I'm planning to build a new house using single skin concrete block (I'm a fan of thermal mass for winter solar gain) coupled with external wall insulation to reach U=0.1. I'm proposing to go for 200mm of PUR/PIR rigind insulation. I'm struggling to find suitable insulation for this purpose. (Please don't let this discussion spread into the relative merits/ageing of PUR/PIR. I have a narrow plot for the size of house and am already facing 450mm to 500mm thick walls.)

Kingspan do an ideal product, KoolTherm K5 EWB, which looks designed specifically for this task, with a mesh on the outside to take a render directly. Unfortunately it only seems to be available up to 80mm, which is clearly not thick enough. In fact Kingspan insulation is generally less than 200mm.

Celotex do a product XR3200 which is 200mm thick and is recommended for EWI, but seems to require an air gap with battens on the outside face, which takes up another 50mm.

None of the other PUR/PIR vendors (xtratherm, ecotherm at least) seem to show anything for external insulation on their web sites.

I'm guessing that it isn't a good idea to form the insulation from multiple layers, but is this correct? (It would allow me to overlap the joints, but be more difficult to fix and provide an opportunity for thermal looping between the sheets.)

Is anyone aware of alternative PIR/PUR suppliers who do something suitable?

Also, should I go with an external airgap anyway to deal with any moisture ingress through failing render?

Thanks,
Mark.

Posted By: passivhausfanHave you actually asked Kingspan if it's available in greater thicknesses?

I think that should be your first step. The marketing department probably just don't imagine people want it any thicker.

Yep, done that and waiting a response.

Also asking Celotex to clarify over the airgap.

Interested in the general opinion here though, especially over whether the airgap is a good idea or not anyway.

Posted By: tonydo it with no air gap -- what will the outside wall finish be? if it was a lime render then it would be self healing for hairline cracks but on the continent they use polymer thin renders routinely without any air gap.

Thanks for the advice.

We have not yet decided on the external finish, so I'm open to suggestions. We will probably want something relatively smooth and I'm open to suggestions.

Saint, any pointers to suppliers for the sliced bunstock? Whisper if you like.

730mm thick sound quite inefficient to get 200mm thick slabs - would end up with 3 slabs at 200mm plus one at 130mm (less 3 saw thicknesses). Ideal for 240mm though.

The renderboard is just a backing board, isn't it? It still needs a render on top of it, so I don't see the advantage over a mesh or metal lath with the basecoat applied.

Mark, the fabricators work out the best yield from the bunstock. It may be a question of taking 240mm from separate bunstock blocks. I'll whisper you the contacts.
As for the render board and I'm no expert it allows the use of thin coat renders

Mark, I whispered you...I think

Mark , what concerns you about using multiple layers ?
Also sheets of EPS I'm currently using for a EWI are 1220mm/610mm , its a good size to work with
Might be worth thinknig about if you're getting stuff cut to size

cheers Jim

Sounds like 100mm thick sheets might be a good compromise , only 2 layers and will allow you to stagger the joins
sealing up some of the gaps
Not really up on thermal looping , I presume its something to do with heat lose caused by the various internal airgaps created by imprecise construction in each layer
Installation advice on the current system I'm using, suggests 4 x 100mm wide strips of adhesive 25mm thick on the back of the insulation board , the board ( 1220/610mm) is then pushed onto the wall and tapped plumb giving hopefully a 50% bond on the wall , similar to dry lining , 2 fixings are used to hold the slab in place whilst the adhesive goes off , so in this situation gaps will occur between the board and wall, but it allows some levelling up .
the next sheet is then butted up tight , or shaped if out of square ( I ended up using a foam gun to fill slight imperfections )

Perhaps it would be better to use a tiling trowel to butter up the boards as this would allow the board to fix tighter to the walls ( if the walls were flat enough) reducing the airgaps
Next layer could be added in a similar way
with larger thick sheets you might get into problems with imperfections in the walls and end up with more airgaps
As walls are rarely flat , small sheet will take-up these imperfections and the scratch coat will level it of again.
Anyway ,food for thought

as mentioned might be worth thinking about what render you're going for as the supplier of thro-colour systems
( Lime and polymer based ) might not like your choice of insulation board
cheers Jim

All these questions are well known to the EWI specialist firms - 200mm is commonplace in Europe tho still beyond the ken of an Englishman.

Chris Hirst [Chris@clan.co.uk] previously recommended on GBF (and by me) and others like http://www.sas-europe.com/ based in my neighbouring village have moved beyond the notorious name-a-price EWI supply/fix specialist format, into supply-only, with generous advice, names of local fitters, and site visits/demonstration rolled into the quote. That way, you get the warranty that comes with their franchised Euro-system, or own-brand assemblage of components, at competitive price - EWI has now become a main-contractor commodity item.

Anything other than EPS or wood fibre is v suspect to me. Only they, amongst the options (also cork) are vapour-permeable. All the foamed plastics incl XPS are not, and are asking for trouble IMHO. Note, EPS = expanded ploystyrene, XPS = extruded polystyrene, much more expensive, and almost matched performance-wise by Platinum-grade EPS. Non-permeability may be why air-gaps are called for. Outer coatings - cement render is rather impermeable - don't even think of it; lime render and thin-coat (commonly 6mm) acrylic (and other patent) renders are permeable and seem to be fine (therefore cheaper) without air gap.