Tags: #architecture #buildingdesign #buildingenclosures #buildingenvelopes #buildingmaterials #buildingproducts #buildingscience #insulation #materialsscience #waterresistance

Here’s proof you can safely design with low-perm WRBs, regardless of climate. It’s all a matter of insulation thickness and placement.

In this series, I show outcomes from 30-day, time-dependent analyses under winter conditions (exterior: 0F, 70% RH; interior: 70F, 40% RH). I’ve selected two wall types – one with the sheathing & WRB inboard of the insulation (XiA) and the other with those components outboard of the insulation (XiB). Neither wall type contains cavity insulation or interior vapor retarders. And both are configured with WRBs having perm values of 10 perms, 1 perm, and 0.1 perm.

Performance is unchanged. There’s no moisture accumulation. So why are we told to embrace high vapor permeance? It’s simple, we insist on hybrid walls with low exterior to interior insulation ratios. This often necessitates needless complexities of interior vapor retarders. We also give short-shrift to exterior insulation. Bad idea. Maximize Xi thickness, regardless of climate. 

Moral of the story: a semi-permeable or impermeable WRB can be used to improve performance in all climates for all seasons. The added benefit is the real purpose of the WRB – improved water resistance.