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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorJulesB
    • CommentTimeFeb 28th 2021
    Hi all,

    I’m hoping to make a start today fixing 100mm wood fibre internally to first floor walls. The walls are rubble and have had a flattening coat of lime plaster. I’ll be fastening the boards with the recommended plaster/adhesive and mechanical fixings and then battens for services. I can’t find any info about whether the boards would normally rest on the water resistant chipboard floor or whether there needs to be a gap (keep any moisture that does make it into the floor out of the insulation?) Am I right in thinking would normally have the insulation off the floor 5mm or so and then battens inboard of it sitting on the floor?

    Thanks very much.
    • CommentAuthorJulesB
    • CommentTimeFeb 28th 2021
    Forgot to say, it’s just adding a bit extra to kiddo’s bedroom rather than a bathroom so no risk of water running on the floor.
    Ideally you want to aim for continuity of your internal insulation, throughout the house.

    So, assuming this is a chipboard floor sitting on joists, I would trim back the chipboard floor to allow me to continue the insulation down the walls below the flooring and then , either cut the board to fit around any joist ends, or use flexible wf batts around the joists. I would then continue the insulation into the rooms below (trimming back the plasterboard ceiling to avoid thermal bridging). . I appreciate that the room below may be for another day, in which case you might finish the insulation above the plasterboard, and trim back the PB at a later date. But, either way, it's best to aim for continuous insulation....

    also consider where you are intending to finish the insulation in respect of the room above, avoiding thermal bridging where possible.

    If you're worried about the odd spillage, say a glass of water on the floor, this shouldn't be an issue. If for some reason you think there would be regular or constant exposure of the WF to water, then that would be an issue. And probably wouldn't be good for the 'water resistant' floor either.

    I'm making assumptions here, i.e. that because this is a first floor room, your room below is also a habitable room. That this is an external wall, which remains external in any rooms above or below...
    • CommentAuthorJulesB
    • CommentTimeFeb 28th 2021
    Hi Chris.

    Many thanks for detailed reply; much appreciated. For various logistical reasons I’d rather not take up the perimeter of the floor but appreciate the quality of insulation will be compromised. One of those not letting best be the enemy of the good situations or whatever people say. The void between the chipboard floor and plasterboard ceiling below is full of rock wool so at least there’s insulation in there. Fully take your point about cold bridge at the edge of floor.

    It sounds as though sitting the wood fibre on the chipboard floor isn’t going to cause any problems as such and will definitely make installing it easier. Will mull it over in the week rather than starting this weekend I think. 👍
    Few other considerations:
    If you cut the board the exact size of the gap it will be difficult to swing into place. (If you are as unskilled at cutting as me, and your walls are as wonky, it will not be an exact fit anyway).

    But you do want an exact fit, otherwise there'll be a draught path from the floor void, round the end of the floorboards and under the skirting into the room, or through the services gap.

    I'd be tempted to cut the board deliberately 1cm short and use a plasterboard footlifter to lift it off the floor whilst fixing it. Then fill most of the gap with squirty foam and finish off with flexible caulk?
    • CommentAuthorJulesB
    • CommentTimeMar 1st 2021
    Hi Will,

    That makes sense - yes, I haven't started cutting these boards yet but giving myself a bit of leeway will definitely be easier. I'll cut them a bit short and foam round as you suggest. Thanks for the tip about the plasterboard footlifter too!

    • CommentAuthorjfb
    • CommentTimeMar 1st 2021
    You can chase services into the woodfibre board if you want to minimize wall thickness and do away with the service gap. But I'm guessing you are finishing with plasterboard over battens?
    • CommentAuthorJulesB
    • CommentTimeMar 1st 2021
    jfb - that's very useful to know. I might have a think about that.

    My current plan is to batten inboard the WF and then airtight membrane over the battens before Wood Wool board carrying the lime plaster. All that's going to take up a fair bit of space but that's the build up that the supplier recommended. If I can save a bit of room that would be good but I think I couldn't have the vapour barrier if no battens.
    • CommentAuthorjfb
    • CommentTimeMar 1st 2021
    Why do you need the vapour barrier? Isn't one of the points of wood fibre board to allow moisture to breath both ways.
    The render behind the woodfibre is normally the airtight layer. (airtight layer outboard of insulation and VCL inboard is normal)

    The suggestion your supplier recommends seems odd to me since you could do away with all the hassle of battens/wood wool board and just render straight onto the wood fibre.
    • CommentAuthorJulesB
    • CommentTimeMar 1st 2021
    jfb - good question. It's this: https://www.pavatex.com/en/products/sealing/pavatex-db-35/ and i guessed it was to limit the amount of water vapour from inside the building reaching the insulation. It's not plastic so I'm assuming that it allows the insulation to dry to the inside at times. There will be a lime airtight layer on the stone behind the insulation.

