying to figure out if it's worth the extra cost. Appears to be a particle board rather than oriented strands.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 9th 2018
     
    Simply that it's a vapour barrier.

    Particulaly annoying because I did my WUFI training in Dublin and a tech rep from Smartply was there. We had a long and detailed conversation about how great OSB esp Smartply is for would-be airtight but vapour-open sheathing - but this controversy raging about whether OSB is reliably airtight, from batch to batch. He confirmed they don't quality-control for airtightness, said they were aware of the problem, wd be holding 'focus group' later in the year with architects, builders, PH designers etc and they'd invite me. Didn't, despite reminder. So what happens is - fanfare - plastic coated Smartply PassivPlus introduced - which lets them out of having to quality-control the actual board for airtightness - but it also makes it a vapour barrier. Duh. Not what the ecobuilding/PH world was (stiil is) crying out for. Still never mind because I've heard the great Peter Warm say 'I've never yet found a PH that failed because of using OSB as airtight layer'. So there. (but I've not got him to reconfirm that!)
    • CommentAuthoradam_w
    • CommentTimeFeb 13th 2018 edited
     
    Update time! So with a lot of help from DarylP, I've had a few more CRA's carried out and even with the 80mm EPS, I'm still at risk of condensation against the OSB layer so that leaves me with no other option other than to use a VCL.

    This then leaves me with another list of unknowns, where should the VCL start and finish? As the ground floor is to be used as a garage/gym and isn't as well insulated, do I just wrap around the walls, across the sealing and under the floors of the rooms above?

    Thanks in advance,

    Adam
    • CommentAuthorgoodevans
    • CommentTimeFeb 13th 2018 edited
     
    Presumably that means that your upper walls also need a VCL. How are you ventilating, trickle vents, mvhr or other.

    I also don't like the idea of a VCL - but I would not want a rotting layer of OSB either. The difficulty comes in that building users abuse their buildings i.e. in winter they close all their vents and don't open windows. As a result VCL's are specified to defend against the worst case.

    Does WUFI give any credit for proper ventilation - i.e. does it allow for the internal humidity to track external absolute humidity plus say 2 g/m3 of moisture from the occupants. Or does it assume a fixed internal RH - such as 66% at 20degC. On a cold day to get up to 66% RH with MVHR you would have to generate a lot of moisture - (I estimate a minimum of 25litres per day for a 150m2 house.)
    • CommentAuthoradam_w
    • CommentTimeFeb 13th 2018
     
    Goodevans - I think the thought is that the garage wont be warm enough as its uninhabited and unheated, it is just the habitable space above it which would need the VCL should I choose to fit one.

    My method of ventilation will be via trickle vent and general window ventilation, specaially in the bathroom. Its not something I've had to worry about previously as the existing house is so poorly insulated but during the external renovation that should change as I'm looking to add 100mm of EPS all the way round before rendering and cladding.

    As for the WUFI, I think our residential expert Tom will have to answer that one for you!

    Kind regards,

    Adam
    • CommentAuthorgoodevans
    • CommentTimeFeb 13th 2018
     
    Without a guaranteed ventilation rate it's more tricky and perhaps you should be more cautious.
    • CommentAuthoradam_w
    • CommentTimeFeb 13th 2018
     
    Be more cautious and add a VCL?
    • CommentAuthorgoodevans
    • CommentTimeFeb 13th 2018
     
    Yes it looks like it - and the same may apply to me, my osb layer has only 37% worth of insulation towards the outside.

    I have the choice of more insulation on top (but I will have planning height issues), moving the air tight osb layer to under the rafters, reducing the insulation between the rafters or smothering it in plastic. grrrr.
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 13th 2018
     
    Posted By: goodevansDoes WUFI give any credit for proper ventilation
    I have often wondered this.

