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Green Building Bible, Fourth Edition
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    • CommentAuthorElliotP
    • CommentTimeOct 3rd 2021
    Hi all,

    I'm converting a 3 bay oak framed garage (9m x 5m) into a dwelling. I would like to try to do the floor, walls and ceiling to as close to PH levels as possible. I won't get to full PH due to the amount of glass going along the front openings I doubt.

    Photos here:



    Anyway, I'm hoping someone would be kind enough to give me feedback on my planned wall build up and also advise if racking is needed? I'd like to avoid it from a thickness and cost point of view given the cost of materials at the moment.

    First off on the racking - the current construction has no racking but it does have several diagonal braces in the walls to keep it plumb. So the question is? Do i need any racking internally or externally. I wanted to avoid it externally otherwise it's gong to push the cladding out too far (beyond the oak beams), but if needed I'm considering panelvent. Otherwise I planned to remove the cladding, simply staple the VCL onto the studs, and then put the cladding back on over the top.

    The wall buildup i've planned (from inside out)

    1) Plasterboard & Skim
    2) Service void (25mm timber screwed through to studwork)
    3) VCL
    4) 100mm PIR
    5) Studwork with Frametherm 32 @ 90mm depth
    6) Vapour Permeable Sheathing Board (Panelvent or something else, or nothing hopefully?)
    7) Membrane (Tyvec housewrap)
    8) Timber Cladding

    Ceiling (from inside out, no service void needed as no lights in ceiling)

    1) Plasterboard & Skim
    2) VCL
    3) 100mm PIR
    4) Joists with with Frametherm 32 @ 140mm
    5) Existing breathable membrane
    6) Existing timber and tiles

    Floor will be 200mm of EPS70.

    My plan is to try to airtight the whole interior by having the PIR meet from the ceiling, walls, and upstands from the floor, aluminium taped together. This will also stop some of the bridging from the majority of oak beams. Then over the top of that, envelope the whole structure in VCL, from the floor, up the walls onto the ceiling. Then get the screed laid on top of that on the floor.

    Any advice on the above would be appreciated.


    Welcome Elliot.

    You said ''First off on the racking - the current construction has no racking but it does have several diagonal braces in the walls to keep it plumb.''

    Neither of the pics seems to show me any diagonals, but I think you are perhaps getting 'no anti-racking *board*' mixed up with 'no anti-racking'. Diagonals are one form of anti-racking, and boards are another. You don't have to have both. You may decide to have a sheathing board for other reasons, but if you have lots of diagonals the sheathing board need not perform an anti-racking function, necessarily.

    As far as air-tightness goes, for the 60-year life (estimate!) of your building I would not use foil tape. I use specialist air-tightness tapes. I find foil tape too fragile particularly when put under any sort of tension.

    Have not yet had time to look in more depth at the lay-ups, except to wonder if PIR in an oak frame 'feels' right, but that's your call, not mine.
    • CommentAuthorElliotP
    • CommentTimeOct 3rd 2021
    Hi Nick,

    Thanks so much for the response. Here is another photo I just grabbed showing the diagonals. There are 8 in total, 4 along the back, and 2 at each side. I'm no carpenter, but it looks pretty well done, and solid:


    Understood on 'anti racking', this makes more sense. So my question is now then; providing the diagonals are good enough (which I assume they are), is a permeable sheathing on the outside going to provide any advantage rather than simply going with a membrane over the studwork, then cladding back over the top of that?

    For tapes, do you have any recommendations? This is an area I've not researched.

    Finally in regards to PIR - I'm open to any advice. I wanted to stop the thermal bridging and reach insulation to PH standards without losing too much width.. I did consider extending the studwork out (I studs?) then cellulose the cavity (as per most timber frame PH examples), but this would be considerably extra work (sheathing each side and a lot of extra woodwork) and space lost (going to 300mm plus service cavity) and cost. I figured easiest/cheapest/thinest way would be to fix PIR to the studs with the 25mm batten to clamp it down, and using that to provide a fixing point for the plasterboard, and providing the service cavity all in one go.

    I'm excited to get started, but want to do it well, of course.

