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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.

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    Hello everyone,

    This topic again...

    I'd appreciate your thoughts on the best way to insulate floorboards from below, and indeed whether I should do this.

    I have a void of 90cm at the front of my property increasing to 1.3m at the rear, with several access hatches of varying size. The floorboards are well finished and have tight joints so I'd like to do this without disturbing them.

    I've read the various threads here on the subject but usually the situation involves a shallower void where the boards are to be lifted, whereas I'd like this done entirely from below.

    Also, any words of experience on the difference this makes to comfort? Any real-life condensation/rot problems? This wouldn't be a DIY job for me and I'm concerned about spending my limited budget judiciously, so want to be certain it would make a significant difference.

    Other info: 1930s semi-detached bungalow, east coast of Scotland. 9" solid brick with painted harling, lathe & plaster inner. Loft insulation and DG in place. Reluctant to do EWI or IWI for various reasons.

    Many thanks.
    • CommentAuthortony
    • CommentTimeNov 7th 2019
    Easy, I would friction fit R32 full depth of joists fibreglass batts, then fix polystyrene sheets across the joists to keep them in place, use nails or screws with big washers or insulation fixings

    Enjoy getting warm doing it then enjoy staying warm with reduced energy use going forwards
    Thanks, Tony.

    Is there anything special to consider around the perimeter, other than keeping the vents clear?
    I'm most of the way through doing this on our new old house, as it made a big difference to our old old house. We could sit on the floor comfortably afterwards, which was nice!

    It's a dusty unpleasant job. Sounds like your void is nicely deep, which is better than wriggling round on your shoulder blades.

    My current method is :

    0 make a big access hatch at least 45cm wide, lay dust sheets everywhere in room above, get lighting etc under the floor, wear dustmask goggles overalls headtorch, do as much cutting as possible above the floor not below

    1 check round under floor for damp rot woodworm and treat accordingly

    2 tidy up any cables, lag the pipes, check air vents unblocked, etc

    3 Leave lots of ventilation space where joists run over sleeper walls. I'm half-filling these gaps with PIR board stuck to underneath the floorboards, rather than running deep wool through them.

    4 I'm using strawberry netting with a wide mesh that I can get my fingers through to position everything. Cut it to size, fasten to first joist by trapping it with a thin batten then stapling through with power nailer

    5 the awkward bit: position a length of mineral wool between joists and loop the net under it to support. Adjust the tension so the wool is secure but as fluffed-up as possible, ideally enveloping below the joists. Clamp the net to the next joist with another batten, tweak the tension, staple through. Repeat.

    6 don't forget any tools down there when you're finished..

    7 seal meticulously around the edges of the floor, draught proofing is a good part of the benefits of the job

    This works for me, as my time is currently low cost. If you are paying for somebody to do this, they might find it quicker/cheaper to rip up the floorboards and work from above. I expect the materials cost to payback quickly, but not if a labour cost is involved.

    Others feel there's a benefit to laying a vapour membrane across the floor, I didn't as I want moisture vapour from the house to escape into ventilated floor void like it always did previously. Your choice!
    • CommentAuthortony
    • CommentTimeNov 7th 2019
    Polystyrene round the perimeter possibly foamed in

    I've reached Point 1 and found some rot! 2 - 7 definitely dissuade me from DIY :wink: I don't have a car or many tools (or much space to store materials) so it just seems easier to pay someone to do it, especially since it's a small house. What sort of a tradesman should I look for? (Child outgrowing chimney sweep size??)

    I'd like to insulate under the joists too as Tony advises.

    Glad that it makes the floor comfortable because that's what I sit on too!
    Posted By: WeeBeastieI don't have a car or many tools (or much space to store materials) so it just seems easier to pay someone to do it, especially since it's a small house. What sort of a tradesman should I look for? (Child outgrowing chimney sweep size??)

    IMO it will be difficult to get someone to do the job and even more difficult to get someone who will put in the attention to detail (i.e. taking care not to leave gaps and cracks in the insulation or not just stuffing the insulation in compressed) after the first half an hour. Without the attention to detail you stand the risk of too many gaps or squashed insulation negating the whole job and the exercise becoming a waste of money. Unfortunately IMO a job best done DIY
    Wee Beastie, if you go to any larger company "insulation specialists (alledgedly)" they will likely refuse to work in an area with less than 1m clearance (have had that refusal previously). So a local joiner would be the best option. We can help with info on how to install, to keep him/her right.

    Either Tony or WiA suggestions are good, and have done both for clients. However with Tony's, it can be a struggle to get the rigid boards down below the floor, and try to manipulate them once down there, but it give a great result. I've made the fixing washers from plywood, drilled holes in a grid pattern, then cut squares. Cheap and effective.

    For AiW's suggestion, I always use a cheap breathable membrane, as it prevents wind washing the heat out of the wool, and can be stapled more securely, rather than needing battens for restraint, but that's me.

    In terms of DIY, I know some people just decide they can't, and that's fine, but in terms of tools, if you went for netting/membrane and wool insulation, you only need...

    - stanley knife

    - a stapler




    insulation (2 layers of this)

    If you do decide to give it a shot yourself, I'm sure we can try to help you work out quantities of materials. Roughly speaking you would need approx twice the floor are for both items. Worst case you buy everything, and can't do it, then you have to pay someone for labour - a joiner would be the trade to go for - think £25/hour.
    Thanks for the advice, it is sounding more do-able DIY with a breakdown of how to go about it. Not as daunting as it first seems. The project is some way off so I have time to persuade myself. Re-wire and finalise a heating plan first which may get some of the wires and pipes tidied up.
    Some floor temperature measurements from last night:

    Floor with wool insulation slung below in nets: 18.9degC
    Floor with no insulation below it yet: 15.4degC

    So I'm very glad to have done all the dusty stuff to get a much warmer floor!

    These were the floor covering surface temperatures taken with an IR thermometer, similar measurements were also taken by walking on it in bare feet!
    • CommentAuthorWeeBeastie
    • CommentTimeNov 19th 2019

    Thanks, good to have a real life report. I 'can't wait' to have this done, but will have to wait as there is wiring and plumbing to sort out first which is part of a plan to extend a little, rewire and sort out the heating.

    Struggling to reach 13C as my living room air temperature today even with the heating on for several hours. Bedroom even colder. Brrr.
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