Double first for wood-fibre insulation as Pavadry wins BBA certificate
NBT’s Pavadry internal wall insulation system has become the first wood-fibre internal wall insulation (IWI) system to be certified by the British Board of Agrément (BBA), and the first of any kind to be certified under a ‘far-reaching assessment’ that now takes account of both internal and external moisture sources.
BBA certification of NBT’s Pavadry system means that for the first time it is possible to specify wood fibre IWI where a project requires BBA certified products. BBA certification is required for all products specified in a project funded under the government-designed ECO (Energy Company Obligation) grant scheme, and is also required by some insurers.
The certificate was awarded after scrutiny by BBA including dynamic modelling of moisture behaviour in the system, on a range of wall constructions. John Albon, head of construction products approvals at the BBA explained that the board carried out a “far reaching assessment” including dynamic modelling of moisture behaviour, and added that they will be applying the same methodology when issuing (and re-issuing) future certificates.
Modelling was to BS EN 15026 : 2007. As Matthew Smith of Pavadry manufacturer NBT, explained, dynamic calculation such as that used for BS EN15026 assesses the impact of a much wider range of moisture transfer mechanisms and sources than alternative methods.
“It includes the impact of rain on the external surface, which is now understood to be a much more significant contributor to moisture levels in solid masonry than occupancy related moisture,” he said.
Smith explained that as well looking at the way water vapour behaves, dynamic modelling looks at the behaviour of liquid moisture and the capacity of materials to absorb and store moisture (for example, how driven rain soaking into the masonry is absorbed and re-distributed , or how occupancy related moisture is absorbed and released by the insulation).
Dynamic modelling calculates the conditions in the wall at hourly intervals across as long a time period as necessary, which can stretch to decades, tracing conditions throughout the year, and assessing the risk of problems that might build up over the long term. “This ‘dynamic’ part is crucial for assessing the impact of wind driven rain, solar drying, and solar induced vapour movement, all of which have strong effects on moisture movement which are impossible to assess with a calculation based on monthly averages,” he said.
Analysis under BS EN 15026: 2007 is therefore more comprehensive than under the Glaser method , the previous norm, as this focuses on vapour movement only, on a monthly averaged, steady state basis, he added.
According to John Albon, dynamic calculation methodology “is becoming increasingly significant for applications of this type”, so will now be standard.
“We apply a consistent methodology to all certificates of a given type, so this [methodology] will be reflected in all future certificate. Those already issued will be updated in the future as part of our normal review and reissue processes, and our Certificates will reflect this.”
Simon Corbey of ASBP natural insulation group congratulated NBT on achieving BBA certification for Pavadry. The certificate offered a valuable addition to the options for specifiers, he added. “When a client requires BBA certification , there is now an option to choose a biomaterial-based IWI product.”
The ASBP natural insulation group also welcomed the methodology employed by the BBA. “We are very pleased the the BBA is giving detailed attention to these moisture issues, and employing a methodology which can reflect the distinctive performance of vapour-open, hygroscopic systems.”
John Albon told Woodknowledge Wales that the BBA is looking more widely at the issue of solid wall insulation. This follows reports (including from BRE) of problems, including moisture problems, arising with some mass-scale installations.
“The BBA is engaged in extensive research concerning retrofit insulation installations and a report will be published in 2017,” John Albon said. He added that the BBA were not concerned about the performance of insulation products themselves, “providing they are installed in line with specified requirements”, suggesting that faulty and inappropriate installations are emerging as the main area of concern.
Pavadry is a wood fibre board with a rigid hardboard face to which plasterboard can be mechanically fixed. It comes in large tongue-and-groove profiled pieces which are bonded and mechanically fixed to the masonry.
According to NBT, Pavadry’s innate physical properties help tackle moisture in a robust fashion, requiring less complicated detailing than vapour barrier-based systems.
The main advantages are in moisture management – in particular, not compromising the potential for the masonry to dry to the inside, while also providing capacity for absorption of moisture produced internally: woodfibre absorbs moisture at lower humidity than most other porous materials, so is ideal for protecting embedded timbers behind the insulation, as humidity at the interface of insulation and masonry is kept within healthy limits.
Drying to the inside is important because driven rain is now understood to be more significant than the effects of internally created moisture sources, and the masonry needs to be able to lose this moisture from both surfaces, Matthew Smith says.
“This is common sense if you think about the amount of moisture absorbed from rain on external surface compared to vapour generated inside. It can be seen in all the moisture monitoring SPAB is doing and we are seeing it with our hygrothermal monitoring of the Pavadry installation at Trinity College in Cambridge. It is also very simple to demonstrate in WUFI the massive difference in moisture load.”
The BBA certificate suggests that: “dynamic analysis of moisture levels should be undertaken for each installation by a competent person.” Smith says that as a supplier they undertake calculations on a project specific basis where experience tells them that modelling is necessary to guide the specification, and enable them to warrant the material build-up. The two most important factors are location (ie external temperature and exposure to rain), and U-value target (lower U-values result in lower temperature at the wall/insulation interface, and therefore higher relative humidity.
“A simple tweak to the specification such as a 20mm reduction in insulation thickness can be enough to bring a proposed build up below the threshold for risk, and ensure a healthy humidity for embedded timbers,” Smith explains.
If masonry is unusually thin (less than standard 215mm brick and a half), and therefore likely to suffer from water penetration, protective measures are required which might include removal of embedded timbers, and an alternative, mineral-based system instead of wood fibre, he added. While the natural properties of woodfibre make it an ideal fit for many internal wall insulation installations, it should not be considered a universal solution without adequate specification guidance.
NBT also notes that the system is relatively dense, which can have advantages for acoustic and thermal performance, and emissions are negligible, as natural lignin from the wood is the principal binder, and formaldehyde content is low .