    Take your point about not needing it but having had a build up recommended I'm reluctant to deviate from it even if it's more work. Having said that the battens and boards are going to increase the wall depth by 55mm (ish) on two walls and it's already a small room. I wonder if the build up that they suggested is adapted from what they would spec for a whole house timber frame where you might be wanting to limit the amount of water vapour getting into the walls in kitchens and bathrooms etc. I'll ask them and post on here.
    • CommentAuthorjfb
    • CommentTimeMar 1st 2021
    pretty sure pavatex do a whole range of boards some of which are designed just to be rendered onto.
    Have you already bought materials?
    • CommentAuthorJulesB
    • CommentTimeMar 1st 2021
    Yes - I'm using Steico Therm boards as the supplier was struggling to get Pavatex boards. Functionally the same apparently. Have got the materials. Most suppliers seem to have long lead times on wood fibre boards at the moment but they arrived last week.
    • CommentTimeMar 1st 2021
    Posted By: JulesBIt's not plastic

    It is "Polypropylene fleece with polyolefin copolymer coating" and I would call that plastic :bigsmile:

    And it appears to have a fixed permeability, so it's not an 'intelligent' membrane.
    Sorry, late to this. Chris in Yorkshire has said most of what needs saying. If you want to avoid squirty foam with WF, collect up all your 'cutting fluff', wet it in a bowl or bucket and use it to caulk the joints, just like caulking planks on a boat - Bolster chisel for wide gaps and filling knife for smaller. It's slow, but it's not Pu.

    JulesB, it will take a relatively short time to cut back the chipboard. You could also pull back the rockwool and do a proper parge coat between the joist ends to make the job all good. If it's a rubble wall there is a good chance of there being air movement in the floor-ceiling void.
    • CommentAuthorJulesB
    • CommentTimeMar 1st 2021 edited
    Thanks Nick. I’ve been thinking about this following Chris’s post. The reason I’m not keen to cut the perimeter chipboard floor is that I’ll end up with a bit of a tricky problem to solve. The floor was lowered previously so it’s not a standard upper floor with joists going into the wall. Instead there’s four timbers bolted to the walls and the joists are attached to those with joist hangers. The edge of the chipboard floor running along one wall is resting on one of these pieces of timber. If I trim it by 100mm to allow the wood fibre to continue down the end of the flooring will be floating unsupported. I might normally just put another joist in to carry the new end of the flooring or some noggins but downstairs is a bathroom and all the plumbing and electrics are currently positioned in the worst possible place. Can’t add another joist or put noggins in without major disruption.

    ***edited because that’s not going to work!***
    Hi Jules, if I'm understanding your post correctly, presumably the wall plate you mention (with the edge of the chipboard sitting on it) will stop the WF board from continuing down the wall anyway?

    Can you post some photos?
    • CommentAuthorJulesB
    • CommentTimeMar 1st 2021 edited
    Hi Chris. Yes you’re right it will. I was thinking I could cut out so it fits around the timber but I guess there’s no advantage to that given the void is full of rock wool anyway. It’s just the edge of the flooring that’s causing a problem. No doubt having timber running around the perimeter of the stone walls and flooring resting on that timber isn’t great from a cold bridging point of view.

    Afraid I can’t get photos under the floor but I’ll post a sketch in a minute.
    How about,

    - lift the chipboard

    - fill any gap between the wall and the wall plate with expanding foam, or if you can get it down there WF fluff (see Nick's post).
    - sit WF board on wall plate
    - screw timber batten(s) to the wall plate, to create a ledge.

    - cut down chipboard so that it sits on the batten

    Or if there's a problem doing this due to services being in the way. How about screwing in the odd metal bracket to take the load of the chipboard edge?
    • CommentAuthorJulesB
    • CommentTimeMar 1st 2021 edited
    That’ll do it Chris. There’s a copper pipe runs horizontally halfway up that wall plate but from memory I could get a 50-60mm deep batten screwed above that which ought to be strong enough to carry the end of the floor. I’m very grateful for your advice 👍
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