Hello I am about to start insulating under a wooden suspended floor. There are plenty of threads on this forum and elsewhere about this topic but the situations are normally not quite the same as mine or the threads a now quite old.

My house is a 1930s end terrace with double skin wall. Outer York stone, inner red brick.

The Cellar at the front of house may eventually be tanked and made habitable but the back of the cellar will remain cold and so insulation is needed to keep heat in the rooms above.The rooms above have been internally insulated with phenolic foam insulated plasterboard on a Gyplyner metal frame system with 40mm cavity behind.

I will be insulating from bellow as our floorboards are exposed and won’t be coming up. I have plenty of room (part proper usable cellar other part mostly dug out) so there are no problems with access.

There are air bricks allowing ventilation of the space and I would make sure to channel these bellow the insulation if required.

Airtightness is a big concern. I want to cut out draughts but also I have a long term aim to make the house as airtight as possible to maybe install MVHR or at least centralised MEV with controlled air inflow.

At the minute I am thinking I would avoid solid insulation and stick to wool / mineral wool insulation as it allows the floor to breath better and can take movement in the floor without later degradation of insulation and airtightness.

Ideally if I was lifting the floorboards I would put down an airtight membrane above the joists then insulate between the joists with wool / mineral wool insulation. Then I would put a layer of insulation across the underside of the joists and use a vapour open membrane under this to allow moisture to escape from the joists and insulation into the ventilated space below…
However:
* I can’t put the airtight membrane above the joists as I am not going to lift the floorboards - Can I run the airtight membrane attached to the bottom of the floorboards and around each joist? The Joists themselves would now be on the warm room side of the membrane but if I am insulating under the joists as well as between would avoid condensation issues?. I have most of a roll of Pro Clima Intello Plus that I could use.
* I will need to plasterboard the cellar ceiling to meet regulations around fire resistance as the cellar is in use. Would this not render the vapour open membrane under the insulation useless?
* The Damp Proof Course on the external walls are just below where the joist ends go into the wall. If I am trying to improve the airtightness around the joist ends and one method is a Parging Coat on the wall around them. However would I be in danger of bridging across the DPC if I do this? Any suggestions to alternatives?
** One possible solution that I have wondered about would be to have the insulation stop short of the outer wall and run the membrane up the side of this to under the floor. This would leave a gap in the insulation at the wall but the room above has insulation that has a 40mm cavity behind it anyway so there shouldn’t be a cold bridge. This would prevent problems with interfering with the DPC and could have the added benefit of allowing the joist ends to be ventilated reducing the condensation risk.

Rear cellar space

I would use inorganic quilt and eps under the joists and no membrane under that , eps against outside walls.

Clean all vents and check down there in cold weather

Front best make decision about habitable then decide what to do as no insulation would be needed,

Be careful about air/wind ingress behind you plasterboard , suggest thermal imaging in cold windy weather

I would rather cold air didn’t get in there, the void behind the plasterboard is on the warm side of thee insulation

Posted By: ajdunlop
* I will need to plasterboard the cellar ceiling to meet regulations around fire resistance as the cellar is in use. Would this not render the vapour open membrane under the insulation useless?

As I understand it, plasterboard, as long as it's not foil backed, is reasonably vapour permeable. But I don't know what its effect would be in combination with the rest of the buildup. My starting point here would be to request a condensation risk calculation from a couple of the major insulation suppliers, for a buildup including plasterboard under the joists and see what they say. They might simply say it is not recommended. You have a bit of a conflict here between satisfying the fire requirements, which means having something fixed to the bottom of the joists, and keeping ventilation to them. Could there simply be a plasterboard layer with some (fire rated??) vents in it?

As I just posted on another thread, I thought the building regs, along with any standard details, discourage any insulation below the joists because it cuts off ventilation to the joists. But that's more aimed at crawlspace type situations; if you have a room in use underneath, perhaps those things are not considered to be applicable - I think you are in a kind of grey zone building regs-wise.