    Thanks again,

    • CommentAuthorrevor
    • CommentTimeOct 3rd 2021
    Have you calculated the u values. To get close to PH standards think you will need more than what you propose in walls and roof.
    Have you gone through LBA Building control is planning involved? Being a dwelling BC will need to be satisfied you meet all the relevant regs. You may need diagonal bracing of the roof rafters. Possible BC may request some structural calculations.
    I have only used one make of tape that was Tyvek acrylic tape. Very good but sometimes peeling off the release paper was a pain there are other tapes which have split release paper which is easier.
    Look at the VAT situation being a conversion you may qualify for 5% VAT I am not sure whether a garage is covered but worth checking and it does need setting up properly
    • CommentAuthorElliotP
    • CommentTimeOct 4th 2021
    Hi Revor,

    Thanks for the info, very interesting on the VAT - I didn't know about this so I'll look it up.

    I spoke to the local planning office, and they've said that no planning is required as it will be dependent on the main house for power, heating, hot water (our utility room is only a few meters away so we're going to plumb everything over to save space and cost). However I do need to involve building control I should think, it's on my list to do next.

    For U-Value, I've calculated at 0.13 for the walls, and 0.11 for the roof as it has a bit more depth. However I've not accounted for studwork in this as I'm not sure how. Either way it will be well beyond BC requirements and hopefully within PH range.
    Posted By: ElliotPFor U-Value, I've calculated at 0.13 for the walls, and 0.11 for the roof as it has a bit more depth. However I've not accounted for studwork in this as I'm not sure how.

    Select use demo version if asked

    This allows you to put stud work into the calculation
    On the layer where you have the studs select timber, studs and beams, structural timber, which puts in a 100 x60 as default where you can alter the values to reflect what you have. and automatically above is added an assumption of insulation between the studs which you can change to reflect the insulation you will use.
    • CommentAuthorrevor
    • CommentTimeOct 4th 2021
    Posted By: ElliotPThanks for the info, very interesting on the VAT - I didn't know about this so I'll look it up.

    I thought you were doing a separate dwelling what looks like you are doing is an extension so may not apply. If it is to house a dependant think there are some special clauses that will allow VAT relief but I am not sure but worth exploring. Will the buildings be linked i.e. go from one to the other without going outside? that could be a factor somewhere.
    • CommentAuthorElliotP
    • CommentTimeOct 4th 2021
    Peter - thanks for that useful tool. Looks like I'm at 0.146 for the wall, and 0.12 for the roof. Info here if anyone is interested:


    Revor - yes it is separate, you would need to go outside to get to the dwelling. Only the services tie it to the main building. I'll have a read up.
    • CommentTimeOct 5th 2021
    Posted By: ElliotPI did consider extending the studwork out (I studs?)

    If you were going to do this, you might well want to convert the studs into Larsen trusses. Google will explain.
    Like this, with one chord being your existing frame.
      Larsen Trusses (2).jpg
    • CommentAuthorElliotP
    • CommentTimeOct 6th 2021
    Hi Nick, djh,

    Yes I did consider this, but I figured this had several disadvantages in my situation:

    1) The time it would take

    2) Cost of additional timber at current prices (both studwork, and would need sheathing each side to hold the cellulose.

    3) Cost of pumped cellulose

    4) Loss of width

    Additionally, is that possible on the ceiling joists? Or would I still need to PIR that anyway?

    I'm open to doing the above, but it will be a fair amount of timber to extend the lot out and down.

    I've kind of made a decision anyway on the sheathing, chances are we will look to replace the external cladding with a composite cladding a few years down the line anyway, so it matches with our future house renovation, so I may as well sheathe it at that point.
    ElliotP wrote:

    ''Additionally, is that possible on the ceiling joists? Or would I still need to PIR that anyway?''

    Yes, if the existing rafters will take the additional load - you need a structural assessment.

    As regards the loss of internal space, the typical use of Larsen Trusses is to wrap the building *outside*. Would that be possible? As it appears to be an annexe you are planning, not a self-contained dwelling, are you constrained to the existing size?
    Hi ElliotP, some thoughts that may be of interest (or not)..

