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Green Building Bible, Fourth Edition
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    • CommentAuthorCrawfW
    • CommentTimeSep 2nd 2020
    As discussed in another thread, I am building a 30m2 garden office. Assuming I figure out how to build the floor, this question is about not bothering to insulate it.

    I appreciate the implications this has for subsequent building regulations approval, but in-terms of heat-loss it should only be worth 15% of the total.

    The key reasons for skipping insulation are:

    1. We are pretty isolated in the country, surrounded by farmland – our garden is a riot of mice, rats and – my particular favourite – snakes. Putting less stuffing down there means less to eat or reside in.

    2. Knowing that everything takes longer than planned, I am worried that fibre will get wet in the time it takes me to get the roof on.

    Am I completely nuts (in relation to these ideas)?
    • CommentAuthorrevor
    • CommentTimeSep 2nd 2020
    What I did was cast a concrete floor slab put the insulation on top and then 22 mm chipboard. This way you can put the insulation in when you have got the roof done. You can build the timber stud walls on top of the perimeter of the slab anchored down with masonry fixings. Not putting insulation in will result in cold feet and create an uncomfortable environment and possible condensation.
    • CommentAuthorCrawfW
    • CommentTimeSep 2nd 2020
    Definitely not switching to a slab at this stage, but appreciate the comments about cold feet and what this feels like - not all about the 15%!
    I would build the floor - OSB on joists then overlay EPS (anything from 50mm upwards then another layer of OSB screwed through to the base layer, no timber battens needed. I would also put a perimeter of timber around to keep out the wild life. I would not use fibre (glass, rock or wood) insulation in this situation. With EPS timing is less important as it is fairly impervious to water.

    What about Staddle stones for the piers to help keep the wild life out? I think they look good as well.

    From the above you can guess that I would insulate the floor - I don't like cold feet either.
    • CommentTimeSep 2nd 2020
    Posted By: CrawfWin-terms of heat-loss it should only be worth 15% of the total.

    Is that still true if the rest of the building is insulated?
    How about using recycled glass slabs in the floor. They can be cut to size and fitted between the joists and I'd imagine they would be impervious to vermin (or at least a lot of hassle for them to gnaw into with not much gain at the end of it.


    It's not cheap but it would be fit and forget (Well, you'd remember it on freezing winter days when your feet aren't turned to ice).
    • CommentAuthorLF
    • CommentTimeSep 4th 2020
    You say "definitely not switching to slab at this stage" ...
    If mice and rats are a problem then having a slab, with no void beneath for them to enjoy sounds a better approach.

    You save money on not having a timber floor on the shed. Have slab smaller than shed foot print so that water from walls sheets down and the cladding drops lower than slab. Insulate under the slab.

    If really too late, i.e you have bought the shed then a rat poison station under the shed that you can check if they are taking the feed sounds like the way to go.

    Second hand or seconds for other insulation needs to keep costs down.
    • CommentAuthorJonti
    • CommentTimeSep 4th 2020
    Concrete slab is the best way to go and it is NEVER too late to do it. Otherwise, seal the void under the shed with concrete blocks and dig a grip/channel 50cm deep around it and back fill with old glass that normally goes to the recycling. I have found this works brilliantly.
    • CommentAuthortony
    • CommentTimeSep 4th 2020
    Note, I have never see such an awful title for a thread - GBF - uncharacteristic
    • CommentAuthorgyrogear
    • CommentTimeSep 5th 2020 edited
    yes - lack of insulants ≡ insolence
    • CommentAuthorCrawfW
    • CommentTimeSep 5th 2020
    So, i've put together an illustration (with units!) and (hopefully) attached it to this post. It shows an aerial view of the garden office, which has a total internal surface area of 30m2.

    The walls have a total thickness of 140mm (11mm OSB, 120mm structural timber, 9mm plasterboad).

    Tables show that C24-graded timber of 140*45 can have a span of 2.34 at 0.25-0.50 kN/m2, so the proposed pier locations seem fine. However -

    a) Would it be worth moving the piers in rows A and B slightly so that the nearby joists actually rests upon them?

    b) Should the joists be single 4.5m lengths supported by piers at half-length, or should they be 2.3m joists supported along the mid-line by another central joist?

    Comments and suggestions welcome, and very much appreciated.
    • CommentAuthorCrawfW
    • CommentTimeSep 5th 2020
    Oops, posted this in the wrong one of my threads!
    • CommentAuthorCrawfW
    • CommentTimeSep 5th 2020
    Tony - Thanking you making me feel so welcome. Wonderful to pay my money to join up and then get told off on my first day. Without challenger, you have managed to make this the least welcoming forum i have ever joined.


