Easy thermally-broken way to add wood fibre boards to *underside* of suspended ground floor, from above?

[YodhrinForge]

Pretty close to deciding I CBA with slabs and UFH and just insulating the existing perfectly good suspended timber floor as flooring has to come up for other work anyway. Easy option would be membrane slung between joists filled with wood fibre, but that leaves the joists as cold bridges, so I'd prefer to use the approach of adding tongue & groove 35mm wood fibre sarking underneath the joists(which doubles as the vapour-open windproof layer) - trouble is my solum cavity isn't gigantic, about 270mm below the joists, and even with a right angle drill attachment etc I can see getting under there with the thermally broken fixings is going to be a pain in the bum. I'll do it if I have to like, but if there were a ready made easier solution I'd be interested to hear about it - some kind of heavy duty plastic L or T bracket that could be hung from the joists maybe? I could screw in diagonally from above but I'm not sure if that's going to be secure enough long-term.

Edited Sunday at 04:22 by YodhrinForge

[JohnMo]

I think you maybe somewhat overthinking this. Have you calculated what bridging you actually get? Then ask yourself are the gains worth it to remove the bridging?

[torre]

We insulated with a windproof membrane like you but thermafleece between. With the membrane well taped at wall junctions it makes a huge difference. We decided the wooden joists weren't enough of a thermal bridge to merit extra mitigation effort.

Thermally broken fixings sound like expensive overkill to me here. How about stapling 10mm XPS tile backer board in strips slightly wider than the joist, from below each joist? That would reduce the thermal bridge and also help support the wood fibre

[Nickfromwales]

There will be nothing left of the boards by the time you wrangle them to the void. We’re currently installing them (22mm) on a clear open roof, and all you have to do is say the wrong word and the corners or T’ / G’s are fecked. 
 

These are a good choice, they are a plus, but you getting these under there, in one piece, to do that job, ain’t gonna happen. 
 

If you think that preventing repeat cold bridging on a cold (unheated) floor will make any difference, stop. It won’t. Just put some strips of XPS on top, and raise the floor by the thickness of that XPS, lifting the infill insulation to the same height.

 

Focus the time, money and energy you intend to use up on under-slinging that wood fibre board, on draught-proofing, as cold air infiltration trumps all the insulation in the world.

 

Don’t waste time and money for only microscopic improvements is my 2 cents. 

[YodhrinForge]

I'm going basically-EnerPHit - "wasting time and money for only microscopic improvements" is essentially the job description lol

 

I appreciate for most people it's not worth the bother, but as I'm DIYing a lot of this project and it's a "generational" retrofit of the building which will almost certainly be my home until I shuffle off, I'd rather do as much as I physically can and get the best possible end result, even if it wouldn't make sense for a commercial project. Adding 35mm sarking to the underside of the floor assembly bumps the u-value from over 0.2 to under 0.18 on the rough calcs which both hits regs(and I'm trying to show willing there whenever I can because there are a few things I want to do that will require the BCO to sign off exceptions, so better to not push my luck) and will actually make a bit of difference in the long run in the context of a highly airtight(MVHR) heatpump-heated home up here in Scotland. If I break any of the tongues eh, I'll just slap some Tescon Vana tape on the join. Lifting the floor unfortunately isn't an option as the doorways are already only just big enough for me to walk through without ducking and I'm avoiding structural work if I possibly can so going around and remaking all the lintels to be a course higher doesn't appeal.

 

@torre I considered something similar to your suggestion about the backer board, but I'm having to be very mindful of maximising the ability of the floor assembly to dry out into the cavity as these old stone cottages only have a small amount of ventilation and being in a conservation area means I can't just go around punching more holes into the wall, so I'd be hesitant to introduce even small elements that aren't as vapour open as possible. That's why the sarking is such an appealing product, being wind-proof and resistant to moisture but still basically transparent to vapour.

[Redbeard]

Is your sub-floor ventilation really excellent? One side to the other, and nice diagonal paths too? Vents at 225 x 150 minimum and preferably 225 square? I have sadly seen quite a few examples of retrofit UFI where moisture is sat on the underside of the bottom membrane due to insufficient 'scavenging' of moisture-laden air below the 'sandwich'. Don't assume that just because you have breathable membranes and vapour-permeable insulation there cannot be problems. This is not necessarily the case.

 

Like others I am struggling to see how you could get your under-layer  of WF under the joists (and fitting tightly together) from above. So would your lay-up, from the bottom, be: 35mm WF, membrane, flexi WF, VCL, floorboards?

[YodhrinForge]

12 hours ago, Redbeard said:

Is your sub-floor ventilation really excellent? One side to the other, and nice diagonal paths too? Vents at 225 x 150 minimum and preferably 225 square? I have sadly seen quite a few examples of retrofit UFI where moisture is sat on the underside of the bottom membrane due to insufficient 'scavenging' of moisture-laden air below the 'sandwich'. Don't assume that just because you have breathable membranes and vapour-permeable insulation there cannot be problems. This is not necessarily the case.

 

Like others I am struggling to see how you could get your under-layer  of WF under the joists (and fitting tightly together) from above. So would your lay-up, from the bottom, be: 35mm WF, membrane, flexi WF, VCL, floorboards?

Almost: 35mm WF(no need for a membrane with the T&G sarking boards), flexi WF, VCL, subfloor(the floorboards evidently went away sometime in the 20th as its particleboard at the moment), LVT or somesuch top layer. Ventilation is definitely a concern as there are presently two vents, one at the narrow front and one diagonally across from it at the wide rear(the best way to envision the property is a wide cottage divided internally into two properties shaped like interlocking Ls - mine is the small front wide back one). I suspect there was a third directly back from the front vent at one point but it was covered by the old kitchen & covered porch extensions, since the porch is getting ripped out and replaced with a new utility room I'll be restoring that one via a duct. Preliminary discussions with architect came up with the notion of using the pipe-converted chimney flue that will be left after I have the gas fire ripped out and carrying the pipe down into the subfloor, changing from cross ventilation to constant draw on all ground vents via stack effect on the chimney pipe(with anti-backdraught cap). That has to be modelled though, to make sure it'll work.

 

I have been thinking over torre's notion a bit more and trying to figure out if I could make the install easier by deepening the joists with 50x50mm rigid wood fibre board "battens" ripped from insulation(ordering a bunch of that anyway for the stone walls) and screwed in diagonally from above, just need to figure out a way to do the membrane underneath - maybe instead of trying to keep it whole and stretch it across crosswise to the joists I could cut it into strips sized to cover the gap between just a few joists and tape them(apply the first one "properly" by nailing off a runner to the wall plate, then secure it to the first joist with tape, then just move across taping each strip off against an extended joist, and then the next strip over that). Lots of expensive tape but easier to manage, and would still break the thermal bridge.

Edited Monday at 06:26 by YodhrinForge