Fill a cavity wall with care, not just insulation.

  • Building Design Expert
  • 8 years ago

During a recent symposium on ‘Sustainable Building Design and Construction’, the speaker posed the question as to what constituted the greatest barriers to achieving higher performing buildings? The overwhelming response returned was that of ‘workmanship’. We can have the most innovative technology ever seen, but if it’s implementation and installation is not of a standard consistent with it’s design, there will be no reprieve for the building performance in use.

Of course legislation is constantly creeping to edge out the inconsistent and poor by introducing post completion testing, as one means of cajoling contractors in the desired direction of high quality uniformity. But we are not there yet and there remain a number of issues to address on the complacency front. At the head of this queue are the practical aspects of enforcement.

Largely, the majority of workmanship issues are covered up as part of the construction process, and even being subject to professional inspection is no guarantee that every avenue of possible discrepancy has been explored and approved, or otherwise. Building design professionals would very much advocate that experience of potential poor construction acts as an indicator for key elements to be monitored closely on the next project. To be fair, some are difficult tasks. But then some should be just like falling off a log, and don’t turn out like that. In truth often the most seasoned professional cannot predict.

There is one particular issue to that pulls me in two directions every time. As building designers we are in the privileged position of being able to dictate, to a large extent, what goes on site. There is this almost god like power that allows us to determine how a building shall be constructed, the form of construction and the materials used, and indeed how those materials are used. Taking a step back, it’s a particularly serious role that carries with it a significant responsibility. To get the right mix of materials, in the right order, for the right reasons. Something that the contractor often blindly implements or, sometimes may simply ignore – then life becomes difficult.

Cavity wall construction has always been a significant issue in on site terms. The principles of construction are relatively simple and straight forward. The implementation on site can so often be so far removed.

Maybe I am unfortunate, or just plain unlucky, but experience of cavity wall construction I find generally produces every ‘don’t do’ in the book: From unfilled and ‘tip-pointed’ perps to wall ties at an angle, or even no wall ties at all. Pointing carried out with a thumb and smeared all over the brick facings, to every bed joint being a different height. Of course the favourite misdemeanour is mortar droppings, not only filling up the cavity bottom, but clinging to the wall ties and sitting on top of the partial fill insulation slab edges. I don’t believe that the brickie doesn’t know it’s wrong, but I always have to ask for it to be put right.

Installing partial fill insulation, to the standard ordained by the insulation manufacturer, is nigh on impossible, or it would seem so for the majority of contractors. For starters the requirement to remove all projecting joint mortar from the external face of the inner leaf so the insulation can sit tight against it is the first stumbling block, quickly followed by the need to tightly butt all joints, particularly around external corners. Then as the main course there is never ending surplus mortar that has fallen down into the cavity as part of the joint wiping process with the trowel on laying the external leaf.

To illustrate the point I recently had occasion to complete the design drawings on an extension for a national chain of restaurants. I followed my design ethos as ever, predicting that a partial fill installation would yield less potential for long term damage than full fill. So having carefully detailed each possible cavity wall scenario, the drawings were issued to the nominated contractor appointed on the clients preferred negotiated basis.

The nature of my appointment required that I visited site on a periodic basis, but not as often as I might have preferred. Consequently the contractor had been on site just over two weeks by the time I made my first inspection. This set the tone for the remainder of the contract : – One word from me and they did as they liked.

The first discovery was that the extended cavity wall had not been toothed into the existing as required, but simply tied with a ladder tie. Then there was the full fill cavity wall insulation, instead of the partial fill board I had asked for. It had rained the night before, and I tentatively guessed that the wall might have been left unprotected. I felt under the course of insulation installed earlier in the morning to find the soaking wet insulation from the night before. “Oh yea” said the site manager “that’s not good is it? I’ll get him to whip that out”. But of course “whipping that out” was never going to happen as it would have meant that a good six, or so courses of brick would have to come down in order to replace it, and that would have jeopardised the finish date which had been carved in stone at the site entrance.

Cavity wall with full fill insulation, and some of the extra features we can do without

Then there was also the issue of countless mortar dropping all the way along the visible insulation top. “Can you make sure that’s cleaned off?” “No problem guvnor. I’ll make sure he does it after his tea break”. You just know it’s not going to happen. Any sniff of a delay and the client will hit the contractor so hard financially, plus they won’;t get near the next job. The client is only interested in the next opening date, and very little else matters.

That job threw up every little thing that influences how I might detail a building. I would love to be able to confidently specify full fill cavity insulation. The benefits of insulation continuity alone would be worth it. However, set against the risks of implementation by an unknown brickie who hasn’t had his bacon sandwich that morning leaves us invariably caught between two stools. Never mind trying to ask him to fill a cavity with care, not just insulation.

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