window reveals-insulating+plastering

[woodgnome]

I have fixed{dot + dabbed} 60mm insulated plasterboard to single hollow block wall,and I am about to set the anglebeads and insulate around the two window reveals.The depth of reveal is 120mm(70mm p/b and bonding,50mm concrete block].My problem is I only have room for 1 cm of insulation before sk.imming -any more and the window frames will start to disapear.I was thinking of slicing 25mm foil backed kingspan down the middleand sticking them up with something that would adhere and seal the two diferent surfaces[waterproof adhesive?]-then skimming over exposed sawn kingspan.....Is this a good/silly idea?...i would welcome any advice .

 

[trowelmonkey1]

tricky one, skimming kingspan isn't going to work though, you can get plasterboard with about 10-12mm of celotex fixed to the back so i guess a thickness of around 25mm is this too much?

do you have a cavity? if so just fit cavity closers and then board.

 

[woodgnome]

There is no cavity-just x-section of insulated p/b and surface of concrete block.The problem with using 25mm insulated p/b is that once it is glued and then skimmed we are then looking at just over 30mm-which is too thick.

 

[1john]

I think you could skim over sawn exposed kingspan, after all you can skim over expanding foam. I dont think it would be very throng though.

 

[trowelmonkey1]

hmmm, yeah no strength at all john.

how about fixing eml onto the reveals then and giving it a coat of sand and cement before skimming it?

 

[marshman]

how about fixing eml onto the reveals then and giving it a coat of sand and cement before skimming it?

That would give cold spot condensation and black mould spots.
the thinnest insulated board I know of is 22mm.

 

[woodgnome]

T-monkey-pardon my ignorance -what is eml.If i did go for sliced kingspan idea-is there anything that i can coat the sawn foam with that will allow a skim to stick to it?

 

[marshman]

I would stick jute scrim on the foam, paint the foam with neat pva then press the scrim onto / into the pva, don't brush the pva over the scrim
as you want to keep all the little fibres lose to bind the skim coat to it.

 

[woodgnome]

Thanks Marshman-Your jute scrim idea sounds like the one.Now,about sticking the foil backed kingspan into the reveals-would I use dot +dab adhesive or something else,bearing in mind that it has to adhere to two different type of surfaces at once ie:foam + concrete{hollow block that has cavities in it but is still a "cold bridge"}

 

[marshman]

I had to use insulated plasterboard on my outside walls so I stuck them on with drywall then drill through and plugged with the little yellow wall plugs
I think they're 4.5 mm then used 65 / 70mm plasterboard screws to secure it, I read Richard c post the over day about using frame fixings for something
similar but I don't know if gypsum will rust the heads of them, maybe Richard will read this and post his knowledge.

 

[1john]

you cant stick ANY foil backed board with D&D adhesive, it will not bond. Use gripfill.

 

[marshman]

Oops, I missed that, ( foil ), John's right the foil won't stick. good job someone is with it.

 

[woodgnome]

Thanks lads-Will go and buy jute + lots of tubes of gripfill later....unless I get posted any more suggestions.Should I prime the two substrates with p.v.a or something prior to gripfill -if so what ratio/drying time?

 

[NickB_99]

.Should I prime the two substrates with p.v.a or something prior to gripfill -if so what ratio/drying time?

I don't think you would want to PVA before using gripfill. I may be guessing a bit here, but I think the adhesive will go off better if it has some suction from the substrate below. A lot of these adhesives (gripfill, no more nails etc) require 1 substrate to be porous.
If you seal with PVA to reduce suction, this may not help.

 

[1john]

Nope, dont PVA, just make sure that you get the gripfill straight out of the tub onto the wall and the kingspan on it straight away, dont let it skin over.