celotex thicker than 100mm for cavity wall?

[acresswell]

Anyone with any experience of putting multiple layers of PIR insulation (celotex/kingspan/xtratherm/quinntherm) in a masonry cavity wall? I’ve twice asked celotex’s technical support for help via email, but haven’t received an answer. I know you can use double layers for roofs, etc... but in a cavity wall they need to be supported by the ties.

Or how about a supplier of boards thicker than 100mm that are suitable for use in a masonry cavity wall?

Thanks

[acresswell]

A bit of background:
We're on an exposed site, so a partial fill to the cavity is recommended.
We'll have a brick outer leaf, so external wall insulation isn't an option.
I’ve also been looking at timber frame (and will continue to do so), but access for a crane (and space to spread its outriggers) would be very difficult,  and we do have a preference for a building system with which we're more familiar, so masonry is currently our preferred choice.

On the plans as at present, we've allowed for a 425mm total wall thickness. 
This could be:
A)  100mm brick / 50mm clear cavity / 150mm celotex / 100mm dense block/25mm plaster
B)  100mm brick / 50mm clear cavity / 100mm celotex / 140mm lightweight block/25mm plaster
C)  100mm brick / 50mm clear cavity / 100mm celotex / 100mm block / 50mm celotex between battens / 25mm plaster

A) seems sensible because it gets the thermal mass in the right place, but it’s only realistic if someone makes thick enough insulation (or I can get info on combining 2 sheets). It’s my preferred option if possible.
B) still has fairly good u-values (around 0.16 or 0.17, possibly better with basalt or GRP ties) and would speed up the electrics slightly because we’d only be chasing socket boxes (etc) into lightweight block. Given that the lack of flat ground means that many of the materials will have to be man-handled into location, the lightweight bit sounds attractive. We'd be reducing our thermal mass, but might get a faster response from the underfloor heating as a result.
Our architect isn’t a fan of the lightweight blocks (e.g. celcon or thermalite)  because he reckons they have a tendency to crack as a result of differential drying, though hopefully by using the slightly thicker 140mm blocks this would help.  140mm instead of 100mm would also mean that any chasing of the blocks wouldn’t leave them “too thin”.
C) seems a bit messy.  The thermal mass isn’t in the right place, and fixing shelves will be awkward.  It involves more different elements, which would probably make it slower / possibly more expensive

Can anyone recommend an online U-value calculator that will cope with A and C?
The builddesk one doesn’t like me... and the celotex one won’t cope with thicknesses greater than 100mm(A) or between battens (C)

[mespilus]

Have you considered:

Durisol?

www.durisol.net

(I saw them at Ecobuild, and have no commercial interest.)

[biff]

acresswell,
          insulation is really about thermal barriers,or the non transfer of heat from one place to another,that is why you have a cavity as well as keeping out the damp.i have built loads of houses with brick external walls and durox or thermalite 150mm inside wall, the problem with the thermalite or durox,is that they are sold as non load bearing,so you have to round off under the joists with 3 courses of brick and use brick under the lintils as padstones. this gave one of the best results on the market at the time,i recall going back to visit these houses and the story was always the same,the fuel savings, be it solid,gas or oil were fantastic.
   to use 2 layers of 50mm kingspan instead of one 100mm should work out more efficent,especially if you had a small consistant gap between the 2 layers say 20mm but keeping that right would be tricky to say the least,if your house is to be built on good high ground and will never be subject to flooding,ie  like on a floodplain, then thermalite is a good bet,i know there are many new types of insulation on the market which claim to be better, but the thermalite finished off inside with carlite browning and finish inside was to me as good as it could get.,just make sure your brickies keep the cavity spotless,any brickie seen to let the snots fall down the cavity and build up on top of the dpc,,race him the road and clean out your cavity immediatly.it is important to make it clear to the brickies at the beginning exactly what you are doing,
                                     good luck           biff

[dhaslam]

Of the three options I would prefer B.   However when you are going so far it is a pity not to be able to go below  a U-value of  .1.     I would prefer a fully filled cavity and as wide as possible, ideally about 300mm.    In Ireland the maximum cavity allowed is 150mm so that is what I have.    Of course the rest of the insulation has to match  but  once the average U-value, excluding windows,  goes below .1  heating requirement is negligible.        

[Baz]

What is the max allowed space between walls? I think above 6 in you have to have 9 in or butresses every 6 ft.
Remember you are building for 200+ years so go for the max insulation. It is often said the tiny gap between the board and wall allows convection ( I don't agree) but consider how to prevent this.
Kingspan is available at 6in and more but one of the problems is joints between sheets which multilayer can cover.
You might want to consider the design around windows to allow for slide up or accross insulated shutters. They will save more by sealing lossy windows than the loss by thinning the primary insulation in that part of the wall.

