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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorjerseyman
    • CommentTimeDec 9th 2019
    We are building above an existing Garage, the Architect has specified eco-joists for the ceiling of the Garage/ floor of the new build.
    For insulation he has specified rock wall between the joists, but as far as I can see, by the nature of eco-joists it is going to be pretty near impossible to get a complete thermal barrier this way. Is any one aware of a standard or recommended approach for insulating eco-joist floors? Or is the rock wool approach OK?

    Thanks for any help
    • CommentTimeDec 9th 2019
    When you say eco-wool, you mean metal web joists? Rock wool is a possibility; it's what we used for our ground floor ceiling/first floor. The main requirement we had there is for acoustic insulation, but if your garage will be cold then you need to consider thermal insulation as well. In that case you'll need to be more careful pressing the wool into the webs to fill gaps. Also think about any wiring and plumbing or ventilation that needs to run in the space before you insulate.

    Rock wool will be a bit messy, it can be very dusty; be sure to wear protective clothing. One alternative would be warmcel. It should fill the space better though it will cost more.
    How deep will the joists be? (How far is the new floor above the garage ceiling?)

    Unless the joists are quite deep, there won't be space for enough wool to keep your extension insulated from the cold air in the garage, so you might want an extra layer of insulation boards under the joists, or a higher spec insulation between them.

    The architect will also be thinking about fire barrier between the garage and the extension.
    • CommentAuthorjerseyman
    • CommentTimeDec 9th 2019
    Thanks for this. The Joists are 250mm, not ideal if just Rock wool , but at least it is good to know that it can be fitted around the metal webs.
    • CommentAuthorgyrogear
    • CommentTimeDec 9th 2019
    No experience of these joists, but looking at them per this :


    there will still be a risk of thermal bridging along the metal webs, because they come nearer to the boundary of the insulation : the web plates look like they come within an inch of the surface of the flange... Taken collectively, the webs will provide a likely route for heat-loss (presumably) downwards into the unheated garage... As DJH says above, some other (additional) form of insulation might be a worthwhile consideration.

    • CommentAuthortony
    • CommentTimeDec 9th 2019
    What U-value are you trying to achieve? Will you attempt in any way to mitigate the thermal bridging of the wood and metal?
    • CommentAuthorGreenPaddy
    • CommentTimeDec 10th 2019 edited

    if you use a wool insulation with a lambda value of 0.032W/mK (I personally like knauf frame therm 32, other makes are available), you would achieve a Uvalue of 0.138 W/m2K.

    That's a pretty good value, and into a garage (unheated space which will not have wind chill or as low temps as externally), I'm not convinced of the cost/benefit of driving that lower.

    You could go for a cheap loft roll wool insulation (0.044) with a 50mm skin of PIR or PUR underdrawing the floor, and that would give you a Uvalue of 0.126W/m2K. That may be more costly, and certainly more work.

    Actually, you might get a little better than above, as I've used timber "I" joists, so a slither of timber rather than open metal straps, but same ball park.

    If the rest of the build is as good as roughly 0.13W/m2K, you'll have a really good build, but focusing on the "easy" part of Uvalues in the chunky bits can blind you to the tricky areas of corners and junctions full of timber, that lead to high psi values for cold bridging for long strips of structure. As you drive down the general Uvalues, so these parts become increasingly significant. Ask your architect to show you details for all those bits (probably 6 to 8 specific details) and make him earn his money.
    • CommentAuthorjerseyman
    • CommentTimeDec 10th 2019
    Many thanks for this Green Paddy that is quite encouraging. I have a site meeting with the Architect, largely to discuss Thermal issues, although the plans did include a fair bit of detailing of the thermal envelope, as the build progresses I have spotted a few areas of concern re cold bridging which I want to discuss.
    • CommentAuthortony
    • CommentTimeDec 10th 2019
    Including through the joists I hope
    • CommentTimeDec 10th 2019
    Apparently metal web joists are pretty much equivalent to solid timber joists thermally. If so then an I-beam joist (i.e. OSB web) might be better if the thermal performance matters, though more awkward for running services. Something to talk to the architect about anyway.
    • CommentAuthorGreenPaddy
    • CommentTimeDec 10th 2019
    Wao. I can't believe easy joists are the same (fractionally worse actually), for thermal bridging as solid timber. I've read they Wolf Easy-joist tech guide, and that's what they say. So must be true.

    If at all possible, go for timber I joists, as they perform better thermally than solid timber (ditto easy-joist), and less timber being used.

    So taking the example of wool 32 as the insulation, for 250mm thickness;

    - solid timber / easy-joist = Uval 0.147 W/m2K
    - timber I joist = Uval 0.136 W/m2K (rechecked this)

    getting towards 10% less heat loss.

    PS...in case anyone happens to suggest filling with glass beads....:wink:
    • CommentAuthorbogal2
    • CommentTimeDec 17th 2019
    Ive just filled the ceiling/floor space between easijoists- not for insulation but for soundproofing- with Warmcel. Staple cheap membrane- I used landscape fabric- onto ceiling then get in firm to pump in the warmcel. Lambda value 0.038 I think.
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