So what is WUFI? WUFI is the acronym for "Wärme- und Feuchte instationär" ("Heat and Moisture transient"). WUFI is designed to calculate the simultaneous heat and moisture transport in one-dimensional multi-layered building components!
YIKES!!!!! How many of your stopped reading at that point! Hands up. I won’t hold it against you :) Ok, so here is what happened in laymans terms.
As you all now the Living House is targeting Passive House certification. Our awesome Passive House consultant, Elrond Burrell from VIA Architecture (he is one of our supporters so I suggest that you check him out, and then get in touch with him straight away as he is a gem!) has been working away really hard on pulling together everything that is needed for our pre-construction review by the also awesome Passive House certifier that we are using, Jason Quinn from Sustainable Engineering (used for work for NASA! – gives you confidence really doesn’t it!)
Well once all our designs officially got into the hands of Jason, he started asking a few questions about our rammed earth walls on the first floor, and if there was any potential for moisture to be absorbed by the rammed earth walls which are currently proposed to be strapped and lined with timber and plywood. The only way to prove that moisture isn’t going to be a problem with these walls is by undertaking a WUFI analysis.
Enter stage right Denise from eZED Ltd. With the generous support of Pro Clima we have been able to engage the services of Denise from eZED who undertook a WUFI analysis for us. In order to analyse, Denise had to accurately build up the composition of our wall structure (right down to the level of information around what is the density of the aggregate that we are using in the rammed earth mix) in the state of the art WUFI software. This software calculates the water movements through each material, based on its material properties and external factors, such as climate (temperature, sun and rain) on the outside and temperature and moisture generated on the inside.
Here are our ‘at risk’ walls. The graphs below with the green and blue lines and areas show the moisture analysis. Relative humidity is in green and water content is in blue.
Unfortunately the results are at this time inconclusive. Poor old Denise has not had quite enough information from us on the finished Rammed Earth product and the results depend on material data which is yet to be determined.
So, at the moment we are in a small world of ‘we don’t know’ which is not where you would normally be with WUFI if you were using a standard wall construction. But since we appear to be one of the first to do rammed earth in this manner in NZ, we are breaking new ground…..again.
The outcomes of the current WUFI are that the biggest concerns centre around the construction type being similar to a Stone/Concrete strap and line, which often suffers from a cold surface temperature that allows humidity to condense as moisture. A possible solution for us is to create a break between the timber strapping and the rammed earth walls. At the moment our internal strapping is designed to be 140mm thick, so we could reduce that to 90mm and form our internal walls as freestanding walls with a layer of continuous insulation against the rammed earth, which should stop any moisture or humidity hitting a cold timber surface. However we also need to make sure that we stop the rammed earth absorbing moisture wherever possible as the rammed earth has structural reinforcing in it (and while it is galvanised to protect it from moisture) we’d rather not have the moisture getting in in the first place). So, to do that we are going to be adding a special, water repelling resin, to the rammed earth mixture. This will mean that any rain will bead on the surface of the rammed earth and run off, rather than being absorbed by it.
A potential solution provided by Denise:
To mitigate the risk for interstitial moisture problems we could treat the rammed earth as a type “cladding” and separate it from the Insulation layer by placing a wall underlay to the outside of the internal framing over a small cavity.