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 September 14 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 September 15 by YodhrinForge

[Nickfromwales]

On 15/09/2025 at 07:05, YodhrinForge said:

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.

So in effect, a continually 'sucking' chimney that pulls freezing cold air through at a much higher rate of knots, by convection, all winter.

 

I'd have thought that would be the opposite of energy efficient?

 

On 14/09/2025 at 17:47, YodhrinForge said:

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.

All I do on a day-to-day basis is >95% PassivHaus standards and above. Current project is borderline EnerPHit refurb, so you've no fear of me 'not getting it' .

 

BCO standards will be severely lower than EnerPHit standards, so I doubt you have anything resembling a hurdle to leap over in that respect; most BCO's simply gawp and quiz when they are on site, typically seeing the types of projects I do for a living for the very first time!! Great that they're inquisitive, terrifying that they do not know much at all about true energy efficient build methodology.

 

My point being, you can invest this time and money on air tightness and reap much bigger rewards, vs microscopic boost on something which takes a huge amount of time / effort for very little measurable gain.

Investing smartly in the correct way, for the maximum sensible reward, is actually the fundamental 'job description' .

 

6mm marmox boards over the tops of the joists, below the floorboards, would be easier and give far better uplift in the floor spec. The joints can be taped and the periphery foam sealed to the masonry with FM330 foam to significantly add to the airtightness of the floor, plus the XPS will be plenty good enough for killing off the repeat cold bridging.

 

Your architects ideas are seemingly innovative, but in practicality are pretty dire. A permanently chilled chimney breast, over two stories, fed from a perpetual influx of damp cold air from outdoors to the under-floor void would be the opposite of my advice. The architect should be advising you to close these chimneys off 100%?!

[saveasteading]

1 hour ago, Nickfromwales said:

close these chimneys off

 

1 hour ago, Nickfromwales said:

stack effect on the chimney

I've just stuffed rockwool into a plastic bag and up the chimney for the opposite effect, to coincide with the oil and rad central hating being fired up.

I've  been up at the chimney pot in the past, and the amount of hot air flying up it is shocking.

 

I had one of those chimney umbrellas you may have seen at exhibitions, but lit a fire and it was gone.

 

I should point out that this house is very draughty. But you need control, not just a hole to outside.

[Iceverge]

Using the chimney to vent the underfloor isn't a good idea thermally. As Nick says it'll cool the house 24/7.

 

Forget rigid boards, impossible to put in there properly.

 

Instead make some Larsen truss type joist extenders with flanges on bottom to tack to the side of the existing joists. If you could rip a 300mm I-joist in half it would be perfect. The OSB webs will be plenty thermal break. 

 

Imaging something like these hanging from below your existing joists. 

 

 

Drop in hardboard  to sit on top of the new flanges. Use acoustic sealant/tape/foam to make it windtight. 

 

Drop in mineral wool insulation. Glue all joists in the flooring deck as your airtight layer. 

 

The process is much like this apart from your Larsen trusses. Cover the ground with polythene if you're worried about moisture. 

 

 

 

 

 

 

 

 

[YodhrinForge]

16 hours ago, Nickfromwales said:

So in effect, a continually 'sucking' chimney that pulls freezing cold air through at a much higher rate of knots, by convection, all winter.

 

I'd have thought that would be the opposite of energy efficient?

If the sarking boards are wind-tight, which they are as they're rated for roofing, I don't see how it would make much difference, as it will all be fully outside the thermal and airtight envelope as is the wall the chimney sits in. We're still talking about 3 pretty small vents feeding one small duct, it's not like it's going to generate a howling gale; back of a napkin prelim calcs suggested it would only be a bit more than double the present airflow which is with only two functional vents, and still below what modern regs spec for with air bricks every couple of metres around the house perimeter. I was also under the impression that a closed up chimney has to be ventilated somewhere regardless and since it won't be from the heated interior I don't see how the solum air is going to be colder than air direct from the exterior.

 

Quote

All I do on a day-to-day basis is >95% PassivHaus standards and above. Current project is borderline EnerPHit refurb, so you've no fear of me 'not getting it' .

 

BCO standards will be severely lower than EnerPHit standards, so I doubt you have anything resembling a hurdle to leap over in that respect; most BCO's simply gawp and quiz when they are on site, typically seeing the types of projects I do for a living for the very first time!! Great that they're inquisitive, terrifying that they do not know much at all about true energy efficient build methodology.

 

My point being, you can invest this time and money on air tightness and reap much bigger rewards, vs microscopic boost on something which takes a huge amount of time / effort for very little measurable gain.

Investing smartly in the correct way, for the maximum sensible reward, is actually the fundamental 'job description' .

 

6mm marmox boards over the tops of the joists, below the floorboards, would be easier and give far better uplift in the floor spec. The joints can be taped and the periphery foam sealed to the masonry with FM330 foam to significantly add to the airtightness of the floor, plus the XPS will be plenty good enough for killing off the repeat cold bridging.

 

Your architects ideas are seemingly innovative, but in practicality are pretty dire. An permanently chilled chimney breast, over two stories, and a perpetual influx of damp cold air from outdoors to the under-floor void would be the opposite of my advice. The architect should be advising you to close these chimneys off 100%?!

It's not either-or man, I'll be investing the time and money on airtightness anyway. All exterior and adjoining walls are being stripped back to bare masonry and will have a lime-based parge/levelling/airtight layer applied before lime adhesive & hammer fixing woodfibre insulation boards. Appropriate tapes, membranes, and where needed liquid-application barriers will tie the lime parge into new passive rated windows & doors and a complete rework of the current room-in-roof attic conversion which will have contiguous insulation and airtightness on the interior of the pitches from apex to eaves(ventilation gaps will be maintained to regs "outside" the insulation), as well as to the ground floor with the airtight layer under the insulation - which in the concept being discussed would be the airtight T&G sarking boards, or in the "normal" method a vapour-open roofing membrane stapled to form "slings" between the joists for the batts. I'm already committed to going whole-hog on this, that's why I'm trying to figure out how to maximise every aspect as much as I can without going *too* mad on costs. 

 

Insulation boards over the top of the joists kind of negates the object, which is to place as much of them as possible *inside* the insulation envelope, which should ensure only the very tips embedded in the walls on the coldest days get close to hitting dew point and even then should quickly recover to a temperature that encourages evapouration and allows them to expell any moisture through the vapour-open buildup.

 

15 hours ago, Iceverge said:

-snip-

All that said, this notion seems much easier to work with than my plan and should work fine with the "membrane sling" airtight concept with only a little bit of faffing. Rough calcs suggest I could hit aroundabout 0.15 with that - could be more, but factoring in the depth of the flange hanging down I think I'd only be able to fit 60mm insulation under the joists with the requirement to maintain a 150mm cavity.

[Nickfromwales]

I like a challenge, but not one that requires 100% of my available life force. 
 

I wish you well on your renovation, and please keep us updated as you progress in accordance with your architects guidance.

 

Are you aware of the retrofit / new build airtightness system from AeroBarrier? May be a good fit for you, to get into any missed nooks & crannies; that’s dependant on what stage you’re at.