Dot dab polystyrene then dot dab plasterboard?

[Johnboy54]

Hello to all.
My logic says this method should be possible
If you can dot dab boards backed with polystyrene, then does it follow that you can dot dab polystyrene and then dot dab boards to that?
My local supplier sells polystyrene backed boards that are 40mm thick in total.
I can buy 50mm polystyrene sheets from them and the boards separately for a total of 25% less.
More insulation for less money!
Are there massive holes in my logic?
Thanks.
John.

 

[roy c]

Nothing wrong with your logic. I have done that in my own house, first of all I had to dub out the walls and scratch it up to a fair line.( Old stone walls) Then I dot and dabbed 1 inch poly on the walls and straightened it up with a piece of 4"x 2" left it to set then dot and dabbed the plaster board on. No problems... I made sure that the poly touched the other poly in the corners so the boards never touched the scratch coat. Depending on the skirtin' you are putting on lift your boards off the floor, but let the poly go down to it ...I left an inch from the bottom but half an inch will do..... Good luck...

 

[NickB_99]

If you can dot dab boards backed with polystyrene, then does it follow that you can dot dab polystyrene and then dot dab boards to that?
.....
Are there massive holes in my logic?

I suspect it won't work.
Plasterboard adhesive works well with plasterboard, brick, blocks etc - probably because the brick/blocks suck out some of the moisture and help it go off.

To D&D the polystyrene direct will prob work (hence why you can D&D polystyrene p/board direct).
But then won't work when you try separately putting plasterboard on top.

I suspect the polystyrene is bonded to the plasterboard using a strong solvent based glue. Plasterboard adhesive is not the same.

 

[NickB_99]

Johnboy - ignore my last post.
Roy C is far more knowledgable and I was only taking a punt

Roy - thanks for the clarification - something learned

 

[roy c]

I think I have put this on before but to those who never seen it I will put it on again.
I was first introduced to dot and dabbing over 20 years ago by an architect friend of mine . He wanted to put foil backed plaster board on to 1" polystyrene and wanted me to dot and dab the bay windows in his house because of a damp problem. He told me he was going to use "Unibond" (which was the trade name and still is for pva) he rollered the unibond onto the board and put the polystyrene onand we rollered it with a dry roller so it was firmly flat. Then we put another sheet of plaster board on top of them and did the same with the "poly" and so on till we had about 8 sheets of poly back plaster board, (They may have been the first ones ever made )
He said we would leave them to firm up so that the poly had stuck properly , which we did and it was about 3-4 days when they were ready to stick on the walls. I stuck them on with a product called "Multi purpose adhesive which wasn't much different to the stuff we use nowadays....But one thing I learned from sticking polystyrene to foil back boards was it is very important to use high quality PVA which unibond was and still is. The reason I say this is because I had a mate who I told about this method and he used "Cheap " PVA and it took a couple of weeks to actually set and bond the poly to the foil back...

 

[Johnboy54]

Thank you so much to both of you.
Roy, you show 2 methods.
Both seem to work.
However, which one would you use if you were to do it again?
I suppose the foil-backed one would give even more insulation.
But would the other give more heat retention by providing 2 air gaps, working on the principle of a 'string vest' would you know?
My house is made of solid sandstone.
Also, any idea about the vapour control layer I read so much about?
My last house had no such thing with no problems. (Walls done by previous owner many years ago)
Solid brick, 1 inch battens, foam-backed boards (not polystyrene) nailed and plastered.
I worry about this vapour control thing.
And pubs going out of business.
From what I've read no house in the land should be dot and dabbed without boards containing one.
Thank you.
John.

 

[Richard C]

I’m all for using ingenuity & being economical but this one could turn out to be a false economy if you get it wrong; it’s unlikely to be as strong or efficient as the real thing & if the work is subject to BR control & inspection, I wonder what the BI will have to say about it It’s not a good idea to attempt dabbing/bonding foil back board either; adhesion will only ever be as good as the foil to the board & the manufacturers recommendations are that it’s not suitable; although I’ve never personally tried it.

It seems like a lot of effort to save 25% of what is not a particularly high materials cost in the first place; surely it makes sense to buy the correct materials in the first place.

 

[Johnboy54]

Hello Richard.
I'm doing the entire house.
It's a saving of about £200.
So, it's worth considering.
John.

 

[Richard C]

Personally I still think a saving of “about £200” on the cost of insulation & plaster materials for a whole house is not a lot given the total cost of the project, even if you are relying on DIY input; there are some keen prices to be had on line. If you’re still determined to do it, I would try a dummy run before committing to buying the lot & bond the styrene to the boards on a flat, solid base first & then fix rather trying to do it on the wall.

I don’t know the extent of the renovation work you’re doing but, as I said earlier, it may not be acceptable to a BI. If you’re doing work that affects more than 24% of a thermal element – roof or external wall, the work is subject to BR’s (Part L1B) & submission; fine if LABC don’t know & you don’t tell them but if your having any other inspection work done they will almost certainly spot it. We’ve had several poor souls on here seeking advice after being caught out during inspections on other work they were doing or, as in one case, a nosey neighbour shopped them.