I am about to IWIing another room in our house with insulated plasterboard on a metal frame.
I'm thinking about what to to between the joists.

In the back of the house I dealt with this by cutting away a section of ceiling in the ground floor room and inserted mineral wool and used an airtightness membrane and tapes. This worked fine but was a bit fiddly and meant the ceiling had to be put right after.

This time at the front of the house I am doing the first floor room 1st so was trying to think of ways to deal with between the joists from above.

Our house is an end terrace so I have to deal with two walls in each room and with that two directions of joists. I am trying to work out how to extend the insulation down between the joists and specifically what to do with the floorboards (these are exposed).

This is easy for the wall where the joists are at 90 degrees to the wall, cut slots for each joist in the insulation then reduce the width of the floorboard along the wall as required.

The problem is the wall where the joists are parallel to it. No slots required in the insulation so that's simpler but what do I do about the edge of the floorboards? They will be unsupported if I have to cut them back to allow the insulation to go bellow floor level.

If it is too difficult I will leave it and do the same as the back of the house when I tackle the front ground floor room.

If I've understood you correctly, you have the following..

- first floor joist running parallel to the wall, fixed to the wall
- floor boards (approx 150mm wide) perp to the joist, with the end of the floor board landing on the wall fixed joist
- you want to cut the floor boards ends off, all along that wall, so there is a gap between the edge of the cut boards and the inside of the joist
- you will then pass insulation vertically down between the cut board ends, flush with the joist face, until it meets the ceiling below.

If that's what you're planning, then I would say those cut floor board tails deff need supporting.

I don't immediately see a better way of getting access to that ceiling/floor void. One question would be...is the floor joist tight against the external wall, or is there a gap once you take pack any plaster etc above. If that's the case I might settle for foaming into that joint, with a flexi tube, to make sure it gets all the way behind the joist (air tightness as Tony mentioned).

If you really want to trim all the boards, and I'm imaging say 50mm gap between board edge and joist inside face, then poss a steel angle (75x75) tucked under the cut tails, with the vert edge of the angle pointing upwards That would then be fixed back to a new timber (or at least blocks at intervals) immed above the joist, and back into the wall.

I assume what you then add as IWI, will land ontop of that, and cover out to the cut board edges?

I would have to cut the floorboards back so there was about a 200mm overhang unsupported.

GreenPaddy:
Answered to your assumptions:
- first floor joist running parallel to the wall, fixed to the wall - Yes, fixed as in joist ends go into the inner brick leaf
- floor boards (approx 150mm wide) perp to the joist, with the end of the floor board landing on the wall fixed joist - Yes
- you want to cut the floor boards ends off, all along that wall, so there is a gap between the edge of the cut boards and the inside of the joist - Yes
- you will then pass insulation vertically down between the cut board ends, flush with the joist face, until it meets the ceiling below. - Yes

There will be a gap between the 1st joist and the wall of probably 10-30mm, if the same as the rest of the house, that I could foam. I could then push some mineral wool under the floorboards in front of the joist and not worry about wicking as the foam and joist should stop that.

I am trying to get my head around your other suggestion if I cut the floorboards. Am I correct in understanding you suggest the metal angle will take the weight of the board ends and this could then be supported between the joists at their ends? When you say the IWI could sit on top of that, what is the that?

Is this really any less work than cutting the ceiling and doing the job from below?

In my humble opinion, doing IWI is extremely invasive in any case, and I suspect the OP is right to be wanting to try and compartmentalise the upset. Whence the interest of researching a feasible methodology to suit the site and conditions.

One of the main renovation criteria being to avoid "killing the green advantage" by avoiding quick-fix works which might bring a short-term gain at the expense of impeding or interdicting potentially higher-value future operations.

(In French this is referred to as "tuer le gisement vert").

gg

Posted By: GreenPaddy1. foam as deeply as possible down behind the joist parallel to the gable wall. 10 to 30mm is pretty good, and will plug all the drafts.

How do you get access to do this without chopping the ends off floor boards?

Sounds good except I am not getting these bits:

Posted By: GreenPaddySo here's what I'd do, now I understand a bit better;

4. Blow in a load of polystyrene beads, to fill the first joist to joist void. Vacuum cleaner in reverse, or some such method that people on here will attest to.

7. do a final to-up of the poly beads at that first joist to joist void.

Air tight ceiling void, with a great insulation value, and no chopping of ceilings or floor boards, not much tuer 'ing of the green gisement, si tu me comprends.

Where are the beads for? I am not sure what you mean by joist to joist void.

Posted By: ajdunlopWhere are the beads for? I am not sure what you mean by joist to joist void.

Along the wall where the joists run parallel, and where you've foamed the gap between the wall and the first joist, then he's talking about the gap between the first and second joist.

Peter_in_Hungry:
We don’t have CWI as the cavity is uneven and the outer leaf is Yorkshire stone so porous so needs the cavity to dry out.
The insulated plasterboard is 90mm and will be fixed to metal battens.

''There is a real and present danger of wind from outside finding its way from the cavity into the void between the metal battens,''

Agreed. I'd suggest you do a lime parge coat first. Min 10mm-ish, and it can just follow the 'wows' of the stone.

Hoping this sketch has attached and viewable. (For "Setcoin", read Section"...I sketch, don't spell)

Would I bother with the poly beads for one short wall, maybe not as PiH mentions above, but this is a repeating feature in a few rooms, so get it right once, you can do for other rooms.

Search for poly beads threads, or ask others how they fill voids. If it's easy, then go for it. If it's a big hassle, then reconsider.

Posted By: ajdunlopTony & Nick Parsons: Why is air movement behind the insulation a problem if it can't get through (because of foaming, skimming of plasterboard, use of tapes when required)

This might help:

https://basc.pnnl.gov/resource-guides/wind-washing#quicktabs-guides=1

gg

I am using 90mm (of which PB 9.5mm ) with an R-Value of 4.06.
How much advantage would sandwiching it with the red brick inner leaf of the wall give?

The original reasons for using battens on the cold side was to over engineer a bit so I could avoid worrying about interstitial condensation as well as straightening out the walls.