    Risking making myself even more unpopular, i do find the incomplete Eco-calculations frustrating. Adding a few parameters, and accepting that a full calculation would be difficult, the proposed concrete slab would embody something of the order of 4 tonnes of Carbon - equivalent to flying 30K miles (~5 return flights from the UK to China).

    By comparison, if this building were kept 50 degrees C above ambient 12 hours a day using electric heating then the additional heat lost by not insulating the floor - even if upped to 25% of the highest plausible total heat loss - would be 230KG/year.

    Or 17 years. A long-time in the life of the garden office you have re-branded a shed.

    And yet not-insulating one area (quite plausibly with an oil-based product that also embodies substantial quantities of carbon) is shocking whilst concrete is not.


    Such comments aside, the more constructive points are appreciated. Staddle stones and channels of broken glass coming up.

    (P.S - Whether or not it is 15% if everything IS insulated is a good point.)
    • CommentAuthorLF
    • CommentTimeSep 6th 2020
    Where do you get 4 te from.
    100 mm 30 M2 slab about 80 kg from Hanson website.
    • CommentAuthorCrawfW
    • CommentTimeSep 6th 2020
    Perhaps i was a little extravagant with my concrete volume calculation - did 8*4 and 300 mm thick (!). That works out at 9.3 m3.

    Circularecology.com suggest that there are about 380KG of embordied carbon per M3, which rounds out to about 4 tonnes.
    • CommentTimeSep 6th 2020
    Posted By: CrawfWif this building were kept 50 degrees C above ambient 12 hours a day using electric heating

    So the temperature inside would be between say 50°C and 70°C ? Not a place I would want to be!
    • CommentAuthorNRDigger
    • CommentTimeSep 6th 2020
    Hi all,
    In all seriousness I'd really like to get to the bottom of the slab question. Clearly the OP has indicated this wasn't a route that he wants to go down, but for others who might come back to this forum, it seems pretty key to me to know whether or not the Co2 emissions embodied in a concrete base/floor would indeed be of this magnitude. As with all green lifestyle decisions, understanding the relative 'badness' of certain decisions helps us take better and more effective action...

    (Personally I was only prompted to begin to take an interest in my own Co2 contribution over the New Year when we tried to work out our family's CO2 emissions over a year and made a New Year's resolution to try for a 15% reduction over the following year. I was amazed that my daily commute to the office was being dwarfed by the emissions from a few months of using a gas boiler to heat the house. Hence my other posts and queries here).

    (I'd like to offer to do the maths, but I wouldn't trust myself!
    • CommentAuthorjms452
    • CommentTimeSep 6th 2020
    Posted By: NRDiggerit seems pretty key to me to know whether or not the Co2 emissions embodied in a concrete base/floor would indeed be of this magnitude.

    There's lots of numbers flying around and it's not always clear how they are derived.
    We probably won't all agree, but we never will if it's unclear what in the assumptions are, so I will put up a straw man.

    CO2 of a slab approx 490KgCo2:
    about 80KgCO2/tonne of concrete [1]
    density of concrete: 2320 kg/m3
    30m2 slab 100mm thick = 7 tonnes of concrete = 490KgCo2

    CO2 of heating approx 416 KgCo2/yr:
    heating on working days for half the year (5*26*8=1000 hours/year)
    U value of 2w/m2/C
    Average temperature difference when heating 10C
    surface area of 5x6x2m box = 104m2
    Heat loss = 1000hr/yr x 2w/m2/C x 10C x 104m2 = 2080kwH/yr
    gas heating 215gCO2/kWh [2]
    2080kwH/yr x 0.215KgCo2/kwH = 416 KgCo2/yr

    So the carbon emissions of a concrete slab are about a years worth of heating

    [1] https://www.sustainableconcrete.org.uk/Sustainable-Concrete/Performance-Indicators-(1)/CO2-Emissions-Production.aspx
    • CommentAuthorNRDigger
    • CommentTimeSep 6th 2020
    Nice straw man, JMS.

    So in terms of [1] there are a few other sources that do put this higher, e.g. http://www.greenrationbook.org.uk/resources/footprints-concrete/ has this as 150kg/tonne

    (so roughly double)

    This is also very interesting: https://journal-buildingscities.org/articles/10.5334/bc.59/galley/24/download/ - if perhaps a bit hard to understand for a diy type like myself
    • CommentTimeSep 6th 2020
    CrawfW said: "Circularecology.com suggest that there are about 380KG of embordied carbon per M3"

    I agree with the number - maybe taken from https://circularecology.com/concrete-embodied-carbon-footprint-calculator.html - especially as it's based on the ICE database. But ...

