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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorCranbrook
    • CommentTimeMay 10th 2020 edited
     
    HI all, whilst a long time lurker, ive now joined the forum

    Im currently building an extension and would be interested to have some feedback on my options

    The framework is to be 6x2, with flat roof also at 6x2, and a suspended floor also at 6x2 (which is supported on one end of the structure by 2 concrete pillars

    My current thinking, is to go with one of the following options (in the walls):

    Option 1: plasterboard > 6x2 stud > osb sheathing > 190mm eps > A thincoat render

    Option 2: plasterboard > PIR partially filled framework (pir thickness tbc) > osb sheathing > 80mm eps > thincoat render

    Ive purposely omitted the vapour barriers as I was hoping this could be a point for discussion. As im in Wales, I will need to double check insulation build ups to meet our 0.21 requirement

    Floor and ceiling insulation also needs consideration

    Any feedback or other options are welcomed
    • CommentAuthorSilky
    • CommentTimeMay 10th 2020 edited
     
    I don't like the sound of it, I think certain materials play well together, if you want to do EPS why not just build with blocks then adhesive to blocks + thin coat, if you want to do timber frame I would use T&G wood fibre boards stapled straight to the frame and some mineral wool bats between the studs. Wood fibre boards can take a thin-coat render, much nicer some wood cladding or a lime render. Here's an example pic of a timber frame build over an existing wall, then wood fibre boards fixed to the frame (with cellulose blown in instead of mineral wool due to complex shape, but bats is easier when designing from new). Look for a website called Ubakus, you can try out your designs there and see how they get on with condensation risk.
      clad.jpg
    • CommentAuthorSilky
    • CommentTimeMay 10th 2020 edited
     
    As you have quite a thick wall in Option 1, another choice could be to use a deep building block (30 / 36 / 42 cm deep ). No additional insulation and a 'reasonable' U-Value for UK climate. e.g. Poroton and render on the block, you won't beat that for simplicity, sound proofing, fire proofing and insulation with thermal mass.. all in one, far less detailing, no condensation risk. It's all 'natural' materials, just clay with bubbles in it, high embodied energy. To get the best performance you are meant to use an insulating thin-bed mortar and insulating render too. What U-value do you need to achieve for an extension in the UK now? EDIT : 0.21 above - apologies shouldn't post late at night when tired
    • CommentAuthorCranbrook
    • CommentTimeMay 11th 2020
     
    Unfortunately it has to be timber but thats definitely an interesting option for future work. I have considered wood fibre boards but ive been skeptical about them with regards to wood and water and how they dont tend to mix well
    • CommentAuthorphiledge
    • CommentTimeMay 11th 2020
     
    We added a first floor to a bungalow using site built 150x47 frames where the timber engineer had only specified 65x38. The additional depth was for extra depth of insulation but it was only a couple of years later I started to regret not making 150mm I beams to cut down on thermal bridging. Unless theres a structural reason for needing chunky timber, dealing with the thermal bridge might be a consideration??
    • CommentAuthorCranbrook
    • CommentTimeMay 11th 2020
     
    Essentially I want to encase all the timber in some form of insulation to prevent/minimise any thermal bridging
    •  
      CommentAuthordjh
    • CommentTimeMay 11th 2020
     
    Personally, I would put mineral wool batts between the studs. Blown-in cellulose is an alternative. The EPS outside is fine. I would cover the inside of the studs with a vapour membrane, probably Intello or similar, and I would use battens to create a service void for cables etc before attaching the plasterboard. Whether I-beams are worth it depends on the U-value calcs, I suppose.
    • CommentAuthorCranbrook
    • CommentTimeMay 12th 2020
     
    OSB sheathing on the outside between the mineral batts and the EPS is it?
    • CommentAuthorlineweight
    • CommentTimeMay 12th 2020 edited
     
    Posted By: CranbrookHI all, whilst a long time lurker, ive now joined the forum

    Im currently building an extension and would be interested to have some feedback on my options

    The framework is to be 6x2, with flat roof also at 6x2, and a suspended floor also at 6x2 (which is supported on one end of the structure by 2 concrete pillars

