Removing ceiling plasterboard to fit insulation

The story is as follows...

Flat roof extension, very little insulation in roofspace. Only way to access the space between the joists to place insulation is to pull down the plasterboard. I've done this already in the garden room section where I pulled all of the plasterboard down, then put fresh plasterboard up over the full area. A time consuming expensive mess.

For this bit I am hoping to work a bit smarter and just take sections of the plasterboard down to access each joist run. As I see it I have two options

1. Cut out a square in the middle of each joist run, then replace that square when finished, screwing a lath in to hold it in place, taping the join
2. Cutting a long run of plasterboard across all of the joists, the replacing with a cut new piece of plasterboard screwed/nailed into the joists, taping the joins.

Regardless of the method the ceiling will be skimmed afterwards.

My current thinking is that '1' would be quicker and easier, cutouts will already be the right size to go back in, no need to prise the nails and bits of plaster off the joists, less messy, able to deal with one joist run at a time, but '2' would mean the plasterboard is fixed to the joists rather than to another bit of plastboard, so might be more stable.

Anyone done this before or able to advise on what would be the best way to go?

Cold roof? how are you going to vent it.

Alastairreid said:

Cold roof? how are you going to vent it.

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That in itself as a long story. In short, I am not. There is no practical way to properly supply through ventilation based on the existing design. I wanted to convert it to a warm roof, but there are a couple of practical reasons why this is not possible, or at least not practical without spending tens of thousands to redesign the roof and move windows etc. While thinking this through I found some building research from Germany that showed that if you can't provide line of sight through ventilation to a cold rood, you are better to full fill it and not ventilate at all, so this is what I am going to do.

You still need a vcl if full filling.

Alastairreid said:

You still need a vcl if full filling.

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In theory yes, but impermeable paint will be my VCL.

I appreciate you concern regarding this, but I have thought this aspect of it through and I understand the science behind vapour transmission and the risks. I was hoping for advice on the plasterboard question specifically.

I think I get what you're getting at. You're using a Kingspan/celotex type insulation I'm guessing. Method 1 will work fine. Cut a square/rectangular hole flush to each joist and slide said insulation into the runs. Fix a lath/timber to each joist and board abutments to allow re-fixing of removed board piece.
If you're skimming the ceiling anyway there's no need to tape the joints as the plasterer will scrim the joints. Stability is not so much an issue as with each method you're fixing to timber which is prone to instability by nature.

Only thing I would mention is don't fully fill the void with insulation. Allow a cavity for a degree of air flow.

There is alternative sub-ceiling solutions(at greater expense) I can suggest if you want, but for me, I think you've already hit on the best approach. Cheap and practical.

Thanks Fenny. I'm using glass fibre, mainly due to cost and ease of installation as I am doing a full fill and trying to fill some awkward spaces. Also concerned that if leaks develop Kingspan or other closed cell insulation new would potential hold water up against the timber. That's also part of the reason I didn't want to go sub-ceiling with kingspan type, but mainly because the ceiling is already low, i'm raising floor to insulate, and I can't afford to lose any more height.

I'm fully filling the void, because there is no way to sufficiently ventilate. I should have posted the link to the article that influenced my decision on this - https://foursevenfive.com/blog/unvented-flat-roofs-a-technical-discussion/

Thanks for the tip on cutting flush to joist.