Not insulation related but you shouldn't be thinking in terms of chasing the blocks. Build in hollow skirting, risers beside doors for light switch and beside windows for alarm etc. This allows for rewiring, adding fibre optic etc during the life of the property.

[rhys]

What is the max allowed space between walls? I think above 6 in you have to have 9 in or butresses every 6 ft.
Remember you are building for 200+ years so go for the max insulation. It is often said the tiny gap between the board and wall allows convection ( I don't agree) but consider how to prevent this.
Kingspan is available at 6in and more but one of the problems is joints between sheets which multilayer can cover.
You might want to consider the design around windows to allow for slide up or accross insulated shutters. They will save more by sealing lossy windows than the loss by thinning the primary insulation in that part of the wall.

Not insulation related but you shouldn't be thinking in terms of chasing the blocks. Build in hollow skirting, risers beside doors for light switch and beside windows for alarm etc. This allows for rewiring, adding fibre optic etc during the life of the property.

Maximum cavity width dealt with in the "rule of thumb" structural part of the building regs "A" is now 300mm.
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdf
Important to get the right ties and the wider the cavity thwe closer the spacing of ties. This is one reason insulated ties eg Teplo are being used more and more.
http://www.ancon.co.uk/downloads/advert/ancon%20teplotie%20.pdf

[acresswell]

Bit of an update:  I finally got a reply today from Celotex to my second e-mail...
Still haven't had a response to my first email - presume it disappeared into the ether!

U-value for option A is 0.13 (using teplo ties), which seems good to me. It's lower than the 0.16 for option B (also with teplo ties) and gets the thermal mass in the right place with no worries about the strength of lightweight blocks or battening

What the email didn't tell me was how I go about using 2 sheets of celotex in 1 cavity...   Anyone know the answers?
both on the same side, or one on either side?  (one on either side would seem to make better use of the silver coating to reduce heat transfer via radiation , but having a slightly ventilated cavity between the 2 would then seem unwise as there'll be more heat loss by convection!)
2 retaining clips (1 for each sheet) or just 1?

Thank you for all your contributions so far:
mespilus, thanks for the suggestion...  I'll have a look at durisol later this evening
biff, if our house floods then most of Worcestershire will be underwater!
dhaslam, I agree that we want to get the U-value as low as possible, but we're already pushing the max external footprint of the property.  Increasing the cavity to 300mm would mean taking an extra 100mm off the inside space which we're not willing to do at this stage. If we were less exposed then I'd also prefer to go for full-fill rockwool  insulation (like the Denbydale passivhaus) but the 50mm clear cavity is important in our situation
Baz, sorry... describing it as chasing was probably misleading. The average socket box is 25mm deep, but I tend to fit 35mm deep ones.  It gives a lot more space for wiring them up (even though I don’t do the final connections myself any more it’s always popular with electricians!), but does mean that I have to recess them slightly (about 10mm) into the wall (more for deep boxes like cooker switches). I’ve no plans to chase for cables.
Interestingly, I’ve always filled the recess behind sockets with a PVA-rich wet mortar before pushing in the metal box.  I always did this because it helped to maintain as much of the strength of the wall as possible (I’ve had some dodgy walls to work with during renovations) and because the mortar helped to fix the box firmly (assisted by a couple of screws), but now that I’m more aware of the importance of airtightness I’ll do it even more, because the mortar should act like a parging layer to reduce air leaks through the back of the box.
Rhys, thank you.  I’m definitely planning on using teplo ties or another insulated tie. Anyone used the Qwik fix GRP ones?

Thanks again,
Adrian

[Kombi]

I put 100mm PIR insulation into the cavity wall of our extension, with the whole wall 350mm thick: 100mm thermal block, 100mm PIR insulation, 50mm cavity and 100 brick.
Fitting the PIR sheets was quite difficult. Because of their thickness, they lack flexibility which means your wall needs to be deadflat with no mortar snort if you do not want to trap cold air. Wall ties also need to be strong enough to ensure the insulation is correctly held in place. From memory (although I may be wrong) Celotex advised to tape the side facing the block wall.

Hope this helps.

Nicolas

[wookey]

acresswell. I'd go for 'A' too.

Put both layers on the inside, offset them to minimise thermal bypass at the gaps. I wouldn't put one on the outside - the point is that any water getting onto the inner face of the outer leaf needs to be able to drip down. I was told the ali foil corrodes if it's against the outer leaf - that's why a gap is always specified with PUR/PIR sheets.

