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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorlineweight
    • CommentTimeDec 11th 2020
     
    I've been carefully looking at the various rigid insulation manufacturers' literature for timber frame.

    In each case they have a PIR type board that they recommend for use with timber frame.

    All of them seem to be quite happy to recommend its use as external sheathing insulation to a timber frame, when the timber frame is forming the inner leaf of a cavity wall with masonry outer leaf. And they all have BBA certs which quite clearly show this as a recommended use.

    Things get a little more vague when you look at timber frame with a ventilated cladding - for example, vertical tile hanging, or render boards. Most of them seem happy to recommend using the boards in a between-and-inside the studs buildup. A couple say it's also ok to use the boards in a between-and-outside the studs buildup.

    None of them (as far as I can see) have a BBA certificate that covers the use of PIR boards in any kind of timber frame buildup except with a masonry outer leaf.

    So why is this? Is it grenfell-related (but I'm nt talking about >18m here)? A lack of demand? Or is there something I'm missing?
    • CommentAuthorGreenPaddy
    • CommentTimeDec 11th 2020
     
    Lack of demand, as you say, and lack of imagination.

    That's how I built my own house 12 years ago, with 50mm rigid insul outside the timber frame, with vented cavity and timber cladding. Keeping the structure warm is something that's been discussed here many times. Means more labour outside, which is cold in winter, and blinding in summer due to reflective foil surface.

    Can't see that PIR etc would be very much more combustible than OSB and the cladding, which would be exposed if the rigid insul weren't there. Fire stop all openings, and I use an intumesent strip horizontally at first floor and eaves, to ensure full venting of the cavity. It passes building warrant/regulations every time, and that is something they are now even more picky about.
    • CommentAuthorlineweight
    • CommentTimeDec 12th 2020
     
    Posted By: GreenPaddyLack of demand, as you say, and lack of imagination.

    That's how I built my own house 12 years ago, with 50mm rigid insul outside the timber frame, with vented cavity and timber cladding. Keeping the structure warm is something that's been discussed here many times. Means more labour outside, which is cold in winter, and blinding in summer due to reflective foil surface.

    Can't see that PIR etc would be very much more combustible than OSB and the cladding, which would be exposed if the rigid insul weren't there. Fire stop all openings, and I use an intumesent strip horizontally at first floor and eaves, to ensure full venting of the cavity. It passes building warrant/regulations every time, and that is something they are now even more picky about.

    It seems a little surprising there's not more uptake when the "warm roof" pitched roof buildup is essentially the same and that seems to be regarded as fairly conventional now.

    I was wondering if it was also to do with the practical limits of fixing through the insulation layer. Where standard buildups are mentioned they don't seem to go beyond 70mm for the outer layer... I've yet to investigate whether there are fixings that can cope with more than that when installed vertically. If 70mm is the limit (which means a u value around 0.15) then does this mean that those who want to achieve a better U value will go with a different type of buildup.
    • CommentAuthorGreenPaddy
    • CommentTimeDec 12th 2020
     
    If you used 70mm PIR/PUR externally, and 140 between studs, you'd get approx 0.115 W/m2K.

    Use frametherm32 between studs (easier to install well), approx 0.132 W/m2K.

    Not saying you should/shouldn't do either of the above, just pointing out possibilities.

    The big benefit of the continuous external layer is the much improved psi (linear cold bridging), but you need to work on the continuity of envelope insulation - same as any EWI approach.
    • CommentAuthorborpin
    • CommentTimeDec 12th 2020
     
    Things have moved. When I first asked BC about TF and external insulation even with a masonry skin, it was a no no (was quite a few years ago). Second house I did use external insulation with a piece of supalux horizontally from frame to masonry between the insulation as the fire stop. Is that a common solution?

    If I was building again, I'd look at using EPS rather than PIR and hopefully find someone that sold interlocking EPS, corner pieces etc. I would like to use a cladding...

    I wonder if the issue is fire crossing the ceiling line with cladding. A fire stop from TF to masonary is a definite barrier (if done correctly). A fire in the cavity is not going to penetrate the masonary, so the only route is inside through timber and plasterboard. Cladding does not provide such a strong resistance to the fire going round the fire stop.

    Of course the risk of a fire *starting* in the cavity of a TF/masonry house is very low, but the risk of a fire starting on the cladding or outside the cladding and getting to the insulation is much higher (I'd suggest) and there is then a greater risk of it propagating above the ceiling line.
  1.  
    Coming with zero knowledge here, but are those boards approved for use as external wall insulation (<18m) with ventilated cladding? If so, would that certificate allow you to build a timber frame building and then EWI it?

