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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorcivhback
    • CommentTimeFeb 5th 2019
    Hi - I've been looking into insulating the suspended timber floor of my Victorian semi and was hoping for some final advice/comments before I crack on with it! I've read plenty of posts on the subject but haven't quite come across what I'm looking to do as it as it covers both a crawl space and basement garage.

    The space is circa 3.7m wide x 7.5m long with joists c225mm deep running width ways. The back half is over a crawl space which I can access from below, the front half being a basement garage converted some years ago. For various reasons I'm not looking to remove the flooring above so my proposal is to insulate from below:

    Crawlspace> Breather membrane stapled to underside of floorboards/sides of floor joists (to both protect the insulation and to aid in airtightness for the flooring above)
    140mm of Knauf Earthwool (speaking to Knauf will provide U value of circa .22) and will give me space for services running within joist structure.
    Supported by netting pulled stapled to side of joist then pulled across and stapled again
    Crawlspace floor loosely covered by non breather membrane

    Garage Ceiling:
    Same as before but with plasterboard applied to underside of joists for garage ceiling.

    I want to maintain a breathable build up as I can't guarantee vapour proof layer on flooring above - airflow is reasonable from below due to garage and crawlspace with a couple of airvents on the side and I've cleared out all the crap that had built up in the crawlspace over the years.

    Looking for general feedback and in particular whether a) airgap between garage ceiling and depth of insulation will give me any condensation issues or whether better alternative to plasterboard (fire rated) b) whether breather membrane above insulation is overkill to aid in airtightness?

    Appreciate any help


    • CommentAuthortony
    • CommentTimeFeb 5th 2019 edited
    I don’t like the void, it is likely to be draughty and cold, if not now then in the future.

    I would fully fill the space between the joists.

    For me vapour barrier on the warm side and no membrane underneath, use strawberry netting

    Double plasterboard staggered joints over garage or fire line board plastered
    • CommentAuthorcivhback
    • CommentTimeFeb 6th 2019
    Thanks Tony - by the void being draughty and cold - the logic of applying the breather membrane to the gaps between the joist ends was hoping to remove that issue (well not the cold bit!) but should reduce airflow when the garage ceiling is back on - crawlspace will still be an issue.

    In respect of vapour barrier because I won't be lifting the floorboards I didn't want to make the joists the only permeable element of the structure hence the breather membrane instead.
    • CommentAuthortony
    • CommentTimeFeb 7th 2019
    Paragraph one, dream on!

    Para2 - if the floor is vapour open moisture will be pumped through it and will condense on the bottom of the quilt or permeable sheet insulation and be trapped above the breather layer likely soaking into the bottom of the joists where more condensation will already be forming. Venting this away fast will be important.

    The breather above the insulation is an intermittent air tightness barrier as I see it .
    • CommentAuthorcivhback
    • CommentTimeFeb 8th 2019
    Thanks Tony - so if I've understood it you believe best logic is to forget the breather membrane above the insulation, and just insulate and support with netting over the crawlspace which should allow airflow to remove condensation, and over the garage full fill depth of joists with insulation and plasterboard ceiling to support it?
    I did this job using wool roll from b&q that comes all sheathed in a thin layer of plastic (the orange/silver one). This is easier to handle while lying on your back in the crawl space and the plastic stops wind getting through the wool to the floorboards. They claim the plastic acts to control vapour a bit, although clearly it doesn't seal the joists.
    I used netting to support the wool, you unroll the wool and shove it in tightly to fill the joists with one hand, unroll the netting under it with the other hand, and staple it tightly to the side of the joist with another hand... a powered staple gun helps.
    • CommentAuthortony
    • CommentTimeFeb 8th 2019
    Yes Daniel, Will’s method looks good, I would use string to temporarily support the quilt and do the netting in a seperate operation. Poly MUST be on top (warm side) if you get that.
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