Why isn't my insulated plasterboard sticking?

[RegB]

Hi guys, I wonder if anyone has any ideas on this. I'm insulating an internal wall with laminated plasterboard, using dot and dab. It's my first time using this technique. I stuck the first board up last night as a test. It seemed nice and solid, but this morning it's loose and obviously hasn't stuck. To be more accurate, the adhesive has stuck to the wall but not to the back of the board. Does anyone have any ideas why this would happen? It's standard adhesive as supplied with the boards (Gyproc dry wall adhesive). The boards are PB over 500mm Celotex with a final foil vapour barrier (so the adhesive is in contact with this foil barrier). Before starting I was sceptical that the glue would stick to foil , which is why I did the test, and it seems those fears have been borne out. Or is there something else I should be doing perhaps? Maybe paint PVA to the back of the boards first? Any advice gratefully received before I do the other boards.

 

[ajrobb]

Have a look at Kingspan K17 for dot'n'dab. With foil backing you describe, you might have something more like Kingspan K18, which is for mechanical fixing. Another point, IF the wall is external solid masonry, you shouldn't be using dot'n'dab anyway.

 

[Richard C]

Whoever advised to dot & dab them has given you bum advice, you cannot dot & dab foil back (Duplex) boards. The foil will delaminate as you have discovered as the only thing holding them up is the strength of the bond between the foil & the board, support will be virtually non-existent & you will also destroy the foil vapior barrier. The type of board your using is designed to be mechanically fixed either with screws or nailable plugs.

For D&D installation, Kingspan as suggested or British Gypsum Thermaline plus with an integral vapor barrier.

 

[RegB]

Many thanks for that reply AJ
Yes, the stuff I have is similar to K18 but it can be used for dot and dab according to the specs. That was why I chose it actually. It's this:
http://www.insulationexpress.co.uk/documents/Speedline Thermal Laminate Datasheet.pdf
Thanks for the suggestion to avoid D&D for external masonry walls. I must admit I feel confused now since I've had quite a bit of advice to say that D&D with this type of foil-backed board is actually a good idea on external walls, and has the advantage over solid fixing that the vapour barrier is not punctured. What's the problem, in your opinion?

 

[RegB]

Hi Richard
Thanks for your reply too. Also very useful. How does using D&D destroy the vapour barrier though? By eventually pulling away the foil layer perhaps? Also, wouldn't using mechanical fixings also destroy this barrier since it involves making holes in it?

 

[ajrobb]

Using d'n'd on external masonry is a problem because the gypsum based adhesive is affected by penetrating damp. Cement-based adhesive would attack the aluminium foil.

I should say I am not an expert but I have a project that needs internal wall insulation too. I wouldn't worry about screwing through the foil into battens; there are much bigger gaps between the boards. As long as you follow manufacturer's instructions you should be OK. PIR is a closed-cell foam and also acts as a vapour barrier. The foil is there primarily to retain the heptane blowing agent in the PIR for a few more years to improve its medium-term insulation. Over several decades, the heptane is replaced by air and insulation of PIR drops down to that of mineral wool.

Another 'advantage' with battens is that you can create sealed reflective cavities behind the insulation, which are equivalent to 15mm of PIR.

 

[Richard C]

Hi Richard
How does using D&D destroy the vapour barrier though? By eventually pulling away the foil layer perhaps?

Yes but see ajrobb’s post as he’s obviously more familiar with foil back insulation boards which, I will admit I’m not. I have used foil back BG Duplex plasterboard extensively where the foil acts as the vapour barrier & BG state specifically it cannot be D&D as the foil bond strength is insufficient to support the weight of the board. I would have though this would be the case with all foil back boards, I certainly wouldn’t like to rely on just D&D without additional mechanical fixings; a look at the installation instructions on link you posted also recommends this.

Also, wouldn't using mechanical fixings also destroy this barrier since it involves making holes in it?

No, it will just make a hole in it which will self seal; but personally I wouldn’t use foil back boards in this particular case if a vapour barrier was required; I would normally use BG Thermaline Plus.

Dot & dab is OK to use on external cavity walls but not solid brick, it causes internal cold spots (where the adhesive is) which are prone to condensation; gypsum adhesive will be affected by damp & foil affected by cement, as stated.

 

[RegB]

Thanks Richard and AJ
What you both say makes a lot of sense and confirms my suspicions about using only D&D on this type of wall. I'm glad I only did one test board. I'll go back to the original plan of battens and mechanical fixings, which seems to make more sense. Thanks to both of you for taking the trouble to help me sort this out.
All the best, Reg

 

[ajrobb]

I suppose the similar argument applies to both PIR and extruded polystyrene. They both seem to be closed cell foams made using insulating blowing agents (heptane and pentane). Both therefore can be expected to lose the extra insulation over the years as the blowing agent is lost.