Envelope Analysis – How to Make Sure Your Walls Won’t Rot

Moisture can be a nightmare for builders. It’s always wanting to leak in, wick up, or infiltrate into a building and with moisture comes rot and mold.

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

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.