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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthora63245347
    • CommentTimeAug 27th 2016
    I'm considering building a new well insulated outbuilding in the garden. Size approx 9m x 5m. It will be used as a summer room and will be very near an existing small outdoor pool. It will have a wet room bathroom in it. No sleeping accommodation. I would like to be able to use it all year round as a home office.

    It will have quite a lot of opening glass facing North West and also North East. These will be Bi-fold doors if we can afford it.

    I'd like it to be insulated well.
    It will have a warm flat roof covered with epdm.

    I'm not sure what the most practical insulated wall construction would be? The outer skin is likely to have some form of shiplap weatherboard as a rain screen. Should it be a normal block cavity wall made from block? Or a solid block wall with EWI on the outside?

    I think I would prefer the thermal mass to be on the outside, so that it warms and cools quickly.

    Any thoughts would be much appreciated. Thanks!
    • CommentAuthorEd Davies
    • CommentTimeAug 27th 2016
    Posted By: a63245347I think I would prefer the thermal mass to be on the outside, so that it warms and cools quickly.
    So why bother with the thermal mass at all? Why not just build a timber building and get more insulation or less wall thickness (so more internal floor area or less external footprint) as you chose?

    If you're having a warm roof then putting the insulation on the inside of a block wall makes for an awkward break in the insulation at the wall/roof junction. Much better to be all on the inside or all on the outside. What insulation do you have in mind for the roof? Can you carry it on down the outside of the walls?
    +1 for timber.
    • CommentTimeAug 28th 2016
    My house is smaller than the OPs 'garden shed'. My house is timber frame.
    • CommentAuthortony
    • CommentTimeAug 28th 2016
    what does insulated well mean? to me it means U=0.1 or less, lots of glass means 3g and reduced opening sizes, flat roof leaves me horrified, min fall 1:40
    Are you in Australia? If in the UK then bi-folding doors facing north seems crazy. They may be very fashionable at the moment but not practical or compatible with being well insulated. Why would you want them?
    • CommentAuthora63245347
    • CommentTimeAug 28th 2016
    Thanks for the feedback!

    I'll investigate timber frame. Sounds much more sensible than what I had in mind.

    The North West face would open out onto the garden. I don't have that much option to change it.

    Other than thick curtains are there any other tricks I should consider in front of the glass?

    Stupid question - Is there a reasonable way of blocking off some of the glass during the Winter with insulation? Eg something fitted into the reveal? Or is this a condensation disaster waiting to happen?
    External insulated shutters - not silly at all!
    • CommentAuthorringi
    • CommentTimeAug 28th 2016
    Firstly how many hours a day will the home office be used for outside of the summer?

    Could you make the home office be one section of the building, then have the wet room etc in the other section, with no connection between them, e.g. two outside doors? (So less space too heat.)

    If you can reduce with width to 4m, I think you will find it cheaper, as joists need to be a lot deeper over 4m.
    What is the ground like? And therefore what type of foundations are you thinking of?

    Does it come under building control? How long do you want it to last?
    You are likely to need Bldg Regs approval, I think.

    • CommentAuthorringi
    • CommentTimeAug 28th 2016
    It may be worth having two separate buildings, a web one and a dry one, so as to get each under the 30m2 for building regs....
    +1 to that.

    Not within a metre of the boundary, is it?
    • CommentAuthora63245347
    • CommentTimeAug 28th 2016 edited
    Slightly hesitant to post the pics, but the more critique, the better the outcome! Here's one version of what I have in mind. The overhang is dependent on what my friendly structural engineer says! It may become a lean to.

    I'm not committed to bi-folds, but I do want to be able to see out, and get out to the garden, and pool. The side with the external door faces out onto the garden.

    Yes it'll need building regs, as I'll also be adjusting the foul drainage. It'll be between 2 and 2.5m away from the boundary, so that I can go up to 3m height with the insulated flat roof. I'll also want to be able to put solar matting on the roof to heat the pool. I already have the matting, but it is on the ground at the moment. I already have 10kW PV spread over the garage and house and an ASHP, so the pool is mostly powered by renewables.

    With this in mind, is it worth trying to make it 2 separate buildings?

    I don't think this needs planning permission, but happy to be corrected?

    I won't use it as a home office to start with, but I want to make sure it can be used as such if I want to.

    The North East window looks out onto the pool. The pool is 8m by 4m.

    Not sure about the foundations yet. I've been given an initial suggestion of strip foundations around 1m deep.
    • CommentAuthora63245347
    • CommentTimeAug 28th 2016
    another pic
    Not as elaborate but we are looking at a SIP kit for our (smaller) garden room.
    • CommentAuthorringi
    • CommentTimeAug 28th 2016 edited
    Just because you need building regs for the drainage changes, does not require you to keep to building regs for the building if you have it under 30sqm. After all, do you care if the roof fails when you get 4 foot of snow…

    Also think about how wet people will get to the shower and changing room without making the rest of the building wet, as the shower and changing room will only be used when it is warm enough for the pool, they don’t need any insulation etc unlike the “office”.

    There are lot of companies selling prefab garden buildings etc, by separating the “wet” into a separate building; you may be able to just buy a prefab. Unless the office will be used for many hours most days, I question the benefit of lots of insulation, so a standard timber frame with 150mm of insulation made airtight may be good enough.

    For the wet building you could use concrete block, with timber fixed to the outside to make it look like the office. But does the wet building even need a rain proof roof and wind proof walls, or would just using nice custom made “fence panels” to the job?

    I don't know how far about the buildings have to be from each other to count as separate for BC...

    For foundations once you get outside of BC, there are cheap options like laying concrete fence post in gravel filled tranches to support what is after all an over sized shed. A lot depends on how much you are aiming to spend on fitting out the “office”, e.g. how much risk you are willing to take.
    • CommentAuthorgyrogear
    • CommentTimeAug 29th 2016
    just wondering how the pool is, for distance from property line ?


    • CommentAuthorCerisy
    • CommentTimeAug 29th 2016
    We were also thinking of an outside work room - (despite building our new home, setting up my drawing board still seems a step too far as it will interfere with Madam's sewing space!) - but I'm concerned about winter condensation. I was looking to do a lightweight timber structure, a lot like the stand alone garage we built from scratch, but with constant ventilation to move the air around. I'll allow for a wall mounted electric heater just in case I get enthusiastic in the winter, which I can leave on frost setting, otherwise I'll keep it simple!
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