    Given this week's revelations about how those certificates are obtained, I'm no longer sure they add much assurance about the fire safety...

    There have been discussions here about how to fix cladding through thick EWI, options like hanging the cladding layer from the eaves, or special long screws, or having fins of OSB sticking through the insulation like the web of an I joist, or burying battens in successive layers of insulation with each fixed to the previous layer's batten.
    • CommentAuthorlineweight
    • CommentTimeDec 12th 2020
     
    Is EPS any better than PIR from a fire point of view?
    • CommentAuthorborpin
    • CommentTimeDec 12th 2020 edited
     
    Posted By: lineweightIs EPS any better than PIR from a fire point of view?
    Don't think so, just prefer it's other qualities and depth is probably not an issue (often the reason for choosing PIR over EPS).
    •  
      CommentAuthordjh
    • CommentTimeDec 12th 2020
     
    Posted By: lineweightIs EPS any better than PIR from a fire point of view?

    PIR has the worst fire combustion products of all the insulations (incl cyanide). EPS is a bit better. Phenolic is better still.
  2.  
    I heard that in a few places but am not sure I understand it. Any material with organic Nitrogen in it (incl wood) will produce cyanide gas when it burns incompletely to smoke, but they all also produce carbon monoxide which is toxic through a similar mechanism to cyanide. It's unclear to me whether either the CN or the CO gas in smoke is worse than the other.

    It seems to me that this misses the point, which is that if you are breathing in smoke, then it's too late to care whether it has more CO or CN. We need to build houses in a way that reduces the risk of smoke exposure happening in the first place, eg by choosing materials with low surface flammability and separating the flammable materials from sources of ignition and from the occupants and their routes to escape, etc
    • CommentAuthorlineweight
    • CommentTimeDec 13th 2020
     
    Posted By: GreenPaddyIf you used 70mm PIR/PUR externally, and 140 between studs, you'd get approx 0.115 W/m2K.

    Use frametherm32 between studs (easier to install well), approx 0.132 W/m2K.

    Not saying you should/shouldn't do either of the above, just pointing out possibilities.

    The big benefit of the continuous external layer is the much improved psi (linear cold bridging), but you need to work on the continuity of envelope insulation - same as any EWI approach.


    Yes...although the recommendation generally seems to be not to have more insulation between the studs than outside of them. A large part of the attraction for me of doing the outside-the studs approach is to remove the condensation worries, and make any VCL less critical.
    • CommentAuthorlineweight
    • CommentTimeDec 13th 2020
     
    Posted By: borpinThings have moved. When I first asked BC about TF and external insulation even with a masonry skin, it was a no no (was quite a few years ago). Second house I did use external insulation with a piece of supalux horizontally from frame to masonry between the insulation as the fire stop. Is that a common solution?

    If I was building again, I'd look at using EPS rather than PIR and hopefully find someone that sold interlocking EPS, corner pieces etc. I would like to use a cladding...

    I wonder if the issue is fire crossing the ceiling line with cladding. A fire stop from TF to masonary is a definite barrier (if done correctly). A fire in the cavity is not going to penetrate the masonary, so the only route is inside through timber and plasterboard. Cladding does not provide such a strong resistance to the fire going round the fire stop.

    Of course the risk of a fire *starting* in the cavity of a TF/masonry house is very low, but the risk of a fire starting on the cladding or outside the cladding and getting to the insulation is much higher (I'd suggest) and there is then a greater risk of it propagating above the ceiling line.


    I did a project recently where we had PIR on the outside of a timber frame (only quite a thin layer though) and lead cladding hung off this. Extended over two storeys. We were asked by BC to install a supalux board there (running vertically, covering the entire area of insulation), but this was only because it was within 1m of a boundary.

    Why the preference for EPS? The problem with that (if installing onto the outside of a timber frame) is that if the maximum thickness is limited by the fixings for the cladding, then EPS is going to give you a lower U-value than PIR. Unless you go for increasing the amount of insulation between the studs. And even if you're happy doing that... it's limited by the depth of the studs, which might be 100mm in some cases.
    • CommentAuthorlineweight
    • CommentTimeDec 13th 2020
     
    Posted By: WillInAberdeenComing with zero knowledge here, but are those boards approved for use as external wall insulation (<18m) with ventilated cladding? If so, would that certificate allow you to build a timber frame building and then EWI it?


    Do you mean EPS boards? Good question - I'm not sure.