    I believe your Uvalues for the build system described in your OP will be as follows;

    wall - 0.122
    roof - 0.110
    floor - 0.14

    I use the BRE convention calculation method, which I use for all my building standards applications.

    So that would permit you to use quite a lot of glazing, but of course the overall performance may be calculated as a whole - not sure exactly how this building will be viewed, whether SAP (so TER versu DER) or just as an extension with default max. Uvalues from the regs.

    I haven't run a condensation calc, but worth checking due to the 100mm PIR internally. I wouldn't normally put more than 50mm internally, especially with 100mm stud (ie. 100mm wool insul). My condensation calcs pass at 50mm internally. Out of interest, a 50mm rather than 100mm PIR internal/underdraw would give you Uvals; wall 0.18, roof 0.15

    From the photo, I think I'm seeing the existing stud wall with weather boarding on the outside? If so, I would consider fixing the 100mm PIR over the external face of what's there already. Why would you need additional sheathing? Then a VPM over the PIR, battens, then cladding. Perhaps the exterior roof overhang is not adequate and so precludes this option?

    Not sure how the slab will be positioned versus the existing wall (digging down, just adding on top). That needs careful looking at as regards DPC/DPM.

    If the roof membrane is a highly vapour permeable type, you can fully fill the rafter void with insulation. Not all breathable roof membranes satisfy this criteria, so should be checked.
    • CommentAuthorElliotP
    • CommentTimeOct 17th 2021
    Hi Paddy,

    Thanks so much for the info - really useful.

    Correct on the exiting wall build up, the problem with putting PIR on the outside is that it will bring the cladding (currently featherboard, but may convert to composite in the future) too far out from the vertical oak beams at the corners of the building. I could possibly put some oak beading on the edges to hide the PIR, but this would still look a bit unsightly.

    For the floor, currently there is a DPM under the existing concrete slab, which comes up to the edges of the dwarf wall. My plan is 200mm of EPS70, then DPM again over that and bring it up the PIR to create an envelope. Then screed over the top of that. Is that sensible?

    Finally with the roof, the roof membrane is https://vaprfree.com/vaprfree/ which has a vapour transmission of 1065g/m2/hr - I don't know if this means it's adequate or not?

    I didn't even consider condensation to be honest. I'm now wondering if I go with 50mm PIR as you suggest. It will be cheaper and easier to work with, however I do need to try to offset the glass, which incidentally is triple glazed with a U value of 0.8.
    • CommentAuthorGreenPaddy
    • CommentTimeOct 20th 2021
    Hi ElliotP,

    I don't see an issue with your description of adding a floor on top of the existing, but I'm imagining steps might be required to get up to the new floor that would be say 300-400mm above existing ground level?

    Looking at the BBA certs for this, you do NOT need to have air gap under the membrane, so can full fill insulation, however, you must ensure drainage of water on the top side, which depends on the sarking/draping/battening method of the roof.

    Here's the cut from the BBA, re. condenstaion. (I don't particularly like their definition of a "warm roof", but you get the idea.)
    Inclined ceiling and insulation (warm roof)
    7.6 For roofs with an insulated inclined ceiling, ventilation above or below the underlay will not be required provided that the passage of moisture by diffusion and by convection is controlled, eg by a vapour control layer or a continuous envelope of insulation with a high vapour resistance. Ventilation may be required if specified by the tile manufacturer or where the roof covering is airtight, as described in BS 5250 : 2011.

    Wall condensation - any decent BCO will require a condensation calc, so even if you know it's ok, you may well need to provide this anyway. My experience tells me we don't get away with gut feel anymore, but by proof that the whole build-up works/is safe. That's why things like passiv haus design systems are effective, as it is a proven, connected process to delivering a design. Not suggesting you go down the PH route, but rather the principle of start-to-finish design.
    • CommentAuthorElliotP
    • CommentTimeOct 30th 2021
    Thanks again for your info, it's really useful.

    I've been thinking about your suggestion to put the PIR outside and this may be a better option. We'e decided we'll re-clad it with composite cladding and cover the oak, which means I don't have to worry about overhang covering the oak beams, so we can PIR straight over the lot, insulating the beams also, and then clad over that. Might not have to be 50mm, but we'll see. Roof will still need to be done internally of course.
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