    My current thinking, is to go with one of the following options (in the walls):

    Option 1: plasterboard > 6x2 stud > osb sheathing > 190mm eps > A thincoat render

    Option 2: plasterboard > PIR partially filled framework (pir thickness tbc) > osb sheathing > 80mm eps > thincoat render

    Ive purposely omitted the vapour barriers as I was hoping this could be a point for discussion. As im in Wales, I will need to double check insulation build ups to meet our 0.21 requirement

    Floor and ceiling insulation also needs consideration

    Any feedback or other options are welcomed


    I'd say it partly depends on whether you are short of space. Your option 1 seems very low risk and straightforward - timber all clearly on the warm side of insulation, and you can use the space between studs to run cables and so on. And you can focus your attention on getting the EPS layer right, and no worries about cutting PIR to fit tightly around frames etc, and fewer worries about what happens at wall/roof junctions. If you are tight on space, then I would consider how valuable the extra 110mm that option 2 gives you is.
    FTR I've done an extension pretty similar to your option 2. That was in London where space is at quite a premium.
    Are you building it yourself?
    • CommentAuthorCranbrook
    • CommentTimeMay 12th 2020
     
    <blockquote><cite>Posted By: lineweight</cite><blockquote><cite>Posted By: Cranbrook</cite>HI all, whilst a long time lurker, ive now joined the forum

    Im currently building an extension and would be interested to have some feedback on my options

    The framework is to be 6x2, with flat roof also at 6x2, and a suspended floor also at 6x2 (which is supported on one end of the structure by 2 concrete pillars

    My current thinking, is to go with one of the following options (in the walls):

    Option 1: plasterboard > 6x2 stud > osb sheathing > 190mm eps > A thincoat render

    Option 2: plasterboard > PIR partially filled framework (pir thickness tbc) > osb sheathing > 80mm eps > thincoat render

    Ive purposely omitted the vapour barriers as I was hoping this could be a point for discussion. As im in Wales, I will need to double check insulation build ups to meet our 0.21 requirement

    Floor and ceiling insulation also needs consideration

    Any feedback or other options are welcomed</blockquote>

    I'd say it partly depends on whether you are short of space. Your option 1 seems very low risk and straightforward - timber all clearly on the warm side of insulation, and you can use the space between studs to run cables and so on. And you can focus your attention on getting the EPS layer right, and no worries about cutting PIR to fit tightly around frames etc, and fewer worries about what happens at wall/roof junctions. If you are tight on space, then I would consider how valuable the extra 110mm that option 2 gives you is.
    FTR I've done an extension pretty similar to your option 2. That was in London where space is at quite a premium.
    Are you building it yourself?</blockquote>

    Yes building it myself

    Space is fine not a problem at all so that's good to hear

    Would you use a vapour layer anywhere? I have tyvek already if needed
    •  
      CommentAuthordjh
    • CommentTimeMay 12th 2020
     
    Posted By: CranbrookOSB sheathing on the outside between the mineral batts and the EPS is it?

    Yes, that sounds fine too for racking. You could use a more vapour permeable board like panelvent.

    BTW, in your option 1, why such deep studs?
    • CommentAuthorlineweight
    • CommentTimeMay 12th 2020
     
    If it was a self-build (ie I take on the risks myself) then I'd say, why bother with a vapour barrier in option 1.

    The timber is all on the warm side of the insulation so there doesn't seem to be any condensation risk to worry about.

    The places where thermal bridging is liable to happen are at the wall-floor and wall-roof junctions, would be interesting to see what your proposed details are at those points.
    • CommentAuthorwookey
    • CommentTimeMay 12th 2020
     
    I agree with silky that whilst what you propose is fine, it makes more sense to do the whole thing with timber and get a fully breathable low-carbon wall. I'd also consider I-joists and blown cellulose for the wall. We are scared of woodfibre in this country because we're not used to it but it works well and is a better long-term plan than covering every building in plastic foam IMHO. You are in Wales so get the detailing right. Decent overhang, expanding beads at windows, right render, or a rainscreen if you are worried about water ingress behind render.