    However, I've in the past done timber frame, with EWI on the outside (rendered EPS type) and without any ventilation gap. In that buildup you can do quite a thick layer of EPS (we did 200mm) because the fixings only have to support the insulation and render layer - not a load of battens, tile hanging etc.
    • CommentAuthorborpin
    • CommentTimeDec 13th 2020
     
    Posted By: lineweightYes...although the recommendation generally seems to be not to have more insulation between the studs than outside of them. A large part of the attraction for me of doing the outside-the studs approach is to remove the condensation worries, and make any VCL less critical.
    For me, I put sufficient insulation on the outside so that the condensation risk analysis put the dew point inside the insulation. I therefore left out the VCL and keep the frame warm.
    • CommentAuthorbxman
    • CommentTimeDec 13th 2020
     
    It is possible that you might not to wise to rely on certificates I had dealings the led me to doubt the veracity of certificates and I suspect those poor souls in north London might have been better served by a more robust system
    •  
      CommentAuthordjh
    • CommentTimeDec 13th 2020
     
    Posted By: lineweightI did a project recently where we had PIR on the outside of a timber frame (only quite a thin layer though) and lead cladding hung off this. Extended over two storeys. We were asked by BC to install a supalux board there (running vertically, covering the entire area of insulation), but this was only because it was within 1m of a boundary.

    I don't understand. The 1m of a boundary regs are to do with the surface spread of flame, I think? I don't think flame spreads over the surface of lead? So what is a supalux board under the cladding supposed to be doing?
    •  
      CommentAuthordjh
    • CommentTimeDec 13th 2020
     
    Posted By: borpin
    Posted By: lineweightYes...although the recommendation generally seems to be not to have more insulation between the studs than outside of them. A large part of the attraction for me of doing the outside-the studs approach is to remove the condensation worries, and make any VCL less critical.
    For me, I put sufficient insulation on the outside so that the condensation risk analysis put the dew point inside the insulation. I therefore left out the VCL and keep the frame warm.

    I think that's the sophisticated version of lineweight's rule of thumb to have at least 50% of the insulation outboard of any material that might rot.
    •  
      CommentAuthordjh
    • CommentTimeDec 13th 2020
     
    Posted By: bxmanIt is possible that you might not to wise to rely on certificates I had dealings the led me to doubt the veracity of certificates and I suspect those poor souls in north London might have been better served by a more robust system

    Hear, hear. The latest revelations about lying and general attitudes inside major insulation manufacturers and the reluctance of French companies to engage just highlight the broken regulations we have.

    The idea that a company pays for a certificate that determines to a large extent what its products get used for is an obvious own goal. I don't know exactly what the answer is, but something that disconnects the funding (which rightly? comes from the companies) from the certification seems to be a necessary component of the solution.

    Plus prison sentences for the leaders of companies!
    • CommentAuthorlineweight
    • CommentTimeDec 13th 2020
     
    Posted By: djh
    Posted By: lineweightI did a project recently where we had PIR on the outside of a timber frame (only quite a thin layer though) and lead cladding hung off this. Extended over two storeys. We were asked by BC to install a supalux board there (running vertically, covering the entire area of insulation), but this was only because it was within 1m of a boundary.

    I don't understand. The 1m of a boundary regs are to do with the surface spread of flame, I think? I don't think flame spreads over the surface of lead? So what is a supalux board under the cladding supposed to be doing?


    This particular point involved quite a bit of confusing back and forth with the BC inspector and I now forget some of the details. But I think it has to be fire resisting, and the external materials facing the boundary have to meet a certain class, and lead cladding doesn't.

    I have to say, B4 of the approved docs I always find quite confusing to interpret, and feel that it could be set out in a clearer way that would make it easier to be certain that you are complying.
    • CommentAuthorlineweight
    • CommentTimeDec 13th 2020
     
    Posted By: djh
    Posted By: borpin
    Posted By: lineweightYes...although the recommendation generally seems to be not to have more insulation between the studs than outside of them. A large part of the attraction for me of doing the outside-the studs approach is to remove the condensation worries, and make any VCL less critical.
    For me, I put sufficient insulation on the outside so that the condensation risk analysis put the dew point inside the insulation. I therefore left out the VCL and keep the frame warm.

    I think that's the sophisticated version of lineweight's rule of thumb to have at least 50% of the insulation outboard of any material that might rot.


