Retrospectively sealing cracks at 10+ year old plasterboard joins

[GP41]

We are embarking on some "airtightening" work alongside installation of ASHP and MVHR - more background here.

 

One of the things that I'm struggling with, is how best to deal with cracks in our relatively recent extension.  Much of it was dot and dabbed and we have numerous cracks, either on the flat face of ceilings and walls, or at wall/ceiling - and sometimes plaster/glulam - junctions.

 

I'm loathed to start pulling perfectly good plasterboard off walls, it just doesn't feel like the most sustainable thing to do.  So how can I fill, and make airtight, those cracks?  I have visions of taking the skim coat back, applying airtightness tape and reskimming - is that ridiculous?  Would I be better with a caulk of some sort, and if so, how flexible and long lasting would that be?

 

Here are some pictures of the sort of thing we are dealing with... 

 

From an airtightness perspective, I actually think our 12 year old extension is in a worse state than the original, wet plastered, 65 year old house.  And in some cases, looking back at pictures I took during the build (I know, if I'd only known then what I know now...) I might be forced to remove sections of plasterboard to carry out remedial work behind plasterboard, in which case I have the opportunity to do a more substantial job.

 

Any suggestions much appreciated?

[Nickfromwales]

Have you filled / sanded / painted any of these cracks before, or is this the 1st time you’ll be tackling them? 

[JohnMo]

2 hours ago, GP41 said:

Here are some pictures of the sort of thing we are dealing with... 

 

They joints look like a bit of a bodge, looks a bit like the plasterboard hasn't been jointed correctly. I.e. it should have been taped and filled etc, looks more like caulk and paint. Caulk has just shrunk away. 

 

I would pull out all the dross and do it properly, either skim or do dry line properly, with tape and the correct fillers.

[GP41]

1 hour ago, Nickfromwales said:

Have you filled / sanded / painted any of these cracks before, or is this the 1st time you’ll be tackling them? 

On many of them it'll be first time. On some of those that are more accessible I've used decorators caulk or silirub. I expect I could remove that though if necessary.

[GP41]

12 minutes ago, JohnMo said:

They joints look like a bit of a bodge, looks a bit like the plasterboard hasn't been jointed correctly. I.e. it should have been taped and filled etc, looks more like caulk and paint. Caulk has just shrunk away. 

 

I would pull out all the dross and do it properly, either skim or do dry line properly, with tape and the correct fillers.

The ones in the pictures have never been touched, other than possibly some decorators caulk for first time pained.  When you say "pull out all the dross and do it properly, either skim or do dry line properly, with tape and the correct fillers." do you mean take it all down?  We do have the added challenge of trying to find a solution that cleanly abuts the glulam leaving its wooden appearance untouched.

[JohnMo]

1 hour ago, GP41 said:

do you mean take it all down

No need for that, go on YouTube and look at dry lining crack repair.

 

1 hour ago, GP41 said:

We do have the added challenge of trying to find a solution that cleanly abuts the glulam leaving its wooden appearance untouched

You need something different to caulking as it shrinks too much over time. Some of the hybrid mastics stay flexible without shrinking and can be over painted. So may be a better alternative.

[GP41]

9 hours ago, JohnMo said:

No need for that, go on YouTube and look at dry lining crack repair.

 

You need something different to caulking as it shrinks too much over time. Some of the hybrid mastics stay flexible without shrinking and can be over painted. So may be a better alternative.

OK, many thanks, I'll see what I can find.  It was more the corners where I was concerned, a lot of the crack repair advice seems to relate to simple hairline cracks in flat surfaces rather than inter-face joints like this that looked like they need a more significant solution that would stand the test of time and remain airtight.

 

Thanks again

[Sparrowhawk]

Was the dot and dab plasterboard then skimmed with jointing compound or with the hard brown (gypsum?) plaster? For the junctions, having tried to doit myself with CT1, I think your best long-term fix is to sand back a bit, and get a plasterer to insert folded paper tape over the join and feather it in.

 

For cracks on the flat surfaces, scrim tape and a reskim or decent feathering in.

 

None of this helps if you've got cold air circulating behind the dot and dab. Do your thermal camera images show it happening, and help you locate the source? You could drill holes and squirt expanding foam in top and bottom and see how much that improves it.

[JohnMo]

59 minutes ago, Sparrowhawk said:

having tried to doit myself with CT1,

Not the correct material or way to do it. For corners you really need the metal backed tape so you get sharp corners (internal and external corners)

 

https://youtu.be/pp7AXv1P1GY?si=nlogtPCzn4psw6XZ

[GP41]

 

11 hours ago, Sparrowhawk said:

 

None of this helps if you've got cold air circulating behind the dot and dab. Do your thermal camera images show it happening, and help you locate the source? You could drill holes and squirt expanding foam in top and bottom and see how much that improves it.

I managed to dig out some old photos and FLIR images.  There are cold patches around the glulam but quite localised - presumably around where it was packed going into the wall, so that could be an opportunity for the drilled holes/expanding foam trick. 

In this room I don't think we suffer too badly with the plasterboard tent problems (as we do in a couple of others...) but there are certainly some problem areas - you can see there is an issue where the insulation in the ceiling meets the wallplate (I thought I had a better picture where I'd actually spotted the specific problem on this wall - I'll need to try to dig it out, but it was basically a flaw in how the insulation finished).

 

 

[GP41]

10 hours ago, JohnMo said:

Not the correct material or way to do it. For corners you really need the metal backed tape so you get sharp corners (internal and external corners)

 

https://youtu.be/pp7AXv1P1GY?si=nlogtPCzn4psw6XZ

That's really helpful thanks.  Did you mean something like this for a flexible, airtight, paintable sealant? Acryrub SWS | Soudal