There's a section in Approved doc C that talks about 'floors exposed from below'; maybe you come under that. It refers you to the guidance in BS5250, which says:

***
8.5.5 Externally exposed floors
8.5.5.1 Joisted floors
A joisted externally exposed floor is shown in Figure 33. Surface condensation is unlikely to occur on floors with timber joists, or on joisted floors where insulation is provided over the joists, due to the fast thermal response. Regular heating is preferable, but infrequent heating may be used with this construction.
Thermal bridging should be minimized, particularly where the floor passes over external walls, and at the floor edges.
Interstitial condensation is unlikely to occur if insulation supports or soffit linings are of low vapour resistance. Where rigid insulants or sheet material supports are used, attention is drawn to the warning given in 8.5.2 regarding water spillage or leaks.
Vapour control layers should not be used with joisted floors because water spillage could cause decay and subsequent collapse.
***

which confirms the need for soffit linings to be of low vapour resistance but doesn't help you in suggesting what materials might satisfy that as well as being fire resistant!

Thanks for all the replies.
djh: if I were to change my approach to something like what you suggest and treat skimmed plasterboard as the airtightness layer and to avoid air movement wicking heat that would still leave the question of joist end airtightness. Ideally I would use a Parging Coat around these, however as mentioned in my initial post I am worried about bridging the Damp Proof Course.
What does anyone think about me using a bit of membrane around the edges of the insulation with a small air gap between it and the wall? The insulation in the room above has a cavity behind it so I wouldn’t be creating and cold bridge.

Posted By: ajdunlopThanks for all the replies.
djh: if I were to change my approach to something like what you suggest and treat skimmed plasterboard as the airtightness layer and to avoid air movement wicking heat that would still leave the question of joist end airtightness. Ideally I would use a Parging Coat around these, however as mentioned in my initial post I am worried about bridging the Damp Proof Course.
What does anyone think about me using a bit of membrane around the edges of the insulation with a small air gap between it and the wall? The insulation in the room above has a cavity behind it so I wouldn’t be creating and cold bridge.

If you were to use skimmed plasterboard, running up to the wall, which was then parge-coated, leaving aside the issue of bridging the DPC is it not then the case that there is not an airtight layer (or vapour barrier) separating the internal heated space from the cavity behind your insulated plasterboard? So, you are potentially allowing damp laden air into your cold cavity, which also contains your joist ends. But also, that cavity is surely likely to be subject to air leaks, which are not isolated from your heated space.

In principle, using a bit of membrane at the edges of the insulation, with air gap, makes more sense to me, because it gives you a continuous airtight/vapour barrier layer between floor and walls. It would just be the practicality of fixing it. I did something similar, with old joists crossing an air/vapour barrier, but I put timber framing and OSB panels between the joists, a very fiddly job that is only going to work if done conscientiously. How would you fix the membrane to the joists - trap it behind battens screwed into the joist sides perhaps?

Posted By: ajdunlopThe parge-coat would be on the wall to make the wall airtight around the joist ends so I think should stop damp getting in behind the insulation. Although I do wonder how long the parge-coat would remain airtight. I would imagine that movement of the joists could crack the coat and reduce its airtightness.

A parge coat or plaster in general are no good as an airtight barrier at any kind of junction, or even a change of direction: basically it will crack at any stress point. So membranes or tapes are good practice e.g. where walls meet ceilings or floors, as well as around joists etc. It's important to leave some slack in the tape/membrane to allow for any movement. There are tapes that are designed to be incorporated into wet plaster, others that are designed to be plastered over, and yet others designed to stick to already-set plaster.

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Posted By: ajdunlopThere are two rows of herringbone noggins perpendicular to the joists. These could be indeed be fiddly, but I don't think impossible with lots of careful taping maybe.

A suitable paint instead of tapes might be a good way of dealing with complicated shapes.

Posted By: ajdunlop

Posted By: djh

Posted By: ajdunlopThere are two rows of herringbone noggins perpendicular to the joists. These could be indeed be fiddly, but I don't think impossible with lots of careful taping maybe.

A suitable paint instead of tapes might be a good way of dealing with complicated shapes.

Like https://proclima.com/products/liquid-seals/aerosana-visconn ?

Do those paint-on products really work ... my instinct is to be very sceptical and put them in the 'too good to be true' category.

I only just stumbled across this product now "Pro Clima Aeronsan", would have saved me massive headaches on my roof ( cimbination of old brickwork, various renders and mortars, OSB - patching over old wood, taping and glueing things together, filling gaps with 'airtight' flexifoam and taping over them ) oh well.. I was aware of air tightness sealing liquids but had just looked away in horror as I presumed that they would be vapour closed.. but Pro Clima claim this as being 'humidity variable' and non-toxic makes it very interesting for renovation projects

yes it seems to be newish ( last 2 years ), obviously getting put on nice and thick in some demos.. www.youtube.com/watch?v=WLam-ZfMZFE