Dabbed plasterboard on a solid external wall - experiences?

[smp50]

Following from another thread:

I've read several times that plasterboard on solid brick external walls, with adhesive (rather than batons or some other system) is unacceptable because the adhesive might or will deteriorate due to the effects of temperature and moisture. I've also heard of a few people (one here, and others mentioned elsewhere) who have done it anyway and had no problems. A plasterer who saw my job casually recommend dabbing plasterboard onto my external walls (as well as the others), although I didn't query him on it at the time so don't know his full opinion on the matter, or if he really thought about it.

I have been plasterboarding a room and have enough board left over to complete the two external walls (both include a window). I'm obviously quite tempted to get on with it and finish the room, but I want to hear more about why it might be a bad idea.

Is plasterboard adhesive ALWAYS going to be vulnerable on these surfaces? Or is it only if the wall has a problem, and if so how big of a problem? As in, should I be able to tell if it's not going to work before I do it, or is it too unpredictable?

Who here has constructed or otherwise experienced plasterboard adhered to solid external walls? How was it?

Thanks,

 

[RogerBoyle]

If Its a single skin wall then you would be better to batten it
For a double skin wall with a cavity then I don't see the problem

 

[NickB_99]

Is plasterboard adhesive ALWAYS going to be vulnerable on these surfaces? Or is it only if the wall has a problem, and if so how big of a problem? As in, should I be able to tell if it's not going to work before I do it, or is it too unpredictable?

Any problem would probably not be evident until at least late Autumn or Winter, when the weather gets colder, and maybe even take longer than this year.

Any wall will allow heat to conduct to the cold outside - the rate will vary on factors - all things equal, a wall with a cavity would normally be affected less than a single skin wall.
The dabs are in contact with the colder wall and so give you cold spots on the plasterboard. If you have high levels of moisture in the air in your room, then this moisture can condense on these cold spots. Over time, this can cause moisture and mould.
This is not a feature of a wall with a problem - it's simple heat transfer.

I don't think anyone can give you a categorical 'it will be fine' as there are so many variables.

 

[stevethespreader]

Is plasterboard adhesive ALWAYS going to be vulnerable on these surfaces? Or is it only if the wall has a problem, and if so how big of a problem? As in, should I be able to tell if it's not going to work before I do it, or is it too unpredictable?

Any problem would probably not be evident until at least late Autumn or Winter, when the weather gets colder, and maybe even take longer than this year.

Any wall will allow heat to conduct to the cold outside - the rate will vary on factors - all things equal, a wall with a cavity would normally be affected less than a single skin wall.
The dabs are in contact with the colder wall and so give you cold spots on the plasterboard. If you have high levels of moisture in the air in your room, then this moisture can condense on these cold spots. Over time, this can cause moisture and mould.
This is not a feature of a wall with a problem - it's simple heat transfer.

I don't think anyone can give you a categorical 'it will be fine' as there are so many variables.

well said newb

 

[dann09]

I'll second that steve,

newb, thats the best explanation of "it" that i've encountered. Keep it clear and simple.

 

[smp50]

Is plasterboard adhesive ALWAYS going to be vulnerable on these surfaces? Or is it only if the wall has a problem, and if so how big of a problem? As in, should I be able to tell if it's not going to work before I do it, or is it too unpredictable?

Any problem would probably not be evident until at least late Autumn or Winter, when the weather gets colder, and maybe even take longer than this year.

Any wall will allow heat to conduct to the cold outside - the rate will vary on factors - all things equal, a wall with a cavity would normally be affected less than a single skin wall.
The dabs are in contact with the colder wall and so give you cold spots on the plasterboard. If you have high levels of moisture in the air in your room, then this moisture can condense on these cold spots. Over time, this can cause moisture and mould.
This is not a feature of a wall with a problem - it's simple heat transfer.

I don't think anyone can give you a categorical 'it will be fine' as there are so many variables.

