Floorboards are up - how do I tackle this?

You may find the only thing holding that sleeper wall in one piece is the weight of the floor. From the pics it looks like lime mortar, sds and 6mm bit will soon shift the mortar and let you pull a brick or 6 out
PIR (solid board insulation) does have better thermal performance than rockwool (100mm PIR gives same thermal performance as 150mm rockwool) but you have to fit it tight (horribly dusty job) and per cubic metre PIR is well over twice the price of rockwool. Rockwool is easier to get airtight and has better acoustic performance, you have the depth to use it.

Gizzydash said:

Thanks - so just to clarify, with a deeper joist I wouldn't need the sleeper wall at all? the span of the joist would just be supported at the external wall plates, roughly 3m span, is that right?

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Yes. Sorry I should have said use a 225mm deep joist. It will course into the brickwork better. When you cut a treated joist or any treated timber you should treat the "raw" end with two coats of hand applied treatment. Generally speaking there are three kinds of insulation available on the market.

1. Quilt insulation
2. EPS (Expanded polystyrene)
3. PIR (polyisocyanurate)

They are listed in descending order of cost and efficiency. PIR is a lot more expensive than EPS or quilt but it offers a much higher thermal efficiency. It can be cut by hand and laid between the joists and supported with roof laths or similar. Quilt insulations can be supported by netting and are cheaper and easier to lay but their thermal efficiency is not as good as PIR per mm thickness.

AiY

Think ill forget about insulation, going through BCO seems a whole hassle for such a minor thing. Take it I just use a decent galvanised nail to nail back down the joists where the previous nails have rusted away to nothing?

Forget BCO and forget replacing the joists. Just do as said above. Staple some netting to the bottom of the joists and add 100mm loft roll between. Silly not to, since you've already done the work taking it all up.

Personally I would use 5mm screws rather than nails. The floorboards will hold everything together.

Gizzydash said:

Think ill forget about insulation,

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No don't do that, it makes a difference. 100mm loft roll is cheap as chips, you only really require netting (garden netting would work) and a staple gun.

Well worth the effort, for both your comfort and your fuel bills.
If you keep the insulation flush with the bottom of the joist, you will still have plenty of airflow underneath for the ventilation.

Agree insulation makes sense while floor is up - is it any concern filling insulation flush to the joist bottom at sleeper wall effecitvely blocking the air flow over the sleeper wall?

It won't be ideal blocking airflow - remove a couple of bricks or (bodge compromise) use some bits of flat tumble dryer ducting between sleeper wall and insulation to maintain airflow while keeping insulation benefits.
You've done the hard work (lifting the boards), making a few gaps in that wall will be a doddle in comparison.

Sorry, hopefully final question on this. Attached pictures are from a joist and floorboards at the side of the room (beside door opening onto old tiled floor). Obviously seen better days - I think this was caused by a chunk of concrete stuck between the wall (below DPC) and the joist and the floorboards butting up to the wall just at/below DPC. The hallway the room joins onto has original tiles and who knows what's beneath, I know it's concrete but no idea if it was damp proofed etc, so imagine the big chunk of concrete was passing damp across and into the joist. The joists and finished floor being right at or just below DPC isn't great i'd imagine and you'd want to ensure finished floor is above DPC but can't imagine there's anything I can do about that now, just ensure when I put things back down that nothing touches the wall or has a DPC around it. So few questions:
I don't think 115x35mm joists are done now - so how do I replace that joist?
Wall plate is rotten at the end - how do I deal with that? Should I be concerned about wood worm (pretty isolated along said wall - all other timbers seem fine).
If I insulate the floor (netting & insulation packed in-between joists) will I not have the same problem in the future i.e. insulation will be touching the wall and act as a bridge for moisture across to the joist?
Thanks in advance

Looks like wood worm. I have a section of wood worm where a historical toilet leak rotted a joist. Worm left the rest/solid wood alone
More qualified people will suggest a fix.

Hi Gizzydash,

I'm not sure what you're trying to achieve.
Do you want insulate the floor? My advice is you should. However, I don't agree that "quilt insulation is your best option because someone has said it's as cheap as chips. It's one option and it is cheap. It is not as thermally efficient as other different types of insulation. Very broadly speaking if you have a thickness of 100mm of PIR insulation you will need about 190mm thickness of quilt insulation to achieve the same thermal efficiency. In addition the current ventilation to the sub void would not meet modern requirements regardless of adding insulation before recovering the floor. Therefore, removing the dwarf wall in the middle of the room leaves no room for error regarding the amount of air circulating within the void. You will need to consider additional ventilation to the external walls. Another air brick to the other end of the external wall should be a vast improvement. So by removing the internal wall and adding one air brick you should achieve a high level of ventilation in the sub void. This resolves one problem but creates a second which is the support for the 110m deep timbers is removed. To overcome this, as I said in a previous post you can use 225mm deep timber joists as replacements. The joists should be treated and regularised. You can fit a standard floor boarding on top or opt for a chipboard floor. I find it ironic that other have advised you there is a shortage of timber within the country have you already have to acquire timber floorboards. I'm aware of this shortage. However, timber is still available albeit you will have a longer wait. The moisture levels in the existing timber at the rear of the room are unacceptable. Generalising a little but moisture content should be 6-10%. You have an opportunity to replace the existing timbers and reassure yourself that the floor won't have to come up again in 10 years time. You also have an opportunity to improve the ventilation in the sub floor and insulate between joists. If you use quilt insulation you're going to need about 300mm depth to meet modern standards. Alternatively you could use 150mm of PIR and achieve the same. PIR can be cut accurately, despite others comments that it can't. If you elect to use quilt insulation due to cost the quilt insulation should not fall lower than the replacement joists as it would have a negative impact on the airflow in the sub-void. You have options and the choice is yours.

AiY

Thanks AiY - options you put forward are nice and clear, just have to make up my mind! Re the rotten joist and board ends butting up against the wall - should if be concerned about wood worm/potential for rot to spread to other timber if keeping existing joists? It looks isolated enough but never encountered wood worm etc before

You can treat the existing joists by applying woodworm treatment and replacing any rotten areas.

AiY