    Just to give you an idea, my internal Temp/RH is 19°C/42%
    External is 4°C/80%

    I just have natural 'Trump' (leaky house) ventilation.
    • CommentAuthorborpin
    • CommentTimeFeb 13th 2018 edited
     
    My $0.02 worth as a layman;

    I plumped for Icyene internally as it is breathable and it seemed to me that even if I did, at worst case, get a small amount of condensation, it would dry out. As it was, I put that much insulation outboard, there was little chance of condensation.

    I would therefore keep it fully breathable and put as much insulation outboard as possible. If you do that all the condensation concerns become void.
    • CommentAuthorgoodevans
    • CommentTimeFeb 13th 2018 edited
     
    Yes I agree but the issue but if you have something very permeable but insulative on the inside (e.g. rockwool) then a layer such as OSB sarking with a only a little insulation on the outside then the rockwool has the effect of making the OSB cold. If the OSB is cold enough condensation will form on the inside surface and the moisture will keep on coming because the rockwool is very permeable. The problem is not that OSB is not permeable it is that it is less permeable than the rockwool.

    In fact its the mismatch between insulative performance and permeability for the different layers that causes the issue. Now if they made rockwool with the permeability of OSB that would be great.
    • CommentAuthoradam_w
    • CommentTimeFeb 14th 2018 edited
     
    Morning All,

    My main concern is that I'm over complicating things and looking at the extension in isolation. Baring in mind that the rest of the house, all 120sq.m of it, is much colder and less airtight than what the new addition will be, which is a modest 27sq.m of habitable space. Surely any moisture is going to get drawn out in to the colder areas first?

    Having said that, as previously mentioned, I am looking to add 100mm of EPS to the outside of the existing house to bring it up to spec but the U value will still be much higher than of what I'm planning to build to the side of the property so maybe it will be ok?

    I guess there is no way to know for sure but I have contacted Passivhaus to see If they could run a CRA for me using their WUFI tool, lets see what they say!

    Edit: I've just come across this link, no VCL and OSB on the internal face, surely the studs would be at risk of condensation? http://www.greenspec.co.uk/building-design/woodfibre-timber-frame-wall/

    Thanks again,

    Adam
    • CommentAuthorIan1961
    • CommentTimeFeb 14th 2018 edited
     
    Edit: I've just come across this link, no VCL and OSB on the internal face, surely the studs would be at risk of condensation? http://www.greenspec.co.uk/building-design/woodfibre-timber-frame-wall


    Although the OSB layer isn't normally seen as a VCL as long as it has at least double the vapour resistance of the external wood fibre board it would still be in compliance with the requirements of BS 5250:2011
    • CommentAuthoradam_w
    • CommentTimeFeb 14th 2018 edited
     
    Although the OSB layer isn't normally seen as a VCL as long as it has at least double the vapour resistance of the external wood fibre board it would still be in compliance with the requirements of BS 5250:2011


    Ian,

    That's interesting, its not something I've heard before!

    In my instance, i'll be using EPS in place of the fibreboard which has a Mu value of between 50-70 so at 80mm gives between 4-5.6m where as the OSB Mu is also 50, its only 11mm thick so 0.55m, no where near double unfortunately!

    Adam
    • CommentAuthorgoodevans
    • CommentTimeFeb 14th 2018
     
    Ian - if the OSB was on the inside face then I think the CR assessments would be better - but the osb layer is half way through makeup and is acting as a vcl where we don't want it to.
    • CommentAuthoradam_w
    • CommentTimeFeb 14th 2018
     
    I would happily move the OSB to the inner face, specially if I can then do away with the VCL. I would even consider increasing the EWI to 100mm if it would make much more difference? Only issue I guess is that I would have less friction between the EPS and the mounting face as its now studs instead of board but I guess its nothing some low expansion foam couldn't cure?