Thanks for explaining the mechanism - that makes sense. I will take it that it's not an inevitable problem, then - and perhaps depending on the local climate, heating regime, ventilation, dab spacing, wall structure and so forth the existence, timing and severity of the problem will vary a lot. Is that right?

Why is it that the same problem doesn't happen with batons, does the wood not conduct heat as well as adhesive?

 

[NickB_99]

I will take it that it's not an inevitable problem, then - and perhaps depending on the local climate, heating regime, ventilation, dab spacing, wall structure and so forth the existence, timing and severity of the problem will vary a lot. Is that right?

Yes, not inevitable, you may get away with it. If you don't dry washing on radiators, have trickle vents in the windows for example - these are things that could help keep your moisture level down.
But if you think of it in terms of probability, D&D may be ok with a probability of 20% success, and batten/insulation will be 99%.
I know which I'd prefer - I hate re-work.
I would have a search on this forum for mouldy D&D (or equivalent words) - there are quite a few I have seen on here over the years!
It will give you an idea of how bad it can get.

Why is it that the same problem doesn't happen with batons, does the wood not conduct heat as well as adhesive?

A good question. You are right - wood does not conduct heat as well as the dab - partly as a result of material, and also the wood is a bit thicker.
When you have dot/dab, the approx 10mm void is just filled with air, which can even move about with draughts (you generally don't manage to seal all round the board). When you batten, then you generally put insulation between the battens too and this is also thicker - i.e. much less heat loss in general also.
If you wanted some science - if you did some looking up on wikipedia, British Gypsum etc you could probably estimate the difference in heat flow for some estimated dimensions with the thermal properties of wood, insulation material, air, brick, plaster etc.

 

[roy c]

I live in an old Victorian house and I dot and dabbed my walls with poly backed plasterboard .But first of all I knocked all the old plaster off back to the stonework then racked out all the joints then pointed them up with weak mix containing w/proofer. Then I scratched them out with the same mix then dot and dabbed them with the polyback. That has been more than ten years ago and have done it in customers houses and have had no come backs. But it is not a cheap fix done this way....

 

[NickB_99]

I live in an old Victorian house and I dot and dabbed my walls with poly backed plasterboard.

Roy - I'd have had you floating the walls with lime plaster

But first of all I knocked all the old plaster off back to the stonework then racked out all the joints then pointed them up with weak mix containing w/proofer. Then I scratched them out with the same mix then dot and dabbed them with the polyback. That has been more than ten years ago and have done it in customers houses and have had no come backs. But it is not a cheap fix done this way....

Iinterested in this solution....
I'm assuming the scratch coat covered the wall before the boards got dot & dabbed - why was it necessary to rake out the joints first and repoint with the same mix?
I'm thinking maybe the mortar was in a bad state - but if it was ok, could you just get away with the scratch coat?

 

[roy c]

Hi newby l raked out the joints as it was the old sand and lime mix and l could of threw a dubbing out coat on it but l didn, t want too thick a coat on it.The pointing mix was like a floor mix (holds together when you squeeze it) minimum amount of water trowel it in to the joints with a small pointing trowel then let it pick up a bit then use a brush to brush it in.lt, s the same method as pointing stone work up but you use a pointed stick to show the edge of the stone.This also helps to keep some of the damp out.When the plaster was hacked off l left the wall for a few days to let any damp or moisure dry out.When the mortar has gone off then you can damp it down and put your scratch on.If the wall is not flat and the stonework is a bit bumpy then l would usually "scut" the muck on to the wall and fill the hollows in.Then let it go off and use a piece of wood just to screed anyhigh spots out. It doesn't have to be plumb and level as you can make the difference up with the board adhesive. Now as l am getting older and still have another couple of rooms to doin my own house l will be haking the plaster off and then put stud work in front of it and polstyrene inbetween the studs and just leave the walls to breathe.l will be sorting this out soon and l will put some photos of the work in an album so you can see the progress..