Post-structural fix dry-lining

heavydawson

(Accidentally cross-posted to DIY, mods: remove it from that thread if needed)
Hey all,
Recently had an issue with horizontal cracking appearing in the upstairs external walls after less than a year in a new build. Had a structural engineer look into it and report on the problem. Turns out there's insufficient anchorage of the roof to the block-work, so we've got a whole bunch of remedial work to do in all of the rooms upstairs. Once all of the structural work is done, we've been advised by the structural engineer that the affected walls should be covered with insulated plasterboard. Given that we already have 150mm full-fill insulation in the cavity, the internal walls are "warm" but I'm worried about interstitial condensation once the boards go up. The house uses MHRV and has a certified measurement of 1.540 m3/(hr.m2). Windows all triple-glazed and 400mm over-lapped rockwool in attic.

I'm open to suggestions on this as regards materials. I've looked into a calcium silicate board as an alternative to the more common dry-lining boards from the major manufacturers but it's quite expensive. However, it does seem far more breathable and should reduce my condensation concerns.
I'll be talking to an architect about this, but interested in hearing others experiences/opinions on the matter before I do.

BryanF

Why was the drylining suggested? Is the structural remedial works bridging the cavity and causing a thermal bridge? Has the architect reviewed the engineers details? What amount of drylining was suggested? Will the engineering contractor's works include any air-tightness remedial works?

heavydawson

BryanF wrote: »

Why was the drylining suggested? Is the structural remedial works bridging the cavity and causing a thermal bridge? Has the architect reviewed the engineers details? What amount of drylining was suggested? Will the engineering contractor's works include any air-tightness remedial works?

Hey Bryan,
Was hoping you'd reply!
So the cavity won't be bridged. There's straps being attached to the inner side of the inner leaf that twist through 90 degrees to attach to the joists overhead. The straps will be coming down the wall between 1.5. to 1.8m. This strapping will be repeated approx every meter. So the wall is going to be covered in them. Technically the bridge will be from the room into the attic space above via the strapping. The contractors works will include costs for repairing all damage to the airtightness membranes that will be necessary for the work. There was no specific amount of dry-lining suggested.
The alternative was to kango the walls where the straps are to be located and re-plaster when done. However, the engineer advised that due to the horizontal cracking is a result of the mortar joint failing, cracking the same area is quite possible again, and so I'm hesitant to simply re-plaster with sand-and-cement and hope for the best.
As for the architect reviewing the engineers details, that hasn't happened yet, as I've yet to bring an architect on-board.

BryanF

heavydawson wrote: »

Hey Bryan,
Was hoping you'd reply!
So the cavity won't be bridged. There's straps being attached to the inner side of the inner leaf that twist through 90 degrees to attach to the joists overhead. The straps will be coming down the wall between 1.5. to 1.8m. This strapping will be repeated approx every meter. So the wall is going to be covered in them. Technically the bridge will be from the room into the attic space above via the strapping. The contractors works will include costs for repairing all damage to the airtightness membranes that will be necessary for the work. There was no specific amount of dry-lining suggested.
The alternative was to kango the walls where the straps are to be located and re-plaster when done. However, the engineer advised that due to the horizontal cracking is a result of the mortar joint failing, cracking the same area is quite possible again, and so I'm hesitant to simply re-plaster with sand-and-cement and hope for the best.
As for the architect reviewing the engineers details, that hasn't happened yet, as I've yet to bring an architect on-board.

Get arch to review air-tightness & thermal details. Obeying the 2/3 1/3 rule would suggest 50mm eps drylining should not cause interstitial condensation on the inner leaf. Make sure the air-tightness is re-tested before drylining, with some attention, you may even improve it..

heavydawson

BryanF wrote: »

Get arch to review air-tightness & thermal details. Obeying the 2/3 1/3 rule would suggest 50mm eps dry-lining should not cause interstitial condensation on the inner leaf. Make sure the air-tightness is re-tested before dry-lining, with some attention, you may even improve it..

Sounds reasonable. I wasn't familiar with the 2/3,1/3 rule, but looks like 50mm will put me at 3/4, 1/4.
I fully intend to get the air-tightness test re-done.

BryanF

Sorry 1/3 or less (& it's only a rule of thumb, you won't find literature on this- your architect can run the Wufi calcs anyway)