To ensure our new building’s walls would avoid moisture problems we did what is called an Envelope Analysis to look at the moisture conditions within the envelope (wall and roof systems) of our building.

Strawbale Moisture Content Over Time in Wall System

We hired a consultant who used WUFI software to model conditions in our walls. The model includes our local climate data and a model of our HVAC system, as well as estimates of how our building will get used: occupancy rates, how many meals cooked, showers taken, etc.

Our wall system is not typical as it includes both a stick frame with blow in cellulose and strawbales. Luckily someone had already figured out the moisture properties of strawbales and published it online.

The good news is that the model showed that in general our wall system will be free of moisture problems. The strawbales showed no accumulation of moisture, with moisture declining over time with some seasonal variation. Our roof system, a more standard system with metal roofing and blow-in cellulose also had no moisture problems.

The challenge is that our kitchen and shower areas could have problems with moisture in the cellulose if we don’t add a vapor barrier or do something to control humidity levels to below 65% relative humidity.

It’s probably possible for us to keep our humidity lower through mechanical ventilation (ie range hoods and venting for the dish sanitizer in addition to the whole house HVAC systems). But prudence would suggest that it’s best to design a building to survive even if the humidity does stay high.

The modelling software showed that if we add a 5 perm vapor barrier behind the drywall in the kitchen and bathroom we should be able to avoid any moisture problems. Another alternative would be a 1/2 inch layer of XPS (extruded polystyrene) foam board behind the drywall.

Now we’ll have to search for an LBC compliant vapor barrier but hopefully we’ll find something that works for us.

So what is the ecological alternative?

We are still looking for the perfect product.

One option is Mineral Wool which is made from rock and has an R-value just a little lower than polystyrene. It is waterproof and does not contain flame retardants (it’s already fireproof — it’s rock) but generally does contain added formaldehyde (which is an LBC red list no-no). So far, we’ve found one brand of mineral wool that has no formaldehyde and is rated for under slabs, but it might only be available in Europe.

For our green roof areas we have found a source for reused polyisocyanurate foam board removed from a warehouse near Kansas City. Polyiso has an amazing R-value of R 6.5 to R-8 per inch of foam, 150-200% of what other foam boards provide. So where we have limited space under the green roofs we can use polyiso rather than blow in cellulose and still achieve the overall R-value we want.

But unfortunately polyiso can’t be used in wet conditions like under a slab, and yet we need to insulate our slab to prevent heat from wicking away into the earth. That leaves us looking at expanded or extruded polystyrene (ie styrofoam or pink board) neither of which sounds very eco. Perhaps we can again find a source for reuse, but we may end up having to buy.

One alternative that looks promising for interior use is expanded cork insulation. The same material as in wine corks and bulletin boards can be processed into a rigid insulation board. It’s a renewable product that can be repeatedly harvested from cork oak trees. The only catch is that its currently produced only in Portugal. We’re not sure if the energy for transport is worth the eco savings for this product but if there is ever a local source it sounds great.

Predicted occupancy levels on a “typical saturday”. This is actually more of a worst/best case scenario – more like a community celebration with 80 people in the building at 2pm

Our hope is to produce a model accurate enough that we can size our heating and cooling systems with confidence that they will be adequate in extreme weather, while not being oversized for general use. In addition LEED requires an energy model to compare against how your building would perform if built to normal code and specifications.

But predicting occupant behavior can be quite tricky, especially in a community setting where the occupants will change from day to day and year to year. Can we count on people opening windows when the weather is nice out? When they do so will they remember to turn off the HVAC system? Will they remember to close the windows when the temperature drops or rises? Will people adjust blinds to prevent glare and excessive solar gain?

The software is actually designed to factor in all of these questions, but we still have to set the parameters realistically. Our first round of data from the model showed our highest cooling load of the year on November 12th. The model was not assuming anyone would open the windows or lowering the blinds, so on a unseasonably warm and sunny day in November your passive solar home could overheat and require cooling. Similarly we were seeing small heating loads in July, which just seemed boggling.

In our next round we have tweaked the parameters to prevent such anomalies and are hopeful to have something more realistic. For instance we told the software “no heating in June, July, and August no matter what” and similarly “no cooling November to March”. We also changed our occupancy loads to better match our prediction of community behavior.

All in all it’s not an exact science, but who said building ever was.