Can you combine two U values for windows on a project?
Where a designer uses a standard single glazed window and then adds a second layer of glass or danpalon product behind this window, how do we combine the U values on a project?
In short, its very difficult for Assessors to combine U values on a project.
The main challenge here is about the ability of the private certifier to get a certificate from your window supplier for a combined system value with your two systems / dampalon product. As it is not a proprietary system your window manufacturer would have to certify this combined system and provide these values to the Assessor if it indeed was an enclosed sealed system. If you could even find a window manufacturer willing to do this for you, it would add considerable cost to the project as it may also have to be approved by the WERS regulatory body.
As an assessor I am able to combine R values for wall elements, but I am not able to combine U values for window elements, due to the fact that the certifier at OC will require a certificate for the overall systems' values from the window manufacturer.
Thus in an ideal world what would happen in this situation, is that the client or the Assessor would be able to get access to the total U values for this proposed danpalon and window module system confirmed from a manufacturer so the Assessor could then input this into the thermal model.
One possible solution is that the project is rated with the highest U value component which is often the danpalon product. The Assessor would use a default value in our system that permits this danpalon product to be installed and rates the window, this would mean using a default window with with a higher U value and a SHGC within 10% (NSW) or 5% (All other States and territories), that is within 10% of the supplied danpalon or better performing layer specifications.
Many Assessors would treat the danpalon / second skin product like a second layer or like a curtain. This would be unfortunate as you would then receive no benefit from the this element in Australia as the regulations for NatHERS assessments prohibits the use of modelling second skins for windows such as curtains and pelmets.
The good news is that we can still rate the project with the danpalon or second unsealed glazed product, and consider it as the window proper which would give the better overall performance. You should then receive a signoff from the certifier if this danpalon product was installed at OC. The additional pane of glass theoretically will be regarded as having no impact on the model or compliance.
As a designer myself I applaud the endeavours from others to break the mould. Breaking new ground in the Energy Assessment industry is not unlike the building industry as a whole, a slow and difficult process.