    Talk to BacktoEarth Mike Wye (probably closest to you?) or NBT about your options.
    https://www.natural-building.co.uk/systems/new-build-systems/
    https://www.pavatex.com/en/application/wall/external-wall-insulation-in-timber-construction/
    • CommentAuthorCranbrook
    • CommentTimeMay 12th 2020
     
    <blockquote><cite>Posted By: lineweight</cite>If it was a self-build (ie I take on the risks myself) then I'd say, why bother with a vapour barrier in option 1.

    The timber is all on the warm side of the insulation so there doesn't seem to be any condensation risk to worry about.

    The places where thermal bridging is liable to happen are at the wall-floor and wall-roof junctions, would be interesting to see what your proposed details are at those points.</blockquote>

    The roof is going to be a warm epdm flat roof with circa 120mm pir insulation. The floor is still tbc but at the moment im thinking of insulating on top of the floorboards with pir.. I think I am going to have an element of unavoidable thermal bridging around the floor area
    • CommentAuthorCranbrook
    • CommentTimeMay 12th 2020
     
    <blockquote><cite>Posted By: wookey</cite>I agree with silky that whilst what you propose is fine, it makes more sense to do the whole thing with timber and get a fully breathable low-carbon wall. I'd also consider I-joists and blown cellulose for the wall. We are scared of woodfibre in this country because we're not used to it but it works well and is a better long-term plan than covering every building in plastic foam IMHO. You are in Wales so get the detailing right. Decent overhang, expanding beads at windows, right render, or a rainscreen if you are worried about water ingress behind render.

    Talk to BacktoEarth Mike Wye (probably closest to you?) or NBT about your options.
    https://www.natural-building.co.uk/systems/new-build-systems/
    https://www.pavatex.com/en/application/wall/external-wall-insulation-in-timber-construction/</blockquote>

    As I already have the timber Im fgoing to stick with traditional studs, but will definitely consider I Joists in future.

    One thing I do need to need to look in to is where and how windows sit in relation to external wall insulation
    • CommentAuthorCranbrook
    • CommentTimeMay 12th 2020
     
    <blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: Cranbrook</cite>OSB sheathing on the outside between the mineral batts and the EPS is it?</blockquote>
    Yes, that sounds fine too for racking. You could use a more vapour permeable board like panelvent.

    BTW, in your option 1, why such deep studs?</blockquote>

    just for a stronger structure really.. already have the timber on site
    • CommentAuthorlineweight
    • CommentTimeMay 12th 2020 edited
     
    Posted By: Cranbrook
    Posted By: lineweightIf it was a self-build (ie I take on the risks myself) then I'd say, why bother with a vapour barrier in option 1.

    The timber is all on the warm side of the insulation so there doesn't seem to be any condensation risk to worry about.

    The places where thermal bridging is liable to happen are at the wall-floor and wall-roof junctions, would be interesting to see what your proposed details are at those points.


    The roof is going to be a warm epdm flat roof with circa 120mm pir insulation. The floor is still tbc but at the moment im thinking of insulating on top of the floorboards with pir.. I think I am going to have an element of unavoidable thermal bridging around the floor area


    Why only 120mm on the roof?
    • CommentAuthorCranbrook
    • CommentTimeMay 12th 2020
     
    Just to meet the u value requirement
    • CommentAuthorlineweight
    • CommentTimeMay 13th 2020
     
    Posted By: CranbrookJust to meet the u value requirement


    Up to you - but with the roof buildup you intend to use, you can substantially improve the U-value with relatively little additional expense/effort, just by increasing the thickness of the insulation to 200mm or so.
    • CommentAuthorCranbrook
    • CommentTimeMay 13th 2020 edited
     
    Actually I need 150mm minimum on the flat roof, not sure why I said 120mm
    •  
      CommentAuthordjh
    • CommentTimeMay 13th 2020
     
    A roof is a good place to start with I-joists. Lighter weight and less need to support things from the web make it more advantageous than walls.
    • CommentAuthorlineweight
    • CommentTimeMay 13th 2020
     
    Posted By: djhA roof is a good place to start with I-joists. Lighter weight and less need to support things from the web make it more advantageous than walls.

