Moisture movement through Bildrite and WRB
Hello, I’m looking for WRB advice. Repairing or totally replacing siding. I’ve been reading old articles/Q&A, but I apologize if I’ve missed available info or have failed to grasp basic concepts.
Please poke holes in my reasoning. I understand that adding a wrb should help with bulk water. However, the remaining balance of moisture from any source might have a harder time escaping with the addition of a new layer. The wall appears to be solid up to this point with the exception of where building practices or errors (missing kickout, siding touching roof or close to ground). But I am comfortable needing those repairs after nearly 40 years especially since the source of that water is addressible. Perhaps the breathability of the bildrite has been the wall’s saving grace and I am about to throttle that.
I understand that using plastic wrb is super common, but my fear is that the practice is born out of price/construct ability and that houses of my era getting remodeled are going to have major, slow to develop moisture problems in 10ish years. Sorry for the extremely long post, but wanted to lay out exactly what I am thinking. Would greatly appreciate any advice, correction on the building science, or even just reassurance that I am way overthinking this.
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Replies
Nick,
The ability of moisture to move through a material by diffusion is called permeance, and is measured in perms.
Bildrite sheathing is between 15 and 20 perms depending on the type and thickness. Building paper is between 0.5 perms and 6 perms, but opens up to 60 perms when damp.
House-wraps vary (For example Tyvek is 77 perms, and Tyvek Commercial is 28 perms), but all are at least as permeable as the Biltrite, so adding them won't trap moisture, or impede drying to the outside.
I don't think the adoption of house-wraps came from expediency. They are in most situations simply a better product than tar paper. They don't deteriorate in the heat the way building paper does, don't tear as easily, and can be taped to act as air-barriers. I'd have no reservations about using them in your situation.
Thanks Malcolm! Really appreciate the response. The idea that a housewrap is higher perm than bildrite, and thus not the wall's vapor bottleneck makes sense. The reassurance is helpful too.
A follow up thought/concern, when the dew point is in the wall or even at the wrap layer. If liquid water gets between the wrap/sheathing, how well would it perform its vapor permeable function? Could happen due to condensation during like mentioned or minor leakage of wind driven rain or who knows. My thought is that water really can't drain anywhere in that sandwich, and that vapor cant move through that space when moist. In such a case, a felt is hygroscopic and might help move the liquid water to where it can evaporate vs. being strictly trapped. Perhaps this is an unrealistic hypothetical but how I imagine small accuulmulations of liquid water is troubling.
I do wonder too if there were any bildrite with interior poly houses that were built originally with tar paper or even tyvek from the early days. Or even houses like mine that got renovated 10-15 years ago. Would be curious to know how they've held up!
Nick,
It's a more forgiving wall assembly than the one which has been the standard through much of North America for decades. The only difference being the Biltrite as sheathing where typically plywood or OSB was used.
Dew points in wall assemblies don't matter until the moisture hits what Joe Lstiburek calls the "first condensing surface of interest" If that surface is impermeable then you get condensation. If it is vapour-open (as your sheathing and WRB would be), the moisture just continues on its way.
If there is any increased drying from bulk water intrusion by using building paper, it's inconsequential. The way to make these walls more resilient, both to leaks through the cladding, and moisture from inside, is to add a rain-screen. That improves them exponentially.