Sorry I wasn't being clear.

From just looking at the R values, PU is about twice as good as Rockwool so 200mm rockwool is equivalent to 120 PU.

BUT.....

as the PU may suffer from thermal looping due to it's rigid nature leaving gaps between it and the blockwork the actual effect of a 120PU insulation layer could be less than 200mm Rockwool.

Agreed.

What i was wondering was are there any ways of preventing the thermal looping.

I came up with two possible ways:

Use a "tile grout" to temporarily stick the PU to the blockwork. The final fixing would be by hammer in ties designed for thin joint blockwork. The tile grout would be applied with a comb giving horizontal stripes. These would help fill and disrupt any voids behind the PU (assuming reasonable blockwork). This method would also allow for a single skin to be built. Then the insulation applied and crucially inspected, before the outer skin goes up.


The other method was to use a thin layer (25-50mm) of rockwool against the blockwork to prevent thermal looping. The PU would then be taped and inspected with any gaps filled before the outer skin is put up.


IF we assume the above constructions eliminate thermal looping, then we are back to a pure thermal resistance calculation.

In this case 50mm Rockwool and 100mmPU give a similar result to 200mm rockwool.

I hadn't considered the air gap. I had assumed (wrongly?) that the PU can sit against the inner skin. It is water proof and the only place water can get across is the joints. In a normal cavity this can be a problem as it is difficult to make sure that the insulation is sitting properly and that there aren't any gaps that throw water inside rather than outside. If I manage to build the insulation layer before sealing it inside the wall it can be inspected and even tested and any problems made good. Would this allow me to have a fully filled cavity?

hummmm

If the gap needs to be 25mm (or 50mm as per building regs) then that puts a major damper on any kind of PU board of less than 100mm.

150mm of rockwool (full fill) gives about 0.25 U, about the same as 100mm of PU and 50mm of air.

That being said, I would guess a 100PU + 50air between two brick skins would be more waterproof than 150Rockwool alone... IF the thermal looping can be eliminated.

I suppose the silver foil would also play a part in reducing the thermal gain in the summer by reducing the radiant part of the transmission across the cavity.

This need for a cavity to keep the water out does make me wonder how externally insulated and rendered systems work. After all they effectively have a fully filled cavity. Do they rely solely on the waterproof properties of the render?

There are no electronic copies available so please, if you want to read the story either buy just that issue or subscribe.

It is, after all, what pays for the administration effort on this forum!

http://www.greenbuildingpress.co.uk/products.php?category_id=7

@ mark & keith:
I think I will get a subscription, congrats on getting a new subscriber! :-)

On the thermal bypass question:

I was looking at methods to insulate the wall and came across polystyrene beads.

There was an article on masonry walls using 200mm of poly bead as a full fill. This would help prevent the thermal looping and could be easy to install on a new build, just pour them in from the top!

Does anyone have any experience of polybeads?

Blown Full.fill
http://www.springvale.com/products.asp?InfoID=525&mySub=517

Partial Cavity Fill - PIR TnG
http://www.xtratherm.com/products/thinrcplusuk.php
Product Brochure - http://www.xtratherm.com/products/literature/thinrcplusuk.pdf
BBA Cert - http://www.xtratherm.com/products/certs/thinrcplusukbbacert.pdf

SafeR is phenolic foam product but has decent section image and better explanation of process for partial cavity fill installation
http://www.xtratherm.com/products/safercw.php

Roofs not Walls but still relevant to thermal bypass, especially in retrofit.
Platinum Warmsqueez is a versatile thermal insulation panel that is specially cut to compress and then expand into position between the rafters and trusses to become self-retaining.
http://www.springvale.com/products.asp?InfoID=526&mySub=518

Sinnerboy,
Joseph's article is excellent, and does draw on some of the research that I reviewed, but where his article remains focused upon wall construction the article in GBM goes somewhat further as it also looks at roof construction, party walls, windtightness and airtightness etc. The GBM article then draws upon a range of research papers and case studies that consider thermal bypass in order propose solutions to the technical problems that are exposed.