    Only really makes sense above a certain span/scale of work, though, I'd say? So maybe not for a domestic extension.
    •  
      CommentAuthordjh
    • CommentTimeMay 13th 2020
     
    Posted By: lineweightOnly really makes sense above a certain span/scale of work, though, I'd say?

    It depends how much/whether Cranbrook wants to increase the thickness of the insulation beyond 120/150/whatever mm although as usual there are alternatives such as warm roofs and inverted roofs.
    • CommentAuthorlineweight
    • CommentTimeMay 13th 2020 edited
     
    Posted By: djh
    Posted By: lineweightOnly really makes sense above a certain span/scale of work, though, I'd say?

    It depends how much/whether Cranbrook wants to increase the thickness of the insulation beyond 120/150/whatever mm although as usual there are alternatives such as warm roofs and inverted roofs.

    They have already said it's a warm roof - so joist size doesn't affect insulation thickness. That's why I suggested, might as well increase it.

    I think the combination of external insulation on walls, and warm roof (ie, insulation on top of roof deck and joists) is a good one, because you can avoid the problem of insulation having to cross over structure at the roof/wall junction*. Easier to achieve genuinely continuous insulation, and no worries about condensation on timber anywhere.

    And the insulation thickness is essentially independent of wall buildup/structure sizing.

    * although, you still have to work out how to support whatever flashing happens at the roof edge, which can be tricky. Or, if there is a parapet, how to deal with that.
    • CommentAuthorCranbrook
    • CommentTime6 days ago edited
     
    <cite>Posted By: lineweight</cite>If it was a self-build (ie I take on the risks myself) then I'd say, why bother with a vapour barrier in option 1.

    The timber is all on the warm side of the insulation so there doesn't seem to be any condensation risk to worry about.

    The places where thermal bridging is liable to happen are at the wall-floor and wall-roof junctions, would be interesting to see what your proposed details are at those points.</blockquote>

    Isn't the warm side of the insulation the critical/vulnerable point? If so wouldn't that mean my OSB could be at risk?
    •  
      CommentAuthordjh
    • CommentTime6 days ago edited
     
    Posted By: CranbrookIsn't the warm side of the insulation the critical/vulnerable point?

    No. The warm side generally experiences pretty much the same conditions as you do inside the house. So as long as you keep the RH below 60% or so everything is fine. Problems start if the RH gets to 100%, because of lower temperature, anywhere that water vapour can get to in quantity. That is typically on the cold side of the insulation, if it occurs, but is only problematic if there is anything there that can rot or decay due to moisture.

    What lineweight said is correct. I'd overlooked that you had said it was a [genuine] warm roof. So in that case you don't need a vapour barrier since vapour can happily flow through the insulation and condense outside if it wishes.
    • CommentAuthorCranbrook
    • CommentTime6 days ago
     
    Most flat roof specs I see, and the few I've completed in the past, all have an 18mm deck on top of the joists, followed by a vapour barrier and then followed by insulation... Would what you are saying then mean that the vapour barrier is actually not required?
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: CranbrookMost flat roof specs I see, and the few I've completed in the past, all have an 18mm deck on top of the joists, followed by a vapour barrier and then followed by insulation... Would what you are saying then mean that the vapour barrier is actually not required?

    Yes, that's right. The vapour barrier is required if the insulation is between the joists, so the outside parts of the joists get cool, or if there is another deck on top of the insulation without ventilation below it. But Building Regs are against the latter configuration anyway, since it can easily result in damage if things aren't done properly.
  1.  
    Isn't there usually a waterproof membrane over the top of the insulation on a flat roof? (Or fancy sheet metal)

    Doesn't the vapour barrier restrict the vapour that would otherwise condense on the underside of the membrane or metal?

    Granted that's well away from most of the timber, but it can still drip or freeze or dribble downslope.
  2.  
    Yes, as I understand it, it's to try and eliminate the risk of vapour getting to the underside of the roof membrane via gaps in or diffusion through the deck/insulation. Particularly an issue with certain types of metal roofing where it could cause corrosion from the underside. For that reason, metal roofing is often ventilated between the insulation and the metal, and while you can do it without, you are always warned that the vapour barrier is then critical.
   
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