Friction fit rockwool sagging between rafters

[daunker]

Converting an existing building, I have been putting 140mm frametherm 32 between purlins and then 75mm rockwool batts below. 600mm c/c I have trimmed the rockwool batts. However there is some sag, some worse then others. 

 

Due to budget constraints I'll not be able to put ceiling finish (timber not plaster due to building movement) for some time. But I do have the 50mm PIR sheets to go under the rafters. Is it strong enough to support it, or will it flex too much? Suggested number fixings per sheet? Was going to use penny washers/insulation discs.

 

(Also while on topic anyone done a timber ceiling finish, and can suggest any good wood options)

[Iceverge]

I would put 20*70mm strapping along the line of the joists to create a service cavity.with 120mm + screws through into the joists. You'll have something nice to fix your ceiling to then too. 

 

A mate recently used T&G pine floorboards for their ceiling. It looks good but lots of care is needed to ensure it is tidy. 

 

 

 

 

 

 

[MikeSharp01]

PIR is stiff up to its strength limit, interesting Youngs modulus I suspect, then just snaps rather than bend so I would have thought it would do it. Why not try a piece and see - you will need some big plastic washers to hold the PIR in place. To fix the ceiling make sure you measure / know the centers of all your purlins so you can find them again once you have the PIR in place.

[Nickfromwales]

I assume you're using 2x1 battens over the PIR? Or what is your plan for electrics to lights? Above insulation and dropped down / through?

Penny washers and the PIR will be ample to hold the wool at bay, just use a couple more and sleep well. May be a good idea to foil tape the joins and foam the perimeter, to stop the fibres entering the air in that room. Not the nicest of stuff to have continuously feeding into the room.

[Nickfromwales]

3 minutes ago, MikeSharp01 said:

To fix the ceiling make sure you measure / know the centers of all your purlins so you can find them again once you have the PIR in place.

Indeed. Set a laser up each end and see if the line stays at the centre of each run. That's a quick way to see if you'll meander off course with fixings. Also needs a map making to avoid the few obstacles.

 

Take LOTS of pics with tape measure in the shot  

[Canski]

In the past I have stapled some 100 mm dpc in rows across the joists to provide a temporary fix. 

[joe90]

Or string?

[nod]

It’s normal to use a strawberry netting stapled over the whole area Cheap and quick 

[eandg]

Where ours sagged I just put some fixing bands across with a wee screw into the joist to hold it in place. 

[ProDave]

It's a shame you didn't use Frametherm for the whole lot, this is much stiffer and does not sag.

 

I did a test piece early on and left it 6 months with no support and it just did not sag.

 

[daunker]

1 hour ago, nod said:

It’s normal to use a strawberry netting stapled over the whole area Cheap and quick 

Have excess black fibreglass insect mesh, will try this! Thank you!

[daunker]

44 minutes ago, ProDave said:

It's a shame you didn't use Frametherm for the whole lot, this is much stiffer and does not sag.

 

I did a test piece early on and left it 6 months with no support and it just did not sag.

 

Was just too expensive, and the trial pack of knauf Omnifit worked perfectly. Then got a great price on a few pallets of rockwool rwa45, 'the same' 🙈🤦‍♂️ and it's the one which sags

Edited July 6, 2023 by daunker

[Iceverge]

TBH I thinks it looks fine. .

 

I would do

 

PIR boards. 

Tape joints and foam to wall. 

3*1 strapping for service cavity. 

First fix wiring. 

Plasterboard

Skim. 

 

 

 

[saveasteading]

Agreed. Do something soon. String is the principle but for a flatter material that nobody would even question beng there I suggest metal strapping., builders band.

OR the plastic strapping used on pallets. That is cheap and could be stapled easily and be near zero thickness.

 

Then you can add a service void when it suits.

[Nickfromwales]

2 hours ago, joe90 said:

Or string?

 

String is king!! Just use a heavy duty staple gun and away you go.

[saveasteading]

13 hours ago, daunker said:

timber ceiling finish, and can suggest any good wood options)

Simple tongue and groove, stained to a colour to suit. It will probably then need an intumescent varnish.

[daunker]

11 hours ago, Nickfromwales said:

I assume you're using 2x1 battens over the PIR? Or what is your plan for electrics to lights? Above insulation and dropped down / through?

Penny washers and the PIR will be ample to hold the wool at bay, just use a couple more and sleep well. May be a good idea to foil tape the joins and foam the perimeter, to stop the fibres entering the air in that room. Not the nicest of stuff to have continuously feeding into the room.

Thanks  I would have liked to. But because its a vaulted ceiling and conversion of existing  the height at the entrance/low side, I'm on my limit for allowance with 150mm insulation and screed, plus coming down 50mm+timber.

 

My plan was where the exposed steels from the portal frame are to have thicker insulation, and box around them slightly oversized, somehow making a feature whilst taking out the cold bridge to allow me to run cables up, and then I would just channel out a groove left to right through insulation required for cables to light fittings which would be surface mounted, which I'm planning to be as few as required (ie larger) Is this allowed? 

 

My alternative is to perhaps run the lights on wire side to side. 

[Nickfromwales]

11 minutes ago, saveasteading said:

Then you can add a service void when it suits.

If you keep the insulation up high, the service voids are preserved.

 

6 minutes ago, daunker said:

My plan was where the exposed steels from the portal frame are to have thicker insulation, and box around them slightly oversized, somehow making a feature whilst taking out the cold bridge

You could just bond Compacfoam to the steel and be as thin as 40mm.

[saveasteading]

9 minutes ago, Nickfromwales said:

could just bond Compacfoam to the steel and be as thin as 40mm.

I'd need a picture. But the easiest solution is to pack the space with insulation offcuts, then box  around.

[Nickfromwales]

9 minutes ago, saveasteading said:

I'd need a picture. But the easiest solution is to pack the space with insulation offcuts, then box  around.

As long as there's no gaps  

The caveat is that the use of a lower performing material requires more of it. Compacfoam or Marmox does the job well within a much thinner profile. Not hugely different in performance per-se, but it's uniform and can be 100% gap / void free so that adds (U) value