Finally Joseph's article could be interpreted as being (slightly) pro-timber frame, or at least an earlier version could be, by contrast the GBM article tries to take a more neutral stance and highlights where timber frame can go wrong as well as masonry - sadly, as the problem is skills based i.e. there is no quick fix.

Mark

Just to answer beelbeebubs query about moisture resistance of cavity fill
"DriTherm Cavity Slabs have a special additive to make the product water repellent"
http://www.knauf-insulation.co.uk/dritherm-cavity-slab.html

also theres a biased but useful comparison of full-fill vs part fill rigid boards
http://www.knaufinsulation.co.uk/products/crown_slabs/crown_dritherm_cavity_slabs/rethinking_cavity_wall_insulat.aspx

Has anyone come accross an alternative to Knauf DriTherm? I'm particularly interested in glass/mineral wool with a conductivity of less than 0.032.

Cheers

Perimeter Plus Blow-In-Blanket http://www.knaufinsulation.co.uk/products/perimeter_plus.aspx Blimey, whatever next? The idea of blowing loose itchy glass fibre around! The patent netting system must be verrry good, because even with harmless Warmcel fibre a certain amount escapes - you can't close off every opening.

Still, assuming it works .... here's the rehabilitation of glassfibre's reputation. no longer the cheapo alternative that's near-useless because of the inevitable gaps between bits of blanket, but blown in and therefore seamless, working its way into every cranny.

Questions -
is it left under compressive pressure, like Warmcel, so it's guaranteed (assuming effective precautions against slump) to stay filling the space completely?
is it therefore airtight-in-depth, like Warmcel?

It has very low water vapour resistance, so is suitable for 'breathing wall' construction - tho not of course with the added advantage of hygrosvcopicity, which Warmcel excels in.

And probably no longer cheap - nice way of upmarketing stuff that costs no more to produce than Rice Krispies - we pay for the marketing and the cost of transporting boxes of air around the world!

Some good stuff from Leeds Met
http://www.lmu.ac.uk/as/cebe/projects/lowcarb4real/lc4r_des_bypassing.pdf

Found at - http://www.lmu.ac.uk/as/cebe/projects/lowcarb4real/

Mark

Going back to the cavity with the PU foam

remember that the thermal performance of the rigid foil faced board may include the low emestivity nature of the foil face. HOWEVER that only works if there is an air space infront of the foil face.

So the cavity may be required for drainage or whatever, but it may also be needed for the boards to achieve their claimed performance!

Timber

This story is now available for immediate (almost) download for a small fee to non-subscribers of the magazine.

http://www.greenbuildingpress.co.uk/product_details.php?category_id=9&item_id=174

i guess you should go sparingly on fixings if you nail your PU insulation - they will be lots of thermal passageways straight from your warm interior bricks. Grout manages to keep tiles up for long enough doesn't it. Maybe fixings which are less conductive than nails would be better. Bit tough to fix it though once it's fallen down! take the roof off, lift it out...

I don't understand the cavity thing. I thought water penetrates bricks so the cavity allows the bricks to dry from the back. So if you fill it with insulation does that mean your bricks and mortar stay damp longer/ all the time? And your insulation is affected. I've read that rock wool is particularly affected by damp.

<blockquote><cite>Posted By: tony</cite>yea but then thermal bypass kicks in!</blockquote>

you mean bypass to the 25mm air gap? Is this the case when you use 2 layers of shiplapped boards with no common joints? Still gaps somewhere!

if I buy that almost immediate download do I get all the answers?

We just did a study on Cold Bridges using WUFI, Therm and Flux which might be of interest to people building partial fill cavity walls http://www.viking-house.ie/cold-bridge

Viking
can you cite the icelandic study? Are you saying that migrating pentane is bad for health, or just the insulation is decaying? If you say PU is 'dirty' are you proposing that your polystyrene is clean?
As for the gap, surely any figures are meaningless without knowing what's going on in the gap? It could be a tiny 3mm gap with fast moving cold air or a 25mm slow moving air gap, that actually insulates better than the brick.