    Thanks,

    Adam
    • CommentAuthorgoodevans
    • CommentTimeFeb 14th 2018 edited
     
    Well - I've done my own calcs assuming worst case
    assumptions:
    150m2 house, reliable ventilation (i.e. mechanical with known flow). Flow 0.3xarea = 45l/s.
    evaporating 11.6l of water per day continuously in all air supply rooms would increase the vapour in the air by 3g/m3 on average (compared to outside air moisture content).

    taking raw weather data from this site https://www.cl.cam.ac.uk/research/dtg/weather/index-daily-text.html
    assuming a barrier such as osb with 37% of insulation outside (63% inside) with vapour permeable insulation each side.

    Under these conditions looking at the weather data from the last 10 years the OSB would be subject to similar RH conditions to OSB stored in a dry, well ventilated, unheated barn. And as far as I know in the UK this is not a problem.

    In reality vapour is created in bursts and mainly in air extract rooms so even more vapour is shipped out further reducing the moisture from the continuous assumption above (or allowing more vapour production). In addition vapour exits through open windows, doors or the house fabric as well.

    In short - If I can convince the BCO - I will be going for no vcl so the OSB can dry to the inside - It would be interesting to measure/log the temp/RH at several points in the makeup and compare to the WUFI model. (my guess is the the internal humidity model of WUFI will be the problem).
    • CommentAuthorgoodevans
    • CommentTimeFeb 14th 2018
     
    Posted By: adam_wI would happily move the OSB to the inner face
    The main problem here is that the osb forms your air tight barrier and locating it here will likely result in service penetrations etc. Perhaps Daril could do a comparison.
    •  
      CommentAuthordjh
    • CommentTimeFeb 14th 2018
     
    Posted By: goodevansThe main problem here is that the osb forms your air tight barrier and locating it here will likely result in service penetrations etc.

    To which the usual answer is a service cavity.
    • CommentAuthoradam_w
    • CommentTimeFeb 15th 2018
     
    Ok...so I thought the only way to settle this discussion for sure was to have a Wufi Analysis performed so I got in contact with Bre (Passivhaus) and to analyses the 3 areas, ground floor, 1st floor and roof, is going to be £1800 plus VAT. Now is it just me or is that insanely expensive? I'd rather just fit a VCL and be done with it!!
    • CommentAuthorIan1961
    • CommentTimeFeb 15th 2018 edited
     
    Posted By: goodevansIan - if the OSB was on the inside face then I think the CR assessments would be better - but the osb layer is half way through makeup and is acting as a vcl where we don't want it to.


    But the OSB is indeed on the inside face of the wall for that build-up diagram that @adam_w linked to:
    http://www.greenspec.co.uk/building-design/woodfibre-timber-frame-wall/
      Capture.JPG
    • CommentAuthorgoodevans
    • CommentTimeFeb 15th 2018
     
    Ian - The advice Adam has been getting here - and in his drawings he has located the osb racking layer outside of the studs with a layer of eps over and cellulose fill inside. Unfortunately DarylP's WUFI analysis indicates a problem with condensation at the OSB layer.

    Some of the options Adam has are...
    a) Move the osb layer to the inside (like the dwg above).
    b) Keep the osb layer where it is and use a VCL on the inside.
    c) Decrease the depth of the studs on the inside (reducing insulation here also) and increase the insulation outside (or similar effect) - this has the effect of keeping the osb layer warmer - but makes it more difficult to fix the cladding.

    It's difficult - I agree with Tom that OSB outside the studs is an ideal place to have the racking and an air tight layer - but it's no good if it rots (and worse - you won't see it so erring on the safe side is indicated).
    • CommentAuthoradam_w
    • CommentTimeFeb 15th 2018
     
    Posted By: goodevans Unfortunately DarylP's WUFI analysis indicates a problem with condensation at the OSB layer.


    Just to clarify, Daryl only has access to the Glaser based software, the quote I obtained to have the Wufi analysis performed was over £2k!

    I agree with everything else though :)

    I think what complicated the situation further is that the ground floor is a garage so not occupied or heated and part of the roof is vaulted and part is not, I never did like making things too easy for myself ;)

    Adam
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press