    Yes exactly. The 50% 'rule of thumb' is stated by some of the insulation makers and allows you to be following "manufacturer's recommendations". Of course you can also choose to do your own condensation risk analysis instead.
    • CommentAuthorlineweight
    • CommentTimeDec 13th 2020
     
    Also, I agree about not just relying on certificates, and there are plenty of products that have no BBA certificate: a BBA certificate in the end is something that you take into account in your general assessment of the reliability of a certain product.

    The reason I asked about it in this thread was more curiosity about why there isn't a BBA certificate for this particular buildup - because that suggests it's not a buildup that is used a lot. And I'm curious why it's not used more, because at first sight it seems like the obvious preference.

    Also of note is that fact that it's not really discussed much in the TRADA "timber frame construction" book. It's mentioned as a kind of alternative option, but none of their standard or example details show it.
    • CommentAuthorlineweight
    • CommentTimeDec 14th 2020
     
    By the way - there's another situation where running insulation externally to timber frame is much preferable: this is anywhere where you have a transition between a conventional cavity wall buildup and a clad timber frame. Because in that case, generally the timber frame will want to sit on top of the inner leaf, and insulation it externally allows the insulation to run in a continuous layer across both buildups. There are cavity closers and things to worry about, but in principle the insulation layer can be un-interrupted and does not need to cross the structural layer.
    •  
      CommentAuthordjh
    • CommentTimeDec 14th 2020
     
    Posted By: lineweightThis particular point involved quite a bit of confusing back and forth with the BC inspector and I now forget some of the details. But I think it has to be fire resisting, and the external materials facing the boundary have to meet a certain class, and lead cladding doesn't.

    I have to say, B4 of the approved docs I always find quite confusing to interpret, and feel that it could be set out in a clearer way that would make it easier to be certain that you are complying.

    Our timber garage is within 1 m of our boundary. All our building control required was that I paint it with fire redardant paint. Since it's next to a ditch, followed by a hedge and then a field I wasn't too concerned about any risks so I just did as asked.

    I'd have thought lead was fairly flame resistant, but maybe it melts at too low a temperature or something. I don't remember what the exact requirement/test is either.
    • CommentAuthorlineweight
    • CommentTimeDec 14th 2020
     
    I had a bit of a look at B4 to remind myself. Essentially it's to do with the risk of a fire on a building on a neighbouring plot spreading across the boundary, and the new wall needs to be able to resist this from the outside for a period of time. In fact the requirement to deal with this isn't limited to things within 1m of the boundary - the basic idea is that you can have a proportion of "unprotected wall" and this proportion decreases, the closer you are to a boundary.

    I'd say, logically, that what's on the other side of the boundary at time of construction shouldn't be relevant, because you should be designing for the possibility of something being built there in the future. Extending that logic though, the property boundary perhaps shouldn't be relevant at all because what happens if the part of the plot is sold off at a later date?
  3.  
    >>>>>"flame spreads over the surface of lead?"

    When historic buildings burn down, I understand that the lead roofing softens and falls off readily, and is found in the ash as a melted mess. I would speculate that thin metal cladding transmits heat pretty quickly, so isn't much good as a fire barrier, even before it falls off?
    • CommentAuthorlineweight
    • CommentTimeDec 14th 2020
     
    Posted By: WillInAberdeen>>>>>"flame spreads over the surface of lead?"

    When historic buildings burn down, I understand that the lead roofing softens and falls off readily, and is found in the ash as a melted mess. I would speculate that thin metal cladding transmits heat pretty quickly, so isn't much good as a fire barrier, even before it falls off?


    Yes, it's entirely non-combustible but not much good in terms of resistance.
    •  
      CommentAuthordjh
    • CommentTimeDec 14th 2020
     
    Posted By: WillInAberdeenI would speculate that thin metal cladding transmits heat pretty quickly, so isn't much good as a fire barrier, even before it falls off?

    I believe that depends on the particular metal. Lead is not a fire barrier it seems, but I understand aluminium for example is. As long as it doesn't have flammable insulation behind it!
  4.  
    It's more complex than that. It is necessary to tread very sensitively here, I would definitely not agree with the statement that 'lead is not a fire barrier.... but aluminium is'.

    Plain aluminium sheet will conduct heat through very quickly, but it can notoriously be used as a composite with other materials.

    BRE tested a large number of composite samples and reported 100% failure. They then tested seven combinations of wall buildup, of which two passed criteria for tall buildings, of which one used PIR insulation.

    The two build-ups that passed, used composites of aluminium with a mineral core, but even this did not fully prevent the fire reaching the insulation behind it.

    https://www.gov.uk/guidance/aluminium-composite-material-cladding
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