How to lower and reframe ceiling to add insulation?

Question

Hi all,

Looking for some advice on the best way to lower and reframe this ceiling so I can improve the insulation.

Existing roof/ceiling structure

* Exposed beams: 190×45 mm @ approx. 1250 mm centres 
* Purlins/battens: 70×35 mm @ 450 mm centres  
* Foil sarking
* Trimdek roofing installed directly over sarking 
* Existing insulation: 35mm Rockwool installed into the 70mm cavity leaving 35mm air gap between sarking and insulation

Room size

Approx. 3.6 m × 2.5m, with an exposed beam running through the middle. I thought this room was "finished" but since the insulation is insufficient, I will have to redo it.

Goal

I want to drop the ceiling by around 70 mm so I can install high‑density R2 insulation batts and improve thermal performance, while still keeping the exposed‑beam look.

I was considering fixing 70×35 timber beneath the existing structure to create a new dropped ceiling frame, something like:

* New 70×35 framing around the perimeter and along each side of the exposed beam for plasterboard fixing. I could also possibly rip 70x45 but it probably wouldn't make much difference.
* A single 70×35 (possibly LVL) running through the centre fixed on each side using joist hangers. I am hoping this is fine for 3.6m span. I could use LVL if standard framing     timber is insufficient.
* 600×1200 insulation batts installed in the new cavity

Here’s a rough sketch for reference:

Questions

* Does this seem like a reasonable way to frame it, or is there a better approach? 
* Is 70×35 sufficient for this span and application, or should I use something larger or a different fixing method? 
* When removing the existing     ceiling sheets, should I: * trim the tops of the wall sheets and add blocking/noggins for fixing, or 
* simply run the new framing over the existing wall sheets?

Here is few more pictures from various stages of works in this rooms if it helps:

Many thanks!

Tom

Dave-1 · Answer

Good Evening @tom138

You seem to be well on the way working out the distances required. Tho I am unsure if you are intending on leaving the existing insulation or removing it and replacing with the new R2 level insulation?

I would be replacing the old insulation and not leaving any air gaps for the best effect. Adding loads to an existing frame is something I try and steer around. Ceilings should be 3.4m and with your math I think you still have 10mm spare :) But still id try to work within the existing framework that is up there. Having old and new insulation in the same area does not add up to a higher level. It creates potential unevenness and pockets of air which defeats the insulations purpose.

Dave

EricL · Answer

Hello @tom138

There are two items that come to mind in regard to your ceiling frame reframe. How thick is the actual insulation you'll be using? Will you be leaving an air gap between the new insulation and the old one? The second item is whether any of those large, exposed members are actually structural roof beams carrying roof loads, or simply decorative/exposed ceiling joists. If they are structural and you intend attaching new ceiling framing or hangers to them, it is always best to verify with an engineer or a licensed builder before adding suspended loads and insulation weight.

In regard to your questions, I'm only familiar with the third one and that is to trim the top of the wall sheets. It is generally cleaner and structurally better to remove or trim back enough of the existing wall lining so the new ceiling framing can fix directly into solid framing rather than relying on the plasterboard edge. I don't wish to give you the incorrect advice in regard to the ceiling framing. I’ll ask some of our seasoned members to weigh in with their suggestions.

Let me call on our experienced members @Nailbag, @Dave-1 and @Noyade for their recommendations.

If you need further assistance, please let us know.

Eric