Cavity ties can be a significant thermal bridge. Use an insulating tie like Ancon Magmatech TeploTie.

I assume you are talking about partial fill polyurethane or polystyrene cavity boards? Rigid boards do not fit well against an uneven inner leaf, leading to thermal bypass where cold air finds its way onto the warm side of the board. This can be limited by using multiple layers &/or taping joints, but bricklayers are not used to using tape & it requires skill/discipline to do it properly in a damp & dirty site environment.

If you use something like the Knauf DriTherm32 mineral wool cavity slabs to fully fill the cavity then you should end up with similar overall performance to PUR/EPS partial fill, but a simpler & more robust build. Avoid the lower density mineral wool cavity slabs as these can slump in wet weather.

Mineral wool cavity slabs are flexible enough to fit tightly against the inner leaf & they tend to knit with each other reducing air movement through joints. Use at least two layers of at least 75mm each & offset the joints to reduce thermal bypass further.

The full fill also prevents mortar dropping down the cavity, so you get less mortar snots on the cavity ties. Any mortar on top of the slabs can be removed before the next layer is fitted. Ideally use a cavity board to protect the top of the slabs/ties & move this up the cavity as each set of slabs/ties is fitted.

David

Joulestherm- I was not permited by BC to go for the full fill cavity as I live in an exposed area of the country, apparently a large area of the west coast of the UK is deemed to be exposed.In this area you must maintain a 50mm air gap inside your cavity. I visited my build every day to remove snots from the ties and ensure all the boards fitted against the inner wall properly and removed as much mortar of the boards as I could ready for the next 1. Just trying to price materials and keep things in order is fairly frustrating at times so good luck. I am just fitting my windows and roof is on at last. Ready to fit my lovely rockwool after first fix, scratch scratch, itch itch, cant wait,
Gusty.

Yep . Joe little here is worth a read on the subject http://www.josephlittlearchitects.com/documents/cavity_wall_paper.pdf

Posted By: fostertomVery good - written as early as 2005, based on much earlier work, and still unimplimented, for vested interest reasons.

Which vested interests are you eluding to?

As far as I can see it comes down to simple economics. Builders may be willing to take a risk on something which is novel & cheaper, but why would they take a risk on something which is novel & more expensive or even novel & cost neutral? They will stay with the well known & well understood cavity wall construction until external wall insulation is cheaper or some specification requirement drives a change.

I don't think U values will be driving any change as a cavity wall is capable of the same or better performance than an externally insulated solid wall. For me the cavity wall appeals because you can fill it with low cost high performance mineral wool insulation. How many EWI solution offer a U value of 0.1 at equivalent cost?

I agree with the issues of build quality. However, in many ways building a wall with a 300mm cavity has more in common with EWI than it does with previous cavity walls & many of the issues are common with EWI (a flat substrate is required to fix the boards, etc). For EWI, quality is addressed by using specialist contractors who train & supervise their staff to the level required. For cavity walls this can be addressed by specifying the wall build-up in detail, discussing it with the bricklayers & the on-site supervision team, & checking progress daily.

I see the thermal mass advantage of a 200mm solid wall over a 100mm inner leaf, but there have been studies which have shown that increasing concrete wall/floor thicknesses beyond 100mm doesn't usefully increase the thermal mass exposed to the room.

David

Posted By: davidfreeboroughI see the thermal mass advantage of a 200mm solid wall over a 100mm inner leaf, but there have been studies which have shown that increasing concrete wall/floor thicknesses beyond 100mm doesn't usefully increase the thermal mass exposed to the room

Perfectly true, when solar gain (via windows) is put into the inner or room face of the massive store, so heat goes in and then a bit later comes out again the same way. In that case it's only the inboard 100mm apx that really plays a part, however deep that lump of mass is.

If that's all you ever expect to do, then a 100mm inner skin is all you can make use of, so CWI away! But you'll only get 1 day max of heat storage, so in a normal sunless winter spell, solar gain can contribute very little - 4 days out of 5 you'll need to be burning fuel, however good your insulation and airtightness is. That's the Passivhaus scenario - 'backup' heating cannot be dispensed with, and how much it's used - really only 'backup', or heavily and regularly used, is extremely sensitive to occupants' discipline in use. In other words, hopeless in a non-Germanic population!

What's more, in this Passivhaus scenario, window area/solar intake has to be strictly limited, because this way of handling solar gain offers little protection against overheating. So a double hit - only small amounts of solar can be collected, and of short duration. That's the best you can expect with only an active inboard 100mm of massive store - e.g. if you install CWI.

Alternatively, if you store your solar gain into the outside face of your massive store (assuming it's an external wall we're talking about), so that heat proceeds uni-directionally from outside to inside, then every bit of its thermal capacity and thickness is useful. In that case a CWI'd cavity is, at least, a thermal complication (it could still work), and also a lost opportunity to add to the available massiveness by filling the cavity with e.g. weak concrete - or using the cavity as the channel by which solar heat is input into the mass store.

A battenspace on the outside of the extg walls, clad with OSB glued and screwed can provide the channel by which solar heat is input into the extg wall's outer face, esp if it's a nice 500thk solid wall. The OSB can serve as a very robust airtight membrane, and all can be made flat, ready for easy EWI application to the outside of the OSB. New windows can be easily fixed and mastic sealed to the outside face of the OSB/battening, and with 200 EWI sits nicely relative to external reveals.

Then, the entire thermal mass becomes part of the internal environment, with 7-10 days storage. That's when it becomes true that you can't have too much massiveness!

What's more, with an effective solar capture/transport away to storage system, risk of overheating the interior doesn't arise, so you can capture as much solar heat as you like, unlike the low limit imposed by Passivhaus's basic physics.

Thought this might be of interest , solutions for fabric build up to beyond Fabric Energy Efficient standard
U 0.15 with 125 mmm cavity ,

http://www.xtratherm.com/publications/Part%20L1A%20Guide%20May%202011.pdf