Spacetherm Dry-Lining, 22/12/10 By Ruairi Kay
Installation of the Spacetherm dry-lining is well underway over the existing cavity wall. This is supplied by A Proctor group in Scotland. Their normal build up for this kind of project, (which has been tested by the BRE!) is to fix Spacetherm F (aerogel laminated to Fermacell board with an integrated vapour barrier) to timber battens fixed to the existing wall. In this case the depth of aerogel is 60mm to give a U-value of 0.15 W/m2 K. We have concerns about this kind of build up in relation to airtightness and moisture management, especially after speaking to Niall Crosson of Ecological Building Systems at the annual AECB conference back in October. The problem with this system is that you are relaying on a butt joints between the boards for integrity of the vapour barrier. As the vapour barrier has been laminated between the Fermacell and aerogel it is impossible to tape or check the joint. So even with extreme care on site you will end up with breaks in the vapour barrier at joints, added to the difficulty sealing around service penetrations. Due to a temperature and pressure difference between inside and outside, (particularly in winter) warm moist air will be drawn through these gaps to the existing wall behind. This will increase the heat lost from escaping warm air but more worryingly may allow moisture to condense on the existing wall behind the insulation leading to a higher risk of mould growth. So, we have worked with A Proctor to design an alternative laminate for this and a few of our other retrofit projects. Instead of bonding the aerogel to Fermacell we asked them to laminate it to 9mm ply. We then fix this build up directly to the wall. The ply allows gives a firm ground for the fixing and also creates a base for the Intello Plus vapour and air barrier that is laid over it. This arrangement gives us the opportunity to seal and check the vapour and air tight layer before we apply any finishes. The elimination of the void behind the boards removes any space for air to circulate and for mould to grow. We then fix timber battens over this to create a service void and finish with plasterboard and skim.
Spacetherm dry-lining
Fixing the boards has not been without its problems. We discovered that the inner leaf of the cavity wall is constructed of hollow blocks, making it difficult to find a secure fixing. The contractor suggested we first fix timber battens to the wall then fix the boards to these with the aerogel notched around them. We were not keen to use this method and have put them in touch with Hilti about an alternative fixing. The guys on site are also finding the aerogel particularly nasty to work with and cannot wash the dust off anything. Given the choice, none of them would work with it again!
One of the guys on site, well covered to avoid the dust, but still got it on his eyebrows!
Comments