Bill Butcher
The walls are going up with a little help from the Russian space programme while Bill flits off to the beach, giving the clients a chance to put their view of the project
Passivhaus diaries, part seven: The walls
I'm actually on holiday in my native Devon this week, but work on the cavity walls will hopefully happening on site in my absence – more details on this below. And while I'm away I'm also giving our clients Geoff and Kate Tunstall a chance to give their perspective at this stage in the build (see box).
On site, the wall is being built in dense concrete block to help increase thermal mass inside the house, with 300mm of Dri-Therm fibreglass insulation batts (three 100mm layers) in the cavity and 100mm of natural stone on the outside.
Below ground we've used polystyrene solid closed-cell insulation within the walls, which will provide stability if there is any ground movement. Also, because it is made using closed cell materials, it will not absorb any moisture if groundwater gets into the cavity. As soon as insulation gets soaked its effectiveness is reduced, increasing thermal conductivity.
Just above floor level we need to join the polystyrene foam with the fibreglass insulation. To do this, we have cut the polystyrene solid insulation at an angle to help control the flow of any moisture within the cavity wall. Because stone is porous there will inevitably be some water percolation coming down the inside of the stone outer leaf. We've put a plastic cavity tray on top of the angled foam, which acts as a damp-proof course and will also take any water that might be blown into the cavity.
The fibreglass batts are woven in such a way that they are layered so any water that does come in through wind-powered rain will not penetrate beyond a minimal surface layer. If we'd cut the polystyrene off square there might have been the potential for some pooling of water, which would have acted as a thermal bridge. The cavity tray itself is just a thin bit of plastic, which won't cause any significant thermal bridging problems.
Instead of stainless steel ties, we will be maintaining the cavity wall using Teplo ties – made of resin and basalt. Developed during the Russian space programme, these are extremely strong and a good insulator in their own right. The basalt ties also have BBA certification, so we have had no objections from our building control officer!
One disadvantage of the Teplo ties is that they are rigid – you can't use them if the coursing is slightly out, so we've had to be very careful in this build to keep courses of block in line with the courses of stone. Two courses of block on one side match three courses of stone very accurately, tied every second course of blocks with a tie. The fibreglass batts fit neatly as they are, very conveniently, two blocks high.
Our choice of wall tie has been carefully modelled in the Passivhaus Planning Package (PHPP) - the design tool for architects designing to the Passivhaus standards. At this level of performance, stainless steel ties would act as a thermal bridge and would create a cold spot on the plaster in the internal wall with the possibility of condensation forming on the internal plaster. The use of basalt wall ties therefore offer good building practice and give us a nil reading for heat transference in PHPP.
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Readers' comments (5)
John Shilabeer | 11 Aug 09 2:50 pm
I am following your diary with interest. May I raise four observations / questions regarding this weeks diary: Alarmingly both the dpc and the tray appear to stop short of the front face of the masonry? Bearing in mind the BS and Building Regulations, is this intended? The tray rises 75mm from exterior skin to interior skin. Given the 140mm/150mm minimum stipulated by British Standards / NHBC, have you secured a waiver to depart from the usual statutory dimensions? To what extent have the issues of capillary action been considered between insulation and tray, given the minimal rise? To discourage water retention/pooling on and around the tray, do you intend incorporating caviweeps to permit penetrating water to discharge? I have an interest, as Cavity Trays specialising in this field. I beleive these are four valid concerns - if I and Cavity Trays Ltd can be of any assistance, we will be delighted to do so.
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Charlie Ayers | 11 Aug 09 3:06 pm
Fullfill insulation can't stop some penetration of moisture which in time may bridge the cavity as specially if the batts are not cleaned properly after the internal block is built if you fitted SureCav Cavity wall Spacer System as a perminant shutter to your natural stone it would form a 50mm cavity and form a barrier against wind driven rain, the cavity would have air circulation and be moisture and mortar free.You can still use what ever thickness of insulation you like. You wouldnt need to cut angled insulation at DPC
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John Carney | 1 Sep 09 3:36 pm
The articles are getting more and more interesting. The wall ties are new to me and seem ideal for this situation. Presumeably they are relatively cheap and strong enough to tie the wide cavity together. I like the detail of the junction of the closed cell insulation and the fibreglass batts - simple and inexpensive. I wonder how this would work on a mass scale - retrofitting the DPC would be practically impossible. Geoff and Kate Tunstall's perspective is interesting and its great to have clients like this to work with. The film will be worth waiting for! Thanks again.
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Sally | 12 Nov 09 6:36 pm
Please can you explain why don't you need an airgap between the inner and outer leaves? Did building control have any initial misgivings? Fascinating read by the way - thanks!
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Bill Butcher | 23 Nov 09 12:03 pm
Sally, cavity walls were originally designed to stop rain driven dampness striking through to the inside of the building. To maintain this principle when filling the cavity for insulation purposes we should; a) Keep cavity clean of all mortar debris. b) Design and install cavity trays and wall ties well. c) Use water resistant and well installed insulation material. The mineral wall batts we are using are vertically layered which will keep any wind driven rain to the outside edge of the 'cavity' making it run out via cavity trays or down into the foundations below ground level.
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