The only way to make this system work properly is to build the inner leaf first, then attach the PIR boards, tape joints, then build up outer leaf. Great care needs to be taken to avoid gaps behind the boards and between the boards, otherwise you won't get anything like the theoretical U-value.

(An alternative is dritherm32 rockwool, full-fill, which will produce similar U-values in a 425mm wall. (0.16). Main advantage is that it's a lot easier to build right. ) The rigid board system is good (better thermally) if you can get it built right.

Do the Qwik Fix GRP ties go long enough for 200mm cavities? And do they have a UK BBA cert?

Baz, I don't know what makes you 'not agree' about the convection-in-gaps issue. It's a serious problem responsible for many cases of buildings not performing in practice as well as the design numbers suggested they should. Use PUR glue to limit convection length.

[acresswell]

Wookey...  a google for PUR glue didn't give me much help...  Is this Polyurethane glue like "Gorilla Glue"?

I can see that it's possible to stagger the vertical joints, but won't the horizontal joints be forced to line up because of the wall ties?

Presumably we should tape the joints of the inner layer of insulation before adding the outer layer and then taping them too. 

Nicolas / Kombi, I can't see how we could tape the side of the insulation that's against the inner leaf of blockwork (there'll be a wall in the way). Presume we will have to tape the side of the insulation that's facing towards the clear cavity. Thanks for advice on wall straightness / snots. I wasn't planning on laying the bricks/blocks myself anyway, and had already thought about visiting some examples of the builders work to check the quality of their facing brickwork... sounds like I may need to try to visit some "work in progress" so that I can see the clean-ness of the cavity.

Once the inner leaf has been built up, what do you reckon about running a thin line of the glue along the top edge of the previous, lower sheet of insulation before adding the next layer above it (as well as putting some on the inner leaf)? Would it "squidge" (technical term) down enough so that there're no noticeable gaps between the boards, but still do something useful to prevent air flow through the crack?

The qwik fix website (www.qwikfixings.com/glass-fibre-wall-ties.aspx) says that their ties are BBA approved for use in the UK and Ireland (testing certificate at www.qwikfixings.com/uploads/Qwik-Fix_Wall_Tie_Summary_Report.pdf).  Their longest tie is 325mm which is suitable for my 200mm cavity.... but has anyone actually used them?

[Kombi]

The outside of the brick wall always looks nice. But not so much attention is paid to the inner block wall, as builders are used to working with wool. I agree with Wookey, you need the insulation on the inner wall and built in the order mentionned. Another battle you'll have on your hands is to get the builder (and his supplier) to deliver undamaged sheets, with complete corners. I I were you I'd be there supervising on the days the works is being carried out. The only way I got my floor insulation done the way I wanted to (two layers of Celotex crossways) was to do it with the builder and align and tape myself the sheets.

Nicolas

[titan]

Have you considered a full fill 150mm cavity with Rockwool. Testing in Denmark has shown that rain penetration through the outer wall rarely exceeds 3-4mm into the Rockwool even in exposed sites  Long ties are available ( I have some)  and it would be worth talking with a structural engineer to minimise the required number and reduce cold bridging. It is also worth using separate lintels  for the same reason. The cavity will require a drain.

[Baz]

The board mfrs sell the appropriate glue.
You could use 1 in of glass fibre between the wall and sheet to take out irregularities.
There is sort of ridged hard plastic sheet that is used for providing a water runoff space when building walls (outside of wall) for basements. That could be used indside the outer wall to provide the gap with rockwool, though celotex is more efficient so would be thinnner.
As these measures will all make the transition of inside humidity to the outside impossible you will need to have internal humidity control measures. Traditional walls breathe, or have draughts to remove the 8 pints per person that can be produced through sweat and hot breath.

[acresswell]

OK, maybe I’m being stupid...  but I’ve looked at the celotex, kingspan, quinntherm and xtratherm websites and can’t see any relevant mention of glue or adhesive...

I’m not worried about the “8 pints of sweat”, because we’re planning to fit an MVHR unit.

I spent a bit of time looking at the “200mm of dritherm” idea this morning. It appeals simply because it’s less complicated.  I’d love to be on-site every day throughout the build (not just on the days that insulation is being fitted) but my work means that I can’t rely on being there on any particular day even though I’ll be able to check on the progress every evening.  Going for 200mm of dritherm would only mean a U-value of around 0.15/0.16 rather than the theoretical 0.13 that 150mm of celotex could offer (assuming it’s installed perfectly!), but it might save us some money to use elsewhere.