    That's only the figure for 'virgin' concrete. I'd expect anybody on this forum to be using a reduced emissions/recycled product that incorporates GGBS or similar, so a bit less than 250 kg/m³ (maybe 230 kg/m³ for anybody being picky) though I'd note that's just the embodied carbon emissions, not the whole-of-life cradle-to-grave figure (which may be bigger or smaller depending on your assumptions). And concrete density is about 2400 kg/m³ if required.

    Slab depths typically vary between 100-200 mm depending on what structural load they're carrying. But they can get a lot more complicated. Mine is a 150 mm slab with ribs of another 100 underneath and an area of 150 mm extra. A smallish shed with slab on ground with no heave problems would only require 100 mm AIUI.
    Yes, interesting numbers!

    For a new build heated building, gas heating is about to be banned. If replaced by direct electric heating, the carbon intensity is now around 150g/kWh but is falling each year. If replaced by an ASHP, the intensity would be about a quarter of this, say 40g/kWh. Further improvement may come by doing the heating at night or other low intensity times.

    Also, building regs/stds will require some insulation, let's assume it ends up around 0.2W/m2/K overall including doors bridges etc, although it could be better.

    Let's also assume a heating season of 2000 degree-days.

    The CO2 of heating by ASHP would be
    2000 deg.d * 24h/d * 0.2 W/m2/K * 40g/kWh / 1e6 * 104m2
    = 40kg/ year

    So the embodied carbon of the slab would be about 10-20years of total heating emissions for the whole building, depending whose number you believe for concrete intensity...

    Interesting to see how decarbonising electricity is shifting the balance away from heat retention and towards avoiding embodied emissions.

    Edit to add: we need to include all the elements of the slab not just the concrete: eg: any reinforcement, insulation, delivery to site, digging the hole, eventual demolition and disposal, etc. And compare with alternatives eg carbon released from felling trees, shipping timber, etc

    Gyrogear will be here soon, to tell us about hempcrete!
    • CommentAuthorgyrogear
    • CommentTimeSep 6th 2020
    Posted By: WillInAberdeenGyrogear will be here soon, to tell us about hempcrete

    Nah, Willie - IMO cob floor, PEX and compost heat is the way to go !



    • CommentAuthorLF
    • CommentTimeSep 6th 2020
    WIA ... I hope there is enough of this low CO2 electricity to cover heating needs.

    So do the government! And also to cover transport needs.

    Discussion here http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16038&page=2 start about halfway down the 2nd page.
    • CommentAuthorLF
    • CommentTimeSep 7th 2020
    WiA - thanks - future gap in electricity is well covered in links.

    Even with reduced heat demand with insulation, there is still hot water, transport etc on top of current loads that will add to the gap. Price will need to go up to stop the hot tubbers !

    Dear all, my Hanson 80 kg number was wrong, I misread , seems it could be as low as 294 kg though. If you follow link on your phone you will see some numbers in very large and very small text.

    I was just doing a sense check on 4 000 kg CO2 number as it seemed high, but this was for 300 mm thick not 100 mm

    Concrete "challenge" was more about rats, damp and rot etc rather than insulation advantages, but the comparison to heating CO2 loads is very informative.

    Hanson number was actually showing 294 kg CO2/100mm (3m3) for "EcoPLUS" concrete but as stated it does not cover all embodied CO2 needed. They claim conventional concrete is 522 kg CO2/3m3.
    This is not pumpable, that took it to about 500 kg/3m3 concrete and for conventional about 1000 kg/3m3

    Good to know that there are lower CO2 concrete methods though when it is required to use it. I wonder what the cost and availability is like. They say "Being able to replace Portland Cement (CEMI) in a concrete mix with Hanson Regen (Ground Granulated Blast furnace Slag or ‘GGBS’) results in a much lower level of embodied CO2 than if ordinary cement was used. Hanson Regen, along with other environmental benefits, also improves the performance of the product, making it more durable and aesthetically lighter in colour. "

    • CommentAuthorRick_M
    • CommentTimeSep 7th 2020
    For this particular project, could you reduce the amount of the concrete needed by getting a few wheelbarrows full of rubble from a local demolition project and tipping them into your slab?
    • CommentAuthorPetlyn
    • CommentTimeSep 10th 2020
    We have used Foamglas throughout our build as it is not attractive to any insects or vermin, non combustible, doesn't take on water etc. We have a pallet of T4 floorboard and also some big bags of 4-8mm loose-fill glass beads remaining if either were of interest to insulate your floor?
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